maxgee

iTrader: (5)

I know the Supra guys and other imports use thicker headgaskets to lower CR, can the same be done with LSXs?

MECHAM

iTrader: (4)

yes sir

maxgee

iTrader: (5)

Appreciate the quick reply. thanks

fast01

iTrader: (16)

Yes it can be done that way, but it's not the best way. By using a thicker head gasket you're losing your quench and increasing your tendency towards detonation. If you have graphite gaskets and want to lower the compression a bit you can switch to MLS gaskets and lose about 2 tenths of a point (.2), but I wouldn't go too much thicker than that. I believe they're already over
.050" thick.

maxgee

iTrader: (5)

I thought lowering CRs decreased chances for detonation?

fast01

iTrader: (16)

It does...to a point. The newer, more efficient combustion chambers are designed with a large, flat area at the bottom of the chamber called the "quench area". This area corresponds to a flat area on the piston. When the piston is traveling up the bore on the compression stroke the piston forces the mixture out of these flat areas and into the actual chamber where the combustion process takes place. This is done violently and the piston should come within .040" of the head. This creates a more efficient burn and has a cooling effect on the combustion chamber thus increasing your resistance to detonation. In creasing this "quench" much beyond .060" or so will forfeit any of the quench benifits and increase the likelihood for detonation.

I got this from the Keith Black Piston website. It sums it all up a little better than I did. Hope this helps!

Excessive cylinder pressure will encourage engine destroying detonation, and no piston is immune to its effects. An important first step is to set the assembled quench ("squish") distance to .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance (the distance your piston is down in the bore). If your piston compression height (not dome height) is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of this piston that would contact a similar flat area on the cylinder head if you have zero assembled quench height. In a running engine the .040" quench usually decreases with RPM to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity across the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, and speeds flame travel after TDC to increase power. On the exhaust cycle, some cooling of this piston occurs due to the closeness of the hopefully cooler cylinder head. The power increase occurs because the shock wave occurs at exactly TDC on all cylinders, every time. It tends to make all cylinders alike and receive more identical flame travel speed. Spark scatter tends to be averaged with the TDC kick received from a tight quench.

The suggested .040" static quench height is recommended as a good average dimension for stock rod engines up to 6500 RPM. Above 6500 RPM, rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or thick head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will likely create more ping at 9.5:l than you had at 10:1. One way to cheat the system is to make sure the piston of choice is light on quench side and to make sure the piston of choice is light on quench side and heavy on spark plug side. As RPM increases the piston tries to **** away from quench surface, allowing a tighter quench at most all RPM. The suitable way to lower the compression is to use a KB Dish Piston. KB Dish Pistons (reverse combustion chamber) are desinged for maximum quench area. Having part of the combustion chamber in the piston can improve the shape of the chamber and flame travel. The Step Dish is sort of an upscale version of our reqular configuration. It allows some piston weight reduction and allows the quench action to travel further across the chamber. It is especially favored when large dish cc's are required.

maxgee

iTrader: (5)

Fast01, thanks for the info! If all goes well, I'm getting a turbo kit next month and doing the homework now for my FI.

fast01

iTrader: (16)

Glad I could help! I'm going FI as well. Should be setting up an STS over the next couple of months. I did lower my compression, but I bought a set of 6.0l heads from a member here to get it down properly. If you look around you should be able to find a nice set for under $400.

maxgee

iTrader: (5)

Yep, found a set for $400 ported and another stock set for $300.

fast01

iTrader: (16)

Good luck with the build. Keep us posted!

maxgee

iTrader: (5)

Will do, will post tons of pics during the install and video of the install....and plenty of shake down runs.