ROG100

So putting GT cams into a early 93 US GTS engine makes it a GTGTS engine 8>)

GregBBRD

I can't confirm or deny the piston dish question...I haven't seen them all, nor is there a distinction from Porsche in either the workshop manual or in PET.

I can tell you this.....all of the GT engines that I've seen apart have the smaller dish pistons.

We are taking one apart, new week. I'll take a picture.


In addition, I think that the heads for the GT engines were hand picked...just like the Euro RS and the RS American head were. I've seen heads with "GT" written on then in red crayon. I'll take a look at the one we take apart next week, for this, too.

928cs

Extract from Porsche document WKD 495.921

Carl Fausett

I agree. We have seen that too.

Just to clarify, I did not say that we saw any modifications in the intake manifold. (see post 7) I said the intake runners in the heads head received some mild reshaping. Maybe thats what you also meant. As to the intake manifold, I have not noticed a difference between the S4 and the GT there, and i thought there would be - specifically the distance between the bellmouth and the outer side cover - what we discuss when talking about intake spacers.

I theorize that, because of the higher CFM through the GT engine than the S4 engine, the effectiveness of the intake manifold spacers is more measurable on the GT engine than on the S4 engine. But thats another topic.

Rob Edwards

2R and 4R intake manifolds definitely have different CSA runners at the mounting surface on the heads, but I don't think that's specific to GT motors, as described below.

I would be interested in knowing the casting number on intakes from '89 GTs, to see whether maybe they were 2R castings that were hand ported at the factory, or the later 4R manifold. manifold. FWIW the intake on my January '90 build 90GT had R3 heads and a 4R intake. And my 8/92 build 93 GTS has R5 heads and a 4R intake. And the 91 S4 intake shown below (from 81M50621) is a 4R intake.

So I think the 4R intake is present on all 90-95 cars, S4, GT, or GTS. Interesting question is what's on the '89GTs? The 2R intake shown below was cast in the 35th week of 1988, and it was on an '89 motor (I assume an automatic but am not certain.)

The comparison below got buried in my '91GT project thread but I think is relevant here.



There are two 4R intakes here, one from Mark's '91 GT, and one from the '91 S4 that I blew up in the Zombie back in January. The refinished intake is actually from an '89 that I got back in a batch of intakes that I had powdercoated for Greg.

I did a quick n dirty measurement across each runner's opening- turns out the 4R intake runners are ~2mm greater than the 2R runners at their greatest front-rear dimension:

All dimensions in mm:

Here's the measured dimension:



You can actually see the difference in the profile of the opening, relative to the injector hole. Same ports (#6 and 7, I think) on each intake

2R: Thicker 'shelf' between the port and the injector hole:




4R: Thinner 'shelf':



The other 4R:




AND- it is also interesting to consider the fit of the intake gaskets- here is the gasket (oriented so the 'TOP' of the gasket is against the intake surface, and centered on the mounting studs, which dictate where the gasket sits..

Turns out (at least for this pair of Reinz gaskets) the overall fit is better matched to the 4R intake runners. The gasket was centered on the mounting holes around port #8 and then #7:

2R #8:




2R #7:


4R: #7:




And a low angle shot of the 4R intake/gasket fit on #7:



BUT: If you line up the gasket across all the runners, there are gaps and shoulders at random intervals around each runner. So I think that unless you're really lucky, the variation in the fit of the gasket to the runner is going to overwhelm some (all?) of the benefit of the bigger runners in the 4R intakes

I have a set of '89S4, 91GT, 93 GTS and the Zombie heads (ported by the folks at Kelly Moss), it might be fun to make some gelflex or blu-sil rubber molds to compare the dimensions of the ports in the inch or two below the manifold-head interface.

Anyone got a link to a Gelflex supplier in the U.S.? I might try the TAP plastics silicone RTV kit:

http://www.tapplastics.com/product/m...king_system/61

FBIII

My 89GT has R3 heads and R4 intake. The heads have GT stenciled on one end.

Carl Fausett

Darn good post with great information Rob. +1

marks gt

My 89gt has r3 heads, and intake. I did measure against the runners on the r2 intake, and the gt intake was larger.

ptuomov

Thanks. Very interesting and useful information.

Port mold experiments are sexy.

I am wondering why they increased the size of these huge holes in later casting revisions? Any idea? There would have been a number of other things that I would have revised.

Rob Edwards

What intake does your 89GT have? I am confewsd.

Jim M.

Differences S4 vs. 89GT: The GT has more precise pistons, smaller well between valve pockets for exact 10:1 compression, high lift and longer duration cams. The GT cams are stamped on the ends no’s are 271.01, 272.01, 273,01, 274.01 the casting numbers are 928 105 271.OR, 928 105 272.OR, 928 105 273.OR, 928 105 274.OR. Lighter and freer flowing exhaust, revised engine computer mapping, idle at 775 vs. 675, rev limit at 6800 vs. 6600, torque peak at 4100 vs. 3200, 326 hp vs. 316 hp. 8" and 9" wheels vs. 7" and 8", 17mm rear bolt-on spacers for wider track, factory rolled fender lips for clearance with wider track, 20mm shorter shift lever. (10% less travel), Limited Slip (40%) and standard Sport Shock/Spring Option, slightly different instruments, Straight Cut racing type ring and pinion. I have both, and the S4 has slightly more torque on the low end for normal drivability, however once the GT hits 4000rpm it’s a freight train and with the close-ratio 5-speed all of the up shifts hit right in the torque peak between 4100-4300. The GT likes to be driven hard and rewards you when doing so. It’s also a much stiffer ride than the S4, and is much rarer than the S4 and therefore worth more. It was a prototype for the 90-91 transition of all 5-speeds being a GT and all autos being S4’s. When you move up into the 90 GT there were slight differences, but have the same engine etc. Here is what was lost and added on the later normal 90-91 GT: 8" and 9" Club Sport forged wheels replaced with 7.5" and 9" cast Design 90. Deleted the 17mm bolt on rear spacers. Fender lips not rolled because of the narrower track. Normally cut ring and pinion gears, 40% fixed LSD replaced with computer-controlled variable PSD, dual airbags added. Oil cooler added below the lower radiator support plus an extra air slot in front spoiler. The 89 90 GT heads are a different part number to reflect a slight amount of hand porting.

marks gt

I believe it is a r3 going from memory. (I could be wrong it may be a r4) I had it off a year ago, and tried to figure out what the difference was with my gt intake, my spare r2, and a gts intake.

Kevin in Atlanta

How does the GT get "lighter and freer exhaust"?

Jim M.

The resonators are smaller and the entire exhaust piping is stainless. Because of the smaller resonators it flows more freely and has a deeper sound when under full throttle.

DR

Hi Jim,

I am still amazed that this post I wrote on Porsche Fans e-mail List eons ago is still directly quoted to this day!

The sad thing is for the life of me I can' remember what year I wrote that.... :-(

Must be close to 20 years ago????

Hope all is well!

Cheers,

Dave

Kudos (once again) to Greg Nichols for having the amazing foresight to archive so many of the posts from the old days on his 928 Tips site!
http://www.nichols.nu/tip042.htm