I've been thinking lately about how unusual the 85-86 intake is and wondering about it's design. The 85-86 intake is quite different than all others on the 928 and the path of the intake air seems long and inefficient to my non-engineer eyes.

I don't remember ever seeing this discussed before on RL but I may have missed it.... what if someone were to make replacement peices for the two large peices on either side that join the T to the intake runners? Would there be any benefit to a smaller peice that allowed the air to make a smooth U turn from the T while splitting it to the 4 intake tubes on either side? Seems like the current design forces all the air into a square box before it exits to the intake runners. Maybe some benefit I don't know about, but seems strange. Would there be less torque perhaps? A setup like this would allow more room for an updated air tube/filter setup like Roger's as well.

 

The long intake runners on the early 32-valve are there so the engine can make lots of torque at low RPM. It hurts the high RPM power of course. That's why in '87 they put a flappy in there so you can have long runners at low RPM and short runners at high RPM and get good low-end torque and high-end HP.

I am sure a smoothed center T-piece would help air flow/turbulence, but the air still has to take many more sharp turns so I don't know how much overall effect it would have.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Actually, the flappy just connects the two plenums. That doubles the volume and helps raise hp. The runner lengths don't really change.

 

The target runner length for the S3 was ~400mm, head to bellmouth in the plenum. Head to valves is ~95mm. Total 495mm/19.5".

The S3 plenums are huge. Lots of volume for high rpm.

From my logging, it appears the 75mm throttle body is the cork which keeps rwhp under 310.

 

The actual aperture of the throttle or the fact that air must go down the U and back up through the throttle plate?

 

I think it's the 75mm throttle plate. I'd like to get my hands on a 80mm '86 ROW 16V TB and see! (AFAIK, '86 is the easiest conversion, but '84-'85 can be made to work.)

The TB/MAF housing is more of a 'J', and is quite large inside. The inside corner could be a problem. Louie Ott suggested adding a hump at the sharp corner to help air go around it.

The openings of the 'J' and 'T', before and after the TB are ~82mm.

 

So you need an 86 euro throttle body? Wouldn't you better off be looking for a pink unicorn who's middle name is Constance? Or am I misreading the rarity of that part?

Why not enlarge the 75 to 80 or 82?

 

Ken,

So the large volume of the plenum is a good thing even though the air doesn't have a very smooth or direct path through it? Maybe I need to post a drawing of what I was thinking of. Not sure I'm getting the message across.

 

I've heard of one S3 TB that cracked when it was being machined for 80.


I want a dual/twin TB with custom J and T to match.

 

Don - I don't think the airflow inside the plenums is a problem. There are nice horns on the end of each pipe, and there's plenty of room for the air to move around in there.


It'd be cool to put throttles on each leg...but you'd have to space each one out individually, as the short legs continue inside the plenums to match the length of the long ones.

Or, make custom plenums, with forward facing TBs on each side... I wish I could weld aluminum...

 

I've often wondered if extrude honing would make a difference.

 

There are a couple of thousand '86 "Euro's" out there. Some of them may even have been parted out

 

How about pistons from the low compression 86 32v?

 

Actually the 85-86 intake runners vary in length quite a bit. This isn't a bad thing as it spreads out the torque curve rather than piling the intake pulses on at one rpm.

As for the 75mm TB being a restriction, the ls1 and ls6 TB's are both 75mm and are not considered a restriction what so ever. A stock ls6 has about 360-370 rwhp and a heads and cam ls1/ls6 car can have easily 450rwhp+ on a stock TB...

 

?? IIRC, not more than ~5mm, measured at the center of the pipe. The 'short' ones have extra length inside the plenums to equal the 'long' ones.

Something is causing vacuum to build right after the throttle plate as rpms rise. Could be the TB, could be the TB/MAF housing.