Is it A good idea to go with A thicker head gasket to lower my compression without hurting the sealing capabilitys.
I have LQ9 heads with 71cc comb. chambers and 14cc dished diamond pistons, on my LS2 402 stroker. I think that should give me 9:1 comp. ratio? I was thinking of going with A .060 cometic head gasket to get me even lower? Please chime in if you have any info on this matter.

 

I am by no means an expert here but would think the level of boost you want to run helps to answer the question. If you don't want to go too with boost I would think 9.1CR is a good place to be.

Just my .02

 

how much lower will that gasket drop you. probably not engough th warrent thicker gasket. i told you that the studs are real important. call arp and as them about L19 like i got.

 

I do plan on using ARP head bolts...

 

My suggestion would be to do a little research on quench... sometimes thicker headgaskets can obtain a desired CR... but could introduce other problems. Notice the emphasis on could. Lots of inputs make it a hit/miss variable... but I would just assume stay away from the guess work as much as possible.

FWIW, I am not a fan of thick headgaskets...

Bill

 

I used Cometic .065" MLS gaskets. The Stock GM Gaskets are .054". Every 6cc is .03 a compression point...if I remember right

 

arp makes different grades standard then L19. L19 are stronger than 1/2 stadard arps! those words from their engineer not mine

lifting heads is what will limit you from max hp assuming your using c16.
(from what i have heard)


BTW LISTEN TO BILL

 

Would .065" be over the stock thickness (.065" + .054")?? How many conpression points will it go down in relationship to raising the head?

 

go to ross piston web site.

 

I agree with Bill about effecting the quench area. Effecting the quench area can be worse than having a higher compression motor. If you have a stock motor your quench would be the gasket thickess minus the deck height. The average deck height I have seen on an LSx is approx. .007". So if your pistons are out of the hole .007" and your head gaskets are .054" the quench area would be .047". For Turbo applications you can go with a little more quench, at least this what the head porter for a (30lbs boost) 7 sec Acura NSX told me. The range varies...some heads are milled .030"-.040" to bring up the CR, which makes the quench area in the low 20;s. I have been told not to go over .060" quench, and I have seen some LS1tech Fbodies at .060"-.070". If you go with a .065" thick head gasket your quench area will be .058" which is not bad for Forced Induction. But if you are 9:1 you might as well try it out before switching to a larger head gasket. You can always replace them. I thought about going to a .054" which would make my CR more like 9.2:1 (my deck height is .014") but I went with the .065" to make it 8.8:1.

Click on link for the NSX
http://www.proheads.com/index.html

 

bill or kevin, how or if it does at all, does dished pistons affect quench??

what would be the diff. if you had two ls1 motors that had same compr.
but one was:
59cc heads w/-21 cc
71cc heads w/-8 cc

i am not sure that those would equal but assume they do???

 

READ THIS!T


THIS WILL ANSWER YOUR QUESTION




Quote:
Excessive cylinder pressure will encourage engine destroying detonation with no piston immune to its effects. The goal of performance engine builders should be to build their products with as much detonation resistance as possible. An important first step is to set the assembled quench distance to .035". The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston 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 the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness to the water cooled head.

If you are building an engine with steel rods, tight bearings, tight pistons, modest RPM and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Turbulence is the main means of reducing detonation. Unfortunately, the operating quench height varies in an engine as RPM and temperature change. If aluminum rods, loose pistons, (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit the benefits of the quench head design and can cause severe detonation. The suggested .035" static quench height is recommended as a good usable 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 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 create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston. Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. High performance motors will see some detonation, which leads to preignition. Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before top-dead-center. Detonation damages your engine with impact loads and excessive heat. The excessive heat part of detonation is what causes preignition. Overheated combustion chamber parts start acting as glow plugs. Preignition induces extremely rapid combustion and welding temperatures melt down is only seconds away!


__________________

 

Wow thats some great info thanks! I think I got it? So Should I use stock thickness or .065 I do have billet rods?

 

So if thicker gaskets are not good for reliablility or performance, why does cometic make up to a .120 gasket for the LS1?

 

It's good info but too generic, and while applicable does not even fit the criteria for the LSx from the factory. Many of the LSx Heads and Cam cars that have some of the fastest (Cartek, LG, ARE, Vette Drs) have milled heads to bring the CR up and some are still using the stock flat top pistons. What you don't really want to screw up is the characteristics of the chamber. Like I mentioned above NOS and FI applications can benefit from more quench. What you really don't want is detonation to far advanced of TDC, this is why head gaskets will fail (and is better to have fail then a piston, unfortunately the LSx ring lands appear to fail faster than the MLS head gaskets under stress).

Notice LSx pistons come out of the hole and closer to the top of the chamber. GM Engineers could have designed the LSx so that it had a ZERO deck height and used a .035" Head Gasket to fit the above criteria. Instead the piston come up out of the hole and the use of the MLS gaskets all but eliminates total head gasket failure.

 

kevin plaese giveme your opinion on post 11# on this tread




thanks

 

OK this is how I see it.
Depends on what arguement you want to make. However, if building a dedicated FI motor I would use a dish piston. The dish in the piston will not negatively effect the quench area. Afterall the whole piston is not dished, so as the piston moves towards TDC the flat part is compressing normally and the dish becomes the fire pocket. If I was building a motor I knew I was going to use for a lot of boost it would be with dished pistons and a smaller chamber (closer to stock). Obviously the more material in the head and the greater heat absorption (hence strength) one could make the same arguement about removing material from pistons too. However today's forged pistons are very strong (you can have them cyro treated also).

Now there is my setup and many others with FI applications that have merely increased the CC size of the head and used flat top pistons with valve reliefs. From what I have seen, this method while not a first choice by the book so to speak, seems to work pretty damn well for the multitude of people using this method. So therefore I would say it is a good method. I always like a contingency plan for example. Lets say I have a forged shortblock and have chosen to use larger CC heads to lower the CR and I add my power adder. Now lets say I want to ditch the car for a new Z06 (not happening but would be cool). So I decide to return the car to NA, selling the TT kit and heads of course. All I have to do is buy some stock heads or AFR heads and out of the box I have a 400+rwhp H&C car that looks like stock and would sell very easily to somebody. In essence, simply using heads and head gasket thickness leaves many options on the table. Versus had I done a big dished piston and with to put stock/afr heads on the car. I would have to rebuild the motor to get good NA performance again. I am not a rich vette owner so I need as many options as possible. In fact last year I had all but sold this car, but a twist of fate allowed me to keep it and now I am ending another project....sometimes I think its more like a curse.

 

hey kev thanks for your reply and time

well i got a feeling that this latest curse is going to have a real happing ending

true, switchinng heads back to smaller cc will bring it back to acceptable compression for NA. in my case with my current 59cc heads, i could throw a set of 67cc heads and drop my compression even more, 20 psi on pump. i think if i went with 73cc i'd be near 7.5 compression



thanks bro