I am considering lowering the compression in my 363 ( 30over 360 magnum block, W2 heads, 590 lift M/P mech. cam) from 11.5 to 1 to 9.5 to 1 so i can use pump gas. It made 549 hp and 450 tq on the dyno and went 10.60s at 125 in my Dart. 3150 lbs, 4.30 gear, 4800 stall Converter, 904. How much power will i loose? I stopped it down to run 11.50s. will it run high 10s-low 11s? Don't mind losing a little but still want to be able to run 11.40s in bad air.

Hard to say since it will depend on the combustion chamber design. I've seen dyno tests where reducing the piston dome actually increased power even though the compression went down. That can happen on a BB Chevy where they use a big dome. So if you are switching from a dome to a flat top piston you might not see a large difference in power.

Are you planning to change the cam to go with the lower compression ratio?

i wasn't planning on it.

I believe dropping 2 points on compression without a cam change to account for this could hurt your torque everywhere. I don't think you have enough cubic inches to just lower the cr without giving up 20 or 30 HP. But it wouldn't be the first time I was wrong about something.

Would you need to drop it down to 9.5:1 to run pump gas?

You can turn down the timing to get you to pump gas levels. Running the N.E. Index? This can get you there. Will take several passes to get the new feel of the car. Might need to cut a tighter light to make up for a sluggish launch( might not even show). But if you back the timing a little, you can try premium,93 octane, and Klotz octane booster. Works well w/o the plug issues of others and makes the pump gas last longer w/o carb/pump issues.

Have you considered water/meth on pump gas?

E-85

I'd spend my money on a cam from a custom grinder long before I dumped the compression and no way in hell would I ever drop it that far.

It's 2019 and you can easily run that compression ratio on pump gas. The issue isn't your compression ratio. It's the cam. I like the MP cams but I'd say look at a different cam, preferably one that isn't in a catalog or on someone's shelf.

Me? I'd call Jim at Racer Brown and tell him what you are doing and see what he says. I run the same CR as you on pump gas. I don't have detonation issues. I also have a dome. It's not an issue.

I've also used Cam Motion for relatively high CR pump gas stuff and never had an issue.

I'm not a fan of giving up CR when you don't need to.

that 590 wants compression...you will have to change it or it will be a dog.the MP 557 would work perfectly.

Thanks for great advice guys, Happy Holidays ! Yes I run some N.E. index races.

I'd shoot for the 10-10.5:1 compression range and others have stated a cam change would also prob be needed to help optimize it.

I ran my old Aluminum headed SB @ 11.3:1 on 92 octane pump gas.

If you could get to 10:1 i think you'd be pretty safe as long as you dont get greedy with timing.

I run 11 to 1 with iron magnum heads, some thought into keeping the air cool and some other tricks should be able to make it live just fine on pump premium as is. 9.5 is very low these days.

Sorry if some of this is obvious?
Tighten your quench to .035".
Reduce your initial spark lead.
Slow up and lengthen your spark advance curve.
Consider a high gear only 2-4° spark retard (45 years old, H/T Bill Jenkins)
Higher number PV.
Step up the mains 1 number.
Bigger ATF cooler.
Retard the cam 4° (watch your exhaust P-V!).

I think POLY wants to build you a moder !!

All these are much better options for the OP than his original idea.

WHy not consider a head swap? get some magnum type heads to replace the W2 . Also you have a magnum block, get an up to date roller hydraulic cam and you probably be fast enough for 10.00 index....

just a thought, im like that...I rather speed up than slow down lol

You may loose 30 HP but gain more torque with lower compression on pump gas
All the iron headed pump gas motors I've dyno that had less than 10.5 to 1 compression, aluminum head motors also, made more torque than HP by 30+ Ft Lbs.
I wouldn't hesitate to run the pump gas with 9.5 to 1 compression ratio as long as you retune the motor for it
Torque moves the mass, not HP HP make it MPH