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.

IF, If, if, you decide to machine the top/crown of your current JE pistons, and put some dish in them to reduce compression ratio,,,,,be advised that for every 2.81 grams of weight removel, you have given yourself 1 cc of volume reduction. Restated; 2.81 gms. = 1 cc.

The reason I'm saying this is, because it is quicker to weigh a piston, than to "pour" for volume using a burret.

Uncle Charlie

Thanks Charlie, that some useful info especially when you dont have a cc stuff but can borrow someones weightwatchers scale lol.

weightwatchers sale

thats funny

Hogwash wrote:what about putting a thicker head gasket on?

that's the wrong thing to do with pump gas. Need quench to stave off detonation and keep flame travel right.

I may be in Montreal in November if you need some help.

dfree383 wrote:

Hogwash wrote:what about putting a thicker head gasket on?

that's the wrong thing to do with pump gas. Need quench to stave off detonation and keep flame travel right.

I may be in Montreal in November if you need some help.

So would like 40 thou or 47 thou be an okay thickness for pump gas? That's what I have.

Low .040's is about as low as you can go safely. With the big bore in a 460 piston rock and contact with the heads becomes a problem any tighter.

Doug Rahn wrote:

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.

You were correct the first time ... the #1018 head gasket is 11.something cc's.

rmcomprandy wrote:

Doug Rahn wrote:

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.

You were correct the first time ... the #1018 head gasket is 11.something cc's.

I see what you're saying Randy, he included gasket volume with his spec. When I did the first calculation on my spread sheet I added in another 10. something cc's to the mix. Spreadsheet I have will figure gasket volume, top ring land, etc.

I wish I knew how to post up Excel spreadsheets on here, so everybody could use them. I have some really good ones, compression ratio, piston speed, gear calculator, 4-link, and a few others.