D-Jet retuning question

Jeff Nelson · 10-04-1999, 10:33 AM

I'd like to find out what the limits of the 2.0L D-Jet FI are wrt to tunablity in support of non-stock cams. The opinion seems to be that it doesn't work. In the D-Jet FI article by Kjell Nelin he talks about mixture adjustments beyond what is availble at the "knob" (which I understand is for idle only). Does anyone know what parameters are changed with a cam that is timed for compatabilty with carbs and why the D-Jet has a hard time coping with these changes?

Dave at Pelican Parts · Dave at Pelican Parts's Garage

I don't know every way that a "lumpy" cam affects the D-jet. As you've noted, there are adjustments that can be made to the system to compensate at least somewhat for having a non-stock engine, but these are somewhat non-trivial to do well.

One of the biggest limitations to the cam is on the duration of the lift of the valve. If the duration is increased to long enough, you wind up with overlap. This is where the exhaust valve is open for some of the time that the intake valve is.

This is effective at high RPMs (and high gas velocities) because the exhaust that is leaving actually helps pull more air in. However, at low gas velocities (low RPMs) it results in some of the exhaust gases being sucked back into the cylinder. This means that less intake air is sucked in. This results in a less vaccuum in the intake, which the FI interprets as more air being sucked in. (Through an open throttle, for example.) It adds more fuel, so the mixture is too rich.

Evidently this effect gets bad pretty quickly depending on the amount of overlap. I hear it is not that hard to wind up with a cam that won't allow the motor to idle because the idle mixture is so rich.

You can compensate for that (at least a bit) by using the knob on the ECU, but the mixture still tends to be over-rich at low RPMs off-idle. And it is also not hard to run out of adjustment on the knob.

I am undoubtedly presenting a simplified view of this. There is most likely more to it than that, but I don't think I've said anything that's completely wrong...

--DD