Building a 540 blower motor. Are most of you runing dished or flat top pistons.
Leaning toward JE 181990 dished for lower compression (8.4:1).
- Gen VI - 9.8 Deck
- Callies 4.25 crank
- Manley 6.385 rods
- Dart 320 ported heads
- Team G intake
- crane 741 cam
- 5 lbs boost
All suggestions welcome.

Dished, 8:1 10lbs of boost. The lower the compressing the more boost &HP you can add before detonation sets in

dished on a blower motor will make more power not because of the comp. but because of the design of the piston

I run flat top pistons with my 540 and @13 lbs of boost when everything is right. I have no compalints and motor makes good power N/A.

a dished piston should not make more power than a flattop piston if the CR is the same on both motors (talking normal BBC combustion chambers).

For a blower motor, though, (especially a roots or screw blower) I say BIG DISH. Generate your dynamic compression with boost.

Dished is the easiest way. Opening up and ccing the combustion chambers is also possible if you want to use flat top pistons. The drawback is that it is tedious to perform and expensive to have done.

Easy to cc chambers.

Get a piece of plate glass and drill a 1/8" hole in it with a carbide drill. Get the piece from a glass shop's scrap pile - needs to be about 6x6".

Lay the head upside down on a table and level it with shims and a carpenters level.

Smear some gear oil on the gasket surface around the chamber. Lay the glass on it. The oil should make a seal (you should be able to see the seal thru the glass).

Take a graduated syringe and suck rubbing alcohol into it. Hold it up and tap the air out like the druggys do on TV. Note the amount of alcohol in the syringe. Shoot the alcohol into the hole (I assume you know the spark plug should be in the head). Heep adding until the alcohol comes up and touches the glass. Tap around to get the air bubble out and add enough to get all the air out. Add the amounts you put in. That is your cc. Check each chamber. They shouldn't vary much, else the valve seats are not cut consistently.

Well, At least we have established that tedious for me and easy for you have equal cc's. LOL!

dish will make more all thing being the same i have tryed it bith ways talk to people at j&e they can tell you on a blower motor you will have better flame travel on a dish less heat on the rings

Last time I cruised the pits at the drags, all the Fuelie motors were running flattops. Since they were hemis, I won't propose to draw a correllation..

BUT the winning Blown Alcohol car was running a Chevy/Rodeck/Pontiac. Had flatties in it. I'd think he'd surely be running whatever dome configuration would give him the best power.

I won't disagree with anything that JE says, but "seems" like a dish would kill the squish effect and that would be enough by itself to keep the pressure peak from occuring at normal ignition timing leads. Seems like a dished setup would require more ignition advance to create similar combustion pressure numbers.

Apples to apples, boost psi to boost psi, and degree for degree on the ignition timing, I would venture to say that the flattop motor would make incrementally better power than the dished motor (assuming the static CR was identical on both motors, which would dictate different chamber volumes to test this theory).

Anyhow, I still agree that dished makes best sense in the application mentioned in this thread.