Dome head compression psi

[BammBamm]

I just rebuilt my 2L with AR 84.6mm dome headed pistons. I’m also using an old 1800 head. My compression psi is 210lbs. Is that normal for these pistons?

[18Fiatsandcounting]

AR sells several different pistons for the 1800/2L engines, with different compression ratios. Which ones did you get?

In general, 210 psi is much higher than the stock compression, so you definitely got a compression ratio boost with these pistons. A stock 2L engine (8:1 CR) might only be 150 psi or so, and I'm guessing your CR is now around 10.5:1.

Note that, all other things being equal, an overbore from 84 to 84.6mm will drop the compression ratio a couple tenths. For example, if you had an 84mm bore with a 10:1 CR and bored it out to 84.6mm with no other changes, the compression ratio would drop to 9.8 or 9.7.

Does the engine run on regular gas without knocking, or do you have to run premium?

-Bryan

[Nut124]

210psig is pretty high. My 1800 has the 8mm dome pistons, 2L head, static CR around 10.5:1 and cranks 185psig.

With 210psig, I'd worry about detonation at mid range engine speeds, between 1500 and 4000rpm, under load. I'd use premium gas.

In the old v8 circles, if facing this cranking compression, they would put in a big cam to lower what they call "dynamic compression" and fight detonation that way. A big cam keeps the intake valve open longer, past BDC, lowering cylinder fill at low/mid rpms.

If you are worried, or notice pinging, you could try retarding the intake cam one tooth on the pulley. That would have the same effect at the big cam in the old V8 world.

Suitable ignition timing curve is important.

[Nut124]

BammBamm, do you know what the intake cam is?