CMW

Rennlisters,

As I have mentioned before my S/C set up is currently limited to about 5 PSI.
The 10.5 CR is something not to be taken lightly for longevity .

So we were asking about the thicker Cometic Head Gasket and if anyone here had any experience with it?

Also has anyone used the thicker Porsche Head gasket and to what effect?

Thanks,

CMW

I guess I should add that our goal is to lower CR to 10:1 IOT experiment with more boost and avoid any potential future pitfalls.

Imo000

If you have the heads off might as well go with a set of low compression froged pistons, they are the best way to go to lowering the CR. They are alot stronger than the stock cast units and with them you can turn up the boost to at least 10psi. The thicker gasket is some what of a band-aid solution.

mspiegle

I don't think we wanted to get that deep into it. Although that is probably the best solution.

Edit: So, where do we find some of those froggy pistons?

Rick Carter

I don't think using a thicker gasket to decrease CR is a good idea.

Ketchmi

We have used Cometic head gaskets with good results, no detonation on 10psi of boost. Using a .074 thick head gasket will drop compression by approx. 1/2 a point. There are tricks to making them seal though, contact us for a full explanation if you are interested.

bd0nalds0n

When my heads were off this spring, I went with the thicker Cometic gaskets. IIRC, they were 2x the thickness of the regular gaskets--but they make a size or two even thicker than what I have on. I chose the thickest they warranted to still have the same clamping force as the standard one--any thicker, and they don't warranty the clamping force, because there's an additional layer.

I spent some time talking to one of the techs from Cometic as I was trying to determine the thickness I wanted. His ballpark estimate was that the CR would be reduced by about half a point, although I don't think that's terribly scientific.

I also figured this might be some additional protection against bending valves if I experienced TBF (again), but this too is very unscientific and probably incorrect. How far do the valves open when they open?

You should expect some weeping of the gaskets after they're installed for a couple heat cycles. This seems to be common. I don't have any problems with leaks.

I will say, however, that I think the car feels noticably slower, although I have not ever dynoed the car. One of these days I should probably do it... My digital boost gauge reads 9lbs of boost right now, but it only displays in whole numbers, so I'm not sure at what point it rounds up. Andy and I think conventional wisdom says it should make somewhere around 8 with the pulley I have on now.

Lastly, I will also point out that I used to have some tip-in knocking if I floored it from a stop. That problem completely went away with the thicker gasket.

I realize this conflicts with an earlier post re: quench/squish, but I'm just sharing my seat-of-the-pants real life experience.

Hard to believe that Cometic would last as long as they have and offer so many different kinds of thicker gaskets if this was such a no no. But I'm not an engineer.

BC

Brian - I think with the thickness you used, with was the 8x one Ithink, you went down more then half a point.

Turn up that boost!

bd0nalds0n

To clarify, I think I used the 0.071 or 0.074 instead of the 0.045 that Cometic lists as stock.

Brendan, I'm not quite sure what you said, but I think I get the point. I have smaller pullies that I will install at some point. I need to get the car dynoed to establish some sort of a baseline with my current setup vs. what most other TS guys are getting, and then see whether any of the tradeoffs between higher performance and reduced longevity are still appealing.

mspiegle

Thanks for the info guys! This is sounding good...

mspiegle

How about head bolts? Is there any reason to try something like ARP?

This all came about because the TB/WP needs to be done, and we figure we can make things "better" by throwing in thicker gaskets while-we're-in-there. Are there any other upgrades that help reliability and longevity?

FlyingDog

Used stock pistons would certainly be cheaper than new aftermarket. How close are GTS piston dimensions to S3, S4, or M28-45/46 pistons? Would a piston swap to readily available (or not as readily available M28-45/46) pistons be feasible?

FlyingDog

Somebody recently recommended against ARP bolts. I think the recommendation was made to Brendan.

Imo000

Ahhhm, changing the head gasket during the WP/TB change is NOT a while we're-in-there procedure. The head gasket is nowhere near the t-belt. If that's all you are doing, I would leave the head gasket alone.

a4sfed928

I was told to stay away from ARP because the stock bolts were designed to the expansion of the block ARP are not.