The story I was told is that the vacuum advance interferes with the way that the REV box processes the signal for spark.

First box I received was defective. It was replaced. Second box would not let the 340 go above idle, plus ran like junk. Talked to Rev-N-ator and was told the solid core wires I was using were causing the mis-fire. My M/P chrome box had NO issues with the same wires. Replaced the solid core plug wires with a carbon core set and the 340 now ran. Went to the strip to test and tune. Ran a base pass pass with the M/P chrome box and carbon core wires. Swapped over to the Rev box and promptly lost 2 tenths and 2 mph. I then followed their tune/timing recommendations and still ran 2 tenths slower than the chrome box. Changed back to the M/P chrome box and matched my first base run. Not happy with the product. Anybody want to buy a Rev-N-ator with only 5 minutes run time on it??