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360 compression ratio with 291C`s?

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BBO UK View Drop Down
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    Posted: Jun/30/2022 at 11:45am
I have been contacted by a Tractor racer who uses a 360 for power, it is a stock `76 with 502 heads but has a Crower 45212 3k-6.5k cam, performer and 650dp carb, with exhaust stubs like a top fuel dragster.

He wants more power and I suggested changing to 291C heads for more compression, problem is I don`t have the figures to work out what final compression we would have, changing from 58cc 502`s to 51cc  291`s?
Are these a direct replacement, on a later `76 360?

Can someone give me an approximate compression ratio swapping the heads, the pistons will remain stock, for the moment. I think the 360 was from a Jeep as he`s using a built TH400 (rated to 1200hp) with a Dana 70HD from a USAF tug !
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ramblinrev Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 12:12pm
With stock pistons, 291 heads should make it 9.0 to 1
Someone can correct me if I'm wrong.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BBO UK Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 12:24pm
Thanks for that, I will let him know, should increase torque a little bit.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic Silver Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 1:09pm
I am not trying to correct Paul because I don't have a 100% for sure answer but take a look at these numbers. DSS offers AMC 360 forged pistons on Ebay. They list the same piston with a 21 cc dish as having 9.2 cr with 58 cc heads and 9.58 with 54 cc heads. 

 Here are some numbers from 350 Chevrolet pistons and compression ratios. Won't be directly comparable to AMC.
Chevy 350 piston with:
58 cc head 10.6 cr
60 cc head 10.36 cr
62 cc head 10.12
64 cc head 9.89
68 cc head 9.47
72 cc head 9.09
74 cc head 8.91
76 cc head 8.75

Another Chevy 350 piston PN# 2256F+.030 with:
58 cc head 11.03 cr
64 cc head 10.26 cr
68 cc head 9.81 cr
76 cc head 9.03 cr

Draw your own conclusions but I would guess between 9 and 9.25:1. It looks like about .1 cr less with every 1 cc larger head chamber size. Just a ballpark guess. I would think an online calculator could give the answer. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic Silver Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 1:21pm
I meant to add that AMC rated the 70 model 360 with the low compression piston and 51 cc head at 9:1, which would make Paul correct. Of course, these AMC numbers may be advertising and merely an approximation.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BBO UK Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 2:13pm
I had forgotten about the 2 different 70 pistons and shouldn’t have as 30 years ago I put some high compression stock pistons in a 360 with 502 heads, and never knew exactly what that final compression was, any guesses Smile
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 4:43pm
502 heads are 60 ccs, not 58 as is posted in many places. The 291 head combo with the low compression pistons will give 9 to 1.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BBO UK Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 5:09pm
Thanks Ken, I’d always thought they were 58cc, but you will have measured them which is always better than believing “what everyone else said they were” Clap
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 5:45pm
Depending on what's in there now, swapping in a set of flat top pistons might be the hot ticket.

I get the following with some assumptions (27 cc piston dish, 0.045" head gasket thickness, 4.265" gasket bore 4.08", deck clearance 0.020") :

60 cc chamber 8.24:1
58 cc chamber 8.39:1 + 0.15
50 cc chamber 9.00:1 + 0.76

Changing to a flat top piston (5 cc valve reliefs)

60 cc chamber 10.24:1 + 2.0

Everything depends on what he actually has now, but the old 343 flat top piston into a 360 engine is a tried and true method.

Then you can add you 291C heads for 11.56:1 if you want to run race gas,


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Jun/30/2022 at 5:58pm
The old PAS said the "big" chamber heads are 57.9 cc, and everybody assumes that was the same to the end of production. It's in so many places on the internet everyone believes it. Everyone knows the internet (no sarcasm) is right! In reality in 74 they dropped the compression a smidge with the 502 heads which are 60.1 cc's. These are in the factory manuals, and officially the spec is 60.12 +/- 1.5 ccs. If you look close you can see the difference.

Edited by Ken_Parkman - Jun/30/2022 at 6:00pm
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