Fuel tuners defeat rev-limiter program

Words and photos by Jeff Burk
8/24/05

t's a good bet that the first phone call Ray Alley made the Monday morning after Alan Johnson and Tony Schumacher stunned the drag racing community with an unreal 4.44/337 lap at Brainerd International Raceway -- a track that has yielded only one nitro record in its history prior to that pass -- was to the engineers at MSD in El Paso, TX.

The much ballyhooed rev-limiter/timing retard device that MSD had developed at the direction of NHRA which had performed flawlessly up to this point apparently had been circumvented by the at least one nitro racer. You can be sure that Alley wanted the MSD folks to reprogram the device to reduce the maximum rpm the engine could turn and to take even more timing out of the engine.

MSD was more than willing to do anything that the NHRA tech department asked, so they put together a team and went to Memphis to reprogram all of the devices on Top Fuel and Funny Cars to meet with the NHRA mandate that the changes be made before the upcoming U.S. Nationals.

What Alley told MSD to do was to reprogram the rev-limiter/timing retard device so that instead of monitoring engine rpm four seconds after a wide open throttle switch activated it, now the engine rpm would be monitored after 3.8 seconds after WOT. Additionally, instead of being programmed to take timing away from the engine at the rate of 50 degrees per second, the device now would take timing away at the rate of 100 degrees per second.

In addition the trigger rpm of 8400 would be reduced to 8300. The nominal amount of timing taken out of the engine is approximately 25 degrees in a couple tenths of a second, which will slow down both the engine and tire speed instantly.

In a perfect world MSD's rev-limiter device would do just what Alley and NHRA intended it to do: keep speeds below 335 mph. Unfortunately for the NHRA and their Director of Top Fuel and Funny Car Racing, this isn't a perfect world. What they obviously didn't know was that apparently Alan Johnson may have figured out how to render the rev-limiter ineffective and even use it to his advantage to go faster.

At Memphis several teams showed me at least two ways they could make the rev-limiter completely ineffective. The first was to put a one-second time delay on the wide open throttle switch that activates the retard device. Since most of the cars use air-activated switches, this was an easy engineering job. What that accomplishes is to change the instant on the lap timeline when the box started looking for engine rpm above 8300 rpm from 3.8 seconds to 4.8 seconds into the lap. In reality the device has no effect on a car making a 4.5-second lap. On a Funny Car they are probably putting in a 1.3-second delay.

Joe Pando (foreground) and Doug Waits, Programming Engineer (background)