I tune 90% of turbo s2000s with thicker headgaskets. This is what customers bring into my shop for tuning, not what I install. I feel more comfortable having the customer running lower compression on their engine while tuning for what its worth. The whole quench debate has been going on for awhile, and is something that as a degree'd ME engineer I can fully appreciate and agree with. With customers running log manifolds scavenging goes to crap, back pressure is high and ultimately the knock threshold is much lower compared to an equal length manifold.

On a side not, a really simple way to determine how well the exhaust manifold/exhaust flows is where the vtec engagement point is set. On a good manifold (FR, Lovefab, etc) I can have vtec engage as low as 4,000 rpm. With a log manifold vtec is usually between 6-7,000 rpm range.

Since the knock threshold is much lower on a log manifold, keeping the stock 11:1 compression is detrimental to making higher power on pump gas. Anything more than 11-12 psi range with a log on a decent size turbo (57/60 trim t3/t4, 3076r, gt3582r, etc) with 93 octane the car will start knocking. You can see it on the plugs, and see it out the exhaust if you know what to look for. Power output is usually around low 400whp range when this occurs, and timing is turned back ALOT in order to keep combustion stable. Same car with the thicker headgasket, can run 18-20 psi range and make another 100whp on the same octane.

Since the f-series and k-series engines are very similar, the knock thresholds are as well. Kpro has an excellent knock sensor threshold in the software for logging knock with the stock engine (i.e stock valvetrain, pistons, etc). Once forged pistons, or aftermarket valvetrain is installed the sensor tends to pick up ghost knock, or noise making the knock sensor read like its getting knock. I have tuned 200+ turbo k-series at this point with kpro. I have seen first hand how the stock 11:1 compression reacts to different turbo set-ups on 93 octane. Good turbo set-ups like FR kits are able to run more boost and timing before the knock sensor picks up knock count, and I confirm this by reading the plugs. Log manifold set-ups like Greddy kits, etc get knock count at much lower boost levels and run much less ignition advance. Backpressure and scavenging make all the difference in what you can get away with.

Getting back to the question at hand, if you are running a log manifold and want to make alot of power on pump gas get a thicker headgasket. If you are running an EL manifold with a efficient turbo set-up like the creator of this post you can get away with stock compression. I am by no means knocking Inline Pro or other log turbo kit companies. The kits are nice, fit well and certainly have a place on the market. People need to realize that there IS a big difference in how backpressure/scavenging plays a big role in knock threshold on a given fuel octane level. You can get away with ALOT in terms of boost/timing/compression when you are in the 1:1~1.5:1 ratio between intake/exhaust pressure levels.