Single TB size for a 2.6L T4, how big is too big (or small)

[Mark the canuck]

I'm out of $ for dual TBs figure and I figure I can do a single cheap system then it's a bolt on when I can afford something better.

I have an 55mm (2-1/8" +/-) and megamanual says 170hp so it's a bit small or I have a line on a new 74mm (3" +/-) cheap but that's a bit big. Too big causes bad idle and low vacumn. My buddy say get it and try restrictor plates.

pic #1 is the 55mm, it looks bigger than it is because it has a tapper startting at 60mm.



Pic #2 this roll of tape the ID is 3" (approx 74mm)



the 914 d-jet runner is a fair bit smaller than an IDF manifold on this end.



But the business end is the same size





Engine spec
T4 914
SDS em-4F (w/crankfire)
78mmx102mm nickies/JE
heads 44X38mm, 12mm plugs.
Cam Raby/web, for big FI engine (I'll have to get the #)
1-5/8" Pete weber (pre tangerine) 4-2-1, most likely will use a magnaflow corvette muffler.

So is this do-able? TB size? plenum.?

Should I keep looking for something like a 60mm? would buddies restrictor plate idea work?
If it would I could try different size plates and test a little....

[raygreenwood]

You are comparing apples to oranges here....and mixing carburettor axioms...which do not work for FI. Dual carb manifold sizing has little to do with plenum and runner sizing. The ITV manifolds have to be bigger to incorporate the volume necessary. The plenum runners are longer so they do not.

Is this a 2 liter 914? And is that the center plenum section ? (it appears to be).
The 2.0 914 center plenum is actually the worst of the bunch. It has large turbulence issues becuase the air must drop down and turn 90*...and rips into four seperate directions. You can fix this with a pyramid shaped splitter underneath the TB....but then that takes out a lot of potential volume.
Those are the three bolt 914 runners which are the larger of the bunch and are perfect up to about 2.1L.
You will not need a TB much more than 55mm to feed upwards to 2.1 to 2.2L. At worst...slightly more....but an excessively large TB is the worst thing you can do for a plenum system. It makes teh throttle plate movement very "peaky" and drops vacuum and vleocity excessively with even slight TB movements. It makes tuning with a map sensor horrible....and makes proper rising rate fuel regulation nearly impossible.

The max TB size you would ever need up to about 2.2L is about 60mm.

Also...restrictor plates are for carburettors. Any restriction in a plenum and runner system would do the same thing it does to a carb manifold.... kill velocity....which is what the whole point of all of this is.....high velocity at the port with excellent atomization.

AND.... this is what I get for eating lunch...working...skimming...all at the same time. I just missed the point that this is for a 2.6L...

But a couple of points...now in reverse:

Those runners will never do. Way too small...and so's the plenum.

Yes...you are pretty close. I'm figuring about 70-72mm for the TB. But...you would actually do better to find ths same area in a progressive TB like from a late rabbit. Much better low end and transitional velocity is kept this way.
Even if you have a large engine......opening a large throttle plate creates a big vacuum drop.......and youwill require approximatley 10 times teh TB diameter....in length....behind the TB plate to get rid of that nasty turbulence plume.

Your runners need to be about 700cc each and the plenum about 1.5 to 1.7L in volume if you want to feed that thing. Ray

[Mark the canuck]

Thanks Ray, hoping you would chime in
Yes it's a 2550cc and it is going into a 914.
More work and money but still do-able.

Then how about the Dual manifolds (and I have stacks) can I plenum them, use the 74mm and I can use the weld in injector holders.

Plenum size? or sizes? bit of design guidance please?

I could do something on the lines of sandeep's bug, but I don't want a turbo.
Plus yes, I'm a carb guy, but I have been successful on 2 engines (2.0 and 1.8 ) in this car. Both with a modified SDS d-jet intake. I have a WB.

[Mark the canuck]

Just read the part on the rabbit progressive and the big TB....I'm looking for something smooth and yet keeping with the KISS method.
I want torque and I'm willing to lose a bit of hp for drivablity.
Stock flywheel.

[Tom Notch]

We virtually have twins..... except I neede to clear the deck

[Mark the canuck]

We virtually have twins..... except I neede to clear the deck

I knew I'd seen this before! The 74mm is the same GM unit as the one I can get.

So have you ever had it dyno'd? do you think it's choking? what injectors?
You don't have that long of an intake from the TB to the plenum.
I have a fair bit of room to play with in a 914.

[raygreenwood]

Nice engine. I would also like to know how its running. That appears to be the bus 2.0 intake plenum which is better than the 914 2.0 by far (as far as turbulence goes).
Adding the spacer/can behind the TB should go quite a ways towards damping the turbulence plume and also adding needed volume to the plenum. Ray

[Mark the canuck]

I could put something like a A frame (shape) on the top of the D-jet plenum and go in from the side or angle. I could use some auto-cad skills about now...

Or if it must be bigger I could do something like this, but the runners here are only 1/8" bigger than the d-jet: http://sdsefi.com/air12.html

[Piledriver]

There was a thread awhile back where Jake compared a 2.0 914 plenum vs a Bus plenum... used the same runners w/rubber hose couplers, so he may not have even tweak'd the 2.0 914 runners so everything lined up perfect.

http://shoptalkforums.com/viewtopic.php?t=102542

The difference was somewhat mind blowing considering the "minor" change, TB and runners being the same, all due to a better plenum design, even though it was the "wrong" volume.

A Vanagon plenum works quite well, the outlets are a better size match for the 914 runners. Needs new mount ears, comes with a 50mm TB.

I plan on widening a WBX plenum initially, but long term, I have an Evil Plan...

I think I have grok'd how slot feed plenums work
(Much beloved by Audi and WC tuners in general, seem to be considered heavy voodoo by most... But I'm pretty sure Audi doesn't use them on their race cars because they LOOK cool)

The following is MY theory of what's happening, so I could be insane, and probably am.
I have never seen a proper explanation anywhere, so don't take the following as gospel:

The inlet area and inlet length a plenum "sees" does not necessarily have to be a TB or ram tube.

It can essentially be the slot down the side of a ram tube, decoupling the actual TB area from the plenum tuning, allowing one to actually tune a reasonable sized plenum for high RPM without using an insane sized TB to get the (significant) boost at the expense of a drivable setup.

The inlet area, inlet length, and the volume of the plenum determine its tuned resonance frequency, so big area/short length... higher frequency (RPM).

One could also easily vary the area of the slot(s) w/o really changing the throttle setting, which would allow for widely variable tuning with one moving part.
(The throttles also do not necessarily have to be UPstream of a plenum, it just usually gets called a tuned airbox)

Note the diameter and length of the runners effects the plenums tuning, as they are coupled.

[Mark the canuck]

OK....I have 2 of the 55mm Honda TB's so have a center plenum before each TB side???

Piledriver

IIRC somewhere it was discussed using 2 smaller plenums (180 degree timed) and 2 smaller TBs as being an optimal solution.

Each 2 cylinder plenum would have to be a larger percentage of the cylinder volume, but this setup has potential packaging advantages on an upright converted ACVW, as 1/3 and 2/4 are connected. (F&R of the shroud, no crossover) A pair of stock TBs should be great, even on a huge engine.

Yes, Chris, the original concept was exactly an intake header.4>1, with a slide-fit dual tube (collector)//plenum to allow easy plenum volume changes for testing.

Jake---You should still try the ram tube setup on the 2.0 bus TB, with the relocated TB. There is no reason that shouldn't work almost as well, and it's easy-peasy as Don says, as well as the length being an easy tuning knob.

Could do this cheap, but a lot of work.

[Piledriver]

OK....I have 2 of the 55mm Honda TB's so have a center plenum before each TB side???

Piledriver

IIRC somewhere it was discussed using 2 smaller plenums (180 degree timed) and 2 smaller TBs as being an optimal solution.

Each 2 cylinder plenum would have to be a larger percentage of the cylinder volume, but this setup has potential packaging advantages on an upright converted ACVW, as 1/3 and 2/4 are connected. (F&R of the shroud, no crossover) A pair of stock TBs should be great, even on a huge engine.

Yes, Chris, the original concept was exactly an intake header.4>1, with a slide-fit dual tube (collector)//plenum to allow easy plenum volume changes for testing.

Jake---You should still try the ram tube setup on the 2.0 bus TB, with the relocated TB. There is no reason that shouldn't work almost as well, and it's easy-peasy as Don says, as well as the length being an easy tuning knob.

Could do this cheap, but a lot of work.

A large single airbox (central) for quad ITBs would be easily doable and tunable to a reasonably high peak RPM if you think "guitar".

The 2.0 runners would likely support far more pk HP if they were SHORTER. One could also cut near the end castings and graft on some larger 90 degree mandrel bends and taper a bit to match.

A large motor will probably like ~42mm ID, depending on desired RPM range of course. Too big hurts everywhere, too small kills top end.

[david58]

Big Welcome to the 10,000 club.

[Piledriver]

Big Welcome to the 10,000 club.

Wow, I spend WAY too much time here

[Mark the canuck]

Did you say quad as in 4?

Congrats on the 10K...you do spend too much time here...helping guys with crazy (ie. cheap) ideas.

[david58]

Big Welcome to the 10,000 club.

Wow, I spend WAY too much time here

we are not complaining