D-Jet Throttle Bodies, D-Jetronic Modification Options

[TC Johnson]

I have seen ads in Excellence and Panorama for enlarging throttle bodies, and have wondered if anyone has found this to be a successful bolt-on method to increase power on a '76 2.0. My engine is stock with stainless heat exchangers and a Bursch exhaust, and I am looking for small steps, not a giant leap at this time.

Since I know that a bigger throttle body may also decrease drivability, I also wonder if anyone has attempted to adapt a CIS progressive two-throat throttle body to a D-Jet injection system, or a larger throttle body from one of the other cars using D-jet injection.

Another idea that I have not seen are double D-jet throttle bodies on custom intake manifolds. There seem to be quite a few injection systems lying around since many have gone to carbs, so parts would probably be easily acquired.

Discuss!

[Bleyseng]

I did the mod so my TB is a 50mm instead of stock 45mm with a dyno'ed difference of 1 hp. The problem is not getting enough air in but rather getting the exhaust out! Different cams ( with longer exhaust duration), careful exhaust porting and a larger exhaust valve will wake up the motor.
Also dyno tuning the A/F mix via the MPS will net some hp.

Are all your FI parts the correct models, matching the year of your setup? Check of Brad Anders site for info as mis matched FI parts robs hp.


Geoff

[ottox914]

I called a guy on that "bore out your OEM throttle body" thing, not sure if it was the same guy, but let me tell you, this guy valued his work VERY highly. Good thing I was sitting down. Not sure how much you'd gain on a basiclly stock motor anyway. The real gains this guy was quoting were on cars w/turbos or LOTS of head work to take advantage of the extra flow. I'm thinking it was like $4-500 or something like that, enough that I could think of alot of other things to spend it that would make more of a difference. Tires, sway bar, better stereo...

[Bleyseng]

That guy quoted me $250 so I had it done locally for $75 including throttle shaft bushings.

Geoff

[airsix]

I went to a 51mm TB and noticed no difference. I only did it because I was building up to the turbo install and needed one that sealed better. $15 junk-yard special.

-Ben M.

[SirAndy]

QUOTE (TC Johnson @ Jan 4 2005, 01:12 PM)

and have wondered if anyone has found this to be a successful bolt-on method to increase power on a '76 2.0.

snake-oil ...

they simply bore out a stock TB to a larger dia.
what they don't tell you is that there's a "venturi" in your TB and by boring it out, you'll loose the venturi effect.
not good ...

plus, like geoff said, the weakest link on the T4 is the exhaust side (the head part to be precise).
if you want more HP, make your exhuast side flow better, pumping more air into an engine that is already choked to death by it's exhaust port is not going to help you ...

(IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Andy

[nebreitling]

as geoff said, until you do something to get exhaust out (cam, porting, header), your gains by putting more air in will be negligible.

de-smoging your motor and putting a header on it will do the most to wake up your engine for now. ought to be good for about 5+ hp.

[lapuwali]

QUOTE

what they don't tell you is that there's a "venturi" in your TB and by boring it out, you'll loose the venturi effect.

I fully understand why you'd want this effect in a carb, but I'm at a loss to understand how it's beneficial in a throttle body with port injection.

[Reiche]

My guess is the venturi "smooths" the intake stream to help accurate metering. Turbulence in the system would probably wreak havoc on Djet.

It also probably helps keep the flow at a high velocity at WOT when the engine needs air most.

[lapuwali]

QUOTE

My guess is the venturi "smooths" the intake stream to help accurate metering. Turbulence in the system would probably wreak havoc on Djet. It also probably helps keep the flow at a high velocity at WOT when the engine needs air most.

Not slamming you here, these are good guesses, but they don't sound right to me. Immediately downstream of the throttle body is a sizeable plenum, which will kill any such effects as far as I can tell. This is esp. true on the 2.0 plenum, which is pretty large relative to the size of the throttle body. I could be completely wrong here.

Slide throttles and roller throttles are used frequently on port injected engines, and neither really has a venturi effect. I know I've seen throttle bodies with no venturi in the, just a straight pipe with a butterfly at one end.

[Reiche]

No problem. They were just guesses. How about another?

It could be something as practical as Bosch only wanting to make one TB casting for all engines from 1.5 to 2.2 liters. The attachments at either end would be universal to the other intake plumbing. The venturi just makes the TB flow well for whatever size they wanted to bore it to.

[DJsRepS]

Go stick that ol venturi thing where your floats go. While our throdle bodies may resemble a venturi any ventruian going on there aint needed as it is such a long way form the injectors down those long intake tubes. Venturies on float carb's use it to create a sighpon effect through the main jets to a carbed motor. In theroy SirAndy is right the venturi effect would change with larger bore but our Djet cars wont care much.

[airsix]

Remember, a venturi does two things. It lowers pressure and it also increases velocity. The benefit of a narrow TB passage might be better throttle response due to higher intake velocity at the butterfly. Just a thought. As was said, once it hit's that big plenum it's going to slow WAY down, so I don't know that speed at the TB has much real effect or not. Maybe.

-Ben M.

[Bleyseng]

Those long runners effect things more than the TB as they push the torque curve way donn in the rpm range. Its one of the reasons the engine falls flat at 5000 rpms.

Geoff

[TC Johnson]

Thanks for all the replies!

One thing that seems to have emerged from the discussion is that enlarging the throttle body without concurrent improvements in the exhaust, particularly inside the heads themselves, will not generate much increase in HP. However, the last post suggested the long runners also affect higher RPM performance (more important on the track).

Looking at some of the Type IV sites finds that some of the higher output engines seem to have long intake runners below the carb or FI. Have any of the lurking engine builders done dyno testing or theoretical calculations on optimum street vs racing runner lengths?

I have not seen any discussion yet on adapting the progressive TB from the CIS system. Has anyone tried that approach?

Thanks in advance for your continuing comments.

TC

[Bleyseng]

The calc's Charles did for me indicate a length of 10.5 inches for runners to set the optimium hp/torque range at 6000 rpms. The plenum also would have to be modifed accordingly. I am going to 103mmm pistons so I would need probably a 55mm or 60mm TB to feed the beast.

Even when you change the exhaust side via larger valves and porting the stock cam sucks. With my 2.1L with ported heads, 38mm exhaust valves,stock cam and Djet tuning I have dyno'd 95hp at the rear wheels, which is about 110hp at the flywheel. For more hp a special cam would have to be installed.

I am going to do just that! A special cam from Jake, along with 103mm Nikkies, modifing the runners and plenum, better valve springs and a ceramic lifters, I hope to hit 150hp. That will be enough for a street/AX machine as it should run upto 6000rpms for its peak hp instead of 4500 rpms stock.

Geoff

[airsix]

QUOTE (Bleyseng @ Jan 5 2005, 07:39 AM)

Those long runners effect things more than the TB as they push the torque curve way donn in the rpm range. Its one of the reasons the engine falls flat at 5000 rpms. Geoff

12psi boost will cure that. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

-Ben M.

[Bleyseng]

I don't have a running 1.7l to install to do that!!! I have a core 1.7 laying around but I still have to tear it down to see its condition (on the back burner for 4 years)

Geoff

[airsix]

QUOTE (Bleyseng @ Jan 5 2005, 10:31 AM)

I don't have a running 1.7l to install to do that!!! I have a core 1.7 laying around but I still have to tear it down to see its condition (on the back burner for 4 years) Geoff

Mine will be a core soon enough. (IMG:http://www.914world.com/bbs2/html/emoticons/sad.gif) I have a feeling it is aging rapidly, but it's fun while it lasts.

-Ben M.