Im looking into a set of the LS6 GM replacement heads. They are 118CC and are supposedly a direct replacement.

If so can I used my closed chamber pistons? They are the stock Ls6 ones.

And my buddy has an L78 in which he needs heads for, so can he use them too. He has the 38.3CC domed piston, that is know for interference problems.

In both engines we are planning on using harolds 272/282 solid, or the 272 flat. both lift out around .530 on the intake and .550 on the exhaust.
The 272 is just .530 on both intake and exhaust. I took the lash out to get the correct lift.

Thanks

I believe the dome size for LS6 pistons is .280 which would be fine with open chamber heads.

Better check; alot of the closed chambers have a more square shape to them, with the dome on the piston similar; while the open chambers are often more oval. The "corner" of the more square dome sometimes hits the edge of the rounder chamber.

BTDT. Kind of messed up a set of pistons that way.

You generally can not use closed chamber pistons with open chamber heads.

Whoa...

First of all, all LS6 heads are open chamber.

Secondly, factory style closed chamber domes will fit factory open chamber heads. Yes, you should check anyways. You should be cautious with the L78 install, but it should work no problem.

I'm doing this right now, but my domes are only 32cc 427 closed chamber pistons, in 049 open chamber castings.

Thanks, Im always worried about the L78 thing. Those pistons seem to always have interference problems.

70 LS-6 heads are 291s. Definately closed chamber.

I thought the LS6 came with the 990's. Maybe I'm mistaken.

Originally posted by TJC:
Whoa...

First of all, all LS6 heads are open chamber.

Secondly, factory style closed chamber domes will fit factory open chamber heads. Yes, you should check anyways. You should be cautious with the L78 install, but it should work no problem.

I'm doing this right now, but my domes are only 32cc 427 closed chamber pistons, in 049 open chamber castings. =

Do not assume a closed chamber piston will fit in an open chamber head.
They will hit in a lot of the open chamber heads at the edges of the piston where it is squared off & need to be checked for clearance.

Monte-73, FWIW I have Speed Pro forged L-78 replacement pistons and #820 open chamber heads with no clearance problems. Factory pistons may be different though as other heads may be. Also the 454 larger bore would increase the chances of interference with my heads. I also have Harold's 272/272 solid cam with adequate piston to valve clearance. The cam is installed 5 degrees advanced per Harold's recommendation (4-6). My pistons were fly cut .040 on the intake side only, but would've been OK (.140 intake valve clearance) without it. It only has .110 exhaust valve clearance so anything bigger than the 272 on the exhaust side would've caused a problem.

Hows that cam run Von?

Is vaccum an issue? Does it sound similar to the L78 cam?

Do you think I would have troubles running the split pattern 272/282.

It has .02 more lift than the 272.

I guess im really interested in the comparison between the new cam and the old L78. Im thinking of either maximizing the compression to NHRA specs and leave the cam stock, or getting the open chambered heads and use the 272-282. THanks

Ps are those the 36.6 dome or the 38.3 dome. They make a couple for the 396's.

Hows it run on pump gas, do you know the DCR?

Are 820's rect port?

The LS6 replacement heads were either 990's (open chamber, 118 cc) or 291's (I have them, closed chamber, 106 cc approx.) You should check for interference just to make sure, but there should not be a problem...I ran the LS6 pistons with both the 291's and 049 heads, and no problem with either one. But I would still check to be sure.

Monte-73; The cam definately runs stronger than the stock L-78 cam I had. The idle sounds identical. I couldn't tell them apart. Vacuum is not an issue. I had it ground on 112 LSA. I have the Speed Pro L2242F pistons (65-69 L-78 replacement). Std bore dome is 39.5cc. .030 over dome is 38.3. Mine are .040 over at 37.9. The #820 heads are open chamber, oval port, with 113cc (spec) chambers. Mine have had the larger valves installed and some bowl porting. I have about .005 avarage deck hgt and my DCR figures out to around 8.2-1 if I remember right. No problems at all on 92 octane. With stock or Speed Pro pistons I think you'd have a problem with exhaust valve clearance with the 282 exhaust duration (it's the duration more than the lift that affects clearance), unless you have a ton of deck height.

I have a LS-6 crate engine, basically it’s the 1970 production spec. short block with open chamber rectangular port heads and a high-rise intake. The 70 chevelle engines had about 11.2-1 with closed chamber heads the crates dropped to about 10.2-1 with the open chambers. Now the 1971 production corvette LS-6 had open chamber heads and different pistons so it only had about 9-1. So unless you have the 71 vette pistons you should have a decent compression ratio.

Joe