Raise Compression

[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.

[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]

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

[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.

[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

[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?

[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.

[Superdave]

.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?

Basically all you are doing is reducing the dish size.. i had the measurements calculated a while back.. I'll dig out a 3400 piston and do it again tonight if i get the chance.


Most shops charge around $65/hr for machine work.. some might take 2 hrs to do all 6 pistons. they'll usually bitch and moan about having to press out the pins and remove the rods.

[34blazer]

meh, i wouldnt go into milling the piston tops to reduce dish vol. youre weakening the upper ring land that just loves to break on these engines. SI valves with an undercut and the ironhead gaskets will help.

[Superdave]

meh, i wouldnt go into milling the piston tops to reduce dish vol. youre weakening the upper ring land that just loves to break on these engines. SI valves with an undercut and the ironhead gaskets will help.

he can't run the iron head gaskets without cutting the pistons.. he'd have .020" piston to head clearance that way..


there is alot of meat on the 3400 pistons above the upper ring land, .020" shouldn't make too much of a difference in the overall strength.

[Monzsta]

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

3700 rpm according to my data logs before the forward clutch began to slip.

That's with a FAFB Pontiac Sunbird converter, ported heads, and a tune. Stock everything else and a 3100 t/b. FYI, a stock W body 3100 converter stalls at 2700 rpm.

[3400-95-Modified]

Yeah, I'm trying to stay away from a 2700, thats why I went with 2095... the stall i have now is just way too agressive.

And for those that think .020 is alot off a piston... Take a set of calipers, set it at .020" if you dont have a set, for reference a standard CD is about .055" thick... so less than half of that thickness... I gotta double check a 3400 piston and I do remember quite a bit of material being up there, and Yeah I was gonna say what dave did... .020 quench is asking for issues.

All this compression talk may take place AFTER this year when the car is getting painted... I'm not totally sure yet on what I want to do. There is still quite a bit to finish up the overall car and if I can get low 14's with this combo right now then I'll leave it alone for a little while.

Ben how much are the SI valves and where can I get them from? You dont have them listed in your store, just in the head assy's

[IsaacHayes]

How come you are not running way faster than Asylum? Is the 4T60-E really that much of a power hog over the 3TXX?

Hopefully it's just the stall eating up a ton of power. I still think a stock one would be good for you, and it would be higher stall anyways with the power you are making.

But more compression always helps with power. That's why the LS1/Quad4/etc have high compression stock. And that's why when they were messing around with engines to get better emissions back when they first started caring, they tried things like lowering compression and those motors horsepower went down the toilet. No whither or not the decrease from 9.8:1 to 9.15?:1 is enough to take away a big chunk of power I don't know. But going higher than stock would be nice too.

[loner666]

This is why Im putting 3400 heads back on if I keep my motor setup. I want my compression back, maybe alittle more since Ill get forged pistons and Ive heard/read the 3400 head flow better then the 3500 when ported for boost, or am I wrong still?

Either way, the decision of keeping this motor when/if my trans goes again is still up in the air. Id really love to put it into an Imala

And Jon you bastard, why couldnt get make a choice on that convertor before I did my trans agian? I could go for a higher stall.............once I have some better rubber for the track. I cant even fully stall right now, which would be around 3k or I make a smoke show all the way to the 1/8. Damn street tires and open diff

[IsaacHayes]

Impala is just more weight! Like 400lbs at least.