mo-pcar

My 928 had an engine replaced years ago and I'm finding out it was not replaced with same MY and type as I was told at purchase. So my question is can GT cams be installed in an S4 engine. I only ask because my GT appears to have a S4 engine (M28/42 81H05548) but certainly performs like a GT (305 rear wh HP - dyno'd). It pulls strong and idles unlike S4's. BTW what is the H in the serial number? That is what year is this engine?

mo-pcar

I see my pic is still my old S3. The car I'm referring to is a 1990 928 GT.

ptuomov

Yes, I believe they can. (Someone who's actually done something like that should confirm, though. Me knowledge of the swap at this point amounts to what one can read on the web.)

Here, by the way, are the lobe separation angles of various combos that once could install (assuming modified S3 exhaust cams):

[wring info deleted]

Vilhuer

All S4, GT and GTS cams are direct bolt on to all '87-95 engines. Intakes and exhaust can be mixed together. Even to level were there is different model cam in all four positions if one modified S3 cam is used. I know one S4 engine where one intake cam was changed to GT cam at local Porache dealer while three other were S4 cams. It did run fine for few years until loose bolt caused some valve damage and engine was changed to another S4 engine.

cold_beer839

If it runs good enough (305 rwhp!!) then I would let it alone.

But in the interest of retaining the value of the car, I would find a GT engine, and there is one for sale for $5500 somewhere in the US right now.

UKKid35

I wonder what effect the weaker S4 valve springs would have on GT cams. I have the opposite issue, I have heads from a GT, including valve springs, but the original S4 cams.

Vilhuer

S4 and GT valve springs are exact same part. At least those I have seen.

mo-pcar

Thanks for the replies. I will also say it idles close to 800 rpm and really comes alive at 5000 (GT attributes). I'm disappointed that I don't have a true GT engine but happy with the performance. I did not mention the Authority Chip or Cat delete but even with those I don't thing the Dyno results could be so high.

Thom

Sorry for bumping an old thread.

Has someone on here experience with GT cams on an early '87 S4 using the 9.5:1 pistons?
I am curious how detrimental half a point less in CR would affect performance compared with the standard GT set up with 10.0:1 pistons.
Can the difference in performance be somewhat minimised by increasing both the cam and ignition timings?

SwayBar

Yes.
Stock 5-speed S4 dyno's at 270-ish rwhp.

Stock 5-speed S4 with GT cams and exhaust dyno's at 320-ish rwhp.

You will not notice the missing 0.5 point of compression.

You can always increase the dynamic compression ratio by closing the intake sooner by advancing the cam timing, but there's no need to do that.

Thom

Perfect, thanks.

If no difference can be felt then this suggests to me that even GT cams are not aggressive enough for 9.5:1 pistons, which is not really a surprise considering they are still negative overlap cams like the S4's.

Speedtoys

Measured and felt are different things.

I don't think .5CR is going to give you 10HP...to be then felt...what you don't feel is the 3-4 it offers.

Thom

The 3-4?

PorKen

Long ago when I was researching S3 cams - GT cams are S3 with a tighter LSA - I understood that the S3's needed higher compression 10:1 pistons to compensate for their long duration cams which bleed off intake charge pressure.

Speedtoys

What do you expect a half point rise in CR to offer?