epfatboy

iTrader: (3)

So I’ve been doing a little research on a LQ9 or LQ4 with 799 heads.

Due to inventory/cash constraints in my area it looks like a LQ4 is going to be the route I take.

My question is which head gasket to use and why.

Since I’m running a dish piston and a cam that won’t have PTV problems (summit pro ls stage 2) why not run something like a .030 compressed and preserve as much compression as I can? I plan on a new set of head gaskets anyway so why not add a few bucks and adjust my compression in a more favorable direction right?

That said where do I stop? I have seen a .026… I know proper quench is a thing but unclear how a dish piston effects the value.

G Atsma

Most LS pistons come out of the hole around .005, BUT that varies with each engine.
You want around .030-.035 quench, which is why .040 gaskets are mentioned often around here.

Che70velle

iTrader: (6)

This has come up a couple times lately. Most important thing here is to correctly measure and find how far your pistons are out of the hole. You gotta use a deck bridge with dual indicator gauges to find this value. Once each piston (gotta measure all 8) is at true TDC, rock each piston until both gauges read the same. That’s your true out of the hole number. You’ll spec your headgaskets from the hole that’s farthest out…they won’t all be the same because of production tolerances and where the rod bearing is positioned on the journal.
What I can tell you for certain here is that Tony Mamo tried .028 quench and it failed at high RPM. .030 would be as tight as I would possibly go in a perfect world if I was shooting for a max-effort dyno record number. .035 is where I’m comfortable at for max-effort street builds. It’s a lot of work to figure, but it’s worth the effort if your going max-effort.
Now I can also tell you that if your not chasing dyno numbers, or looking for a max-effort build, you’ll see no difference in going with .040 quench vs .035.
A dished LS piston will not matter vs a flattop LS piston in determining this value. The portion of the piston crown that will actually position itself undear the “closed” part of the chamber…where quench is measured and it actually matters…is flat.

epfatboy

iTrader: (3)

Chevelle, thanks for all that!

This is a JY build soooo, maybe I’ll stick with a .040 and call it a day.

I probably will try to measure how far out of the hole the pistons are…

epfatboy

iTrader: (3)

WHOOPS, Found an LQ9 and bought it.

So maybe I do care a little more about PTV or maybe I care a little more about Compression Ratio?

The cam is a .226 dur @..050 and .600 lift on intake, I believe the clearence is there with a .040 gasket but plan to measure, it's for sure there with a .051, a friend did one and does not have his measurements anymore but said he had more than plenty. I just don't know if PTV is as important to me as Compression and pump gas ratio.

with a .040 and 799's (64.5 cc) thats just shy of 11.4cr
with a .051 and 799's thats just shy of 11.1cr

I'll probably run 93 octane most of the time but would rather plan on using 91 octane as I'm sure I'll find myself screwed if I don't. I read about and know folks who are running up to 11.5 on 93. I feel thats about right and I'd feel comfortable pulling a bit of timing at 11.4 just to be safe. I don't feel that way about 91 octane and 11.4, I'd want to pull more timing than I think I'd like.

So any thoughts on just using a .051 so that I don't increase CR too much for pump gas as well as knowing ptv is good? I know effcient quench = improved detonation control... Does a .051 create that "inefficient" of a quench that 11.1cr has the same risk as .040/11.4 combo?

and not for nothing this is a JY build... 11.1 isn't doing this old bullet any favors as it is, never mind 11.4 so I really think .051/11.1 is where I'm heading unless I'm missing something?

G Atsma

A .040 HG with .005 out of the hole is only 10.54:1 compression.

epfatboy

iTrader: (3)

64.5cc

G Atsma

OOPS forgot about the 799 heads!
11.39:1 with those heads.... so yeah....