Combining two stock TBs

[Dannyho]

This question might be in the wrong place because it's almost an EE question but here goes. I'm building an intake manifold and would like to use two TBs from dodge stealths on it, aka 3000gt. The ECU I plan on using has output for a stepper motor, but in this case I would have two. I'd be able to control the stepper as much as i want but I cant for the life of me think of how to wire the two of them together. I've found a site that has a lot of model specific information on these motors . . .

http://www.stealth316.com/misc/dsm_tech_manual.pdf

I know it's a large file, but the pages I'm talking about are pg 156 - 165 they have everything about how the motor works, more than enough information. If it's possible to use these ISC servos I'd like to.
If anyone could help me it would be greatly appreciated.

Thanks!
Dan

Also these throttle bodies have a coolant input that has an open passage when the engine is cold and closes when the engine gets warm, I'm not planning on using these at all.

 

[LateApexEE]

Just to clarify, are you wanting to use a stepper motor for actual throttle plate control or are you referring to a stepper idle control valve?

 

[Dannyho]

I want to use them for idle control valves. I'll be using standard mechanical linkage for the throttle plate. so basically dual idle control valves.

 

[Fabrico]

Why double the idle control? Each would flow half as much air as designed for and sloppiness would double.

 

[Lou Scannon]

Not an EE myself, but I seriously doubt there is a robust way to drive two steppers with a single output. If you think you need more flow than one valve can provide, you could try enlarging the internals.
FYI, twin throttle bodies are quite common on V8 engines, GM applications use stepper motor idle air control, so you might be able to find yourself a ready made solution instead of cobbling one together yourself.

 

[LateApexEE]

I agree it is pointless to use double idle control valves. Just block one off and use the other to handle idle control duties.

 

[Dannyho]

well the throttle bodies are for a 3L engine and they're going to be two of them on a 5.2L engine. so they're not going to be flowing half as much air as normal. they'll be flowing 87% they're normal. The problem with trying to block off one and just compensate the other is that the engine is going to be a split plenum type, and while there will be a crossover tube in the plenum, I dont think the airflow will be distributed at all. from what I can tell a stepper motor just works by energizing different coils at different times to turn the thing, so maybe it would be possible to double the current then run the two steppers in parallel? next step will be to call the ECU manufacturer I guess.

 

[LateApexEE]

On a properly calibrated setup, the idle control valve won't have to flow very much air during normal idle operation. Even taking into account your larger displacement engine and split-plenum design, the single idle valve will work. I normally get an engine to idle by itself through fueling and ignition timing before setting up idle control tables, this places less reliance upon the idle valve for a stable idle.

Now, if you had some special reason for very high flow idle control valves (rally style anti-lag system for example) then it may be worthwhile to think about double idle valves. Otherwise, keep it simple!

 

[franzh]

The early 80's Corvette with the crossfire engine had dual throttlebodies, but only one had the idle speed control motor. The other was blocked off. The idle speed. The crossfire had a common plenum and there he one was sufficient to control was no problem with mixture differentiation.

Franz

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[Dannyho]

well that certainly clears things up. It seems contrary to my intuition. but that is exactly why I ask here. Thanks for your input.

 

[MWPC]

Open the butterflies on the TB without the idle stepper a little and you should not have to worry about distribution. Then the stepper will just be a fine idle speed adjustment.

 

[Dannyho]

the stepper motor in this case opens a valve that bypasses the TB, its not connected to the throttle plate.

 

[LordMalaK]

good luck. GM tried it a few years ago- was called 'cross fire injection' and was discontinued because it was junk.

The problem your facing is keeping the throttle bodies balanced throughout their operating range and won't be as simple as tying the plates together with a common linkage, you'll have to control the IAC's individually to keep the intake air balanced between the 2 sides or your gonna have idle problems as well as mixture imbalances, and when the IAC passages start to carbon up (whick won't be identical between the 2 throttle bodies you'll need to compensate one or the other. You'll prolly end up completely closing the IAC passages, tying the 2 throttle plates together with a common linkage, then repositioning the TPS to some point to convince the ECM that the plate is closed (even tho it's not- to allow the engine to idle), and hope that both throttle plates carbon up evenly,or your back where you started.

I'd say your best bet is to take a lesson from BMW and run 2 seperate ECM's (one for each side of the engine).

 

[MWPC]

Dannyho,

That is the way the few TB's that I have work on were setup too. But they all had screw adjustable throttle stops, some under a pressed in cap. I have not done dual TB's but on singles I usually adjust the throttle plate such that the stepper is just open a couple of steps when the engine is warmed up and idling under optimum conditions. That way the stepper is not hitting bottom routinely and the engine should not stall if the ECM over reacts or if a little carbon builds up in the stepper passageway. If you start out with both butterflies open evenly or the one without the stepper open a little extra you should be okay. Will you be running seperate closed loop systems from each plenum cylinders?

 

[LordMalaK]

MWPC-

Do you ever use min idle spec ?? It basically is a mode in the ECM that forces a fixed timing and closes the IAC completely so that you can adjust the closed throttle position (and base TPS position), otherwise your gonna get starting problems, idle speed fluctuations (hunting), off idle hesitation, incorrect idle maps, etc.

Just curious is all

 

[Dannyho]

I'm still debating on how I want to set up hte cylinders. I'm pretty sure i can say that two ECMs is out of the question, but as for the rest i'm just not sure yet. As i understand it, the IAC is made to compensate for extra engine load, things like air conditioning and the like. But I do plan on tuning this engine right. The cross fire injection was a throttle body injection and i'll be running MPI, so that might help me out I think, I've always seen TBI as nothign mroe than a glorified carburetor anyways. How i set up the engine will come down to how much i want to throw at a computer for it i guess. But if I can get away with just using one IAC then I will. The more i think about it the more i see how it would work.

 

[mattsooty]

You wont be ablt to set up with 2 ECUS, BMW manage to do it because they have a CAN link between the controllers.

Forget about how hard these guys are making it out to be it doesnt really matter whether the thing idles or not, you just have to be a bit canny with the throttle pedal - or keep a throttle plate cracked open slightly and let it find its natural idle speed. Its not like you a have an FTP-75 to pass...

MS

 

[MWPC]

Dannyho,

I agree with Matt, go for it. The small passage between the plenums will allow it to balance at idle. MPI will take care of the mixture.

Lord,

I have not used the min idle spec. I just kept increasing the butterfly opening until the aldl said the stepper counts were down to 2-3. I understood that the ECM would recalibrate for the base throttle position on the next restart.

 

[LordMalaK]

MWPC- you might try this on an MPI systen

Don't try to wire your IAC's in parrellel, just close em off completely and run a pipe between the 2 throttle bodies BEHIND the throttle plates and rig a single IAC motor to feed that pipe. That should keep the idle problem managable. Don't forget to run your inlet hose into your air cleaner.

Then tie the throttle plates together with a single linkage and only use 1 TPS to tell the ECM what the throttle is doing.

The computer will think you still have 1 throttle body (even tho' you have 2), but you'll still have to calibrate your connecting linkage. It's similiar to carb balancing on a multi-carb motorcycle.

Good luck

 

[Fabrico]

The cavity and air passages for the IAC on most cars are somewhere between quite and extremely, simple. There is no reaon you can't relocate an IAC to the balance port or other shared air flow. Many "foriegn" cars have separate or external IAC's. BMW used a hose mounted unit for years. This might also open the door to using a different or larger IAC.