Due to a switch in heads, I may end up with 14.22 compression , instead of 15.25 that I could run with new pistons with a higher dome.
So what difference have you seen on the same combo just changing compression? This is a 550 cube 572-13 combo on injected alky. The new heads can't be cut enough to get up where I want to be, which would be 15.25 to 15.5/1

I don't "think" you will see 20 to maybe 25 HP difference with the lower compression ratio, but I have never race with straight methanol, let alone with that much compression: confused:

I don't have test numbers to give you.
But, IMO the power loss with CR is bigger at low CR and is less the higher you go. For example, 11 CR to 10 CR is a larger percent loss than 15 CR to 14 CR.
I am sure there are many other factors (cam duration, etc.). But I would not be too concerned if you can build 14.2 CR.

Not sure it applies but in the max wedge 11.5 to 12.5 was 10 HP according to Chrysler back in the day. My 499 Indy head motor had 15.19 compression, o-rings and copper gaskets. One of Kenny L motors when he was at Herbs shop.

Hopefully someone will have direct results. When I cut the heads from 67 down to 62 cc I know I picked up power, and I would guess over 20 hp, but there was so much ch wrong and different that I have no hard evidence.

IF the larger chamber frees up flame propagation you may not see any loss;.....maybe gain?

I ordered my terminator from James Monroe (KillerRons)a month ago, and we discussed this and his thought was there wasn’t any or enough hp. gain over 14.7 to 1 warrant running that much compression, you might give him a call for some more input. Doug

Long time ago a bud did a cyl head swap on a 74 440 RV motor ...

From the stockers to a set of mildly shaved 516s ....

The performance gain was HUGER than lil Organ !

There is math for this, but it doesn't account for valve shrouding or dome intrusion in the flame path.
http://victorylibrary.com/mopar/otto.htm

depending on how much combustion chamber is cut down,it will have negative results with head flow if the head is cut down into the seat erasing the chamber above the seat. as stated above power decreased at the higher the compression number goes up.....

IMO one thing you are overlooking here is you will now have an aluminum block not a cast iron one. Aluminums ability to wick away heat far exceeds that of cast iron. You can "get away" with significantly more compression ratio with an aluminum block. Similar to the difference between an aluminum head and iron head, only larger. IMO staying at the same static compresion ratio from iron to aluminum will cost you power. How much depends on cylinder head, combustion chamber and piston design. It will also be more difficult to build cylinder pressure in an aluminum block compared to an iron one. Just something to think about. If I told most the dynamic compression ratios we run with aluminum blocks most folks would be scared

Al, I was just thinking the exact same thing, especially after owning 3 Alum block cars.

Greg, in the end there will be a difference for sure, but I don’t think it’ll be too significant to worry about. Over time you’ll be able to offset the difference in other ways with the cars overall performance. Time will tell! Good luck brother.

Thanks for all the replies. At this point my math says 72cc heads and +6cc dome pistons at . 047 clearance piston to head at running temp will put me at 15.32/1. That is . 32 higher than the iron block.
I am guessing a piston with +6cc dome may be the limit in size before messing with power. If I can run 15.32, hopefully that could offset the power loss from going to an aluminum block . But with a weight savings of 104 lbs, that in itself is equal to 30 hp roughly. The car and driver will weigh 2920, goal is 8.5000 ET at 3,000 da.

I remember talking to someone back in 2002 that said above a certain compression ratio the need to go fatter to keep the motor out of detonation eats away at potential gains in power. Methanol supposedly has an octane rating of 108 at stoic, but methanol cools the charge five times more than gasoline. I read in an alky tuning book that a blown application with high boost will run at 3.8 AFR to live, where stoic is 5.6. that's about 30 percent extra fuel used to raise detonation resistance enough.
when I stroked the 528 and went to 550 cubes and upped compression by cutting the heads from 67 cc to 62cc, I had to go up in fuel ratio a lot.At 15/1 I had 210 cranking compression. The power the motor made was real good . But I started eight percent fatter (.104) than a slightly rich tuneup with 38 nozzles and a main bypass of . 112 that ran well in the 528. By pull 14 I was down to a bypass pill of . 80 and each increase along the way gained power. That was the fattest pill I had so we called it a day. The last two pulls we only gained about 5 hp . So now that I think about it the connection between going fairly rich and high compression was showing up. A call to Monroe is in order. I need bigger pills anyway.

Do you have a high speed lean out valve ? How much can that effect can it have on horsepower and top end ?

Yes I do. Spring loaded poppet valve type. It was set to come in at peak torque, .064. Didn't have to change it, as the opening was below stall speed of 6200, andAFR was very even.