Rob Edwards

Just for posterity, here are GT vs. R1 GTS rods- exact same length.

Compression height on a GT piston is 42.8 mm, compression height on a GTS piston is 39.3 mm (my measurements..) . So with an S4/GT crank stroke of 78.9 mm, vs. GTS stroke is 85.9mm, the diff is 7 mm. So if the deck height is the same, and the rods are the same length, the compression height of the GTS piston is 3.5 mm less than an S4 piston.

heinrich

Thanks Rob. Why would ordinary old 5-litre rods not work perfectly fine in a GTS engine?

Rob Edwards

Dunno- are there clearance issues with the thicker shoulders on the GTS rods against the girdle?

Also, the GTS (R1 anyway) rods are a hell of a lot lighter than the GT rods, might create a problem with balancing?

GT rod: 846 grams

GT piston: 616 grams

GTS rod: 710 grams

GTS piston: 524 grams

GT crank: 25280 grams

GTS crank: 28436 grams.


So a set of GT rods and pistons is 11696 grams, vs. 9872 for GTS parts. That's a 4 pound difference. But the GTS crank is ~7 pounds heavier.

I won't pretend to understand all the factors in engine balancing, and I don't know how much the earlier forged S4 rods weigh, but it seems to me that you might run into balancing issues (?)

Then there's the matter of the cost of GTS rings....

heinrich

I have all those parts. And while my R1 rods are perfect (I have then in my hands) Greg has promised me he will personally shove them up my tailpipe if I use them. So... assuming R2 rods are heavier as well, is there a reason to not use 5-litre rods? Anyone? Bueller?

brutus

Rob may be right about the balancing you have to ask Why would Porsche design and produce the GTS rod if the old 5 liter rod would work just fine ?? You might need a bunch of heavy metal added which is expensive. I wonder how low the compression would be using GTS pistons on a 5 liter crank with S-4 heads or 85-86 heads ?

Vilhuer

Balancing and compression height are two big issues. 5L and 944 rods are so much heavier that I suspect even ultra light aftermarket pistons would require added weight into crank counterweights. In any case its possible to calculate if its needed when GTS and replacement rod big and small end weights are known. Formula is in archives in some old stroker thread.

So easiest option is to get custom GTS compression height pistons and either 2R or light custom rods. Without suitable pistons everything else is useless.

GregBBRD

And then there are the people who will tell you that if a piece of steel (connecting rod) makes 100,000 cycles, it will last forever....

Porsche added material to the R1 rod, right were the beam meets the cap section.

This additional material no doubt added some stiffness to the rod, however the location of the additional material would make one think that Porsche must have been seeing cracks, in this area.

Magnafluxing some of these rods would be interesting....

daveo90s4

Brutus, my calcs suggest that GTS pistons mated with S4 crank would yield static CR of 9.18:1. This on the basis that S4 and GTS are both 10:1 and S4 and GTS heads have the same chamber vol.

My logic as follows:
S4 swept vol = 619.9 cc / cyl so at static 10:1 CR unswept vol = 61.99 cc / cyl
GTS swept vol = 674.9 cc cyl so at static 10:1 CR unswept vol = 67.49 cc / cyl
S4 crank and GTS pistons swept vol = 619.9 cc/cyl and unswept vol = 67.49 cc / cyl so CR = 9.18:1
Caveats as above.

cfc928gt

Weight and balance being an issue, how much more do R2 rods weigh over R1 rods? Does adding the addtional weight into an early 5.4 engine by swapping R2s for the R1s without some sort of rebalancing create problems? The TSB doesn't mention anything about it other than saying to swap the R2s for the R1s if more than one rod nut is removed so does this mean it's not an issue?

Vilhuer

1R and 2R weight is same. No need to do anything to crank.

cfc928gt

Thanks Erkka. So if installing the R2 rods what else should be changed?

Shark_Week

what a loaded question

Cosmo Kramer

Heinrich

Are you looking for a way to lower the CR to run more boost? If you are using a S4 crank I would think you would want to mate S4 rods to it just to ensure that there are no issues with thickness, weight etc. The GTS pistons are lighter as stated above so not sure how that would affect how everything balances out.

heinrich

Thanks Ted ... nope.

I'm looking for a way to run my GTS crank and not pay $2,000 or even 1,000 for rods.

If that is the only option, I will not use the GTS crank.