SRF Dyno Test ResultsUpdated: Aug 19, 1999
Dyno Results: These results are from a DynoJet chassis dyno test performed on my Spec Racer Ford in January 1998 . Peak torque was 116 ft-lbs, and peak horsepower was 100 hp. The dyno correction factor was 1.02 on that day.
While the horsepower number is the most frequently quoted figure for an engine's performance, I really consider the torque curve to be the most important graph to indicate the performance of a car such as the Spec Racer Ford. The engine torque is multiplied by the driveline gear ratios and then acts through the distance of the rear tire radius to produce thrust at the contact patch of the tire. That's what makes you go.
At 4000 rpm, my car has about 115.5 ft-lbs of torque. At 4800 rpm it has about 108 ft-lbs, and at 5600 rpm it has about 86 ft-lbs. I always look at the engine torque at three rpm values: 4000, 4800 and 5600 rpm. I like to look at the torque values at 5600 rpm since that is around the normal shift point, so it gives me a feeling for how well the car is pulling at shift time. Similarly, the 4000 rpm torque is about the value that you get immediately after you shift gears, so it represents how well the car pulls at the lowest rpm that you normally use. The 4800 rpm torque is halfway between the other two rpm values and represents how well the car pulls in the mid-range. Looking at these three torque values gives a pretty complete picture of how the engine is running across the spec racer useful rpm range. You can convert torque values to horsepower values by the following formula:
HP = ( Torque x RPM ) / 5252 Leakdown and compression checks are very popular diagnostic tools, but don't just assume that high leakdown or low compression means that your engine is a dog. It may still be very competitive with less than ideal leakdown and compression. My engine had some pretty bad compression and leakdown numbers, but is still has good power. Here were my compression readings (cylinders 1, 2, 3, 4): I've seen DynoJet tests on about 17 different engines, and the range of corrected torque readings have been as follows:
Dyno Tips: It takes a few minutes to get the engine and drivetrain warmed up on the chassis dyno, so you will need to take several runs before the data becomes consistent. I like to just run the car on the dyno for about 5 minutes before taking any data. Make certain that you have a strong fan blowing air into the radiator or you will overheat the engine. In first and second gears, the inertia of the engine and gearbox will reduce your horsepower readings. Third gear is a good choice for getting accurate data. All of my data is always taken in third gear. By the time you get up into fourth and fifth gears, the drivetrain losses are significant and reduce the horsepower. When you start making data runs, go full throttle at a little over 3000 rpm and run it up to 6000 rpm. Do that over and over until the results stabilize and give power curves that are the same within 1 hp.
Using the Dyno data: Using the engine torque data, along with the transmission ratios and the tire diameter, it is possible to calculate the rear wheel thrust in each gear. For the following plot, I have ignored the variations in efficiency of the different transmission gears and have also ignored the effects of windage and oil whipping in the transmission at high speed and have also ignored the rotational inertia of the engine, tranny, wheels, etc.. The five curves in the plot below depict the rear wheel static thrust that is available in each of the five gears. If you divide the thrust in pounds by the weight of the car, you will find the acceleration in g's. The dashed line on the plot depicts typical forces due to rolling friction and aerodynamic drag.
In the plot above, note how quickly the available thrust drops off after the intersection with the next gear. Armed with speed versus rpm data, you can easily determine the rpm in each gear at a specific speed. The ideal shift points are at the speed where the available torque is greater in another gear. That's where the curves intersect. For maximum acceleration, the plot clearly shows that it is very important to shift near the optimum rpm and not run the engine too far past the optimum shift point. For example, in the Spec Racer Ford, keeping the engine turning above 4000 rpm but below 5600 will get maximum acceleration. There will be cases where race strategy is more important than maximum acceleration. It might be problematical to shift in the middle of a corner, for example, and the evil of running the engine up to 6000 is better than upsetting the car. But when it becomes a drag race, stay between 4000 and 5600 as a general rule. Another thing to consider when hunting for your shift points is that your tach may not be telling you the truth. For example, my Autometer tach is 100 rpm off at 6000 rpm. The available thrust from the engine is opposed by the rolling friction of the tires and bearings, the aerodynamic forces acting on the car, the mass (weight) of the car and the rotational inertia of the engine, drivetrain and wheels. Therefore, if you instrumented the car with a g meter, you would find that the car does not accelerate as much as the plot above shows, since the effects of the rolling friction, aerodynamics and rotational inertia were not accounted for. Here is a plot of the car's acceleration that has been corrected for the effects of typical rolling friction, typical aerodynamic forces and typical rotational inertia. This should be pretty close to what you would measure with a g meter in the Spec Racer Ford at wide open throttle acceleration:
You can estimate how quickly, or slowly, the car is accelerating by realizing that 1 g acceleration will change the car's speed by about 22 mph per second. For example, at 110 mph, which is about 4500 rpm in 5th gear, the spec racer is only gaining 1.3 mph per second at wide open throttle. At about 130 mph, the available thrust from the engine is exactly the same as the losses due to rolling friction, aerodynamic drag and rotational inertia, which means that the car can't accelerate any more and it has reached it's top speed.
Mathematical Modeling: For anybody interested in mathematically modeling the Spec Racer drive train, here are some fairly accurate polynomials that were derived from my latest dyno runs, after an Enterprises top rebuild. The following second order polynomial approximates the DynoJet chassis dyno torque reading at a specific RPM. This polynomial is valid only in the range of 3800 to 6000 rpm:
You can convert torque to horsepower with the following expression:
The following polynomial approximates the drivetrain loss as a function of speed in MPH, which was measured as negative horsepower on the DynoJet chassis dyno. This expression is valid from 0 to 130 mph:
On the DynoJet, you measure the drivetrain losses by running the car up to a certain speed and then simply pressing in the clutch to disconnect the drivetrain from the engine. As the speed slowly decreases, the dyno measures the total drivetrain losses due to the transmission, differential, CV joints, bearings and tires. The speed in MPH can be related to the RPM in each gear with the following constants:
For example, here are the calculations for 5000 rpm in third gear in my Spec Racer:
All of these calculations are within 1% of the August 4, 1999, DynoJet chassis dyno measurements on my car. The flywheel horsepower, as would be measured on an engine dyno, is approximated by adding the drivetrain hp loss to the measured chassis hp. For example, with my car in third gear at 5000 rpm, the drivetrain loss was 5 hp and the chassis hp was 103. So, this engine would be expected to produce 108 SAE corrected hp on an engine dyno. Richard Shelquist |
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