Landseer

Same basic engine.
85 and 86 were detuned a bit, but reportedly have slightly higher performance camshaft profile.

87 S4 got knock sensors and higher fuel pressure to accomodate a more aggressive tune.
Lots of minor differences to many parts, like wiring harness.
Incremental changes to computers.
Different intake pipe arrangment
Different exhaust configuration, mainly that the pipes are split instead of side by side.
"Upgrades" if you call them that for S4 (X pipe) fit.

Like Mickster says, 85/6 is tuneable with minor modifications.

87 has some auto trans mods but 85./6 is just about as good.
And some front suspension / brake / overall styling changes of course.

Search the forum for more comparisons.

HP modification is probably the last thing I would do on a purchase of either car. So much is usually gained by repairing vac leaks and bad sensors that dial the timing to safety zones by default. That's generally a discovery process that eats about 4 to 5K parts beyond purchase price and takes a couple months for a hobbyist.

Once running well, for what reason does 20 or 30 hp more hp matter, realizing the earlier and later cars will be about equivalent if you chip the earlier one? Why go further with either?

SteveG

The OP may not be still looking, and my info is mostly second hand, but . . . it comes from a good source.


RE: the detuned '85: I mentioned to ROG that I had (sold) an /85 5-sp and it "felt" faster than the GTS auto. Neither has been on dyno or stop watch. Keep in mind that the '85 is several pounds lighter. He said the S3 cams will liven up the GTS; and I think, Ken's chips and a shark tuner can do more.

I think bang for your buck is an '86.5 and talk to Ken about his mods which are $$ reasonable. You don't really sacrifice that much with an '85.

Air bag and IIRC, the digital dash came in after '89. If you don't want to get into changeing cams, the '89 GT is the object of desire of some, but not all.

PorKen

85-86 (S3) cams have the same lift and duration as GT cams but they have a wider LSA (lobe separation angle). S3 cams trade more low rpm torque for less high rpm HP which is better for regular driving.

S4-up got higher fuel pressure, but smaller injectors (19# vs. 24# on S3). I think this is mostly for emissions (better fuel atomization at higher pressure).

Mike Frye

I don't think anyone has mentioned the other (and IMO significant) difference between the early S3s and the 86.5: The dual exhaust. The early S3 has a 2-into-1-into-2 system that constricts it. Even so, with Ken's chips my '85 dynoed at 292hp at the wheels and over 300ft-lbs tq with an early chip version. I think I'm over 300 now but haven't had it back yet. I still have stock cat-back exhaust and I keep blowing it off my X-pipe.

The dual exhaust of the S4 plus the added lightness and Porken possibilities of the OB is a great combo.