No IAC, Idle Help

[TD2593]

Im running the Fast SEFI and have no IAC, 160 lbs per hr injectors. I cannot seem to obtain a satisfactory idle the motor has a very very large plenum space due tot he IC mounted directly to the top of the sheet metal manifold. The motor wants to surge slightly and if you tap the throttle it will die when the revs come back down. Cold start is not so great either. The motor in question is a 397 inch Twin Turboed V8. Ive drilled a very small hole in the TB it really seemed to help out alot. Im having to screw the throttle blade open too far to get a idle. Thanks

 

[bobc455]

As far as the cold idle goes...

I don't know what you have for a TB, but perhaps you can make a provision for a solenoid that kicks the blades open a bit more during cold starts / warmup. It's primitive, but can be a substitute for the lack of an IAC.

-Bob Cunningham
bobc@gnttype.org

 

[Craig Smith]

With the 160s it can be tough to get a good idle. But, it can be done.

What I have found quite frequently with 160 lb/hr injector setups is that reducing the injector opening time to around 0.5 mS allows you to tune for a better idle. The pulsewidth can never be any smaller than the specified opening time, and occasionally an engine will want less pulsewidth at idle than what the user has set the opening time to. Any changes made here will affect the entire tuneup, though.

Another thing I have learned is that there are different 160 lb/hr injectors and some exhibit much better controllability at idle than others. The Bosch blue-tops seem to be the best of the bunch.

You may also try reducing fuel pressure. This will allow for longer pulsewidths at idle but still deliver the same amount of fuel. This gives the air/fuel charge a better chance to mix properly and it tends to help idle quality. This will also affect the entire tuneup.

Hopefully some of this will help you out. Good luck!

 

[TD2593]

Thanks Craig, Bob, when I get the dragster back in the shop Ill try those changes. The Throttle body is a holley 95 mm unit. and I do have th bocsh blue top 160's. Fuel psi is at 40 static with a Aeromotive belt driven pump. Craig do you think that the excessive plenum volumn will cause problem with surge controllabilty ? How will the effects of cam position sensor location effect idle. Ive got it at 80* btdc now. Thanks