Raceboy

Machined 1mm from the top. Middle part of the piston has not been touched. They sit lower in the bore.

Thom

Stock torque settings on stock studs?

Raceboy

Stock studs, torqued to 100 Nm (factory suggests 90degree iirc).

V2Rocket

so you have ~1mm deck clearance now?
stock headgasket? that's a wide quench/squish area, nice to know that the engine can tolerate at that kind of boost.

great numbers. do you know what your static CR is with the pistons like that?


running the numbers a 944S piston under an 8v head is right about 9:1CR, and a 951 piston under a 944S head is about 9:1.

my math puts a 944S piston with 1mm deck clearance and stock bore/stroke at about 9.7:1.

Raceboy

Yes, that was approximately my calculation too, 9.6:1.

Markus with his Jägermeister 951 and 968 turbo engine had even more quench area (he used 3mm Cometic iirc) and that engine worked well until something went wrong with the boost control when it was already on Duke's car.

Thom

Any reason other than the used stock headgasket to torque the head slightly tighter than the factory spec?

Raceboy

Nothing else other than experience of my friend Kert (cmf here in Rennlist). Also as a former automotive industry quality manager I am a bit sceptic in all that degree stuff when it comes to torquing things. Either use stretch gauge or specified torque in controlled environment (lubricant, temperature etc). But that's just me.

Regarding the build, I guess my project has proven that budget build of 16v turbo is possible. I have covered ~8000 km's in the current configuration and with all things done correctly (i.e. don't reuse headgasket lol) 500hp is realistic goal for durable street engine. Not talking about prolonged track performance, just fun street motor that can withstand some spirited driving and regular dragraces

V2Rocket

8000km at 400+ hp on a used headgasket is pretty impressive nonetheless!

Thom

Didn't you use 951 pistons initially though? I remember pics of "drill valves" to cut the pistons... Are S pistons forged?

Not sure of what it takes to overtighten a headgasket, but I initially used Raceware studs on my 3l that I slightly overtightened and the headgasket leaked within the first start up. Upon replacement it showed no sign of damage, but probably the rings had not enough room to expand when hot?
Been running stock studs and an original 968 turbo headgasket since, no leakage with 1.6 bar of boost.

Raceboy

I don't think it was due to overtightening, my guess is that the headgasket was just damaged (the areas in the coolant jacket were concave after distilled water froze last october). Didn't think much of it back then and reused it, obviously it was a mistake. Glad it was a cheap mistake lol.

Regarding pistons, yes, I modified 951 pistons first but decided to use 944S ones after finding out during assembly that the 951 pistons had the iron coating worn off more than I would have liked.
944S pistons are not forged afaik, they look like a usual cast piston.

Voith

I think this is more due to high CR than reused headgasket..

Which gasket will you use next?

Raceboy

Lol, you may think what you like, I do things by empirical data.

If CR is so important why even stock cars blow headgaskets?

Voith

I have never seen an stock car head gasket where all rings came out deformed that much. Looking at that gasket it was a matter of time for other cylinders too which to me suggests peak cyl pressure exceeded hg capabilities all across the board.

Will you use cometic for the new one?

Raceboy

Nope, will use stock one.

And I have seen plenty of similar failures, this is the most common failure mode if block is open deck type.

Raceboy

Meanwhile, before the headgasket went, we shot a few photos with my friend who is a professional photographer.