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Exner Expert 19,174 posts. Neil passed away 18 Sep 2015. You will be missed, Neil!
Posts: 19146
   
Location: bishop, ca |
This article is from the 10/03 Auto Restorer, and the formula for determining the
proper size (CFM) for your carb(s).
People like to brag about the CFM's that their "Double-Pumper" is producing, but,
in all likelihood (of course, not in YOUR car...) "their" carbs are flowing too much
fuel for their engines to consume efficiently.
The proper CFM formula is simply : engine CID x engine's maximum RPM that you're
going to be running = ??? divided-by 3456 = !!!! CFM @ 100% efficiency x 85%
real-world efficiency = #### CFM that your carb(s) should be providing.
E.G.: for my D500 (which is bored out to 330 c.i.) : 330 x 5500 = 1,815,000 divided-by
3456 = 525.17 x 85% = 446 CFM.
At 5,000 RPM (more reasonable in the real world: 406 CFM.
BTW, in 2003, a US gallon of Premium cost around $2.00
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Expert 5K+
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Location: Big pimpin' | I like to experiment with real world experience, not believing what I read in mag. articles, but tis fun to work out a few formulas on the desk here.
However, change only a few numbers like the RPM range and CID, and you'll find the 'ol "Double Pumper" will come into play yet again.
Edited by dukeboy 2012-08-24 6:14 PM
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | how do you calculate to factor in a supercharger? |
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Expert 5K+
Posts: 8443
Location: Perth Australia | Well, by that my poor old dodge will be excessivly over carbed when its all going again.
2 x 500cfm eddies on a 318 poly
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Exner Expert 19,174 posts. Neil passed away 18 Sep 2015. You will be missed, Neil!
Posts: 19146
Location: bishop, ca | Mick; it will be grossly over-carbureted, but your car will probably operate fine, on
the rear carb's primary throats, and on the front carb's primaries.
When you put your foot into the carbs, there may be too much gas delivered,
depending upon your camshaft's grind and the engine timing (remember the
discussions on how to hot-rod a distributor, and to select the proper vacuum advance
for it?).
Here's the rest of the article, by the obviously-know-nothing author.
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | i'm seeing that EXACT same formula *all over the place* on the internet, it obviously can't be coincidence...
(okay, some divide by two and then divide by 1728 instead of dividing by 3456, but algebraically IDENTICAL...) |
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Expert
Posts: 3396
Location: Chestertown, NY ( near Lake George) | Cylinder head design and camshshaft timing also have a great effect on carburetor cfm requirements for maximum performance, using that formula my Max Wedge race car at 426 cu in would be way over carbed with 2 -750 cfm Carters, not so in this case, as the formula works out to about 750-800 cfm.
Edited by Shep 2012-08-24 8:19 PM
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Expert 5K+
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Location: Perth Australia | Very informative article.
But, it does not take into account any engine mods (which is fine for the car he is doing).
I am not sure of the total recipe for my engine yet and I have probably started at the wrong end, but, The basics of what I
want out of my car and how it will be used will determine how it ends up.
To be totally honest, the engine is getting dual carbs mostly because I like the look of them.
I will be trying to set the engine up to run relatively better than stock and I am planning a cam change and other
improvements that I find and/or can afford.
I am not looking for monter performance or an engine that tries to throw you out the window at idle, just a good street engine
with a bit of poke.
I believe the 57/58 dual quad plymouths were carbed at about 800cfm total, so I will be 200cfm above that, but I am not
totally sure of engine displacement (318 or 350?).
Still a bit of research needed on my part.
At this stage, my engine is on my engine stand (Split the tranny off it last night) ready for another bath before I start pulling it
apart to see whats inside, but this is more for my "my barn find" thread.
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Expert
Posts: 1352
Location: Springville, CA | Gearspear,
rule of thumb: at 8lbs boost, volumetric efficiency = approximately 125% to 150%. Also, it's kinda hard to over carb a blower, 'cause it stuffs the mixture in and the vacuum signal is very large!
Loyd |
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Expert
Posts: 3034
Location: N.W. Fla. | GearSpear - 2012-08-24 5:27 PM
how do you calculate to factor in a supercharger? I've seen one, had an adition to the one above involving the CID of the blower. |
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Expert
Posts: 3034
Location: N.W. Fla. | An easy one to remember is CID X 2 X .8+ CFM In reality many factory engines are over/under carbed. 2 Ford examples is the 351C-2V with a 2 bl of only 350 CFM & the 302C-4V with a 750 Carter Thermoquad. |
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Exner Expert 19,174 posts. Neil passed away 18 Sep 2015. You will be missed, Neil!
Posts: 19146
Location: bishop, ca | Mick, your camshaft grind, the timing, and the porting/manifolding/exhaust system of your engine will complement or restrict
the efficiency of its fuel consumption.
Progressive linkage is your friend. You may well be able to cruise around on the primaries of both carbs, getting better
MPG than a single-4 carb, which has larger secondaries than the second-carb's primary throats.
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | i'd say that ttotired stated it best...
MOST people, or should i say *ALL* of them, that are putting "huge" carbs on top of their motor, "crowning their motorhead", are doing so *NOT* because the engine "needs it", but because they LIKE THE LOOKS OF IT...
the WCFB sitting on top of my '61 Hawk Stude 289 is only spec'd at 385 cfm...
the engine redlines at 5200, so that calculates out to a 370 cfm target by the formula at 85%, or 390 cfm at 90%...
i've never had ANY problems with her being "under-carbed"... 385 really does seem to be "enough"...
(i've had problems with high-idle dropping out sooner than it should, but never had any "under-carbed" issues...)
the same exact engine is used in the blown '63 Avanti R2 - she's stock with an AFB spec'd at 625 cfm...
so do you use 125% to 150% in the formula to estimate proper carb size?
that would calculate out to 540 cfm (125%) to 650 cfm (150%)... |
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | i think back to the SonoRamic Commando cross-ram set-up...
"tuning" proper induction really is an *EXACT SCIENCE*...
i personally have ZERO doubt that "over-carbing" results in INEFFICIENCIES...
just as experimenting with "short tubes" and "long tubes" on the SonoRamic made *DRAMATIC* differences in performance...
but to what point is a carb "too big"? if the formula calculates out to indicate 400 or so, obviously a 600 cfm is not going to hurt anything, but let's be honest, i highly suspect that a 950 cfm double-pumper would be a "waste of money", even if she "looks pretty"... |
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Expert
Posts: 1352
Location: Springville, CA | The major problem from over-carbureting is that the vacuum signal is severely reduced at higher throttle settings a due to lack of airflow through the carburetor venturi. This means that the air-fuel mixture becomes increasingly lean at higher throttle settings. Most people try to correct this by going to larger jets, but this will not work as there is not enough vacuum to pull additional fuel through those larger jets. With a properly sized carburetor you should not have to change jets by more than one or two sizes to correct a mixture probem.
Loyd |
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Expert 5K+
Posts: 8947
Location: WHEELING,WV.>>>HOME OF WWVA | two cars leave the factory with the same C.I. motor . one has a 4 bbl , the other a 2bbl . all internals in both are the same . the only exception to the similarities , one has dual exhaust ! the question
is which one is mis-matched .............. which theorem is max cfm and which is minimum cfm ? --------------------------------------------later |
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Exner Expert 19,174 posts. Neil passed away 18 Sep 2015. You will be missed, Neil!
Posts: 19146
Location: bishop, ca | What are the CFM on both carbs, in this hypothetical?
Doesn't the factory show different HP and RPM ratings for the engine, with either of the two carbs
installed on it?
Edited by d500neil 2012-08-25 6:57 PM
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Expert
Posts: 1352
Location: Springville, CA | the answer is neither! |
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Expert 5K+
Posts: 8947
Location: WHEELING,WV.>>>HOME OF WWVA |
Mopp 1206 000 Dyno Test Carburetors On Our 383 Six To One
Carburetor Dyno Test - Six To One
Dyno Testing Carburetors On Our 383...
From the July, 2012 issue of Mopar Muscle
By Dave Young
Photography by Dave Young
We've all heard rumors and opinions about what size, style, or manufacturer's carburetor is better for the engine in your Mopar. At Mopar Muscle we've tried just about every combination on the cars we drive and race. And while we feel we have a pretty good idea what size carburetor works well for a given combination, a comment we often hear at cruise night is that a certain engine is "over- or under-carbureted." You may have your own opinion about what size carburetor would be best for your Chrysler engine, but an engine dyno is the only real way to see which size carburetor makes the most torque and horsepower. This month we'll test six different sized Holley four-barrels on a Mopar big-block to see which performs best.
Since we had a 383 Chrysler big-block on the Superflow engine dyno at Auto Performance Engines, we decided it would be a good time to settle an age old question: What size carburetor is best for the Mopar 383, the most widely produced high-performance big-block? From the factory, the 383s with the highest horsepower were rated at 335 horsepower and came equipped with a 650-cfm Carter AVS. Our 383 is basically the 335 horsepower combination, with flat-top pistons and 906 casting cylinder heads. At just over 10.0:1 compression, this is a great pump gas engine, which we've modified in a similar way many 383s are modified.
In stock form, the big-block Mopar engine is one of the toughest engines around, and pre-1972 versions all have a forged steel crankshaft. Our 383 is a '68 version, with a forged crankshaft, windage tray, stock oiling system, and stock connecting rods with ARP bolts. The only deviation from stock is the .030-inch overbore and appropriately sized flat-top pistons. We topped this engine with 906 heads that were treated to a multi-angle valve job, new valveguides, and Comp valvesprings to match the .500 inch lift flat-tappet camshaft. Knowing this combination would like the upper rpm flow of a single-plane intake, we chose an Edelbrock Victor 383 intake manifold.
A carburetor's size is dictated by a rating called cubic feet per minute (cfm), which is the amount of air the carburetor is capable of flowing at wide-open throttle. The cfm requirements for any given engine can vary depending on a lot of factors, so choosing the correct carburetor from the huge variety available can be a daunting task. It would also be a daunting task to test all of the carburetors available for your Mopar on the dyno, so we narrowed it down to six different Holley models between 650 and 1050 cfm. In our experience, carburetors from Holley, Carter, Edelbrock, Quick Fuel, and other manufacturers all work very well if properly tuned. For the purposes of this article, we'll be testing all Holley carburetors since we had a variety of that brand on the shelves. Follow along and we'll show you which size four-barrel carburetor performed best on the big-block we tested.
1. Holley 650-cfm vacuum secondary
Starting small and working our way to carburetors with higher cfm ratings, we installed a Holley 650 vacuum secondary unit on our 383. It may look a little rough, but it works fine. This is the smallest carburetor we'll test, and is the same size that the factory installed on '68 383 HPs like the one we're testing. Using an Innovate Motorsports oxygen sensor to monitor air/fuel ratio, we made our first pull, but power was down from our initial break-in pulls. This is a clear indication that our 383 wanted a larger carburetor, like the 750 we used for break-in. On its best pull with the 650, our 383 made 368 lb-ft of torque and 356.9 horsepower.
2. Holley HP 750-cfm mechanical secondary
The next carburetor we bolted on our 383 was a Holley 750-cfm unit with mechanical secondaries and both front and rear accelerator pumps (hence the term double-pumper. And while the extra accelerator pump and mechanical secondaries of this unit won't necessarily help peak dyno numbers, the 750's larger cfm rating means our big-block should get more air and fuel, and make more power than the 650 did. Our first pull with the 750 showed definite improvement, and the best dyno pull with the 750-cfm carburetor installed netted 391.6 lb-ft of torque and 387.6 horsepower.
3. Holley 780-cfm vacuum secondary
Stepping up a little in size, we next bolted a Holley 780-cfm 3310 series four-barrel onto our 383 to see if it would further improve our engine's torque and horsepower. Vacuum secondary carburetors are known to be more economical than carburetors with mechanically actuated rear throttle blades, and arguably improve drivability as well. And while neither of those factors matter much while attempting to achieve the best power and torque on the dyno, they can be important once the engine is installed in a car. We figured the 780 would perform about as well as the 750 and it did, but at 381.3 lb-ft torque was down by ten. Horsepower was also slightly lower, at 380.5, so our engine liked the 750-cfm Holley HP carburetor a little better than the 780. Are we verifying that you can over carburerate an engine? With 30 extra cfm for this portion of the test, it seems so. But one drawback of this carburetor is the lack of a rear metering block, making precise air/fuel adjustments more difficult. So without the tunability of rear metering, we'll have to see what happens when we go larger.
4. Holley HP 850-cfm mechanical secondary
Taking the next logical step, we decided an 850-cfm carburetor may just be what we needed to break the 400 horsepower barrier with this engine. Bolting a Holley HP series 850 to our engine, we made a couple of pulls and adjusted jetting for the proper air/fuel ratio. This carburetor improved torque and horsepower at every rpm, and sounded crisp and responsive. Even so, it fell a little short of our 400 horsepower goal, but it reached the one horsepower per cubic inch benchmark by making 393.2 lb-ft of torque and 388.2 horsepower. Our data indicated that this engine liked the 850-cfm unit, and we'd already gained more than 30 horsepower over our baseline with the 650 installed.
5. Holley 850-cfm mechanical secondary
We had another Holley 850 on hand, so we decided to see if there was much difference between two models of the same size. With a list number of 4781, this 850 is a non-HP version, and has a choke horn installed whereas the 850 HP doesn't have a choke horn. Otherwise, these carburetors are very similar, both featuring dual metering blocks and accelerator pumps. After tuning this carburetor's main metering circuit through a series of jet changes, we found that 87 jets in the front and rear, with the power valves blocked off, made the most power. On Auto Performance Engine's dyno, our 383 big-block with the Holley 850 double-pumper made a best pull of 392.5 lb-ft of torque at 4,500 rpm, and 401.4 horsepower at 6,100 rpm. Since our 383 seemed to like it each time we bolted a larger carburetor on, we decided to see if we could over-carb the engine with a Dominator.
6. Holley 1050 mechanical secondary (modified by Quick Fuel Technology)
Knowing that we'd likely reached our point of diminishing returns, we decided to test a 4500 series Holley 1050 Dominator that had been modified by Quick Fuel Technology just to see what would happen. For our mildly modified, relatively small-displacement big-block, a 1050 Dominator is a lot of carburetor, but since our results so far showed this engine liked larger carburetors, we decided to give it a try. Of course, the 4500 series has a larger flange than our intake, so we utilized a two-inch tall tapered spacer to make it fit. After a little tuning, it was apparent that although the 1050 ran well on our engine, it didn't make quite the power of the 850. On its best pull during this test, our 383 made 393.4 lb-ft of torque and 398.9 peak horsepower, indicating that while the 1050 is a good choice for many big-blocks, it was just a little much for our 383.
Results
The results of our testing make it apparent that the 383 we tested liked the 850-cfm size carburetor better than the others tested. And while the 1050 Dominator did seem a little much for the 383, it certainly performed well, making nearly the same horsepower, and slightly more torque, than the 850, but is a substantially more expensive carburetor. As a result of our testing, we determined that larger carburetors generally do make more power on a big-block like ours, and that proper carburetor tuning is an important factor as well. And, while we were testing for peak power and torque, our results show that even with a smaller carburetor, the engine made good power. Since unleaded gasoline is still over three bucks a gallon and smaller carburetors are known to be more economical for daily driving, you may want to choose the carburetor for your car based on economy, not peak power. So when is too much carburetor too much? When the dyno numbers stop going up!
Read more: http://www.moparmusclemagazine.com/techarticles/engine/mopp_1207_ca...
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Expert 5K+
Posts: 8947
Location: WHEELING,WV.>>>HOME OF WWVA |
Counting Carbs - Carburetor Comparison
Choosing The Right Carburetor For Your Engine
From the July, 2005 issue of Mopar Muscle
By Kevin Shaw
Photography by Courtesy of the manufacturers
0507 MOPP 01 Z CARB
Now, we here at Mopar Muscle are all about big cubes, low track times, and nasty plumes of rubbery smoke clogging up the air, but, at the same time, our nonsensical, barbaristic shenanigans need to be tempered. While building a crazy, rip-snortin' engine isn't rocket science, it sure isn't as easy as burning toast.
Take carburetors, for example. They are basically like the nozzle attachment to a really big shop vac. The right nozzle will give you maximum concentrated airflow, while the wrong one will be too restrictive or simply not focused enough. If you bolt on a carb too restrictive, you'll be starving your engine of the necessary air/fuel mixture that it needs, and, transversely, if you slap a 1,150-cfm Holley Dominator onto your 318, you'll be drowning your small-block to death with too much anatomized air versus a properly mixed balance.
What we're looking to do is offer you a basic (the key word here is basic) guideline in choosing the proper size carburetor for a more-than-stock Mopar. For those of you who are restoring your Mo' Power, you already know what you need, the factory already did it for you. But for the rest of us who are a few fries short of a Happy Meal, this is for you.
You need to know what your engine is asking for. Meaning, what is the optimal airflow mixture needed for your specific engine? Here's how to find out. First, you need to come to grips with what kind of engine you've built: a stock/street configuration, a high-performance engine, or a purebred race engine. With that in mind, we've provided the mathematical equation to determine what cfm you will need based on 1) your engine's cubic inches (ci), 2) the maximum rpm in which ideal airflow ought to occur, and 3) the percentage of volume efficiency (ve).
ci x maximum rpm = ideal cfm
3456
ideal cfm x ve = real cfm
If this looks too much like high school algebra, let's do an example together. Let's say we have an imaginary 440 that makes its optimal power at 6,000 rpm. But the 440 is going to be used for street and strip use, making it a high-performance engine. High-performance engines use roughly 85 percent to 90 percent of its air volume intake at full throttle; stock/street engines use 80 percent to 85 percent; race engines use 90 percent to 95 percent. Engines using 100 percent or more are usually supercharged or turbocharged, and we're nowhere near that neighborhood.
So plugging in our variables, let's see how our numbers come out.
440 (ci) x 6,000 (rpm) = 764 (ideal cfm)
3456
764 (ideal cfm) x .90 (ve) = 688 cfm
What this means for us is that our imaginary street 440 Wedge requires a 688-cfm carburetor. Yet, knowing that nobody makes a 688-cfm carb, what are we to do?
Going too small strangles our engine, so that's not an option. going to a 700-cfm or 750-cfm carb is our best bet. But how much larger can we go before the engine suffers? That is ultimately based on the intended use of the engine, coupled with the driveline characteristics and weight of the vehicle.
If the vehicle is lightened for race-readiness and stocked with steep rear gears and a performance-oriented transmission, then the combination will allow leeway in the final airflow intake. A slower, heavier, street driving-bred vehicle will fall short of needing the extra cfm and, ultimately, do little good. The high-performance application will push the air volume efficiency more towards the 100-percent mark, pushing the final cfm number more towards our previous ideal cfm number. That is why Dick Landy's cross-rammed Darts were sucking in 1,560 cfm with 426 cubes, while the massive 440 Dodge Polara needed only a meager 625 cfm to push its hulk along.
Now that we've determined what cfm our motor requires, we need to choose what kind of behavior we'd like our engine to exhibit. The carburetor regulates throttle response, launch, and idle speed, making for a difficult decision. With a four-barrel carb, options are nearly endless. In keeping with our theme of boiling down selection, we've reduced this choice to two: vacuum secondaries and mechanical secondaries. due to the vast array of available intake manifolds with either standard Holley or Carter bolt patterns, the brand of carburetor you choose for your motor is solely based on 1) what look you want 2) what performance you desire, and 3) what intake you choose.
Carburetors with vacuum secondaries are usually found in stock applications. The vacuum produced by the engine sends a reading or vacuum signal to the carburetor, alerting it of any drop in engine vacuum. If a drop occurs (during acceleration or excessive load), the rear barrels will open up to ingest additional fuel and air, combined with the running front barrels. Vacuum secondary carburetors offer the best average fuel economy since the rear venturies are only functioning a fraction of the time. Yet, many find the lag in reaction from vacuum secondaries detrimental to quick acceleration and response. In addition, most high-performance engines with aggressive camshafts sometimes fail to produce the necessary vacuum to operate the vacuum secondary carbs properly.
This leads us to mechanical secondary carburetors. Typically referred to as double pumpers, mechanical secondary carbs operate their secondaries off the throttle linkage. In stock applications, they usually are set to respond at a 40-percent throttle, allowing for mild acceleration and hard launches all from the same carb. It is undoubtedly why most street racers and competitors demand the double pumper. For more insistent launches, a double pumper can be reconfigured to have all four venturies flowing equally with no lag between forward and rear barrels. this carburetor setup will offer less than stellar fuel efficiency, but that's the price you pay for old school horsepower.
All of this math and ado is for squeezing the most optimal performance out of your Mopar. One needs to pay attention to the needs of the motor built. If aftermarket heads, cams, or intakes are being used, your engine is going to want more than the equation demanded. We're not saying it's hocus pocus or black magic, but having a definite feel for what the engine will demand is a fine talent to have. A bigger carb might not be necessary, but if you have the motor for it, then, by all means, go crazy.
Read more: http://www.moparmusclemagazine.com/howto/mopp_0507_carburetor_compa...
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Expert 5K+
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Location: WHEELING,WV.>>>HOME OF WWVA | John Dickey: Picking the Right Carburetor for your Engine Combination
Dragzine
by Dragzine on July 23, 2010
Author and President of AED Carburetors, John Dickey
About John Dickey and AED Carburetors
AED Performance was founded with one basic premise in mind; manufacturing of high quality components at competitive prices. Their background includes over two decades in the automotive aftermarket field in design, building, and research and development of high performance components. AED manufactures a complete line of fuel system components, including carburetor kits, billet & birchwood spacers, CNC billet metering blocks & billet baseplates, and replacement parts. Their R&D facility consists of 3 flowbenches, Mustang dual eddie current chassis dyno, Superflow engine dyno, test engines, CAD-CAM design modeling equipment, and complete CNC machining facility.
Choosing the Right Carburetor
Picking the right Carburetor size for your car & engine combination is paramount in being successful – whether you’re a professional racer or just want the right carburetor for you’re Saturday night special. There are three main parameters that correctly determine your baseline selection. From there you need additional information to fine tune that selection. Let’s start with the big 3.
1 – Cubic Inches
First on the list is cubic inches. Simple parameter for total cubic inches, including any additional bore and stroke.
2- Next is Total RPM Range:
Here you need to know both the minimum and maximum RPM range. Minimum rpm is just as critical as maximum rpm. Foremost, you need to be concerned with the minimum rpm that you have to accelerate from. Several additional pieces of information come into play here – such as transmission type, and converter flash rpm. For example, the exact same engine in a car with a 3500 rpm stall converter will want less carburetor than one with a 5500 rpm stall converter.
It’s all about acceleration – a larger carburetor that might make more torque & power on the dyno will usually run slower ET’s if the engine can’t accelerate properly from the minimum rpm. Remember, engines are not happy when they run below max torque rpm! That’s why the correct converter should flash to roughly 100 to 200 rpm above max torque rpm. Gear ratios also play a role in this calculation, as the car weight does. The heavier the car or higher the gear will affect the time spent in different rpm bands. This would require a carburetor sized smaller than a lighter car, with more rear end gear (numerically higher).
Camshaft & the cylinder head (intake port volume) also plays a key role in the rpm range. In most cases the camshaft and cylinder head dictates the rpm range. Here’s where you can get confused easily. In many cases, the bigger the cylinder head (intake port volume) the less carburetor the engine wants; while assuming two similar engines with the same rpm range. A good example is a standard small block Ford & small block Chevrolet. The Ford factory head volume being smaller requires more carburetor than a typical Chevrolet. We have proven this through dyno (engine & chassis dyno) as well as racetrack testing. It comes down to port velocity & choke points (usually pushrod area on head). The induction track only wants one choke point to control velocity, which is usually the cylinder head or carburetor.
3 – Power Level
The third consideration is horsepower. A lot of the previous questions are about determining how much power your engine makes & at what rpm; now let’s get technical & crunch some numbers. Your engine needs a certain amount of airflow to reach a certain power level efficiently. From our 25 years of dyno experience, we’ve found that it takes 1 cfm to make between .7 & .8 horsepower. The more efficient the engine, the less airflow it requires to make the same power.
AED has their own in house CNC machining equipment for aiding in their carburetor designs
The basic formula for carburetor sizing & CFM consumed is (CFM = CID x RPM x VE ÷ 3456). Here CID = Cubic Inches; & VE = Volumetric Efficiency. This is just a rough estimate, as the VE number is the basic efficiency or cylinder filling of the engine. This formula should only be used as a starting point on carburetor sizing, then all the other factors we are discussing will move this number up or down. Let’s take a typical small block Chevy 383ci stroker engine that makes max power at 7000 rpm (assuming 108% VE), plug in the numbers & you get a CFM requirement of 838 on an engine that should make between 590 & 670HP – using our above formula for horsepower per cfm used.
Street motors might be 90% and a great race engine might be 120% (Volumetrically Efficient). The basic “cfm formula” can be figured out simply, but it’s just a piece of the puzzle and all of the previous observations are equally as important. We have found that you would be better off using a carburetor that flows this amount of air at a lower pressure drop (less restriction) like 1.2? to 1.3? of manifold vacuum at max rpm. This usually makes more power, but might affect acceleration in a negative fashion. A good example of this is a light car with a loose converter & plenty of gear – with a narrow rpm band – that might actually run better with closer to 900 cfm. This is only if the carburetor is setup correctly with maintaining the correct fuel curve & proper signal to the booster, which will sense airspeed changes efficiently. Have we confused you yet? Don’t forget all the other factors that go into carburetor selection mentioned above.
Explaining the CFM myths in more depth – Real Carburetor Airflow Numbers
Years ago the Society of Automotive Engineers provided a standard for airflow that’s still used today. For 4-barrel carburetors that number is measured airflow at 1.5? pressure drop (manifold vacuum in inches), or used on a flow bench is 20.4? water. For example, a 750 cfm carburetor should flow 750 (cubic feet per minute) at this pressure drop. A carburetor has fuel flowing through it, which actually displaces some airflow (usually around 8%) so this should actually be flowed with fuel or wet-flowed.
In 1990 we built our own custom computerized “Wetflow-Bench” to properly measure carburetor airflow & fuel flow. We can measure CFM, pressure drop, air/fuel ratio, pounds of air, pounds of fuel, and correct for weather conditions. We can also wetflow the carburetor throughout the entire rpm range, giving us insight to the complete fuel curve on all throttle bores. The problem comes when some carburetor manufactures do not use SAE numbers. We’ve had many carburetors through the years claiming to be one number and some would be well over 100 cfm less. Some manufactures would use 28? of water to flow carburetors, and hence the totally unrealistic claims. We don’t play the airflow games as we feel it’s a real disservice.
As you can see, determining the correct carburetor selection isn’t easy. It’s really part science, part knowledge, and part experience. """""There is no real formula for absolute carburetor size""""". I’ve worked on a spreadsheet for years that won’t always work. The proliferation of all the new cylinder heads and manifolds makes selection even tougher. Bottom line is always get good knowledgeable help when selecting carburetion, just like a good camshaft they can tie you’re whole program together properly & make a big difference in performance. It’s always less expensive to do it right the first time.
I hope this helps in getting a better handle on the important things to consider when choosing the right carburetor for your application. If we can be of any help we have a great tech department that can supply the correct AED Performance carburetor for your specific application & budget. Check us out at www.aedperformance.com or call us at 804-271-9107.
Sincerely,
John Dickey
AED Performance
jd@aedperformance.com
Edited by 60 dart 2012-08-26 2:33 PM
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | some very good reads...
and "ecstatic" to see that the 1050 fared WORSE OFF than the 850...
making me wonder if all of the "dualies" i see at car shows are also WORSE OFF  |
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Expert
Posts: 3396
Location: Chestertown, NY ( near Lake George) | Mid range performance and driveability are always sacrificed with any " over carbed" issue, 2 small 4bbls with progressive linkage running on the rear carbs primarys are the best option in any case with a stock or near stock engine. I would add, if one insists on dual quads.
Edited by Shep 2012-08-26 8:13 PM
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Expert 5K+
Posts: 6203
Location: Big pimpin' | Yeah, and the same MM Mag did a shootout with intakes. Wanna guess which one made the MOST TORQUE AND HP???
Hint: It starts with a "T", and it's wasn't a "Torker" either. Out performed a 1050 Dominator on a single plane intake too! I was impressed. just goes to show ya' not everything that looks good on paper works out that way ion the real world.
Edited by dukeboy 2012-08-26 7:48 PM
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Expert 5K+
Posts: 6203
Location: Big pimpin' | Copied and quoted from another board by my buddy Ron.
Here's some data to chew on....several builds coming off the same dyno, with the same dyno operator (Dwayne Porter) using the same correction factors. Translation ; the results are DEAD NUTS reliable....no overinflated magazine bulls**t !
I've yet to see one of these magazine beauties make it into a car to back up the horsepower claims with a timeslip. I guess that makes me a skeptic....oh well it is what it is
Here are some engine combos with dyno numbers/CFM @ peak HP to illustrate ;
(1) 446 E-head > 535hp/540tq with 830cfm dp'er (Proform 750DP) = 760cfm
(2) 493 E-head > 563hp/592tq with 850cfm dp'er (Holley 4781 850DP) = 785cfm
(3) 572 Indy head > 715hp/720tq with Quickfuel1050 (4150) dp'er = 930cfm
(4) 572 Indy head > 722hp/725tq with Holley 1150 Dominator = 960cfm
The comparison between number 3 & 4 are interesting ; this is my 572 and we tried to see if the engine really wanted more carb. Despite a 100cfm increase in carb sizing, going from a 1050 (4150) to an 1150 Dominator it only made an additional 7hp even though the engine was ingesting 30cfm more air. What I learned from this was that throwing more carb at it doesn't allways translate into bigger numbers....and lots of air equals a small amount of power if the engine doesn't need it. In most cases if the carb is sized appropriately going bigger will just soften up the throttle response and make the driving experience less enjoyable.
I can say that the smaller 4150 Quickfuel 1050 was waaaay more responsive than the Dominator. Smaller venturies mean increased airspeed and sharper throttle response.....allways has, allways will !
I'm not even sure that 7hp would even show up on the timeslip at 4100lb raceweight and there was no way i was going to trade off a razor sharp throttle to find out.
**So if after reading all that you're still convinced that your stock or mildly modified 425-450hp Big Block still needs an 850 carb then i guess my response would be..... good luck with that. |
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Expert 5K+
Posts: 8947
Location: WHEELING,WV.>>>HOME OF WWVA | couldn't one still explain a 7HP increase as optimum performance ?-----------------------------------------------------------later |
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Expert
Posts: 3396
Location: Chestertown, NY ( near Lake George) | Not when it comes at the very top of the rpm range and kills mid range throttle response. |
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Expert 5K+
Posts: 6203
Location: Big pimpin' | 60 dart - 2012-09-05 12:59 PM
couldn't one still explain a 7HP increase as optimum performance ?-----------------------------------------------------------later
Depends on how you want it to run to achieve that 7 HP. just because there's a cam out there that will make 700 Hp with your 361, doesn't mean you will like driving it to the cruise night. |
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | and besides, when it comes right down to it, i'm sure we would *ALL* rather have LOW-RPM *TORQUE* than hi-rpm "horses"...
"horses" are for 'marketing', to "sell" a car to a FEMALE, lol...
"horsepower sells cars, torque wins races"...
didn't Virgil Exner use to say something along the lines of "Horsepower is how hard you hit the wall, torque is how much of that wall you take with you", or something to that effect...
does anyone remember the actual Exner quote? |
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Elite Veteran
Posts: 1060
Location: Building incorrect cars since 2000!! | It looks to me someone had to much CFM's in there highlighter when they went to town on those articles. Looks like Stevie Wonder mark those babies up :laugh |
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Expert 5K+
Posts: 6203
Location: Big pimpin' | 60 Plymouth - 2012-09-07 9:37 PM
It looks to me someone had to much CFM's in there highlighter when they went to town on those articles. Looks like Stevie Wonder mark those babies up :laugh
That's CFC's there skippy..LMAO!
Edited by dukeboy 2012-09-08 7:27 AM
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Expert 5K+
Posts: 7400
Location: northern germany | GearSpear - 2012-09-06 8:02 PM
"horsepower sells cars, torque wins races"...
i never heard that before! maybe in a class that does not allow transmissions?
actually when it comes to racing the saying goes: "do not trade an ounce of horsepower for a pound of torque" |
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Exner Expert 19,174 posts. Neil passed away 18 Sep 2015. You will be missed, Neil!
Posts: 19146
Location: bishop, ca | Ask and ye shall receive....
Enzo?
http://www.starquestclub.com/forum/index.php?showtopic=116574
Or Carroll?
http://www.gencoupe.com/engine/27054-horse-power-sells-cars-torque-...
Or both?
http://www.yellowbullet.com/forum/showthread.php?t=214901
Edited by d500neil 2012-09-18 8:09 PM
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | 1960fury - 2012-09-18 7:12 PM
actually when it comes to racing the saying goes: "do not trade an ounce of horsepower for a pound of torque"
did you pluck this out of you hind quarters?
i have NEVER heard ANYONE say this before...
and i can't seem to find it ANYWHERE via Google either... |
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Expert 5K+
Posts: 6203
Location: Big pimpin' | GearSpear - 2012-09-19 6:05 AM
1960fury - 2012-09-18 7:12 PM
actually when it comes to racing the saying goes: "do not trade an ounce of horsepower for a pound of torque"
did you pluck this out of you hind quarters?
i have NEVER heard ANYONE say this before...
and i can't seem to find it ANYWHERE via Google either...
As with most of what 1960fury says...... |
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Expert 5K+
Posts: 7400
Location: northern germany | d500neil - 2012-09-18 8:06 PM
Ask and ye shall receive....
well i did not ask but you can find absolutely everything in the www if you search for it. just for fun i googled "the world is flat" and i actually found proof that this is true
Edited by 1960fury 2012-09-19 9:56 AM
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Expert 5K+
Posts: 7400
Location: northern germany | dukeboy - 2012-09-19 7:51 AM
did you pluck this out of you hind quarters?
i have NEVER heard ANYONE say this before...
and i can't seem to find it ANYWHERE via Google either...
As with most of what 1960fury says......
says the undisputed heavyweight champion of talking bs. whats up? still frustrated that you can't even change a b&t dust boot without bringing it to a machine shop? 
Edited by 1960fury 2012-09-19 9:55 AM
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | 1960fury - 2012-09-19 9:40 AM
you can find absolutely everything in the www if you search for it.
i tend to agree...
but that doesn't change that when i searched for your quote, i couldn't find it anywhere
but "horsepower sells cars, torque wins races" can be found all over the place
keep me out of this squabble between the two of you, i'm just sayin' i can't find your quote anywhere else...
my relatives were fairly big names in racing back in the 50's (for Studebaker)...
all i'm sayin' is that i'm not "new" to racing slogans and EVERY racer i know will take a pound of torque over an ounce of horsepower any day...
horsepower figures really are for SELLING CARS to men who want toys, not to men who will PUT THEM ON THE TRACK...
the torque aspect is part of the fame of the beloved Slant Six 225...
it didn't have high horsepower numbers, but the torque curve on that bad boy was extremely impressive...
not that the Slant Six was used for "racing", but anybody that ever had one knows it can't be beat by other six cylinders and it did hold its own against the smaller eights of its day...
Edited by GearSpear 2012-09-19 10:21 AM
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Expert 5K+
Posts: 7400
Location: northern germany | i guess it depends on type of race and vehicles that are raced. but generally speaking hp is more important than torque in a race car.
http://craig.backfire.ca/pages/autos/horsepower |
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | dang near *ALL* "racers" will disagree !!!
granted, to INCREASE hp generally implies an INCREASE in torque...
you can't have one without the other, the two ARE *RELATED*...
i was once told that if you built up two different engines to the same "exact" HORSEPOWER curve, that they would also have the same "exact" TORQUE curve...
i myself have always been a HUGE fan of *LOW-RPM* TORQUE...
don't care what horsepower it has when it's wound up and throwing rods, give me LOW-END *TORQUE* and i'll take each and every one of you at the light
Edited by GearSpear 2012-09-19 1:34 PM
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | a ver small and quick read, geeky, but i'll take geeky opposed to pure "opinion" - http://www.rubydist.com/Family/Power.html
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Expert 5K+
Posts: 7400
Location: northern germany |
well not just pure opinion. did you bother to read the link i already posted? here is another one:
http://performancetrends.com/blog/?p=7
anyway this does not proof anything for every article you post i can post another one and vice versa. |
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA |
ps - that link is saying that TORQUE wins races, not HP...
both the "ricer" and the "redneck" hypothetical engines are the same exact 500hp, the ricer peaking at 337 lb-ft and the redneck peaking at 573 lb-ft...
from the article verbatim, "In drag racing, the Ricer would have lost the race, but on a racetrack, the difference could be quite minor."
so i guess all we can really do is agree that it depends on the type of race and the vehicle being raced...
heck, i used to have a 500cc rice-rocket that would win "races" against any kid in a mustang or firebird or trans am or camaro that got snotty with me at a red light... when i traded that sucker in on a 600cc four-valve dohc, those races were no longer even "close" by any stretch of the imagination...
so if we REALLY want to get down to brass tacks, we need to talk about RATIOS of weight to hp or weight to torque...
took a while to read through that link, a very good read for those with some free time |
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | 1960fury - 2012-09-19 1:56 PM
did you bother to read the link i already posted?
not at the time of my post, but have since...
did you read it? he also said that the RICER would have LOST the race
i think we can just agree that it all depends on the type of race and vehicle...
"car guys" have been arguing horsepower versus torque for DECADES, it will NOT be "solved" here, we both know that... |
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Expert 5K+
Posts: 7400
Location: northern germany | GearSpear - 2012-09-19 2:03 PM
ps - that link is saying that TORQUE wins races, not HP...
both the "ricer" and the "redneck" hypothetical engines are the same exact 500hp, the ricer peaking at 337 lb-ft and the redneck peaking at 573 lb-ft...
from the article verbatim, "In drag racing, the Ricer would have lost the race, but on a racetrack, the difference could be quite minor."
so i guess all we can really do is agree that it depends on the type of race and the vehicle being raced...
heck, i used to have a 500cc rice-rocket that would win "races" against any kid in a mustang or firebird or trans am or camaro that got snotty with me at a red light... when i traded that sucker in on a 600cc four-valve dohc, those races were no longer even "close" by any stretch of the imagination...
so if we REALLY want to get down to brass tacks, we need to talk about RATIOS of weight to hp or weight to torque...
took a while to read through that link, a very good read for those with some free time : )
actually that article comes to the conclusion "high average power wins races". anyway even top engine builders seem to disagree
http://www.hotrod.com/techarticles/hrdp_0401_torque_horsepower_guid...
so i guess we can not settle this here |
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA |
this one states my point all along -
1) if we didn't have gears, torque is more important
2) the torque peak is more important for "launching the line"
for every article you post i can post another one and vice versa
i wouldn't be so sure about that, i've been searching for a few hours and it's closer to about for every ONE article you find, i can find FOUR or MORE to say the opposite
but yes, we simply MUST leave it at this, "it depends on the type of race and the type of vehicle"...
any "debate" past that is FUTILE for both of us (no matter who can find MORE links than the other, lol)... |
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA |
i wouldn't quite go that far either...
engine builders agree that if you OPTIMIZE the HP curve, that TORQUE will fall into place...
you can not INCREASE one without INCREASING the other...
how far "undersquare" or "oversquare" you go just defines if you want your power band closer to idle or up there on the high end where rods are thrown into the glove box...
my dad was a drag racer, so yeah, i am biased toward torque...
yes, i acknowledge that if you are driving in a boring circle and you are shifting CONSTANTLY, than HP is more important BECAUSE OF THE CONSTANT SHIFTING...
but i submit that once you throw "shifting" into the mix, than it's really no longer about the CAR, but the person DRIVING that car
i can "launch" a rice rocket without the front wheel capsizing...
that doesn't mean that "everybody" that tries to "race" a rice rocket will be so LUCKY... |
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Expert 5K+
Posts: 7400
Location: northern germany | GearSpear - 2012-09-19 2:21 PM
i wouldn't be so sure about that, i've been searching for a few hours and it's closer to about for every ONE article you find, i can find FOUR or MORE to say the opposite
but yes, we simply MUST leave it at this, "it depends on the type of race and the type of vehicle"...
any "debate" past that is FUTILE for both of us (no matter who can find MORE links than the other, lol )...
funny i made the same experience only that i rarely found anything that says torque is more important in a RACE car:)
but yes this will lead nowhere as most internet discussions. lets agree on "it depends on the type of race and the type of vehicle"
Edited by 1960fury 2012-09-19 2:32 PM
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FwdLk56's Original Account
Posts: 894
Location: Midwest USA | i'm going to post a "vote" thread just for fun...
it will be curious to see how many of us Forward Looker's vote one way or the other...
i'll post a similar thread on a Studebaker site and see if we get similar results...
if i had to put money on it, i'd bet that it's more 50/50 than either of us will expect it to be...
and yeah, if i HAD to put money on it, i'd bet that "torque" wins with a very VERY slight edge, like by a single-digit percentage |
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Expert 5K+
Posts: 7400
Location: northern germany | well... yes... i believe that is why you found more entries for torque. here is the question you posted:
while the two are mathematically related and you can not increase one when building an engine without increasing the other, which of the two do you feel "more important" when it comes to engine design?
reading that question i personally would vote for torque. the question was in RACE engines. daily driver: torque. race only: power. ok enough said. i'll vote anyway |
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What size CARB for your CAR ?
