EFI -- Plenum question

J

Jeffyoung

Well-known member
The EP TR8 build continues.

I'm relatively convinced the EFI is a better option than the four barrel, given 12 years of experience with these motors.

In reading the rules, it looks like I have to retain the stock intake manifold and the stock throttle body. What about the plenum? Free with the rest of the injection system? And if yes, is that true even if the throttle body is presently cast into an appendage on the plenum?
 
Always reference the technical glossary beginning on page 156 of the GCR. In this case you want the definition for "manifold" I think. You will be disappointed, sorry. Can't wait to see a TR8 on track again. Going to be very cool.
 
Been doing this long enough I should know to do that...lol...

I am mobile and can't get the GCR on my phone -- is the plenum defined to be part of the intake manifold?

Thanks man.
 
Page 171

Plenum – An induction system chamber generally interposed between carburetor(s) or air intake(s) and ducts feeding ports.

EP Spec Line for TR8

Since it's running a "limited prep" motor, I would assume you can't touch anything in the manifold, plenum, or throttle body, unless specifically stated in the spec line, but there is no reference, other than compression and valve lift.
 
There is a conflict there also. You can port the TB but cant change the size of anything.
Short story , No you can not touch anything after the TB to the intake ports and may port match to the 1 in rule ofcourse.
 
After a deep dive into the rules on EFI for Prep Level 2, I would suggest this is not so simple (is the Plenum "free"?).

1. Plenum is a separately defined term in teh glossary, different from manifold. Clearly they are two different parts under the rules.
2. In the "INduction" section of the P2 rules, it basically says air has to go through the stock throttle body, and that the intake manifold has to stay stock except for port matching. Nothing about the plenum.
3. IN THAT SAME section (under "Induction") it says that the "fuel injection system is in all other respects unrestricted" or something like that.

So the question is whether the plenum is part of the fuel injection system or not, in my view. If not, then it doesn't say you can.....

The definition of fuel injection system would suggest no, but the placement of it in the "Induction" heading suggests yes.
 
Hi Jeff -

On the injected TR8, is the plenum physically separate from the rest (runners etc) of the intake manifold?

(Guessing yes, otherwise the idea probably wouldn't have arisen. Googling pics of TR8 intake manifolds hasn't helped as there are many non standard versions out there. Certainly some V8 manifolds, including my '80s Corvette, are multi piece.)

I think that the rule was written with a typical inline 4 one piece plenum and runners manifold in mind with the (unstated) assumption that plenum + runners = manifold. I think it's been therefore been assumed that the "no porting" rule applies to the whole thing.

I'd also go so far as to say that you've entered a Catch 22 situation - either modifying the plenum is already illegal, or else you've found a loophole which once recognized will be slammed shut.

Cool project, regardless!

Al Seim
HP VW Scirocco 1.6
 
Yes, exactly right. The plenum is a separate piece, a small box really, that bolts onto the top of the intake manifold.

I do think that right now, if the plenum is separate, there's an argument it is free. I also agree that "allowance" might be slammed shut if exploited...lol.....

Car is coming along. I dipped it last March and it's had some minor rust repair, seam welding, etc. Body panels are now done so it is off to the cage man shortly.

I hope to have it running late summer, shake it down the end of next year, and make a run at Indy in 2017.

This will be a full tilt build. To the max. I've been racing these cars a long time and think I have a pretty good idea what works and what doesn't, and with the modelled/expected power output I think it can be competitve in E Prod. But we will see.
 
Jeff, I hate to counter your argument since it will be great to see your well prepped TR8 out there. In my opinion you can only gasket match as well as some minor improvements to the throttle shaft. It can be argued two ways: that you have a 2-piece intake manifold or that the intake and plenum are separate-but in your case the plenum is also the throttle body casting/housing, so either way you are limited.



 
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