Raise compression with head gasket?

[david86tr]

I am about to put a set of ported heads on my car and was wondering if anyone had exp. with uping the compression ratio to 9.0 to 1. With rjc head gasket kit. I plan on getting a larger turbo like a pt67 or a 70 in the very near future and dont want to run more than 21 to 24 lbs on race gas when at track and 15 to 17 on street. I'm trying to make it a low low 11 or high 10 second car with a conservative amount of boost.



mods now are
te44
gbody 21 row stock location
55lbs
forged internals (no girdle or caps)
340 hotwired
cam (not sure which one)
and other small bolt on downpipe exhaust.....

 

[bison]

Your not going to get 9.0:1 unless you run no gasket. With the thin RJC it should be around 8.4:1. You need to get the pistons at zero deck and run a .040in gasket and you will be over 9.0:1. To go further you need custom pistons.

 

[Boost231]

whats the stock compression?

 

[bison]

whats the stock compression?

Its right around 8.0:1 from GM.

 

[Boost231]

thanks my builder told me mine was right around 8.4:1

 

[david86tr]

so.... is 8.4 :1 noticeable over stock? anyone running higher compression or know someone who has? any input would be appreciated. Im just trying to go fast with lower boost levels

 

[GNVYUS 1]

I had my block decked and heads milled and even with the RJC gaskets my comp was not 9:1.

 

[bison]

The one i had in my blue car had the pistons at +.004. I used the thin cometic and 47cc heads and it was around 9.1:1. I think the dish on the pistons was 27cc. Bore was 3.82in. Not positive on the dish on the pistons and i dont have the pistons here to measure the dish. I used an online calc and got my numbers.

 

[bison]

so.... is 8.4 :1 noticeable over stock? anyone running higher compression or know someone who has? any input would be appreciated. Im just trying to go fast with lower boost levels

Anything is going to help over the stock 8.0:1 if the engine is built to handle it.

 

[jasjamz]

The one i had in my blue car had the pistons at +.004. I used the thin cometic and 47cc heads and it was around 9.1:1. I think the dish on the pistons was 27cc. Bore was 3.82in. Not positive on the dish on the pistons and i dont have the pistons here to measure the dish. I used an online calc and got my numbers.

Would you say this was worth the $$ spent to do it?
Did the sound of the car at idle change any?
Did you feel a change in drivability?
Do you need to mill the intake and compensate in the valvetrain?

Interesting thread!!

 

[bison]

Would you say this was worth the $$ spent to do it?
Did the sound of the car at idle change any?
Did you feel a change in drivability?
Do you need to mill the intake and compensate in the valvetrain?

Interesting thread!!

I dont have a before and after on the sound since i dont build them under 9.0:1 anymore. Driveabilty increases with added spoolup, better mpg, and more power and torque everywhere. I didnt have the deck milled that much. Instead i had the rod center to center lengthened a little to get the pistons at zero+. The one in my black car (sig) is a stocker with thinner head gaskets. Its a brute on the street at 24 psi. Especially in the cold weather we had last night up here. I think its worth it for most guys if they are trying to go 130mph plus.

 

[jasjamz]

Stock Or Aftermarket

I see stock short block in your sig. So you can do this using a stock rod? I think I want to try this 9 to1 idea asap!

Keep the info coming guys, Thanks!

 

[david86tr]

That is good info bison thank you it seems though that is the way to go.

 

[bison]

I see stock short block in your sig. So you can do this using a stock rod? I think I want to try this 9 to1 idea asap!

Keep the info coming guys, Thanks!

The one in the black car is a stock shortblock with thin RJC gaskets. Its 8.4:1. I have done nothing to the bottom end of that engine. The last few that i had machine work done to were 9.0:1. You can do it with a stock rod but you have to bush the rod and relocate the center to center on the small end a little which will lengthen the rod a little. Ive done it with stock rods long ago but i cant remember the bushing i had them use. They will be full floating when done.

 

[JDEstill]

When I was screwing mine together last year I got some of the old GM steel shim head gaskets. At the time I figured this took me from 8.0:1 to 8.7:1; the shim gaskets are 0.018" compressed. Haven't really got many miles on it yet, so I can't really report much, but so far so good. Back in the late 90's I did swap shim gaskets for stock, and noticed a reduction in throttle response and low end performance. I could run a pound or two more boost on pump gas however.

Part of my reasoning for using the steel shim gaskets this time around was the dynamic compression ratio, which is partly a function of the cam. This seems to be an important point with n/a cars, where bigger cams need higher compression to maximize performance, and maintaining a good dynamic compression is one of the ways they do that. With my 214/214 cam the 8.7:1 static compression gave me a dynamic compression that is just a hair higher than stock, so I figured it was a pretty good number, and should still run pretty good, esp. around town which is where I do almost all of my driving.

John