with a 4" stroke sb what do you think would be the point of diminishing returns?
I once read in the mopar journal that 13:1 had the most advantage. beyond that gains were very small.

There's no one size fits all. It's totally dependent on the rest of the combination.

4" stroke 4" bore small block using indy 230cc head and indy intake with 1100 dom, .700 lift roller cam. drag race n/a

The curve of a hemi piston slows down the burning of fuel. Many ran that ran 13.5-1 changed to 12.5 which allowed the use of 110 octane instead of 112or 114. Lower octane burns faster then higher grades .

Many comp engines are built at 16.5:1 compression. Like said before it depends on what gas you want to run. You will be wanting 116 or better fuel when you get over 15:1.

My 421 W8 was 16.5:1 and it preferred 118NOS fuel which was illegal in the class I ran.

Leon

k, I guess I was doing that backwards. thought I could pick the cam and fuel to match the compression. 110 race fuel (gas)

Based on the ideal cycle, the plateau starts to occur in the teens. But this is based on an ideal cycle, not taking into account fuel, operating conditions, etc. But it gives you the general idea that the substantial gains are already realized once you get into the race gas region.

dave......this might give you a little example of cam/compression change had on my 4 inch w5 combo...

I went from a 260/266 flat tappet cam 416 with 11.8 compression w5 headed combo that ran a best ever of 10.38 at 129 and change.
Swapped in a 422 shortblock, that ended up being 13.3 compression, and added a 700 lift roller that was 273/280@ 50 cam shaft.
same heads, headers, carb, intake, gears and convertor. So pretty much the only real effective changes was going from 4.07 bore to 4.100 bore, the compression bump and the bigger roller over the smaller flat tappet. Rest of the combo remained the same.
Best with those changes was 9.85 at about 135. So about 5.5 tenths and 5 or 6 mph.
The new shortblock of course was fresh, and had 043 ring pack, but it yielded a significant difference. Picked up about a full tenth in 60 foot, suspension remained identical, as did tires/wheels etc.
Hope this might help.

should probably also add, current owner of car took off 150 pounds,down to 3200 pounds freshened shortblock again, went to a more aggressive roller that is 276/282 I believe,same lift or close, and bumped compression to just over 14 to 1, and picked up additional 5 mph and 2 tenths, with everything else unchanged..same gears,tranny convertor, carb, intake, headers, suspension etc. The car really seemed to like the much more aggressive profile of the camshaft, over the quite mild profile I ran that was easy on springs and was somewhat streetable. All three cams, the flat tappet and the two rollers were all specc'ed by Dwayne Porter.

For a race engine you'll want the most compression possible without hurting power. Sometimes a large dome will reduce power or deep valve pockets will reduce power.

Max power on race gas would be 16:1 compression with a flat top piston but you rarely get that lucky.

that's what i'm really tryin to figure out. I know there is a point the extra compression is not worth the little gain. the engine has to work harder driving the next piston up on compression. don't want to use exspensive race fuel neither. like to run it on 110. looks like 16:1 is the platue as far as diminishing returns. thanks

You also have to factor in the value of the increased gains vs. the diminishing thickness to your wallet for fuel. If you plan to race a lot, factor in the cost of these expensive higher octane slower burning fuels needed to support that extra compression. Going higher than what the average 110 octane fuel will support (13.0:1 or so with aluminum heads) will add 25-30% or more to your fuel budget which is constant and not a one-time expense like the initial build.

If you want to stay with Sonoco 110, 13.5-1 sounds like the right compression and you should be able to use a flat top depending on cc of the combustion chamber.

David Vizard's book "how to build Horsepower" had an inverted triangle chart that gave you the 'order of magnitude' from old to new CR...I have it somewhere but from say (for example) 9 to 11 was much higher percentage than from 11 to 13 which to some extent is mathematical 11/9ths is a higher proportional increase than 13/11ths. The higher you went the lower the gain, while it's true as you increase the pressure you also increase the speed of the flame travel, but at some point you hit diminishing returns and design limits of the engine itself.