Raise Compression

[WrathOfSocrus]

I have been thinking of doing something similar to that myself, but I wasn't sure how well it would work. If u take the amount that the piston protrudes off (.020) it slightly reduces the volume of the piston. Then the volume lost from the difference in gasket adds to it. It seems about 3.5cc lost from the gasket and I'm sure a bit less lost on the piston. In the neighborhood of 5-6cc total.

The only problem is if you use that setup and a large cam, there could be valve interference if u have to shave the head down. .020 on the gasket and another .020 shaving the head is a bit of space lost. I guess clearances would have to be thoroughly checked.

Dave, you say it would cost around $200 to have the pistons shaved? At that price it would probably be worthwhile to find new pistons. If you know someone who could machine them for cheaper I'm sure it would be a much more appealing modification to try. Unless you were including the price of the new gasket and head bolts, then that's not so bad.

[3400-95-Modified]

.020 isn't going to be enough to mess it up, I think I still have plenty of room in my lifters to accept more preload.

Either way as noted, converter is getting swapped soon and we'll see what happens there.

Also Dave, how can taking -.020 off the pistons and only dropping to a .040 crush gasket raise compression... isnt that a double negative there... or does the .020 off of the ring make the CC's of the piston low enough to raise it up with .020 off the heads as well? In other words, what is the piston CC's after the .020 is off them?

[MY91GT(Z)]

i didnt have any alignment issues w/ my heads bein shaved .020 on stock lower end and cam i just wasnt sure how it would be w/ ur cam thats y i suggested .010-.015 to keep it safe

[Superdave]

Well that plus the stainless should get me to 9.46... I would be happier with that and then I can still spray on top of it.

I ran the CR checker with 30.7 as the cc's and it came up as 9.36 and Ben said the SI valves are worth .1, so thats how I got my number.

I'll see how this converter wakes this car up and go from there.

.020" is about as much as you can shave off the heads.. any more then that causes gasket alignment issues. Also keep in mind that the more you shave off the heads also slightly changes the valvetrain geometry.

IMO once you have the heads off it would probably be cheaper just to pull the pan and push the pistons out the top.

it is possible to have .020" shaved off the tops of the pistons, it'll cost around $200 and if you use the .040" iron head gaskets you'll get closer to your goal of 10's.. or just throw in a set of Camaro pistons which sit about 0 deck and give 11.x:1 compresion.


if you want to get deep into it, search Summit for pistons with the same size bore as the 3400.. then compare compression height and pin diameter.. you might be able to find something that'll fit in a 3400 and give you want you want. That's how i found a few possible setups for the 3500 (305 SBC and 3.3 Buick V6)


power increase from compression alone isn't that drastic.. i dropped mine from 10.4 to 9.8 this year and made more power.

[3400-95-Modified]

brake torque is your best guess at stall speed or as close as you'll get, shocking it is a lower stall typically.

[IsaacHayes]

How do you test where your stall speed is again? I tried "flashing" the converter today and it broke the tires free at 1500 rpm. And I can peel out from a stop without brake torquing it, but if I do, I don't take off as fast and the tires don't break free...

[3400-95-Modified]

Well that plus the stainless should get me to 9.46... I would be happier with that and then I can still spray on top of it.

I ran the CR checker with 30.7 as the cc's and it came up as 9.36 and Ben said the SI valves are worth .1, so thats how I got my number.

I'll see how this converter wakes this car up and go from there.

[onefastV6]

Ahh here we go:

Stock 3500 Combustion Chamber: 32.4cc
Milled .020": 30.7cc or 31.7cc

Either way, you will need pistons get the compression over 9.5:1
I believe milled .020" you will be about 9.2:1

BTW: I didn't have any issued getting the LIM on and sealed.

[3400-95-Modified]

Whats the deck height of a camaro piston? Anything over .02 and its cutting it close... less and its not enough power, or just below optimal... stock 3500 is .05 quench, 3400 is .04

[IsaacHayes]

I forget what is the quench with camaro pistons and 300 heads? Too much right?

[3400-95-Modified]

Thank you....

Also was it difficult at all to get the LIM on with them milled 20?

[onefastV6]

I will have to do some digging through 2 year old posts, but If I recall we did CC my heads after they were milled .020"

[3400-95-Modified]

you dont have a set of 3500 heads CC'd with .020 off do you? because technically thats the only number I need to get an estimate

[SappySE107]

9.16:1 is what I got. SI will give you about .1. I don't know what shaving .020" off will do but I will have a general idea once I finish a set of 3400 heads I am porting for a customer. He wants .020 off and I already cced all the chambers (street/strip).

[3400-95-Modified]

is the CR calculator correct in saying I currently have 9.15:1? And what would the SI valves get me, and how far can I shave a 3500 head?

Bottom line whats the highest compression I can get with what I currently have?