How to lower compression from 13.2 to pump gas

Hi Guys

If i put new pistons in what else do i have to do? balancing , cams swap,etc. Sorry im new to the bbf thing any advice would help

Thanks Drew

Here is my setup
521 CID
Compression 13.3
Intake Valve 2.190
Exhaust Valve 1.760
Rod 6.700
Cam Lift Intake .741
Cam Lift Exhaust .715
Intake Duration @ .050 268
Exhaust Duration @ .050 274
Combustion Space 86.91 cc
Bore 4.395
Stroke 4.300
Heads Edelbrock Victor CJ
Rods & Crank are Eagle
Pistons are JE
DIVE A BLOCK
4 BOLT MAINS
JOMAR STUD GIRDLE
EDELBROCK VICTOR 4500 INTAKE
MOROSO RACING PAN
1050 cfm DOMINATOR CARB
Mezeriere tall water pump
all ARP studs
Danny Bee Belt Drive Kit
msd flying magnet crank trigger

If you can get a set of dish pistons that are the same weight as what you have now, you won't need to rebalance. You could get with one of the guys that are Diamond piston dealers, because it's gonna take a pretty big hole on top to drop that C/R down to 10:1 or less and Diamond will make you whatever you need.

With the aluminum heads and larger cam, you can get away with 11:1 ish and run on 91.

Have you thought about e85?

Run some dynamic compression calculators and see where the real compression currently is, and go from their.

Without knowing other variables in the formula, I took a guess at them and a 20cc dish will get you approximately 9.97 to 1.

dfree383 wrote:With the aluminum heads and larger cam, you can get away with 11:1 ish and run on 91.

Have you thought about e85?

Run some dynamic compression calculators and see where the real compression currently is, and go from their.

Good idea there...E85 will support a buttload of compression on the street, and it's almost as cheap as bottled water.

Dave, what are you doing up at this ungodly hour?

I'm up at 4:30 every week day..... Mans got to work...

dfree383 wrote:I'm up at 4:30 every week day..... Mans got to work...

did tha coug teach you that

dfree383 wrote:I'm up at 4:30 every week day..... Mans got to work...

Heard dat. Me?....Livin' in hotels....Boredom sometimes sends me screaming through the lobby.

dfree383 wrote:I'm up at 4:30 every week day..... Mans got to work...

You're late if your workin' with me! Mon-Thurs I'm up at 3:45am, and if I work on Fri it's 2:30!

dfree383 wrote:With the aluminum heads and larger cam, you can get away with 11:1 ish and run on 91.

Have you thought about e85?

Run some dynamic compression calculators and see where the real compression currently is, and go from their.

Ya i live in montreal canada we dont have that stuff here e85 fuel i found some pistons for sale in racingjunk would these do ? JE PISTONS 170879
Bore Size: 4.390
Oversize: 0.030
Dome Volume: -39.0
Weight: 568

Thanks
Drew

39cc dish will yield about 8.75 to 1.

Doug Rahn wrote:39cc dish will yield about 8.75 to 1.

oh wow still with those heads i have, they say its Combustion chamber volume 75cc? i though i would give me at least 10.1

Don'y go too crazy with lowering the compression. I'm running 11.9:1 with a smaller cam and I'm sure a heavier car an having no issues on 93, so I would think that if you needed to run 91 you could get by with 11:1 and still have a very strong engine.

9 or 10:1 is just too low for that cam and also for the aluminum heads.

Greg

what about putting a thicker head gasket on?

Hogwash wrote:

Doug Rahn wrote:39cc dish will yield about 8.75 to 1.

oh wow still with those heads i have, they say its Combustion chamber volume 75cc? i though i would give me at least 10.1

Did I read your specs. wrong? I did the calculations based on 86.91cc. If the combustion chamber is 73cc then the C/R will be about 9.60 to 1 with a 39cc dish.