Kevin in Atlanta

Cams I know.

What about the heads and block? Heads and manifold are matched 4R.

Just curious what else?

ptuomov

Erkka will know! ;-) Some intake port matching maybe by hearsay, cams, and the ECU calibration for the engine.

Rob Edwards

Cams, smaller piston dish (supposedly true 10:1 compression), different LH and EZK maps, and head/intake port matching.

ptuomov

I think that at least from early '88 on, both the S4 and GT got the same 20cc pistons that brought the compression close to 10:1. The dish diameter is the same as in the '87 S4 pistons, but the depth is closer to 7mm instead of close to 9mm. So at least for most GT's, the pistons are the same what you'd find in the contemporary S4s. Someone with actual information can confirm this.

928cs

Same list from the Information Technik booklet made when the '89 GT was introduced.
+ "M28/47 85" stamping, instead of "M28/41 81".

Black Sea RD

Calling Mr. Greg Brown...

Carl Fausett

In addition to what I have already seen mentioned in this thread, we have also seen some very slight intake runner shaping on the GT heads. More than mere gasket-matching (although they did that as well).

Mongo

Pictures of the runner shaping in comparison to S4 would be great to see.

FBIII

Not engine components but part of the package: small resonators and dual tipped muffler.

terry gt

Con rods as well

ptuomov

No real difference. Older S4's have the better PPF conrods, the GT's have mostly the worse cast conrods. But that's just because of the production dates, I think.

ptuomov

Where? As far as I know, they matched the ports and the intakes around the gaskets. Is there any other difference anywhere in the intake manifold?

The later manifolds seem to have gotten a thicker or more elastic coating that hides some of the very poor quality casting they did. But I don't think that's a GT vs. S4 issue.

(Opinions and guesses, not facts.)

DougM

Are you referring to just the '89 GT? Because I think '90-'92 the only difference between the GT and s4 is the trans.

Kevin in Atlanta

In this case, a 91 GT. Engines are different.

Rob Edwards

Yes, the connecting rods changed in the middle of the 1988 MY (starting with engines 81J00726 (5-speed) and 81J06740 (automatic). Not sure whether that was done at the same time as the different dish pistons introduced in February of 1988, as the service info blurb on the pistons says they weigh the same as the earlier S4 pistons and are interchangeable.

AFAIK there is no GT-specific connecting rod, I think they're all the same from mid-'88 through '91.

For all intents and purposes I think the cams are the only important difference. if you take a set of GT cams and stick them in any mid '88 to '91 S4 motor, and stuck a pair of PEMs with a GT map in the brains, you'd never be able to tell the difference vs. a 'real' GT motor.