Vilhuer

That is cheap.

ShawnSmith

...and then there's the piston's Greg has made based on the 968 piston as a starting point....

Rick Carter

What would be the CR with S2 pistons and 85/86 32 v heads?

Vilhuer

Around 9.5:1 with 95.25mm stroke crank and 5.85" rods. Any other combination and things start to get interesting. Stock S3 crank and custom lenght rods would give low 8:1 or high 7:1 for example.

Stock valves are too small I think and part of the fun from larger displacement is lost.

Rick Carter

Thanks; looks like it may work for me as I would use the stock crank and have boost. Mark I'm interested and will try and get in touch with you Monday.

Cheburator

what about a GT fitted with these - what displacement and compression ratio will it have?

what about a GTS with these - 5.9ltrs, but what compression ratio? could be potentially interesting, given that my class is restricted to 6.2 ltrs, and i have 968-valved GTS heads sitting in their box...

mark kibort

That would be a great engine with a stock GTS crank. but, a lot of work. I think you could make more mods on the GTS to make near the same result for a lot less hassle. big valve heads, have the GT cam reground for more lift and some dyno tuning could get you in the near 400rwhp range!

mk

Vilhuer

S4/GT crank + custom rods + 104mm 944 S2 pistons = 5362cc and around 9.5:1 CR depending on what numbers are used.

https://rennlist.com/forums/928-forum/344250-head-and-piston-top-cc-numbers.html#post4073230
https://rennlist.com/forums/928-forum/455702-sbc-swap-pros-and-cons-who-bought-the-renegade-mounts-and-kit.html#post5828260
http://www.csgnetwork.com/compcalc.html

GTS crank + custom rods + 104mm 944 S2 pistons = 5838cc and around 10.2:1 CR depending on what numbers are used. Big intake valves would need larger holes on pistons and CR would suffer a little as 944 S2 has 37mm intake valve.

Closest to 6.2 using commonly available parts is 100.5mm 1st oversize S4/GT pistons + custom rods + stroker crank = 6045cc and around 10.6-11.6 CR depending on what pistons and numbers are used. That would be interesting engine. I would build it using 100mm stock size GT pistons to keep cost even more down down to earth and give later option of rebuild using 1st oversize if necessary. Result is 6045cc and CR very good for racing 11.2-11.5:1. Basically you would need $2500-3000 Moldex crank and $1500 custom rods. All the rest would be stock GT parts. Only trouble is that much of the piston skirt would need to be removed so that they would work in block at BDC. This means other way round ie. stock crank and 104mm pistons is much easier and it costs less very likely also. Down side is lower displacement.

I have 8 Mahle 944 S2 104mm standard size tolerance group 1 pistons which obviously have full skirt. Plan is to try them into 104 bore stroker block before putting KS 968 in there for actual stroker engine build just to see how such setup would work. If normal S4/GT or even GTS crank works together with them it would be slightly cheaper way to get larger displacement than full bore stroker.

Cheburator

Mark,

I was thinking - bore the spare GT block out to accept the 968 pistons, put the GTS crank and 968 valved heads, with GT-reground cams into it and hey presto, I would have a monster 5.9ltr engine making stupid bhp (for its size) and still have a steel linered GTS -block + GTS pistons with less than 20k miles lying spare in the garage in case the Gods of Nurburgring demand their due.

Machnist charges - to bore out the block - in Bulgaria are stupidly cheap. Building the 2 x GTS heads with new ceramic seats, new guides, new seals, tri-angle cutting of the new 968 valves cost me around $700

Thus, it could be a possible path...

Vilhuer

And he has correct tools and experience for Alusil? In any case your largest problem is to find block which has enough material for 104mm bore.

Cheburator

This guy has been building race engines for about 30 years - mainly BMW ///M stuff but has also been trained by Mercedes to work on alusil blocks. He has already bored a S2 to accept 100m pistons...

Why would I need custom rods - could I not use my GTS rods with 968/S2 small skirt pistons?

Vilhuer

Sounds really good.

944 S2 and 968 pistons have smaller compression height than 5L or GTS pistons. There is difference between 5L and GTS also. IIRR 5L crank and 150mm stock rods would be more than 2mm below deck level at TDC and GTS more than 1mm. This would kill CR ratio also. Besided GTS rods are worst possible stock rods to use, doesn't matter if they are late '94-95 MY ...2R version.

Mark Anderson

if you are looking for displacement I have the following

Scat stroker crank $2,000
Oliver 5.85 rods $1,000
968 pistons $ 400

this will give you 6.5 liter

Abby Normal

I used 951 pistons and that made it 8:1

Cheburator

Too big...

Only allowed 6.2ltrs max displacement. That's why the 2 Zakspeed Vipers run on 8 cylinders only