Hilton

The small resonators were standard on S4's in 89 too (at least on manuals), and all S4 exhaust piping is stainless end to end.

The GT did get a different twin-pipe rear muffler for a sportier note - however I'm not sure if anyone has ever tested back pressures of that versus the stock S4 rear muffler?

Vilhuer

I think some '90-91 got GT pistons when earlier style stuff ran out. Same thing with larger intake. So in theory at least late S4 should on average make more hp and tq than early ones.

Reason for larger late style intake was that in tests early GT engine made IIRR 324hp with normal S4 intake. There are obviously differencees between individual engines and to get 330hp certified some part change was needed. So Porsche had to spend $50K for new intake casting mould to get 330hp DIN (326 in US SAE hp).

UpFixenDerPorsche

ptuomov

Interesting that the bigger intake runners make more power with gt cams. I would have not guessed that.

Hilton

There's a different piston for GT?

There was a change mid-year in '88 to the pistons to the ones with the smaller dish and cutouts - I thought that was the only piston change in the M28.41/42/47 engines?

the flyin' scotsman

Never looked that close until Robs post; the S4 intake I have in the spare parts bin is a 2R casting which IIRC was off a '87 engine.

The intake off my GTS (intake refresh project) is a 4R casting; I'll check the port dimensions.

Hilton

2R will have the small ports. The change from 1R to 2R was the additional strengthening around the mid-pipe where the flappy is located.

ptuomov

Rob -- Thanks again for the excellent measurements on the flange side. A good deed never goes unpunished, so my question to you is whether there are any changes on the runner inlet / trumpet side between casting revisions?

Vilhuer

That's good question. Do they mean mid '88 MY piston style change is same as GT piston or is there separate high CR piston style for them. Nowdays PET lists just one style pistons for all engines but '89 GT document says there is difference between S4 and GT.

XS29L9B

+1

UpFixenDerPorsche

Yes ptuomov. Bellmouth pics below.

The differences are easily seen in the metal, so to speak, but a bit harder to get on camera.

Focus on the volume within the bells. Same with the runner exits.

BTW, several runner exit angles do not align with the intake port angles ie the centreline axes do not line up, so intake air is being directed at the sides of the port instead straight down the throat of the port.

Images are deliberately large.










Note the different location of casting numbers c/- GT manifold

Rob Edwards

The last pic- I assume the 3R refers to the heads from that particular S4 motor?

UpFixenDerPorsche

No Rob.

When the manifold was off I had not anticipated I would be doing this type of comparison. The S4 pics are likely from a Rennlisters car, and only intended for comparitive purpses.

My S4 is an Oct 88 build, M28/42 motor.

Oops. No not the heads. It seems to be a casting model number on the manifold. 1R 2R etc

Rob Edwards

Got it, on the manifold. I'm just doublechecking because I have never seen an S4 manifold with 3R embossed on it- it seems like they went straight from 2R to 4R, sometime in late 1988 or early 1989, which is about the time the '89 GTs started being produced. So I'm not convinced that a 3R intake exists. But I would be happy to be mistaken about that.

UpFixenDerPorsche

Oooo ho-ooooh!

I took the 3R from the picture post title.

But 2R is staring at me from the casting.

I'll change the before anyone else notices.....