Idle Speed - Manual Override Switch ?

[gravelguy]

Any thoughts on how to electronically hold the idle speed at an elevated RPM for drag race launching? I'd like to just flip a switch at the start line and have the idle jump up to a steady 2200 RPMS or so (my car has a 3200 stall convertor). I'd turn the system off after the finish line so as to not disrupt the engine management during the run. The 4x4 jeep guys have several products like this, mostly used for winch operation (the higher idle keeps the battery charging even with the high electric winch load).

http://www.offroadengineering.com/index.html

My second thought is to route a hose after the MAF to an intake manifold location after the throttle body. Basically bypassing the throttle body, like the IAC does. An inline solenoid valve could be opened or closed at the flip of a switch, bringing more air to the engine. The computer will add fuel to raise the idle speed. The goal is to leave the line near the same RPM on each run. Maybe I just need more practice with the right foot...

 

[XLRVIII]

while I dont launch at 2K or above {i launch at 1K)

I did this same thing using my SCT chip with the 4 position switch.

I have 2 slots programmed with the same tune positions zero and 1.
Position 1 has the "idle speed in drive" set at 1000rpms, "0" position has near stock idle speed in drive.

I prestage the car, flip the switch from "0" to "1", wala! instant,consistant 1K idle speed in drive.

From what I have read 1K is about as high as you can set this scalar unless SCT "increases this limit" for you in the software database.

I have had great success running this system for consistant launch RPM's.

If you are looking for a more "consistant" launch, this could work for you as well.. that is.. IF your PCM "can" use a chip.. some later PCM's have the Jport/Chip port disabled...which is very EASY to "enable".


I might try and chunk a higher idle in drive RPM for testing purposes, but personally I actually like the 1K launch RPM.

IF you cant get any higher than 1K it would still work and would be a great way to ensure you leave at teh same RPM each time.

Of course the TC is going to "flash up" to it's stall speed, but leaving at 1K will make that "flash" alot more consistant as well...and will "relieve you brain" from all the starting line drama/distraction.

hope this helps.

 

[gravelguy]

Your setup is pretty cool. I'm glad I'm not the only one looking for brain relief at the start line. I have an Xcal2 from Blue Oval Chips, and rather not add a switch chip. Others will learn from this post though.

 

[driller]

Warm Idle
This one tends to be more about emissions. Warm idle occurs if the engine starts and ECT rises above 185°F, we come up to a stoplight, or place it back into neutral. The computers is programmed with a target RPM for idle, typically it’s around 672RPM. Typically the computer is struggling to achieve its target RPM. 255°F is the magic number to start complaining about overheating. Let’s break it down by system. Fuel is now closed loop with HEGO switching. We are now aiming for 14.7:1 A/F ratio for perfect emissions, enrichment will occur only if the ECT is above 225°F. There will be no EGR operation at these RPM’s. Timing is mildly advanced and will begin to retard after a minute. We need hot exhaust gasses to help complete combustion and converter operation. Air is pumped into the converter, but will dump to the atmosphere after several minutes to prevent overheating the converter. The Throttle Air Bypass will be controlling idle, we need a nice smooth idle. Not to high with A/T transmissions to prevent creep at stoplights. Idle will increases 100-200RPM if the ECT or ACT is over 225°F. This RPM increase will help burn the rich mixture cooling the combustion chamber. The faster RPM speeds up the accessories, specifically the water pump and fan. Belt fans will speed up, and electric fans will need more juice from the alternator.

http://fordfuelinjection.com/?p=64

The IACV controls the idle along with timing and fuel. The question is what would happen with more air is allowed in at idle? My thoughts would be the PCM would then start to close the IACV valve and regulate fuel and timing to compensate - thus negating any additional air.

It would need to be programmed by the PCM.

 

[Frankie X]

Your IAC has two wires going to it: a solid Red, which is a keyed nominal 12v. power source, and a White/Light Blue tracer, which is the IAC control feed from the PCM. The PCM controls engine idle RPM by pulsing the ground of the IAC in a variable duty cycle on that wire.
-hot idle (~675RPM) is achieved by ~30-35% duty cycle.
-high idle, cold or hot, is achieved by increasing the percentage. Ford specs show 1600-1800RPM = 70-80%.
-providing that W/LB wire with a switched path to ground would equal a theoretical 100% duty cycle, wouldn't it? This would pull the IAC to its maximum opening and hold it there, as long as the ground "short" was present. Maximum IAC opening will give you ~ 2100-2400RPM. You could probably control RPM more accurately with some form of variable resistance on the ground "short" you've created.

The PCM should still be able to deal with the increased RPM effectively enough to avoid any problems; the only drawback would be that it would see the other control path as an error, turn on the MIL/"Check Engine" light, and set code P1504: "IAC control circuit short to ground". You'd need to find a way to deal with that.

I realize claiming to solve your problem by tapping into a single wire sounds awfully simplistic, but this might be something worth experimenting with.

 

[logres]

Your IAC has two wires going to it: a solid Red, which is a keyed nominal 12v. power source, and a White/Light Blue tracer, which is the IAC control feed from the PCM. The PCM controls engine idle RPM by pulsing the ground of the IAC in a variable duty cycle on that wire.
-hot idle (~675RPM) is achieved by ~30-35% duty cycle.
-high idle, cold or hot, is achieved by increasing the percentage. Ford specs show 1600-1800RPM = 70-80%.
-providing that W/LB wire with a switched path to ground would equal a theoretical 100% duty cycle, wouldn't it? This would pull the IAC to its maximum opening and hold it there, as long as the ground "short" was present. Maximum IAC opening will give you ~ 2100-2400RPM. You could probably control RPM more accurately with some form of variable resistance on the ground "short" you've created.

The PCM should still be able to deal with the increased RPM effectively enough to avoid any problems; the only drawback would be that it would see the other control path as an error, turn on the MIL/"Check Engine" light, and set code P1504: "IAC control circuit short to ground". You'd need to find a way to deal with that.

I realize claiming to solve your problem by tapping into a single wire sounds awfully simplistic, but this might be something worth experimenting with.

Dude...you are way too knowledgable...

 

[XLRVIII]

The solution I posted would be the "cheap easy" way out.

The BEST solution to this would be to install a transbrake in your 4r70W then wire it to a MSD "two step" device.

The two step would have two seperate rev limiters, one LOW limit for use when you are holding the transbrake button {you can set this to ANY RPM you like} the "high limit" on the two step would be your regular rev-limiter which would be used when you "are not" on the transbrake button.

Transbrake engages 1st gear AND reverse when the button is pushed..which allows you to reach your TC actual Stall Speed, then when you release the transbrake button, reverse is released and your car DOES A WHEELIE!
(haha) well... MOST do.

This is how most drag racers do it, if they are seeking a particular launch RPM and they are not running in what is called a "no electronics/footbrake" bracket race.
Those are the cars you hear that "hit a stutter box" when they are at the tree and more times than not when they launch they do so.. "wheels up"!

If your running in NO-E/Footbrake pretty much "all of this is going to be illegal" by definition of the class.