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56D500boy

Posted 2025-12-30 6:30 PM (#642459)
Subject: Visual comparison of low deck and raised deck early Dodge hemis

Exner Expert 10K+

Posts: 11182
5000

Location: Lower Mainland BC

.
And naturally by "hemi", I really mean "double rocker"

Previously, I have compiled info from Willem Weertman's Chrysler Engine Book regarding early Dodge car hemis, 1953 to 1957:

As seen from the table above, the earliest Dodge hemis had a deck height (distance from the centerline of the crankshaft to
the flat surface of the block that the head sits on) of 9.29". This covered the 53 and 54 241 engines and the 1955 270 engine.

When Dodge needed more displacement, they decided to go to a longer stroke (3.80" vs 3.25"). Apparently that was going to cause
internal grief of some kind (rod angles? rod to block clearances? etc). Their solution was to recast the new blocks with a taller deck height,
i.e. 10.38" vs 9.29" (1.09" taller). The affected a few things like the water pump manifold and the intake manifold widths but apparently
that wasn't a big problem.

While I knew about the deck height changes, I never really saw real world examples of this until this week I watched a couple of videos
by Jamie at Dead Dodge garage, one for a 241 tear down and one for a 315 tear down. I have captured images from these videos
to illustrate (hit me on the head with a hammer) the difference. (NOTE the distance from the water pump manifold opening in the block
in each of the two (241 and 315 engines) to the bottom plane of the head. You can pretty much see that extra 1.09" deck height.

And now I know.

(I know lots (most? all?) of you know this stuff but I am still learning. Too soon old, too late smart.

Edited by 56D500boy 2025-12-30 6:37 PM

(241DodgeDoubleRockerShowingShorterDeckHeight.jpg)

(315DodgeDoubleRockerShowingTallerDeckHeight_2.jpg)

Attachments
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241DodgeDoubleRockerShowingShorterDeckHeight.jpg (98KB - 108 downloads)
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315DodgeDoubleRockerShowingTallerDeckHeight_2.jpg (134KB - 151 downloads)

Mopar1

Posted 2025-12-31 10:57 AM (#642466 - in reply to #642459)
Subject: Re: Visual comparison of low deck and raised deck early Dodge hemis

Expert

Posts: 3185
2000

Location: N.W. Fla.

The 241 advertised C/R is 7/7.1:1, but on a build on the imploded Webrodder site one measured at an actual 6.19:1!

Powerflite

Posted 2025-12-31 4:18 PM (#642470 - in reply to #642459)
Subject: Re: Visual comparison of low deck and raised deck early Dodge hemis

Exner Expert 10K+

Posts: 10542
5000

Location: So. Cal

You can typically expect to lose around 1/2 compression point from using modern head gaskets, so that would put it at 6.5:1. 6.2:1 isn't too far off from that, but with the shorter stroke and smaller bore, the loss from the thicker head gasket may have an bigger effect than it does on the larger engines. That's why I try to tell people that if you aren't willing to pay for custom pistons in these engines, don't even bother. But if you do increase the actual compression up to around 10.5 to 11:1, you will notice a HUGE improvement in power and response from using a better cam in it that can take advantage of that compression. Your only other option is to use stock pistons with a supercharger of some kind, but keep the boost ratio below 8:1 or so.

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