Dennis K

Up until a couple weekends ago, I had never seen a GTS crank or 968 pistons in person. I had always been curious about the 8 counterweight GTS crank and the short-skirt/big diameter 968 pistons. Jim Morton acquired both of these items recently and after we cleaned them up a little bit, I had the chance to snap some pics. I thought all of you might appreciate a little automotive part **** . . .

Here's the 8 counterweight, 85.9mm stroke 928 GTS crank. Note the cutouts at the two counterweights on both sides of the center main journal - this provides clearance when they drill the crank. Also the GTS crank has a longer snout than the S4 crank.






Here's a 104mm 968 piston


Comparison of 928S4 (left) & 968 (right) pistons - look at the difference in skirt length & compression height!


S4 (80's) vs. 968 (90's) vs. CP Piston (state-of-the-art)




Underside of the 3 pistons


Closeup of the forging on the CP piston (this thing is LIGHT). This is destined to live in the heart of Jim's monster 928 motor.


Ring pack comparison between S4 & CP


Eight 968 pistons ready for their next mission in a "mild" smoggable motor.

IcemanG17

Great pics.............cool stuff too......hmm if my math is correct.....968 pistons on a GTS crank would make 5.83L or a 356 inch motor.....very nice.....but how long rods would you have to use to make it? I'm guessing something custom....since 968's had an 88mm stroke, or will GTS rods work? I don't know how long GTS rods are?

86'928S MeteorGrey

Sweet. That is some candy.

Vilhuer

Rods would have to be slightly longer than standard 150mm. GTS rods have exact same dimensions as all other 928 rods.

Those 968 pistons are Mahle's. I suspect they will have to be modified to fit in at bottom dead center if plan is to use standard 95.25mm stroke crank. With S4 and perhaps also GTS crank there wouldn't be any problems.

FBIII

Boy those pistons have short skirts. How does that affect bore life?

ptuomov

Cool pistons.

The CP x-forgings must be great for high-rpm NA engine. Their web site has a couple of nice presentations about piston design.

Am I eyeballing that the CP pistons have centered pin? I was just learning about that stuff last week, and what I think I know now is that offsetting the pin will lower the maximum intertial loads near TDC and allow one to run higher rpms and/or lighter parts.

For a turbo engine, the stock S4 pistons are actually pretty damn good, as long as one runs high enough octane gas. I believe they can take all kinds of abuse with no problem, except high RPMs.

123quattro

Strange, I can't see any of the pictures.

Here is what my stuff looks like in the 200 20v.

Eagle rods good for 200 hp/cylinder.


Custom Diamond pistons with ceramic tops and teflon skirts. They directly copied the Audi ring pack design, but went from 81 to 82mm bore.

Tom in Austin

Hey, while we're talking about cranks, what exactly is a "knife-edged" crank? Is this visible in the pictures here?

123quattro

The counterweights have a V cut into the leading edge. Normally it's just a blunt face. It helps cut down on windage losses.

stock.

knife edged.

heinrich

Beautiful shots Dennis .... I hear you are the parts clean-up KING!! :chers:

danglerb

I forget which brand of rod it was, big end clamped down on one of those flat plates to measure stuff, pushing down on the piston end I could make it bend enough to touch the plate. The impression I got from the mechanic was that this was not a good thing.

GregBBRD

Dennis:

You're eventually going to find out that those are not 968 pistons, but S-2 pistons.....big difference.

John:

Chinese steel rods making 200 horsepower per cylinder=Dreaming

123quattro

They are the exact same rods as in this engine. I think they made 900+ on the dyno last year. I forget exactly how much. No one has ever broken one yet.

Castrol 034 MS Top Shop engine

GregBBRD

Chinese steel varies significantly, from day to day. You can get "lucky" on one batch and then have "crap" on the next. Pretty much why really serious people stay away. You'll almost never find Chinese steel pieces in an engine that needs to "earn" back the money it cost to build....usually only used in "hobby" engines.

mark kibort

Those middle ones are S2? (like 85-US 928s??) looked like S4, 968 and some custom, no skirt 968s

Here is a modex stroker crank!