Which head gasket would be better for an NA-T application; a stock 1.3mm TT head gasket or a 2.5mm gasket from SRT? What's the pros or cons to having one gasket almost twice as thick as the other?

 

There isn't an *advantage* so to speak from the thickness, it's more based on how much boost you want to run as it alters your compression ratio. A thicker gasket will give you lower compression which will allow you to run more peak boost.

It's more like a trade off, if you want to make max hp then ultimately a 2.5mm gasket will allow you to run the most boost for any given octane level. The price you pay is that the rest of the rpm range (bottom end for example) will be less responsive and make a little bit less power. Most high hp non-forced induction cars run a very high compression ratio which requires high octane fuel to avoid detonation, but this allows them to make big power with out a turbo or supercharger. A turbo/supercharged car on the other hand runs lower compression so that the boost can be turned way up and make big power through forcing air into the motor instead of just using atmospheric pressure (NA) with a high compression ratio. Did I confuse you yet?

If it was me and I was building a drag car or would be running high octane (race gas) all the time I would go with the thicker head gasket and turn the boost way up. If I was building a street car/daily driver I would run the TT gasket which will lower you down to roughly stock TT compression ratio and run it so you still have some bottom end responsiveness as well as a nice top end hit.

 

Actually, I think running the TT headgasket only drops the comp ratio down to 9.5:1 on a VVTi GE engine. The 2.5mm HG drops the CR down to around 8.7:1 or so. At least I think that's what it was the last time I did my calculations. I could be wrong though.

The non-vvti GE motor has lower comp than the VVTi motor. So, on those putting a 1.3mm GTE HG on will result in a CR very close to the 8.5:1 of the stock GTE.

 

That sounds about right, going through the calcs for a 2mm comes up with roughly 8.9:1 on a Vvti motor so you are probably right with a stock TT gasket being in the low/mid 9's. You can still run a TT gasket if that's what you already have (still much better than stock) and you will be fine at moderate boost levels with a nice bottom end.