Proper headgasket choice

[Chiba_kun]

Just a few quick questions...

Stock headgasket thickness on an RB25 is 1.1mm correct?

I've been told that going with a headgasket that is thicker than stock can sometimes cause detonation. Is this true?

What headgaskets are you guys running? AND, who did you go through to get them?

I like the Cometic HG price, but I know a guy who has blown 2 of them in the past month and 1/2 on his 500+whp Evo8. I wont be putting down quite so much power, but I'd still rather not have to go through what he is.

Should I spend the $200+ on an HKS or Apexi?

[Imissmyturbo]

I don't think RB's really have HG issue's with the stock ones. What kinda power are you looking to make?

[themadscientist]

a thicker head gasket would lower the compression and raise the detonation threshold (the reverse of what you were told). The RB25DE has high compression for a stock turbo engine, 9:1. This is fine over here where 100-octane flows like water ($4.00 a gallon water) but stateside I think 92 is the best you can get right? With that in mind I would lower the compression to at least 8.5. And before someone swoops in and starts telling me about all these drag cars that run 11:1 and huge boost let me remind you that those cars are not street driven, run on race gas, have a squad of top-notch wrechers keeping it alive. For the street and pump gas I would'nt play with high compression and boost.I would reccomend dropping coin on a quality gasket. HKS, Apex and Tomei make excellent peices. I always get the stopper type vs the bead type for maximum strength. You are replacing the gasket for insurance, why get the cheap policy?

[skillzilla]

I have a 1.2mm GReddy hg, got it from takakaira. The shipping was fairly quick, and I was happy with the quality.

BTW we have 93 octane in GA, which is $3.39 a gallon water

[Wulfgang]

The guy that told you that about detonation was correct. Although increasing the headgasket thickness DOES decrease the compression ratio, it also ruins the squish band. If you don't know what a squish band is, then you can Google it, but suffice it to say that the squish band is the PRIMARY detonation reducing feature of the combustion chamber.

The purpose of the squish is to cool the combustion gases so that they do not reach detonation threshold temperature. By increasing the headgasket thickness, you make the squish volume larger so that it does not sqeeze the gases as much. The gases thus move slower, transfering less heat to the walls and are therefore more likely to detonate.

[Joe]

but, if you are running more boost, then you are getting more pressure in the cylinder wich *should* let the squish do its work, correct?

[Wulfgang]

Not necessarily. Velocities are dependent on volumetric effects (geometry), not density. For example, if you fill a cylinder with 0.5 L of air and squish maybe 0.1 L of that air out of a squish band that occupies say 1 sq. cm at TDC, then over the 10 ms or so of the stroke, you've created a velocity of (0.1 cm^3 / 1 cm^2) / 0.01 s = 10 cm/s. Increasing the density (by running more boost) will not change that velocity.

Since squish works by creating large velocities, it is not a good idea to increase piston clearance. If you can find a way to use a thicker headgasket while keeping the squish geometry the same (for instance by planing the head or decking the block), then I would go with that.

But like I've said before, it is my opinion that aftermarket headgaskets are completely unneccessary for boost below say 20-25 psi. If you're blowing headgaskets, then you're probably detonating a little bit. Fix the detonation and keep the OEM headgasket. It's really easy to blow a headgasket even on a stock KA with no boost if you overheat and detonate. But that's just my opinion.

[Wulfgang]

Btw, I just pulled those numbers out of the air. Obviously your engine is screwed up if the squish is only making gas move at 10 cm/s.

[themadscientist]

must high-boost motors here have the squish area ground out and the chambers polished.

[Wulfgang]

I see that JUN does that also, but I cannot figure out why that would help. The only way that makes sense (to me) is if there is no other option to reduce compression ratio.

There was something about Ford/Cosworth experimenting with squish, and in every case the engine with a properly designed squish area outperformed the open chamber designs. But of course those were all NA.

[BoostFab]

I've been told that going with a headgasket that is thicker than stock can sometimes cause detonation. Is this true?

no! it's the opposite. thicker HG will decrease the posibility for detonation.

What headgaskets are you guys running? AND, who did you go through to get them?

I like the Cometic HG price, but I know a guy who has blown 2 of them in the past month and 1/2 on his 500+whp Evo8. I wont be putting down quite so much power, but I'd still rather not have to go through what he is.

i'm working assembling the my rb25 with a 1.5mm Cometics HG, after a long unsuccessful search for one form HKS/Greddy/Apexi. can't beat cometics great price.doesn't matter how good the headgasket is, bad-tuning will break your motor.

[themadscientist]

My buddy used a 1.8, Apex if I remember well. stock turbo, GT-R intercooler and SAFC 300hp on the chassis dyno. That is of course on Japanese high octane.