Sitting here contemplating my winter engine build and I thought I would try to get some input. I have a set of 915 heads and was wondering about how much compression I could run on pump gas? What I have gleaned from reading about quench, is that it reduces the tendency to detonate with higher compression ratios and that somewhere around .039 to .049 is the ideal range. If I use a flat top piston at zero deck and my closed chambered heads with a .040 head gasket, I end up with approximately 11.28:1 for my compression number. Will this work with 92 octane pump gas or will I need to run an additive or racing gas? I'm hoping to be able to use my heads since they have been mildly ported and completely gone through. Am I just wishful thinking? Do I need to save my money and buy a set of Stealths?

That crazy high compression ratio could only work with an equally crazy big cam. This will make for a somewhat radical mill for the street. This sucks for when you just want to cruise it. Not everyone spends 100% of the time at WOT.
A 10.0 to 1 with quench and a moderate cam is a far better approach. Fighting detonation with race gas, limited ignition timing, methanol& water injection....it all works but why not build a simpler and more reliable street engine?

I could use a thicker head gasket or a piston farther in the hole, but then I lose my quench.

You could use a piston with a reverse dome. The ones that I have considered have a flat top on the quench area with a dish on the other side. Sorry, I have no part #s for them but 440 source lists them. They have a compression ratio chart to help calculate too.

For pump gas I think you want to stay below 10.5

I like Frankenduster's idea on the reverse dome pistons

Icon makes a 12 cc dish piston. Mill the block to a zero deck with that piston, use a 0,040" head gasket, and fine tune your cylinder head cc to the compression ratio you want.

80 cc head will be 10.0:1
76 cc head will be 10.3:1

Your cam will make a difference on what you can do on CR/pump gas, but this gets you in the ball park of where you want to be.

Those heads should be about 79-80ccs stock...... add a few valve jobs and its 82-83ccs. with a zero deck and .040 you should be closer to 10.7 not 11.28. Im running a flattop SRP with .020 gasket and .022 deck and I figure it at 10.3. Im running pump 93 with no problems.

I would suggest absolutely no higher than 9.5 & that is with good quench (.035"-.040") & 040 is max. Gas is crap (& getting worse) & you want a cushion so you can run a fast ign curve which is critical to getting max power and far more important than pushing the (CR) limit & having to compromise the curve. FYI published gasket thicknesses are all over the map, measure exactly what you have as that is critical for getting quench, (above .040 quench effect goes away very fast)

from my experience 9.5:1 with iron heads and pump gas is all i want.

Should have no issues at 10 to 1 and tight quench. Personally I would not settle for less than 10.5 comp. It drives me crazy knowing I left something on the table. The higher compression will give you better MPG and make up for the difference in 87 and 91 octane. Set you quench right at .040. If you need a few more CC then open up the chambers around the valves. With tight quench you just don't need as much ign timing, it is not a compromise to back down around 38 total and 18 at idle when that is all the engine needs. A flat top is the best piston you can get for flame front and also reduces the need for extra ignition timing advance. The best plan is a flat top, tight quench and open the chamber if needed to get 10.5 compression.

Best is to get the quench dialed in as close as possible to .040". .045" max. .049" quench still helps but tests posted here shows the benefit is dropping pretty fast. And use the dish piston as needed to get CR where it needs to be.

Any sustained hard running puts a lot more heat in the head vs a short blast. That heat will want to make it detonate/ping. Also, gas quality may vary as mentioned. 1/2 point too much compression can make life miserable and may hurt the engine if run hard. It is very hard to fix once the short block is done. I'd suggest tight quench and 9.5 max CR... then run the cam you want, time it for best performance and have fun.

I built one with 9.7-1 .040" quench. Combustion chamber depths were all equalized. Small 214@.050" cam 3.23, 906 heads, stock intake, carb, exhaust. It's normally pretty decent but will rattle in hot weather at 36 degrees total lead.
Doug

currently i have my 3rd open chamber head pump gas big block on the engine stand. i know from experience 10:1 or greater won't work with pump gas; good quench or not. stay between 9:1-9.5:1 with a good tune-up and quench between .040"-.050" to be safe.

Rapid Robert and I agree on this with you

First time ever!

There are people running 11:1 with pump gas.

I'm more of a cylinder pressure kind of guy.

Being in Denver, I try to plan the build around known fuel quality, purpose, gearing, trans, and especially altitude.

If I can get 93 octane at sea level, I generally try to target no more than 180 psi with iron heads. Without running the specific numbers, I'd say 10:1 mechanical compression is about it, then juggle some cam timing numbers to get close to target. Cams are a lot easier to change than pistons.

I've heard of unicorns, reliable Fords and trustworthy politicians, too.

Not with factory 440 Mopar heads.
Doug

i live at 750ft elevation and have found 180psi can be touchy. i would treat 180psi as the upper limit.

they must be throwing an extra $20 bill in the plate on sunday morning.

You can do it but not with a flat top, get a piston with a D shaped Dish to lower the compression but still give you quench/squish , and personally you don't want more than .045 , try to keep it as close to .040 as possible. 10.0 would be good with iron heads.

Fixed it. You can not do it with open chamber heads, they suck and no reason to run them unless you have to for a certain class you choose to run.

Yes, 180 would be the limit and even that would depend on variables previously mentioned. I hate leaving power on the table.

My old 440 I built that I used to have in my 63 had 10.0 comp with 906 heads and I ran 92 pump all the time in it and it ran fine with no ping and 38 total timing. I used the KB quench pad pistons and built a nice .045 quench in it. But it was a real pain to use open chamber heads and quench pad pistons as you have to measure all the head cyl chamber depths and do alot of work getting them all right. As all the cyl head chambers are not the total same depth and it takes alot of work. I wont ever run them again as I had them so I used them. But to use 915 closed chamber heads and a flattop piston is the way to go. You can run 10.0 comp with the iron 915 heads if you build it with good quench and use the right cam to keep cyl pressure at no more then about 180. I did it and it worked great but I dont reccomend going higher then 10.0 with iron heads and pump gas. Ron