# Lower Compression on 48 heads and SRP pistons



## jtwoods4 (Dec 22, 2011)

I have an ginal 400 block with the original 72cc #48 heads. I picked up a set of SRP pistons for the rebuild. I plan on rebuilding a very hot street engine. The specs on the pistons say that the specific pistons I have will make 10.9:1 compression on 72cc heads. This sounds like to much compression to run on 93 octane gas. What do I need to do in order to lower the compression so my original heads will work with these pistons?

Here is a link to my pistons:
part number 153982
PONTIAC - Pontiac 400/428/455 Flat Top - JE Pistons


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## BearGFR (Aug 25, 2008)

First, cc those heads yourself to make SURE of the chamber size. Pontiac heads are known to vary from factory specs. Do the cylinders need boring and if so, by how much?

Just "seat of the pants" here, SRP's web site says those pistons have 5 cc's of clearance in the valve pockets, so if those heads really are 72 cc's and everything else about your engine is "Pontiac normal", then with standard bore you'd have 10.033:1 compression and with a +.030 overbore you'd have 10.159:1 compression. Both are too high for 93 octane, in my opinions. Everything else staying the same, the "right" way to get compression down would be to use different pistons - a set with a D-shaped dish in them with about 15 cc's of volume would put you right where you'd want to be for 93 octane with a +.030 overbore, 13 cc's with a standard bore. If you were to zero-deck the block, which I recommend, then kick those dish volumes up to 19 cc's and 17 cc's, respectively.

Bear


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## geeteeohguy (Feb 2, 2008)

I agree. Change the pistons.


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## jtwoods4 (Dec 22, 2011)

Anybody wanna buy some SRP Pistons?


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## ViperT4 (Jul 18, 2010)

Sorry for the threadjack, but is something different about old engines that makes those CRs too high for 93 octane? Are you adding something to the gas to run in the old engine which lowers the octane rating, or is it because new engines are EFI? The stock LS2 11:1 and you can run that on 93 all day long. Just looking for education.


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## Rukee (Feb 8, 2007)

It's the aluminum heads that dissipate the heat so much better. Add a set to the older engines and you can bump up the CR a full point.


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## BearGFR (Aug 25, 2008)

:agree on the aluminum.. Plus technology exists today to actually model and even see in much more detail what happens during combustion, and this has allowed the shapes of chambers to be optimized to induce turbulence to the mixture as well as reduce or eliminate "hot spots". Both help to keep an engine from going into detonation.

Bear


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## jtwoods4 (Dec 22, 2011)

SRP Flat Top Pistons for Pontiac 400 Engine | eBay


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