Here it is, sorry I lost the spread sheet described at the bottom, I will work on making it again (computer virus hit me last year).
The Robitronics dyno is a very nice piece of equipment and can be very useful to a knowledgeable user. The problem is that most people that I see using the Robitronics or any flywheel dyno for that matter just don’t know what to look for. Even the most experienced motor men can find themselves just staring blankly at a screen with tons of data not knowing what to do with it. Big Jim Greenemeyer prefers the Competition electronics dyno because it tells him the information he needs without the clutter of too much information.
Many neophytes come on bulletin boards asking about the RPM of a motor, thinking this is what they need to know. Experienced racers try to tell them that is not really what to look for but they can relate to RPM the easiest. A semi-experienced racer knows it is power and not RPM that makes the car go, but many just look at the peak power produced which is only slightly better than asking what the RPM of a motor is. Most racers with the Robitronics dyno will tune their motor for the peak RPM, the maximum power, and the maximum torque numbers and total spin up time but do yourself a favor and don’t even look at these. They are next to useless believe it or not, more on this later.
If you are new to the Robitronics dyno I recommend using the view data screen on the menu to look at your data. Its fairly hard to relate the graphs to actual performance, even for the most experienced motor tuner.
I am going to limit the scope of this work to stock motors because they are what I have the most experience with. The most important thing about tuning a motor to perform on the track is actually knowing what you need the motor to do. Simply saying “I need a faster motor” can mean many different things. Often this statement doesn’t even have anything to do with RPM. By knowing what you need a motor to do you will have to approximate the amp draw of your motor in the situations that you feel that you need more power. If you constructed some type of speedometer you could work backwards and find the RPM range of the motor you also need more power, but I find that a good guesstimate will get the job done nicely.
Here are my guesstimates for 6-cell stock touring car and offroad racing. If you feel that you need some more out of the corner punch try to increase the power at 25 amps. For more of a top end feel shoot for maximum power at 15 amps. For overall “goodness” of a motor look at the power at 20 amps. And also the average power between 15-25 amps is a good indicator of the motors performance on track. If you tune to these parameters and feel the motor is a little soft coming out of the corners bump them up a few amps and try again. I stress again the importance of tuning a motor for the range that you are going to run it.
Now that I discussed what to look for when tuning a stock motor I am going to discuss the downfalls, shortcomings and quirks of the Robitronic dyno and its results. First lets discuss the peak power number and why it is not very important. Peak power is the power at one specific point in the power curve. When you are racing your motor is constantly changing RPM, even when going down a very long straight away. So you are probably lucky if you operate at one specific point 1/100th of the time it takes to make a lap. Secondly look at where peak power occurs, in stock motors running off 6 cells it is almost always at 35 amps give or take an amp. Just about the only time your stock motor draws this much current is when you peg the throttle coming off the line. In my experience in offroad and stock sedan a 20 amp average amp draw is fairly typical.
Although I find the peak power number fairly useless, using it is not all that bad the next mistake is far worse. Tuning for the peak RPM is just about the worst mistake that a motor tuner can make. The flywheel dynos load is provided by the acceleration of the flywheel. When the flywheel has reached maximum RPM the load is very small because the acceleration is very small. The power required to keep a flywheel spinning at 30k RPM is only a few watts. However when a car reaches maximum speed there is rolling resistance between the tires and ground, wind resistance and mechanical resistance in the drive train holding the car back. One day I may design a flywheel dyno that also has some sort of braking resistance so it provides the best of both worlds.
Spinup time can be somewhat useful in determining how quickly the motor can pass through its entire power band but it can also be quite misleading and I recommend that most people avoid using this number because its redundant if you use the power numbers to your fullest advantage. I say spin up time can be confusing because you can have two motors with virtually identical power bands where motor 1 takes 8.13 seconds total spin up time and motor 2 takes 5.84 seconds spin up time. (See the picture) You will notice that on motor 2 all of the extra time it took to spin up was at the very end of the power curve. Remember from before that it is impossible to get a motor in a car to operate in this range anyway, so this tells us that this extra spin up time reported by the dyno doesn’t mean a thing! You can check the elapsed time that it takes a motor to accelerate from a 30 amp draw to a 20 amp draw and use this number, but that is a fair amount of work and again it is redundant.
To figure out how much total drag is on a car find out its speed and you can work backwards to get the RPM of the motor at this point. Then you can find out the wattage the motor puts out at this RPM and viola, that is the drag of your car at maximum speed. I have created a spreadsheet to calculate the rollout, the tire RPM and motor RPM using your car’s speed, tire diameter and gear ratio. (attached).
