[DogMeister]

I was hoping to get the throttle body set up before going with a controlled distributor, but variation in the tach signal makes sense as a cause.

[Lee]

Originally Posted by DogMeister

I was hoping to get the throttle body set up before going with a controlled distributor, but variation in the tach signal makes sense as a cause.

I installed my first EFI system about 7 years ago. I started with the HEI (Chinese) providing the tach signal. The system ran horrible! Adding a filter made little improvement. Then I added the distributor - clouds dispersed, birds sang,... I was amazed at how different it ran.

About a year later, I took a 2-day seminar at FAST. This was discussed there in more depth. Tachometers are HEAVILY buffered, so they do not bounce around. But if you look at the actual signal, there are all sorts of extra spikes. The ECU needs to respond QUICKLY, so it can't be buffered - so those spikes throw the fuel calculations WAY off.

Then the O2 starts sending rich signals, and the EFI tries to compensate for that, while still using erroneous RPM readings for calculations.

Once you have the ECU controlling timing and fuel, it will start using the timing to smooth out transition periods (going in/out of drive, compressor kicking on/off, abrupt throttle changes, etc.) The timing changes can occur virtually instantaneously, whereas the IAC and such will lag slightly behind. So the ECU will buffer the changes with timing, then restore the timing as the IAC and such settle into place.

I won't install an EFI system unless the owner agrees to let me install the matching ignition system as well.

Good luck