James Bailey

to have decent compression ration you need to use the 85-86 USA 32 v piston. The out put of a 4.7 is not much less than a 5 liter much of the increased power is from bigger cams and headers exhaust work that usually gets done at the same time. If you have a running Euro 4.7 S just throw it in the car and drive !!! Odds are you will have more car than you can use at least for a year or two There is no substitute for track time just go do it !

RicerSchnitzzle

Agreed, I wouldn't do a hybrid until you had a reason to replace the 4.7L bottom end. A lot of effort for 30 more HP. However if you have a blown block or lots of time on your hands, it's fun. Of course with bigger cams, headwork and larger TB, the 5.0L will give even more power.

kccampro

All good info. It's not that I need the fastest 928 on the block; it's more that I wouldn't want to spend the time and money freshening up the 4.7 when I should have used the S4 block sitting next to it. Sounds like just using one of the Euro S motors is the right move.

Stan.Shaw@Excell.Net

I have an extra 85/86 block already bored to 104mm and an extra 8 944S2 pistons (as I recall). Let me know if you have any interest. The block would need to be lapped to match the pistons.

danglerb

Hey Jim, I'm Baaaack!

Like Jim says you need the 85/86 short block to get good compression with a Euro S head, the S4 I think has more of a dish to the piston top which requires a smaller vol in the head. Even if they were the same though, you should be able to sell that complete S4 for enough money to buy two good 85/86 short blocks.

All of the dyno testing I've seen so far suggest that using the 5.0L 85/86 short block vs Euro S 4.7L, that the gains are more than just the cubic inches would suggest (106%). Some of the dyno numbers do have other changes, but others clearly show just the short block change as more than a 10% increase, and thats plenty of bang for the buck.

I would agree, if the motor is ok to run as is, or at least without pulling the heads, I would drop the sucker in and start racing. You have two Euro S motors, whichever doesn't get used now you could take your time and rebuild as a spare.

danglerb

Failure modes would be blown head gasket, or a crack in the cylinder?

If the triggering event is detonation, doesn't the thinner wall have some cooling advantage in preventing detonation in the first place?

Maybe even more ability to expand if detonation occurs without cracking?

If someone had a spare junk block a curious experiment would be to use maybe a hydraulic rig to find the burst strength of various wall thicknesses.

toofast928

Going from a 100mm bore to 104mm is equivalent to boring .157" of an inch.

Stan.Shaw@Excell.Net

Agreed, been there, done that, and didn't get the 30hp right away either, you need exhaust and headers to get that, based on my knowledge of two other builds that got to 300rwhp.

Vilhuer

I tried Belzona 1111 chemical metal and used salt to build 8-9mm thick deck plate at top 1-2mm below deck level but effect was minimal. Cylinder towers are still .050mm out of round when heads are mounted and block is basically useless. When block is bored to 104mm its critical to use correct stress plate to simulater cylinder head. Factory WSM manual doesn't mention this as its probably not needed when boring block to .50mm oversize even if its good idea to use it then too.

Fastest928

Erikka.

You point out an absolute essential step in the process...any block bored to 104 without a deck plate torqued correctly is just short of useless.

danglerb

First toe now is in the water, I bought a set of 4 92 968 KS pistons in tolerance group 2 104mm, now looking for 4 more.

heinrich

Therein lies the problem. I had a full set of 8 same-tolerance, same manufacture identical 968 pistons on sale for years. All I got was spoilt brats trying to drive the price down, all of which I turned down. I recently decided to sell my GTS crank and 8 968 pistons to a very deserving Rennlister, and gave him the same price those brats were pushing for. ... Dangler, you're gonna have a hell of a time finding 4 more 968 pistons in the same group as those!

danglerb

Tolerance groups are 0, 1, and 2, anybody know what that works out to in size or weight differences?

*** found this, tolerance group 2 or 104.00mm [Tolerance group 0= 103.98mm], maybe tolerance group 1 is 103.99mm?

Maybe I could do cylinders 2, 3, 6, and 7 with one tolerance group and 1, 4, 5, and 8 with another?

heinrich

I always thought I would paint them rainbow colours so the net effect in there would be a lighter, white spectrum mix resulting in faster piston speed and lighter load. I would recommend against running two tolerance groups in every other cylinder. In order to stay balanced, you may want to saw each piston in half and weld them together

James Bailey

If the block is bored to fit the pistons it makes no difference as long as you put the correct piston in the fitted bore. Weight is a separate issue and the balancing by removing material would take care of that. Porsche I believe stated that you could run one 96 mm piston and seven 95 mm in the same engine as long as the weight is balanced if you had one scratched bore and needed to go oversized. Note also that tolerance groups overlap...