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Fine tuning DFCO

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I am trying to fine-tune my DFCO settings. Without it, I was getting some pretty wild swings in my Lambda as the engine decelerated and fuel puddled on the ITB blades (injectors are above the blades) and the walls around the blades. With DFCO enabled, this has all gone away and the engine behaving much better.

For those unfamiliar with my other posts, the engine is a Big Block Ford FE (468") running a Borla EightStack ITB that looks like Weber 48IDA's. The injectors are located above the throttle blades.

My ECU (Performance Electronics 8400) provides three settings for DFCO other than to enable/disable the feature.

1) Minimum TPS%. I have this set for .9% Sometimes on a light cruise, it is not unlikely to see the TPS at 1%. I felt the .9% would indicate the blades are closed and the motor is in Overrun.

2) Max RPM. This is now set to 2000rpm. The car idles between 800-900. I tried 1700 and found times the engine would stall/die. I've raised this now to 2000 but still see the occasional stall. I am thinking this is caused by me pushing in the clutch while in the 'Enabled' mode and the engine dies before the ECU can shut off DFCO.

3) RPM Delta. Set to 100RPM This is to keep the engine from 'hunting' and turning the setting on and off. At this level, DFCO is turned off when the RPM drops below 1900 and won't re-enable till it goes above 2000

Any suggestions on how to fine-tune the action of this feature so I don't experience the engine stalling? The gearing of the car is such that 1700rpm = 54MPH, 1600RPM=48MPH. At those speeds, I don't like to allow engine overrun as it tends to get jerky below that point. I've gotten into the habit of pushing the clutch in and with this new-found DFCO feature, that habit may be my problem.

Looking for someone with more knowledge than me on this. I followed Andre's webinar on this feature, however, my ECU doesn't have as many bells and whistles as his example.



I don't know the answer for your issue, but I would try increasing the RPM delta first, then raise the enable RPM (is that "max RPM"?).

Lastly, can you tune your < 1% TPS cells where Engine Speed is above 2000 to have as close to 0 fuel as possible?

If the ITBs are not electronically actuated, and you have no idle air motor fed system of hoses to post plate in the ITBs, the engine is going to be prone to stalling when you clutch in. Some method of air control would be a huge benefit.

You're perhaps using ignition timing to help manage idle to some extent, and if the target is 850, when you clutch in at 1500+ RPM you may be operating in an area of the idle ignition control strategy which called for a massive reduction in timing advance, causing a sudden loss of engine torque which may cause the engine to decelerate too quickly for it to catch before stalling.


The engine is equipped with an IAC, but at idle, with the injectors above the plates and TPS at 0%, the only fuel it will get is what leaks around the throttle blades. Probably not the best design, but it is what it is. It does work and maintains a respectable idle.

The IAC does use ignition timing to control the idle, in addition to bleeding air. It is allowed to take out up to 15 degrees timing, which for this engine, could put it to around 0-5 degrees.

I believe it is the DFCO fighting with the idle circuit when I push the clutch in, but there has to be a way around this, just haven't figured it out yet.


Still trying to learn the ideosyncracies of the ECU & program. DFCO enables when the TPS drops below 1% and engine rpm above the max amount. However it stays enabled until the rpm drops below Max - Delta. I understand why you need the delta, I think it is that Delta area that if I push the clutch in at that point, I'm dead.

My typical cruise RPM is around 2400 so the area for the DFCO to be effective becomes very narrow.

The guy who wrote the program for the ECU is no longer around so it is very difficult to sort thru the programming and try to understand his logic. Not sure if other ECU's work the same way or not, I don't have enough experience

Your comments really help to sort thru this. I need to drive the car to Paso Robles


Paul it sounds like you have a good handle on what's making this twitchy.

I think you've likely done a great job to make the setup run as well as it does to this point, and you're getting to the limit of what you have in front of you, so don't beat yourself up.

It likely takes too long for fuel to puddle and leak through the almost closed throttle plates and reach the combustion chamber to save the engine from stalling when you reintroduce fueling below the DFCO lower RPM limit.

If relocating injectors isn't feasible, shutting DFCO back off and accepting the nature of the system won't let you optimize result lambda during conditions where throttle is almost shut, may be the next best option. There may be a more advanced bandaid possible, but I don't know what that ECU can do, and ultimately you're relying on fuel leakage around a plate which is not going to be an ideal situation.

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