Anybody tried double D-jet air boxes and TB before?




/Lars S

Mine's a little different than what your picture shows.
I made my own plenums.
1911cc with two 1.7L TBs.

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We have a racer out here running 4 diet throttle bodies

Oh that's more professional!
Mine is also a 1911cc, havn't started it yet, any D-jet tuning tips to make double TB's work?


/Lars S

Wideband AFM and set up the MPS so you can make adjustments using the info on Brad Anders' site.
You'll need to modify the bypass screws to close further since there are now two. Otherwise you won't get the idle down low enough.

Thanks!
I thought of making the linkage so that one TB is fully closed until 2/3 throttle like a 2 step carb. After 2/3 throttle resistance will be added to the CHT for enrichment.
Up till 2/3 hopefully good MPG .


/Lars S

I made mine so both are at the same opening at all times.
Being so far apart and only joined with a small hose, making them progressive isn't an option but it might work with your setup.
The only downside I've noticed is a sensitivity to slight input changes at very light throttle settings.

I'm not sure but I think you may want to cut the cyl 1/2 runners so they are the same length as your 3/4 runners, you could then use the cutoff section of the runner to connect the 1/2 TB manifold to the 3/4 TB manifold?

This should make your setup and flow to each cylinder bank more even. The way it is right now I would expect cylinders 3/4 to have higher flow than 1/2 (even if the 3/4 TB is closed and 1/2 TB wide open CYL 3/4 would still have the 3/4 TB manifold acting like an air reservoir while 1/2 would have to pull their air all the way through full length runners from the 1/2 TB manifold.)

I don't have fuel injection on my Type IV but from an even flow perspective that should make things easier, neat project though

So what do you gain with Double Throttle Bodies and Two Plenums?

I can understand that if I bumped the Stock Displacement of a 2.0, I'd get higher vacuum at the same RPM (making no changes to valve and cams). I'm assuming that this higher vacuum gets out of the range of the MAP sensor so the computer doensnt' get signal to send MORE gas?

If I split the airflow into two seperate throttle bodies, I can see how Pressure-Drop or Vacuum would be reduced quite a bit for the same airflow since I'm flowing only to 2 cylinders.

Is this the idea?

I am in love with this hack.
Please explain why and the benefits.

I just see twin turbos, but please don't get side tracked on that.

Rich

To be honest - I dont know yet. This is an experiment I set up, could not resist to make it since I had all the stuff and it is a super easy modification (so far).
The Engine is a 1911cc with standard 1,7 heads and std 1,7 D-jet, not started with the double arrangement yet. Its a street car, Im not looking for race performance.

The theory is thet the larger displacement needs more air (and fuel) and hopefully the two 1,7 airboxes and TB's and can give some more air. Fiddeling with the MPS, CHT and fuel pressure will bring more fuel. I may have used a single 2,0 TB with manifolds instead if I had one.



Yes lower vacuum sensed by the MPS means more fuel as far as I understand.


/Lars S

Yes the arrangement is not balanced (yet). It was wery handy to use the original brackets for the left plenum so that one is in its original position. However it would not be to complicated to make the installation almost symmetrical.


/Lars S

Why not split two plenums down the middle at the seams, and weld the two half pieces together to make one plenum with two throttle body openings?

It would be simpler, and the intake runner lengths would not change.

More volume in the plenum is part of this solution.
So adding that volume with two plenum a is one way of doing that.

I am still curious as to why someone would do this?

More cc's needs more air.

/Lars s

Yes one combined plenum sounds like a good idea!

/Lars S

Neat idea, but I don't see any benefit in your case. The small increase in displacement, maintaining the same long restrictive intake runners, and I assume the stock fi cam make it a fun science experiment that will probably be a pita to tune with little to no benefit.

Thanks,
Im afraid you might be right but I can't resist to do it.


/Lars S

If you move each TB close to the heads like you have on the 3/4 Cyl side the shorter runner and closer TB may help out?

At last, after running in my new 1911 Engine I have now tested the performance of the double D-jet arrangement VS single stock D-jet:

Acceleration on plain road from 37 to 68mph in 4th gear was one second faster with the double arrangement (10 VS 11 seconds).

Tested several times on the same road with both systems, no wind etc.
My double system has an extra fuel enrichment at 3/4 throttle (by fooling the cylinder head temp sensor), my stock not.

Looks like Im removing the double arrangement due to the minor advance... also it is blocking access to #3 spark plug...

/Lars S

Cool! Thanks for sharing
I'll add this to my list of "potential ways my 1.7 can go faster" (right behind McMark's turbo).

This is fricken cool Good on you for trying things and reporting them here.

The volume of your plenum can cause you some issues, too. I do not know how much and at what volume, but a larger plenum would be like a big room. If you open a door to a large room you feel nothing. If you open a door to a small room, you feel it more (if that makes sense).

Another thing is runner length. Longer runners make for more torque. How long and such depends on your displacement and other things.

When I was designing and building my intake system for the bus I had to take all of this into consideration. It helped that someone else had tried similar and their plenum was visually way too small and yielded crappy results.

Thanks!

I had serious ocillating idle problems with my double system...may have to do with the plenum wolume, I cured i by making the idle-mixture (over)rich, lets see how it reacts when going back to single D-jet.


/Lars S

What about just using the 2.0 plenum? it is larger, then just add the 2.0 throttle body. I don't know the size difference between the 1.7 and 2.0 throttle bodies.

Yes thats a good idea, I think many have done that on 1911cc's. Best would be to use the larger 2.0 intake pipes also (and heads of course).
I just happened to have another 1.7 d-jet plenum and throttle body and wanted to make a test.

/Lars S

Neat to see that you actually got a power increase from the larger TB area. I increased the size of the TB on my latest build as well but was not sure if it made any difference. I have run a 1.7 plenum/TB/Runners on a 2.0 before and it definitely restricts the power. From what I recall (and don't quote me on this)

1.7 TB ~37mm (also smaller runners and much smaller plenum)
1.8 TB ~45mm (same size runners as 2.0)
2.0TB ~46mm (largest stock TB, same size runners as a 1.8 but intake layout causes turbulence. Also 3 bolt runners vs 1.7 and 1.8 4 bolt)


The 2.0 intake requires a bit of hacking to fit on a 1.7, and still has its problems.

My solution, still looks stock and gets you a total throttle plate area pretty close to what you get with dual 1.7s, and also you keep equal length runners with the larger diameter.



1.8 plenum with the TB opening stretched to 52MM, + modified 50MM vanagon TB:
total throttle plate area 1962mm^2

on top of a 2056, does not seem to be lacking for air on the top end at all

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Good ideas JamesM!

How do you match the vanagon TB to the D-jet TPS?


/Lars S

You install megasquirt!!!

The throttle shaft size and shape are the same, though the vanagon TB the TPS side of the shaft does not extend as far out, looks like there would be enough to engage the d-jet TPS though. You need a plate under the TPS to be able to mount/set the angle correctly (same thing I had to do with the TPS I used on my Megasquirt setup). The Vanagon TB pulls in a different direction, so you need the throttle cable arm off of a 1.8 TB as well, basically just undo the nut holding the spring and arm on the 1.8TB and it all just bolts directly the Vanagon 2.1 TB. Got to love how interchangeable VW stuff is!

Thanks JamesM!

Are we talking about a TB type 025133067A ...?

/Lars S

Let me see if I can dig up the part number again. It is the one from the very last vanagons, 2.1L watercooled

Yup, that's the one!

Gowesty sells ones that have been rebuilt to a 52MM bore as well, but then you lose the cool stock wedge throttle plate.

Thanks JamesM!


/Lars S