Thread: Street compression ratio w/pump gas

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Hi again u guys,
I put my 540 project off since last year, now I am getting interested in it again.
I have a bowtie gen 6 block zero decked, and a 540 balanced rotating assembly consisting of an 4340 eagle crank, 1 pc int balance, h-beam rods, and SRP small 10cc dome pistons.
I wanted to run factory iron heads to make the motor look stock, found a nice LS6 pair on ebay awhile back, they are casting number 3994026. They intake volume on these heads is approx. 320-325 cc's. I cc'd the chambers and found approx. 114 cc. Stock they are 118cc, so it looks like they've been shaved. If I assemble the motor now, the static compression comes in high at 10.7:1.

At this point I think I can either change out the pistons to flat tops and rebalance the rotating assembly, fly cut the tops off the existing pistons and rebalance the rotating assembly or just get different heads with a bigger chamber. Any suggestions. I am leaning toward a gm factory iron head engine, since it's going in a '70 vette.

So now this leads me to wonder just what is a good target compression ratio for this size engine that's been zero decked and has iron heads?
Thanks for your help, Ed

Are you asking if 10.7-1 compression is too high for pump gas, If so its not too high for premium 93 octane pump gas. You are safe to about 11.5-1, being safe, or even a bit higher 12-1 with an iron headed motor on premium pump gas. The 12-1 is pushing your luck with the fuel and with the use of iron heads.

well you can open up the head chambers on the old heads or buy world iron heads that flow better then yours and would have a bigger chamber or run a bigger cc head gasket then you do not have to screw with the bottom end 10.5 is ok with good pump would have a lot to do with were your at with dynamic CR with your cam

Yes, I thought about opening up the chambers a bit. The question is, how much can they be hogged out and would it be even worth all this effort, I don't know..?
I would like to stick to gm heads if possible.
Thanks

Originally Posted by ekitch

Yes, I thought about opening up the chambers a bit. The question is, how much can they be hogged out and would it be even worth all this effort, I don't know..?
I would like to stick to gm heads if possible.
Thanks

well i would not use the word hogged out they can be open up to the head gasket bore size .and re work around the ex side you can pick up some CC how many ?? 118 cc or more

over 11:1 you will have to start looking at duration dwell to bleed of pressure and you will lose bottom end performance.12:1 is definitely gonna put a hurt on the octane requirements. 10.5:1 is ideal for a street performance big block and not have to worry about higher than 93 with a good street performance cam.

Well I am going to try and keep it at around 10:1 due to only 91 octane availability here in CA. It used to be 92 octane when gas was only $1.50 a gallon, maybe 8-10 years ago. Now there is talk they may just eliminate the 91 octane too. I would have to use some kind of octane booster if they did that..

I owned a '68 427 Vette, oval port closed chamber heads, 11.36:1 measured volumetric CR. Had to have 50% race gas with 50% 93 octane. Also if you lighten up the front end a tad your weight distribution will improve and so will your handling. Aluminum heads will also get you further away from detonation issues. JMO.

Kitz

Originally Posted by kitz

I owned a '68 427 Vette, oval port closed chamber heads, 11.36:1 measured volumetric CR. Had to have 50% race gas with 50% 93 octane. Also if you lighten up the front end a tad your weight distribution will improve and so will your handling. Aluminum heads will also get you further away from detonation issues. JMO.

Kitz

Thanks Kitz, I have decided to go with aluminum heads after all. There are just too many advantages to pass up. I just had the iron LS6 heads magged and cleaned, no cracks they are ready for a build up and are for sale...have pics so make offer if anyone is interested. They are stock for a 1971 LS6 Chevelle.
I have heard that dart heads will fit the car with hooker super comp headers with there raised 0.300 exhaust ports. I don't want to hack up those nice headers or build custom ones. And with the new darts larger chamber, 121 cc, that will give me an exact 10:1 CR. The other option is the edelbrocks, they will work too.

That sounds real good to me .................

Kitz