I'm starting to second guess myself on this build. Compression with the Neon pistons worked up to 10.9:1 or so. I thought that would be too much for an intermittent weekend racer that drives up and down the west coast to races.
The target compression with a different piston has been arbitrarily set at 9.8:1
I don't need overheating problems climbing the passes out in the desert.
Where does high compression become a problem on pump gas? I run 10.4:1 on Premium in the Alfa with no difficulty.
Building too much cylinder pressure is a concern as well with the shorter duration intake event of the Erson I initially chose.
This car isn't a regular commuter. I drive it to work "Once in a while."
I'm concerned that I'm going to need a different cam for this build. I don't particularly care if the "Idle Quality" is poor.

CJ
3.430" x 4.440", 198 Rods.
Offy, Erson RV15M, Holley 600

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Part of Tyrde-Browne Racing

Well, some people can tune for 12:1 on pump gas and some people can't make 9:1 work. It seems you have some experience here.

I have run up to 11:1 on 91-93 octane with a cam as small as 228 @ 0.050" duration, but it did ping and run a bit hot in the head. I'd advise a 240-250 @ 0.050" cam with 10.9:1 and you should be fine. My 64 has 10.5:1 right now and was fine to CA and back and road racing on premium. The idle is decent because of the high comp.

Lou

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Home of Slant6-powered fun machines since 1988

My own goal with our slant is to make power with the 91 octane crap we have in California, so 9.0 to one is it for me. If I add a little spray like I am planing I can always add some good stuff to the tank. I "might" drive this car to work now and then when it's nice out too.

It seems clear to me that you are talking about accurate ratios, not the about, probably or should be kind. So that is what I am talking about too.

I have been round and round with this on so many v-8 and v-6 engines and that is where I am at with it. It cannot be tuned to make max power on too low an octane by lowering the timing, it will make less than the correct timing with the correct octane will. The cam will not allow too much compression either, not for max power. Killing the bottom end with a big cam does not make a high compression pump gas engine, it is part of the combination for a race gas engine. The engine will still need to run in it's peak range under full load and need whatever octane at that point, regardless of the cam.

Having said that, I have run up to 10.0 to 1 with 91 octane in well setup engines and for normal driving and even fast highway cruising they were all fine. Backing down the timing more than 2 or 3 degrees is not an option or good idea, it will run like crap and get hot while doing it, making the problem worse. It is better all around to run the correct timing, which will keep it cooler. I would strongly recommend a cooler thermostat to help with detonation, and and enough radiator and fan capacity. Run it between 160 and 180. An electric fan would be nice, a big conventional fan takes too much power away for racing. You can change the electric fans setting to raise the engine temp for racing if that helps too. A trans cooler helps keep the engine cooler too and is always a good idea for desert driving.

I would target a ratio around 10.0 to 1 for a car like this with the correct cam for the power band you need, not trying to "make up" for the compression ratio to run pump gas. Time and jet it right with the correct octane, keep it cool and try to keep your foot light for normal driving with pump gas. You may have to back off on hills and down shift like you would when towing. A little click or two under light load is nothing like full on detonation at max power.

You could also use some left over race gas now and then, maybe add a gallon or two to the tank for hot or hilly streches of highway in the summer. I've driven the I-5 in southern Oregon during August enough times to know what that can be like.

It sounds like you are realistic about this, I think you can get what you want without much trouble.