18:1 C/R verses 21:1 C/R in measured cylinder pressure

[john8662]

I know I could probably find the math somewhere after digging around forever but.

I'm considering an 18:1 build up within this and next month and want to make sure this is a good idea and why. So, in an engine with 18:1 compression that is fully broken in and the rings are seated and sealing their best, what compression should someone see doing a compression test?

Now, looking at the other side, what does the 21:1 compression 6.5 stock see on a fresh broken in engine? I'm thinking 400psi all around, but I wanted evidence.

Thats it for now, this is just one piece of the puzzle for me.

[eracers999]

John:

I havent hooked up a compression guage to mine yet, but remember that there would be a variable that plays in, and that is the timing chain or gear. The looser the chain the less roll over comp. The only accurate way i can think of to measure this would be to get a comp reading on a fresh stock comp rebuild with a gear drive, and the same on a 18:1 with a gear drive.

Kent

[john8662]

OK, lets assume apples for apples, in theory what would be the difference in measured compression between an engine that has the 18:1 pistons and one that has the 21:1 with the engines being identical. Or, what in theory or running the numbers equal for the compression?

Basicaly sounds like a lower compression 18:1 engine is just like a worn engine, that has lower compression, wrong?

[rjschoolcraft]

Assuming a full charge of air in each cylinder (which should be ok, since when cranking, volumetric efficiency should be close to 100%), a 21.1:1 engine should see about 295 psig cylinder pressure cranking. An 18:1 engine should see about 250 psig.

Take 14.7 psia pressure in the cylinder, multiply by the compression ratio, then subtract 14.7 psia to get gage pressure.

[a5150nut]

The idea of the 18:1 is you run more boost at operating levels. Thats where the HP gain kicks in.

[john8662]

Talked to Benny at Avant. It's approximatly 350 to 370psi, so about the same as a worn 6.5L. But, that compression is likely to remain constant, the lower compression means less wear. I think the main points of the 18:1 were the ability to handle more boost, quieter, smoother overall operation, especially at idle, possible longer block like and head gasket life.

Loan officer is gonna love me this month...

[Mark Bajus]

18:1 does nothing for boost without additional fuel. If you swapped pistons for 18:1 without changing anything else boost might even drop as cylinder pressure and heat would drop.

18:1 gives you more capacity for boost without sending pressures through the roof. With 18:1 you can stuff more fuel/air into the cylinder, creating more power at the same peak cylinder pressures as 21:1