Thread: 6x-4's or 8s?
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Old 10-16-2021, 08:29 PM
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Quote:
Originally Posted by pastry_chef View Post

I'm impressed with a person running 11 to 1 compression with cast iron heads on pump gas with a mild cam. I've seen it claimed but how legit?
LOl, I am on a 8 + hr work road trip from the WI/IL border to AL, so being a bored pickup passenger, I will take a swing at that…..

Some could be legit, it is easy to miss estimate the compression though when the engine is near to slightly over 11:1. A few cc’s swings the compression quite a bit, if they didn’t check all the volumes it is a good bet the SCR was over estimated.

We have build some dual fuel propane/pump gas engines with high compression up around 11:1. Octane for propane is well over 100, so we raised the compression to utilize the octane more. They were a little picky on pump gas but not as much as we expected. Never did it with a Pontiac, but I think the size of the cam matters a ton. In the 11:1 small cam scenario I think it is opposite of what people talk about with “bleeding compression”. A very very small cam the engine can’t breath well enough to pump as much compression as one might expect. It may pump a bunch of compression on a compression check but doesn’t use it efficiently at all. If you put really big flowing heads for the cid, with a big CSA and combined it with a small cam I think the same thing would start to show. The octane requirement would raise, but not as much as one would expect.

I have seen that same thing play out on some of the old 60s era high compression 2 bbl engines,. An Pontiac example would a factory 389 2bbl 290 HP. The one we had would run better on less octane than it’s higher HP version but had the same compression.. More cam (still mild) and it made it worse. What you don’t want in a streetcar is so much compression that you run out of head flow for the cams power band trying to get the compression under control.