924Board.org

[-nick]

Hey guys,
I’m still undecided on what to do about the intake. My latest thought is to cut the stock manifold leaving the runners very short, then weld a square plenum onto the runners and have the throttle body directly across from (and between) the #2 & #3 runners, facing the left fender. Then I can run a smooth 90* pipe (or two 45* angles more likely) from the TB to bring the charge to the IC.

The good points- the welding will be pretty simple. And the main reason is to just fit everything! I want to keep the AC, which means I can’t move the alt, which severely limits the possibilities. Quick revs using short runners. It will be easy to weld venturi’s onto the runners inside the plenum.

Bad points- using such a short runner length (to make everything fit) worries me a bit. I’m imagining the torque loss off boost will be noticeable. I also need to make sure the #2 & #3 runners have a wall to bounce the pressure waves off of and don’t bounce off the throttle plate opposite them. Some careful measuring should tell me what the tolerances are, but I’m wondering how much distance there should be between the throttle body and where the pressure waves will be focused against the plenum wall?

Anyway, some more general stuff-
From what I can gather, runner length isn’t as much a concern with forced induction, but plenum size is. The story seems to be to use a plenum that is 100% (or just over) the engine displacement- which would certainly be larger than the stock 931 plenum. I’ve heard some talk of a lag in throttle response using a large plenum.

From what I've found, it looks like the rule is to make the plenum so that it extends an inch or so past the outer runners. This is to provide better airflow to the #1 & #4 ports (equalizing them to the airflow of the inner ports).

Last, there appears to be some controversy over the usefulness of venturi’s in the plenum with forced induction. I’m also wondering if what protrudes inside the plenum is “counted” as runner length?

nick

[-nick]

Just for completeness, here are some various shots of 931 intakes-

Racing's:


Racing's venturi pic:


Random pic?


Pic from the net- nicely made, but the pressure waves are going to be screwed up by the difference in the distance it takes them to bounce off the back of the plenum?


The stock GTR intake:


The stock GTS intake- which has a strange plenum with that extra length of pipe connecting the "main" plenum to the throttle body.


nick

[Raceboy]

-nick: For optimum performance "Maximum Boost" suggests venturi-style runners.
And as for Racing's new intake design, haven't you considered diffusor-style? To make things more clear: http://www.bufkinengineering.com/intake%20manifolds.htm

Here's some info and pics of this solution.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[Manning]

Folks like Jon Milledge and Lindsey are using tapered plenums for the 951 to equalize flow into the cylinders, as described in the article raceboy posted a link to. I don't think pressure waves are as much of a tuning issue on a forced induction motor as they are on an NA, since you are stuffing the plenum full of air rather than relying on the motors pumping action. Actually, there are periodically huge debates about this on Rennlist, maybe do a serve for "intake" on the 924/44/51/68 BBS.

http://www.jmengines.com/throttle.htm

http://www.lindseyracing.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=LR&Category_Code=944INTAKELINDSEY

http://www.speedforceracing.com/productsporsche_intakemanifolds.php
_________________
Michael Manning
1988 924S Undergoing weight loss program

[endwrench]

Nick,
I would think something like Racings first design would work well for you. It is not optimum but it seems your space constraints are going to limit you. You might also consider installing an internal diffuser creating a sort of dual plenum. This should also eliminate some of your concerns about pressure wave tuning.

This is the way I have it pictured in my head. Assume you are using 4" pipe as a plenum. Picture a 2 1/2" piece of plate the same length as the plenum welded into the plenum as a wall directly in front of the throttlebody opening basically sealing it off from the runner side of the plenum. OK, now you cut a 1" or 1 1/4" slot in this plate the full length of the plenum. This is your diffuser. I am sure there are some logistics to this that will need ironed out such as runner airhorn to diffuser plate space. Not sure how much you would need. Anyway it is an idea.

I plan to build a diffuser style intake for mine if I ever get around to it!

Todd

[Racing]

The first thing to have in mind that is often overlooked imo is intended use.
Fabbing a new intake becomes an issue out of a couple of reasons.
1/We´ve learned as a community.
The OEM 31 intake is an OLD compromise by now.
No matter how you look at it the 31 intake is basically just an offspring of the NA idea.
We don´t built turbo manifolds that way anymore.
What that gives is that more or less any step in the direction of what´s more commonly known today WILL render positive results.
However...there´s absolutely no point what so ever in designing a 600hp intake for a 300hp motor.That´s just pure folly and a damn engineering exercise to prove that it can be done.

2/Seing that we indeed BUILD turbo engines way different than back in the 80s(or whatever)that basically needs to be adhered to as far as i´m concerned.
What IS the design objective?
If it is to design an UNOBSTRUSIVE intake for a 300hp+ 31...
Well,then i´d go on record and say KISS.
No need to reinvent the wheel.
As stated...the first design8or more to the point second actually) that wound up in my 31 would most def do the trick.
Two of it´s prime advantages being the increase in plenum volume,and the increase in TB area.
In short it made for a much nicer,more torqy,impression when driving around town on a daily base.(bet ya didn´t believe that huh Nick )
Anyways.
In such a scenario,i´d STILL opt to "redesign" it and make good use of a diffusor.
Basically this can be done from regular square alu tubing.
Much like i´m currently doing for the merc.

Not that we really experienced any and all that much trouble with pressure waves of the design in the pic,just...that it isn´t the way it´s done-when done properly.
Did it work however...yep.

[Racing]

Speaking of which Nick...
Lag in throttle response with large plenums?
Yeah,to an extent.
But...start by gettin rid of the damn CIS...and you´ll have a "new" motor right there...and the lag that comes from a larger plenum and an EFI will not by far be the same as the stock setup.

[endwrench]

OK, so my design would be a little on the crude side but should work but a better design might incorperate a wide 'V' shaped channel welded to the side of the round plenum (with the throttle at the 'V' point) and the diffuser slot cut into the side of the plenum tube. Still simple but more elegant.

What do you think of going with a smaller main plenum if space constraints dicate it? Could the volume of the diffuser plenum(don't know what you call it?) be counted as part of the overall plenum volume?

I will actually be doing a front mount throttle like yours (Racing) so I plan to adopt the tapered channel idea but I am wondering what would be best for Nick's side throttle setup.

Also Racing, I was wondering why you ended up going to shorter runners on your setup with the silicone connectors? Clearance problems or something else?

Todd

[Raceboy]

[endwrench]

Ahh, makes sense. Good tip too!

Todd

[Racing]

Yup.
Vibration.
En engine produces amounts of it.Even so in intake tracts asf.
So..we opted to have the entire plenum suspended in rubber.
Also used a bracket from the OEM block point to mid ships plenum.

Space constraints.
Yeah.
Exactly,and the motor STILL ran well...
See what i was talking about above?
What i mean is that of course we could have optimized the intake and just cut into the body metal as needed.
Reasoning tho...what would be the purpose?

[Racing]

[J1NX3D]

hi nick, have a look at the setup on this al holbert 944 turbo cup car

_________________
'86 944

[Peter_in_AU]

Dear Abby

OK, for the rest of us, would someone like to explain the whole diffuser thing? I understand that we are trying to equalise the pressure at the individual ports. Is the diffuser the tapered section on top of the main plenum in the link posted by Raceboy? What would one of those plenums look like if you sliced it in half?

I'm wondering whether that "dual plenum" design is very different to many "single" plenums I've seen where the plenum is tapered towards the rear cylinder and the throttle-body is at something like 45 degrees to the base of the plenum.

Signed
Confused.
_________________
1979 924 (Gone to a better place)
1974 Lotus 7 S4 "Big Valve" Twin-cam (waiting)
1982 924 (As featured on Wikipedia)

Learn to love your multimeter and may the search be with you

[Racing]

I´ll try to.
On a NA motor u usually design a high performance intake by the way the engine digests air.
In VERY general terms that means that the plenum gets smaller and smaller the further away from the TB you get.
Normally by 25% per cyl.

Of course one can design a turbointake the same way,but a turbo engine and it´s more "static" approach of digesting the air renders bigger freedome in intake design really-again from a performance perspective.

Major pitfall of boosted intakes is gettin the pressure uniform over the runners-as you clearly are aware.
So...
What we do is use some type of diffusor to equalize the pressure as best as we can.
Ergo..the reasoning of a "feeder" plenum of smaller volume ahead of the main one holding the entry for the runners.
What one does is to design a slit if you want where the feeder plenum reaches the main one.
Slit is to be kept of greater area-WITH consideration to turbulence-than the throttle body to avoid pressure drops across that segment and letting the TB still be the thing that controls the amount of air entering the main plenum in practice.

Got any clearer?