I've got one of Tony's 'spare' S4 pistons to compare with, but it looks like the top of the GTS piston is slightly shorter than the S4, measured from the piston pin. Even though my pistons have 'bowl' mods on the top, I don't think the tip of the piston was machined. Last time I was down at 928 Intl., Mark pulled a new GTS piston from the box for me, and I compared the two. They looked the same height-wise side by side.

This might be why the combo fits... I know the crank still has its original journal sizes and the rods look unmodified. If Garrity finds something different with the rods, then he'll tell me.

 

The pin is in a different location between the S4 and GTS pistons. I have no idea how your engine was assembled to use the S4 rods and GTS pistons.

 

The way I see it, my most likely option for lowest cost and performance gains, is to use the GTS crank I have with GTS rods and pistons. If I can use S4 or other 32V pistons and/or rods, that is undoubtedly the way I'll go. I know that the GTS piston has a shorter skirt and higher mountpoint for the rod. Bill this is interesting that you have GTS pistons and S4 rods. In theory the rod is shorter and mounts higher, meaning that this is a lower-compression combo than GTS or S4. However there must also be increased side loading on the piston as a result. I wonder if I can somehow have my 5-litre pistons modded to give me good compression with stock 5-litre rods and GTS crank.

 

True, but GTS piston prices: OMG! Plus, custom rod costs too, or, stock S4 rods & Cometric gasket

 

It would seem that the only way to run any 5L components on a GTS crank would require either facing off the crown of the 5L piston and/or resizing the big end of the rod to decrease the beam length. I think you'd be removing a lot of metal in either case, and if it were physically possible (meaning the parts actually survived the machining) its seems that you would be left with some seriously weakened parts. If you have access to a GTS piston and rod combo, set it next to an S4 combo and you would be able to physically see how much shortening you would have to do.

 

OK. So it seems my project will remain on the shelf a while longer then

 

I'll do this eventually and also compare them to 951 and 968 pistons and rods. Don't seem to have time to do it any time soon.

 

I don't have my rods back yet to compare overall length, but this is what the pistons look like.

 

the ring lands look roughly the same height/thickness and given the cast S4 pistons withstand some boost then the GTS piston should withstand it even better as it is forged. do rods mainly fail in compression or under tension? i cant remember exactly but i think its under tension ie over revving. with that in mind how much of an issue are the GTS rods being slightly 'wimpy' by comparison to others. to me the biggest advantage with more modern rods and pistons is the weight saving.

 

I just compared the stock S3 32v piston/deck height compared to a 951 piston in the same block on the same 928 rod (factory standard bearings, both rod & crank). 951 piston sits 1.9mm lower in the block, not 5mm as previously posted.

Using an S4 head to reduce the combustion chamber size & bring the compression ratio back up, 1mm or so needs to be removed out of the 1.9mm to get an approx. 8.4:1 compression ratio: surfacing of block/head (.9mm or so combined), or custom rods 1-1.9mm longer. I'll check tolerances & such, but 1mm of total area to be removed from the block/head is not a lot. I'm investigating surfacing the head to it's maximium, then taking the rest out of the block to get my total 1mm or so.

When building my last 928 motor, the OEM ring set said for 100mm bores: 944, 951, 928, S4, GT, GTs if memory serves.

 

Its funny that you posted this. I just spoke to the red car owner last night about his current projects.

We didn't discuss it, but I know his first or so engine was a regular S4 engine with the 951 pistons. Now I don't think he even surfaced anything, but he was extremely happy with that engine and the fact that he safely ran ALOT of boost up ro 20psi I think.

He siad it was a bit weak on the lower RPMS because of the lower compression, but after 4k, a screamer.

 

Marcus Hutchinson? I sold him his pistons & recommended that route for him, though he had custom rods made though, unsure if they were longer or not. Also, Marcus' car was an automatic.

You know it was stolen right?? bummer.

 

H,
I believe, but not sure, that the S3, S4/GT and GTS rods are the same length. The longer stroke of the GTS is handled by moving the ring stack and rod pin up in the GTS piston. Not a shorter rod. The S4 rods you have may work with a custom piston and nikasil the block, or GTS pistons. The S4 rods are probably heavier than the GTS rods so depending on the weight of the pistons you might have to use heavy metal to balance the crank. That's expensive. The S4 rods held up well in Todd's highly boosted, S4 so rod strength doesn't seem to be a problem.

 

No, not marcus. Its a gentleman named Todd, in Green Bay, WI. Where most of the powerful 928s are from.

 

Marcus did the same thing though back in 2000. His car was easily mid 500rwhp with a modified FAST set up & built 951 engine with custom rods.