Don Carter

I have a complete 89 long block with TBF. The 86 that is available threw a timing belt due to the water pump locking up, and bent some valves according to the owner.

I have the 89 head bolts, just wasn't sure if they were threaded the same as the 86 studs.

It's all sounding a bit more than I want to take on at the moment.

BC

Not a big deal. Just get the 86 car, throw on the 89 heads and intake, and run ethanol.

Lizard928

If you notch the pistons slightly then you will as Erkka says have 10.6:1.
You can run 94 octane and be OK with tuning. But I would recommend GT, or S3 cams.

Vilhuer

Minimal cuts will result above 11:1 CR. I had mine cut as much as I dared to get 10.6:1. This resulted virtually safe setup which will keep valves intact even if belt goes. Piston top shape might not be ideal but added valve safety is obviously good thing.

Based on dry trial with bare head I think S3 intake will work with S4 heads, at least after some minor mods. So S4 heads can be used to make high compression S3 engine without geting into trouble of integrating electrics from different MY's in cases where car is S3.

Flint Ironstag

Don, have you had a chance to look at the '86 TBF car? What's the condition of the rest of it?

BC

Vil -

you put 86 intake on an S4 engine?

Vilhuer

No, we just tried if it would fit. Seems it does, at least after little modifications. Plan was to use it without side pieces and replace them with custom air to water intercoolers but there isn't enough space for useful thickness setup.

Hilton

This surprises me - the Australian S3 engines are the special lower compression "smog motor" 9.3:1 (sold to Switzerland, Australia, and as an option in Germany only).

Or did the Aussie in question (was it John Gill?) do it with a US long-block?

This is of particular interest to me, as I have a US '85 long block sitting in the garage at the moment (was a complete engine - pulled the heads etc. off it last weekend).

I'll be tearing it down further for the block (just over 8mm cylinder thickness, yum), but if the rods/pistons combination can be used in another S4 along with the S3 cams (modified to fit S4), to make a high compression motor (we get 98 RON here - which is certified to be 98 at the pump and tested), I might be tempted.

mark kibort

after thinking about this the valve pockets might be too shallow due to the combustion chamber differences. and if the valves are in a different starting position relative to the piston face, are there other differences in the heads too? we know that the piston wont touch the intake valve as it chases the piston down the hole on the intake stroke, but who knows about the exhaust valve as the piston is coming up? it usually stays open a little past TDC in some cases. PLUS, the real reason for the piston cuts is for rotating the cams when the engine crank is locked in position.

Hilton

Interesting - quick check on a compression calculator shows that with Australian '86 pistons, compression is in the order of 11.1:1 to 11.5:1 depending on the volume of the S4 heads

(data taken from https://rennlist.com/forums/928-foru...c-numbers.html, using stock 5L stroke of 78.9mm).

Using US 85/86 pistons pushes the compression about a point higher, and over 12:1.

Vilhuer

I have US S3 pistons which have really large valve pockets made into them. Thus CR is 10.6:1 instead of over 12:1. Crank and rods are from S4 and they have been blanced to work together with pistons, GTS heads have 39mm 968 intake and stock '87-95 928 exhaust valves, head gaskets are normal 1.4mm thick replacement parts, cams are S3 set modified to fit '87-95 head and both intake and exhaust have 0.5mm removed from base lobe. This setup has worked fine for past 10k km's. Currently I'm changing cams to stock lobe S3 setup as those modified cams have tq and hp so high in rev range that they aren't really suited for automatic car.

hans14914

Reviving this thread from earlier in the year.

*IF* tuning was not an issue, will the S3/S4 hybrid work? I am interested in potentially using an 85 short block with 88 heads with the intention of getting a bit higher compression. How deep would the valve pockets need to be re-cut for the larger S4 valves (or at all since there is less lift on the cam). If the S3 cams were modified, or for sake of discussion, a base-circle reduction on the S4 cams and lash-caps to get comparable lift to an S3, how much would the pistons need to be modified to clear the valves, and is there enough meat in the S3 pistons to remove that much material?

Thanks,
Hans