Thread: SBC Flatt and Dished Roller and Flat

Tweet

All else being equal would there be any changes to ignition timing and spark plug gap between these two diffrences in the similar 9-1ish build engines, understanding the dished pistons reduce compression.

SBC 1
Flatt top pistons and Flat Tap Cam

SBC 2
Dished Pistons and Roller Cam

Goal is best racing performance

Your question is a bit confusing. If they both have "9-1ish" compression, then the dished pistons would not reduce compression, would they? So, the answer would be "no." But the roller cam would out perform the flat tappet cam.

I guess then a better question would be how would you get more compression then 9:1 out of a flat top piston and flat tap cam. Rod length? Heads? without changing the top of the piston ie dome how high of compression can any other changes get you?

Any idea the purpose of dished pistons? if a choice for flat or dished wouldn't you choose flat for increased compression on a carburetor engine?

Originally Posted by racer7077

I guess then a better question would be how would you get more compression then 9:1 out of a flat top piston and flat tap cam. Rod length? Heads? without changing the top of the piston ie dome how high of compression can any other changes get you? ?

You are lacking a basic understanding of how a motor works and probably need to do some reading.
The camshaft has nothing to do with static compression ratio. It will, however, alter dynamic compression ratio. Rod length has nothing to do with static compression ratio. It will, however, alter dynamic compression ratio.
If using the same piston, additional static compression ratio can be achieved by using a different thickness head gasket, cutting the block decks to decrease the piston deck height or by changing the cylinder heads to units with smaller combustion chambers. Combustion chamber volume in the heads will determine how high you can go on static compression ratio with any given piston crown design. A 355 SBC with 42cc racing heads and flat top pistons will have a static compression ratio in the range of 14.0:1

Maybe I can begin helping you out with an explanation of how to figure static compression ratio. There are 5 values which must be used.
1. volume in the cylinder expressed in cc's
2. volume in the combustion chamber expressed in cc's
3. volume in the piston crown expressed in cc's
4. volume in the head gasket expressed in cc's
5. volume in the piston deck height expressed in cc's.
First, you add all the volumes together. Let's say we have 727, 64, 6, 8 and 3 for a total of 808 cc's. Now, removing the cylinder cc's of 727, we add the remaining volumes and find 81 cc's. Now, if we divide 808 by 81, we find a 9.975:1 static compression ratio. Cubic inches can be converted to cubic centimeters by multiplying times 16.387

Originally Posted by racer7077

Any idea the purpose of dished pistons? if a choice for flat or dished wouldn't you choose flat for increased compression on a carburetor engine?

Dished pistons are chosen to lower static compression ratio with any given set of values on the other 4 values. If you were going to keep the heads, gasket thickness, piston deck height and cylinder volume that you have, but wanted to lower static compression ratio, the only other variable would be the piston crown volume. Whether the motor is carbureted or fuel injected has no bearing on the static compression ratio. You can build either one to anywhere between 6.00:1 and 16.0:1