Did the car actually feel slower coming off Turn 2 or did the motor just bog for a second? The car ahead seemed to pull away coming out of the corner, but maybe you didn’t roll into the throttle fast enough. Although, now that you think about it, your crew chief did make some carb adjustments for that new track you just raced at, so maybe the mixture is a little rich. Of course, it’s also possible that you never got the timing just perfect after switching to that new distributor. Maybe that’s why the car felt “flat” out of 4. Or did it? The other racers just look a bit “snappier” coming on to the back straight. How long has this motor been in the car anyway? Maybe it’s time for a refresh but, then again, it seemed to pull well on the top end.
We’ve all done this. But in this day and age, there is absolutely no reason to be sitting in the pits wondering how well your engine is running or how much power may or may not be left in the tune-up. And, the best part is, you don’t have to track test four days a week or pull the motor every race to get the data you need to make the big show on Saturday night. What are we talking about? Chassis dynamometers and these cool data acquisition devices aren’t just for big-dollar race teams anymore. In fact, you’ve probably got a chassis dyno in your town that you can rent for as little as 75 bucks an hour, and you can make and measure changes to your car without having to guess or “feel” the results.
A chassis dyno, in the simplest of terms, is a device that measures horsepower and torque at the rear wheels of a car. To fully understand how a dyno works, we asked the DynoJet engineering team directly, who build and produce one of the most well-known and respected chassis dynamometers in the United States. Prepare for some math… “Power, in mechanical terms, is the ability to accomplish a specified amount of work in a given amount of time. By definition, 1 horsepower is equal to applying a 550-pound force through a distance of 1 foot in 1 second. In everyday terms, it would take 1 hp to raise a 550-pound weight up 1 foot in 1 second.
So to measure horsepower, we need to know force (in pounds) and velocity (in feet per second). Dynojet’s inertia dynamometer measures power just in this way. The dyno calculates velocity by measuring the time it takes to rotate the heavy steel drum one turn. The dyno measures force at the surface of the drum by indirectly measuring the drum’s acceleration. Acceleration is simply the difference in velocity at the surface of the drum from one revolution to the next. The force applied to the drum is calculated from acceleration using Newton’s Second Law, F=MA, (F)orce equals (M)ass times (A)cceleration. Power is coupled to the drum by friction developed between the driving tire of the vehicle and the knurled steel surface on the drum of the dynamometer.”
"In this day and age, there is absolutely no reason to be sitting in the pits wondering how well your engine is running or how much power may or may not be left in the tune-up"
And that gives us horsepower, which is great, but how do we read torque? “When an object rotates around a point, the object’s speed of rotation depends on both an applied force and the moment arm. The moment arm is the distance from the point of rotation to where the force is being applied. Torque is the product of the force and the moment arm. For example think about trying to spin a drum by wrapping a rope around the drum and then pulling on the rope. If the rope is wrapped around a drum of 1-foot radius and pulled with 550 pounds of force, the resulting torque is 550 foot-pounds. The torque on the dyno’s drum can be calculated by multiplying the force applied by the drum’s radius. However, engine torque is not equal to the dyno’s drum torque because the gearing through the drive train changes the moment arm. The change in the moment arm is proportional to the ratio of engine speed to drum speed. Therefore, tachometer readings are necessary to calculate and display engine torque.”
Got it? Good. If not, just try to think of a chassis dyno as a rolling road, a torque wrench, and an engine dyno all wrapped into one device. It’s a racetrack that stays still, and it will allow you to get precise, accurate horsepower and torque data from one run to the next, allowing you to make and measure small changes in your tune-up and combination. Like an engine dyno, you’ll get an abundance of useable information, but unlike an engine dyno, you will also learn about the rest of your drivetrain in the process. And, worry not, because a quality chassis dyno will be as reliable and repeatable as you need it to be, given that everything on your vehicle stays the same. Which brings us to…
If you are looking to test engine modifications, such as timing changes, carburetor tune-ups, or anything else related to horsepower and torque production, it is important to remember to keep everything else on the car consistent in order to achieve reliable, repeatable, and accurate results. Said another way, you only want to change one thing at a time on the chassis dyno, and you want to ensure that no other variables are influencing your results. If your objective is to test timing curve changes then that should be the only thing changed between runs. Do not, and we repeat -- do not -- make several changes at once and hope for the best. One change may add 10 rwhp and the other may lose 6, but you’ll only see a 4-rwhp difference on the graph. And then you’ll just be guessing.
It is important to remember to keep everything else on the car consistent in order to achieve reliable, repeatable, and accurate results.
As important as it is to only make one change at a time, it is also important to remember to keep other variables out of the equation. That means tire pressure must remain consistent from dyno run to dyno run and dyno session to dyno session. Additionally, you want to strap the car down to the dyno using the same mounting points and the same tension on each strap.