Dyno Results On Your GTS Project!

[MKIV]

Wanna hear something crazy? I ran 18psi on my RB20 on an RB25 turbo. The boost creeped to 20psi at times.

:|

[Skym]

Headgasket is changed when fitting bigger turbo, boosting up. I was told by tuners, over 1 bar.

If you have seen the seal difference (frayed seal with stock headgasket, goes from thin to thick), you would fit a metal headgasket that has consistant thick seal around cylinders. Also stock headgasket can be crushed by head easily with higher engine temps which happens on racetrack, but metal headgasket has some resistance to this.

[MKIV]

What's the recommended headgasket for the 20/25's? I hear Tomei makes a 1.1mm metal head gasket?

[Skym]

Depends on what compression you want. General rule of thumb is if want to run high boost (up and over 20psi) reliably with stock bottomend or forged bottomend on RB20DET, thicker head gasket to lower compression (1.8 Tomei for forged or 2mm HKS metal headgasket for stock piston) to get it into 7's with compression (similar compression to low compression version of Group-A RB20DET engines with forged pistons that run around 24psi all the time with T3/T04E turbo for around 448rwhp and are reliable for 3000km before rebuild is needed, which is the same rebuild km on RB20, RB25, RB26 with wet sump). But you sacrifice offboost performance with thicker headgasket.

Factory use 1.2mm, 80.5mm gasket for roughly 8.5:1 compression on RB20DET, which produces good off boost performance but limit's boost level that you can run reliably.

Rough compression, thickness of headgasket with stock pistons, aftermarket Tomei pistons is mentioned here for RB20DET -

http://www.tomei-p.co.jp/_2003web-ca...20_gasket.html

RB25DET runs higher compression (like a NA engine with higher compression pistons) and when combined with VVT, extra capacity gives it better off boost performance and I think uses a similar thickness (1.2mm) gasket to lower compression -

http://www.tomei-p.co.jp/_2003web-ca...25_gasket.html

With RB25DET you fit GTR crank, conrods, forged RB26 pistons to lower the compression. I think it's 20psi with bigger turbo (similar to GT3076R turbo), that's the reliable boost level for RB25DET with metal headgasket before dropping compression.

I noticed many tuners have come to the same conclusion (after blowing engines up) that anything over 20psi on RB engines, it's recommended to change pistons to forged to prevent ringland failure. With E85 or similar fuel, that limit can be extended to over 30psi (RB20DET with metal headgasket) due to cooling affect of fuel (cooler cylinder temps, cooler piston temps, higher resistance to knock which is one of the causes of ringland failure).

[r33_gtst]

^lots of good info

[MKIV]

Bigger/thicker head gasket = Lower compression? Interesting

[Skym]

With compression it's a combination of piston crown design, thickness of headgasket, dome shape in head. Running a thicker headgasket makes that area between piston crown, head bigger hence lowers compression.

Here's a diagram showing what I mean with thicker headgasket, etc and formula to work out compression -

http://www.tomei-p.co.jp/_2003web-ca...02_gasket.html

Dome like shaped piston crowns are used on F1 engine pistons which helps to suck air into engine at higher rpm -

http://jdm-insider.com/Blogs/Eric/wp...7/11/sema5.jpg