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As part of my intake refresh for my 89GT, I acquired a spare S4 throttle body to scavenge some small parts of the linkage assembly.
After a night of scrubbing and bathing in Brakleen (the good stuff), I realized that despite having the same part number cast into the housing, they are in fact different. S4 GT GT on the right
The S4 housing is actually in nicer shape and I'd prefer to use it. But that difference in the casting serves as a stop for a tab in the GT lever assembly which is not on the S4. My GT lever pieces are all pretty from being re-plated, but it looks like the parts wont mix and match.
I've found a thread talking about a "GTS" housing vs the S4, but my car pre-dates the GTS by a fair amount.
Anyone know the purpose of this change? Are there any functional differences? Could I otherwise use the entire S4 housing and lever assembly on my GT intake?
Short of that, anyone have the later style "GTS" throttle body laying around they'd like to part with?
"The S4 housing is actually in nicer shape and I'd prefer to use it"
Given Porsche's aerodynamic airflow analysis to optimise the lower plenum shape for each motor, I'd say a "nicer shape" is hardly a solid tech reason upon which to base your decision.
Why not just buy a Red car with a loud pipe? Everyone knows they're much faster.
Never mind - accumulated performance benefit will be part of a suite of other 'small' gains.
Well that is certainly part of the question. However, I can find no other differences than the casting boss and the lever assembly. FWIW, it seems like this was a running change as I'm seeing a few other used throttle body housings around that also have this configuration. Unlikely they are all GT or GTS parts.
For more power the throttle housing can be modified with larger 80mm butterfly valve.
Here some pictures from my work. Porting the throttle body creates a lot of aluminum chips.
Åke
Last edited by Strosek Ultra; Jun 28, 2021 at 11:49 AM.
For more power the throttle housing can be modified with larger 80mm butterfly valve.
Here some pictures from my work. Porting the throttle body creates a lot of aluminum ships.
Åke
As part of my intake refresh for my 89GT, I acquired a spare S4 throttle body to scavenge some small parts of the linkage assembly.
After a night of scrubbing and bathing in Brakleen (the good stuff), I realized that despite having the same part number cast into the housing, they are in fact different.
...
The S4 housing is actually in nicer shape and I'd prefer to use it. But that difference in the casting serves as a stop for a tab in the GT lever assembly which is not on the S4. My GT lever pieces are all pretty from being re-plated, but it looks like the parts wont mix and match.
I've found a thread talking about a "GTS" housing vs the S4, but my car pre-dates the GTS by a fair amount.
Anyone know the purpose of this change? Are there any functional differences? Could I otherwise use the entire S4 housing and lever assembly on my GT intake?
Short of that, anyone have the later style "GTS" throttle body laying around they'd like to part with?
Thanks.
I am not the expert that you are looking for, all I can offer is a couple of clues. But first, what year S4 is the throttle-body from? As far as I know, GT and S4 throttle-body assemblies were the same-- at least PET currently shows the same part number for '87 through '91, S4 or GT: 928-110-093-03. And no other part numbers are listed for the individual parts, other than the throttle-position switch.
So if there are differences, then I think they would be year-related, not GT vs S4. And if there are differences in casting/machining with corresponding changes to the throttle mechanism, then it is still the same finished assembly part#. What that says is that you need to keep the parts together with the casting that they came from, and use one or the other-- but do not try to mix and match to make a Franken-throttle assembly.
And since this is a GT, then I would strongly advise sticking with the original GT parts if at all possible.
Also not the answer you're looking for, but I agree with Jim. Stick with the GT parts.
There are subtle differences year to year in the castings, but this is the first time I've heard that there is actually a functional difference. AFAIK all the non-steel castings for S4, GT, and GTS engines are identical. Later cars had casting numbers that were different, such as the intake which has the casting number (not part number) suffix 2R or 4R depending on the year, but the mold was the same.
With that said, I am wondering what's wrong with the GT casting that made you even consider replacing it. The throttle body is invisible and pretty indestructible. The little scoop looks like someone ground it, perhaps after Porsche shipped it, perhaps to clear something that was re-installed wrong?
The castings are not sitting exactly the same on the table, but from the angle, it looks like the one on the right has a larger rear (bigger butt??) , which would be a bigger plenum size.... Ken
Yes, they weren't sitting perfectly flat, although I tried to get them even. I think any apparent difference in size is just the camera angle, despite my best efforts. Hand-held iphone
I don't know what year the S4 plenum is from...I bought it here in the forum recently.
The reason I bought it was that I was trying to recover one part of the lever assembly, the piece with the plastic drum that the heavy spring goes on. The rest of the plenum just came with it. When I received it, I noted after cleaning that it was in nicer cosmetic shape than my original one. My original lever piece with the plastic drum was cracked on the edge by the plater. Still functional. but cracked. It matters not that you can't even see this part. If you saw how I have been doing this work you would understand-there isn't a nut, bolt, washer, clip or other part that has not been replaced with new or re-plated.
Anyway, despite the identical part numbers on the plenum (not a casting number) they are most definitely different in the cast tab that serves as a stop. In addition, the lever assembly has some differences that only work with the cast tab they belong with. This includes the part with the plastic drum, so I am SOL on this one short of buying another plenum with lever assembly of the correct vintage. Based on what I see being sold, I'm pretty sure this was a running change and not S4 vs GT at this point.
I went back after my original GT plenum with a brass wire brush, Dremel with brass brushes, and copious brake cleaner. It cleaned up pretty well but there is still a little aluminum oxidation visible here and there. I put all the GT lever parts back with the GT plenum (new bearings, new TPS, new ISV, new hoses, new elbow, etc.), slightly cracked plastic drum and all. It will annoy me, but it's the part that came with the car. This isn't an attempt at a concours restoration, think more like a resto-mod. It is supercharged after-all.
Sorry - didn't mean to challenge your reasoning - I just missed the point I guess. Your restoration and plating looks great. My question was mostly about the casting, which also looked fine so it was puzzling that you wanted to change it out.
But I am still very surprised that the throttle body differs from year to year or GT to S4 and that is a useful data point for sure.
No worries, and I didn't mean to sound defensive. I was just trying to explain it's more of a restoration than a functional "fix" needed. The change in the casting and assembly does have me curious though.
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Throttle body differences S4 to GT
