UpFixenDerPorsche

GregBBRD

But it has that really sweet curved boot that joins the two, which "straightens out" the airflow.....

Trust me, once you sit down and try to design a better manifold than Porsche did, you really begin to appreciate what they did in that compact little area.

UpFixenDerPorsche

Hi Greg.

Intuitively that is what U would expect but fast moving air doesn't work like that.

It takes the shortest path between high and low pressures, which is the short turn in a curve. The slowest air is that on the outside of the curve.

mark kibort

you are saying there will be less of a pressure drop across it if it is partially open due to more "parallel" appearance of the TB butterfly? I don't buy that.

ptuomov

In my opinion, in the S4 throttle body element, what happens after the throttle plate is a lot more important than what happens before the throttle plate.

1989porsche928

are you going to replace the bearings while it is off?

UpFixenDerPorsche

Not sure what you mean Mark?

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UpFixenDerPorsche

Too late.

UpFixenDerPorsche

Could you expand on that pls Ptuomov?

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ptuomov

Just voicing the opinion that a 74mm throttle plate is going to flow enough air without much loss for 350hp or so engines, even without exactly optimal angle. So who cares if the entry angle is not exactly right. (Not conceding that it's not exactly right, by the way, since ideal straight air vanes are not facing the flow exactly.)

However, what might be going on terribly wrong in that S4 manifold is the air exiting the throttle plate "venturi" (if that's what you call it given the lip at the top) and then trying to make the turn to the secondary pipes leading to the plenums. If I were you trying to modify the throttle body, I'd focus on how to facilitate the air exiting the throttle blade and turning up.

Again I tell you, it is easier for a camel to go through the eye of a needle than for air to turn up into the secondary pipes inside the 928 S4 throttle body element.

UpFixenDerPorsche

LOL. Funny you should mention that

At WOT the airspeed is abt 60-70mph.

Air then hits the closed end of the TB plenum - which looks to it like a T-junction, and in the reality that we can't see, it's not pretty - something like this: turbulence and swirling = energy loss:




So I thought .... hmmm ... a splitter should smooth flow a little:
(did I mention I built ships in a bottle).





Involved a lot of CAD work:
(Cardboard Aided Design. )

ptuomov

Might work in principle, I see what you're doing.

How are you fabricating that piece such that it doesn't get blown away with the extreme pulses as the throttle is closed and opened at high rpms?

UpFixenDerPorsche

It's a piece of that aluminium sheet. I used cardboard to to get shape right so it would fit the curvature at the back of the plenum.

Then cut out the aluminium piece and bent it to match. This is the piece shown in the pic.

When bent and curved like that it is very rigid, so the only issue was attaching it securely.

Did this with a pair of PK screws in each tail, screwed in through the plenum wall from the OUTSIDE.

Some RTV silicone under the screw heads when tightening, and around the threads on the inside.

Then more RTV to completely seal the top annd bottom gap all the way round. It's an excellent flexible adhesive. Result was that I got no signs of movement even with a lot of hard pushing and pulling. Worst case failure: it will clank and rattle but will go nowhere.

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