Rev Limits

A new guy who was testing at Willow (he rented Phillip’s car) was asking about Rev Limits for our Mazda GT. In a nutshell they aren’t any. A rotary motion, as opposed to the up and downs of piston’s and valves, can take a lot more RPM since the stress is always angular. It’s the same reason turbine engines can routinely spin at twenty thousand RPM. Now this doesn’t mean we’ve beaten back the laws of physics or anything, it’s just that the concept of ‘this far and no farther’ doesn’t quite work for us like it does with pistons and valves.

In physics the stress on our rotating parts will increase at the square of the RPM’s. So the stress, and increases in heat, will start to spike dramatically in our motors between nine and ten thousand RPM. But it’s not going to fly apart as soon as you hit, or even exceed, ten thousand either.

Prior to the Jerico this lead to some interesting trade-offs. Because the gaps between the gear ratios varied from gear to gear in a regular transmission, you could stretch the motor a bit in certain gears, to insure that after the shift you would remain in the power-band, which trades some longevity for performance. If you look at the old ‘Best Thunderhill’ video, for example, you’ll notice my stretching the rev’s well past the shift light for a lot of that race. One of the many things I like about the Jerico is that there is less of a premium on stretching out the RPM, meaning you can be competitive without having to spend two to four grand on the motor each year. The reason is that the gears are evenly separated and close. If you are above 8.5 you’ll at least stay in the power-band (best thought of as seven to nine) after an upshift. An advantage to going higher is still there, and you can and should use a few more RPM’s, judicially, when you are in a tight fight, it is just a lot less rewarding than it was before. So if you want your motor to last, recognize that the trade is between longevity and spinning really fast (above nine), which leads to the obvious question, “why waste that mojo in practice.”

Myself, I routinely shift a little shy of nine thousand during practice and most of qualifying, (I’ll try a bit harder in qualifying during laps two-four when the tires are at their best) and will push to between 9.2 and 9.5 (my shift light is currently set to 9.2) routinely during the race. I am willing to stretch all the way to ten to get by somebody where I know I can stay ahead after the pass. After having motor problems early on, once I learned this along with some other engine goodies, I’ve found either a magic engine or my paying attention to all the details you need to increase longevity are finally paying off. My motor was put in the car at the beginning of 2006, when dyno’ed midway through 2007 it was stronger than any of the other motors (at 220 to the wheels) and it still feels good at the start of the 2008 season. All with nothing more than routine maintenance.

Now you can, and I do, adjust those numbers a bit for conditions. For example, at Willow Springs this weekend (Feb '08) everyone was running a little higher EGT's than normal. This can be caused by any of a number of differences in conditions or just the quality of the fuel. A lot of the time we can just change the jetting to make the carb run richer, but when I tried it the car went way too rich. After changing it back I decided that during most of practice and qualifying I'd shift a little lower just to keep the EGT's at the top end of third gear below 1,700.

One other huge difference between our Mazda GT’s and your daily driver is that, realistically speaking, we have a minimum RPM as well. In fact, it is easier in some cases to damage a motor by spinning it too slow than by spinning it too fast. The reason is that a peripheral port, like we have, isn’t happy at all at low RPM. Casually stomp on the throttle at four thousand and you’ll regret it. You’ll neither move, nor will the engine be very happy with all the coughing and sputtering.

Bottom line is, once you get the engine between seven and nine thousand RPM; try to keep it there and it’ll be happy as a clam. If you are in a fight and don’t mind a little extra wear if it means winning, push it to ten K. If you’re starting out of the pits, be gentle with the throttle until it stops sputtering and pay attention to what RPM it’s sputtering at. If you get caught out on track and lose RPM, you’ll need to know where to nurse the throttle since it’s likely to be the same range.

Above all, when you get out of the car, be able to tell the mechanics what the gauges were saying when. I know it's hard to do with so much to pay attention to, especially at first, but the engine you save will, not might, be your own.