Air Leak!

[Piledriver]

Wow, huge difference, almost like the wrong butterfly is installed... Amazing it ever worked like that.

[Max Welton]

Well, it may not have. I did use parts from 3 different assemblies and this is certainly not the combination that was together before the engine rebuild.

Anyways, it's working good now. But what a hack!

What do they say about stupid ideas that work?

Max

[Steve Arndt]

Why not adapt a throttle body that actually throttles?

[Max Welton]

It's my daily so the car needs to stay in service. But yes, I would be totally up for something I could graft onto the stock runners.

Max

[Steve Arndt]

Easy enough to weld a flange on the plenum to adapt. That opens up options of having idle bypass built in to the throttle + A TPS.

[Max Welton]

Any suggestions? I would even consider chopping the "nose" off a stock TB and replacing the business end with something better made.

Max

[Piledriver]

A t4 DJet or Ljet TB should be ~easy to graft on, tapered seating surface (fits on a cone)
Has its own air bypass screw, uses a pair of m6 screws for retention with a rubber donut gasket as the seal.

The Djet and later LJet T4 TBs have the drive tang on the other side to run a switch or TPS, but you can adapt the non-switch units pretty easy.
(They have the TPS/switch plate mounting bits, just no output shaft, some 1/4" rod and a 3 minutes with a dremel will provide a tanged floating extension, currently running that setup using a GSXR TPS)

The vanagon WBX ones are nicely made but 50mm... are somewhat progressive with a fat side built into the butterfly so only one side "opens" at small throttle settings. My setup seems to prefer the 45mm std tbs, either will fit the widened Vanagon plenum I'm using.

The WBX ones have an idle switch plate/shaft that can be used to mount/drive a TPS, or just use the switch if you are happy with MAP accel only.

[Corysvdub]

You can make a center section and weld a throttle body to it. That's what I did anyways.

[aircooledtechguy]

That throttle plate doesn't even look close to fitting. Are you sure it's not mounted backwards on the shaft?? That could explain the poor fit. . .

At the least, I would look for a replacement T-3 plenum. Otherwise, I would look to fabbing a replacement plenum that uses a later style TB with an IAC stepper and such. They are cheap in wrecking yards and would be no problem to fab-up.

[Steve Arndt]

Grab a 50mm TB off something that rotates the right way and clears everything. Make a flange and weld it on. My main consideration would be adapting to work with the stock throttle cable.

You don't gain much by going with ljet, vanagon, etc. Still no TPS (pot type) and they are all old by now anyway. Volvo and VWs from the mid to late 90s adapt easily and are in the 50 to 55 mm size.

[Max Welton]

I make my own cables out of 1/16" wire rope so length would never be a problem.

Hey, I seem to remember that there is some actual science relating to the plenum volume. If I'm thinking about changing that, I should probably understand it. Any of you guys know what I'm talking about?

[Steve Arndt]

Yep. Search the forum for old Ray Greenwood posts related to plenum volume and relationships.

[aircooledtechguy]

Grab a 50mm TB off something that rotates the right way and clears everything. Make a flange and weld it on. My main consideration would be adapting to work with the stock throttle cable.

You don't gain much by going with ljet, vanagon, etc. Still no TPS (pot type) and they are all old by now anyway. Volvo and VWs from the mid to late 90s adapt easily and are in the 50 to 55 mm size.

If you carefully measure everything and choose your TB carefully, (a) you will not increase/decrease the plenum volume by much (not enough to notice) and (b) still be able to use the factory air cleaner assy.

[Max Welton]

I'm assuming the increase in displacement has already disturbed the perfection that is the type-3 plenum. But I do want to keep everything under the cover where it belongs.

Ahh, it's academic in the near-term anyways. The car is my daily and there's no backup vehicle. As long as it's working reasonably well I won't mess with it. Many other things need attention.

Max

[aircooledtechguy]

I'm assuming the increase in displacement has already disturbed the perfection that is the type-3 plenum. But I do want to keep everything under the cover where it belongs.

"Perfection" is a very relative term. While there is lots of physics, science and even philosophy that comes into play when building and creating car parts to be "right" and "perfectly designed", cars are very forgiving things. I would virtually guarantee that a minute change in displacement will have no noticeable effect from the driver's seat. Nor will it have any noticeable affect on tuning.

Case in point #1: many, many years ago, I designed and built a CIS injection system for an upright converted type4 that I first made out of steel/aluminum. Everyone who "knew better than me" said it would not work and would not run well and have tuning issues. . . It ran fine, but ran out of breathing too early due to my runner sizing. So I made another plenum out of fiberglass that mated to IDF manifolds. It ran very strong and smooth, but looked like hell, so I bolted on carbs to that motor and never looked back at CIS.

Case in point #2: Several years ago, I had a friend who desperately needed an engine to get to a new job he was about to start. He had no money but had a parts stash of used parts. Over a weekend, he and I built a "TP 1600" out of 100% used parts. "TP" meaning "Tri-port": SP on the left side, DP on the right!! Now all the "experts" will tell you that a motor like that should run poorly and certainly be un-tuneable. Not so. It ran just fine like any other 1600 and lived just fine for about a year before he replaced it (still running) with a new 1776cc that I built for him.

The point is this: People do a whole lot of talking and speaking philosophically about a lot of stuff. 90% have actually done very little of what they speak and there's a big difference between talking and doing. On paper, a lot of ideas don't appear to work well, but in reality-land, they work out just fine. There is power that you can only see on a dyno and there is what you can feel by the seat of your pants. Dynos mean very little unless you are doing R&D or need a very high state of tune. Seat of the pants is what 99% of us around here use.

Ahh, it's academic in the near-term anyways. The car is my daily and there's no backup vehicle. As long as it's working reasonably well I won't mess with it. Many other things need attention.

In your spare time, weld-up a replacement plenum similar to the one you have now with a real TB that works. Maybe add one with an IAC stepper. Make it how you want it so it all fits. Then one afternoon, replace it and see how un-scientific and completely forgiving your car really is. I think you'll find that there is a LOT of over-thinking going on.

My .02 FWIW. . .