Once Tim was comfortable, we ran five sets of three runs, averaging the maximum horsepower at the rear wheels as well as the amount of horsepower of resistance during coast down at speeds of 85 and 100 mph. The numbers for resistance show as negative horsepower figures due to the dyno measuring "backwards" from the way it measures output. The average horsepower for the runs using the standard quick-change rear end was 283.88 hp with coast-down averages of 16.70 hp at 85 mph and 23.93 hp at 100 mph.

We then removed that rear end and installed a quick-change that had a different type of low friction bearings and low friction seals. Almost all of the internal surfaces were polished, especially the gear faces. The car was once again positioned on the dyno and all running gear angles (transmission, drive shaft, and pinion) were returned to the same as in the previous runs. The first run we made with the new differential told us that we had stumbled upon something significant.

After a few warm-up runs to heat the engine, transmission lube, U-joint bearings, rear-end lube, and axle bearings as in the first runs, we did the first three-run set. We were surprised to see a significant gain in maximum horsepower as well as a loss in coast-down resistance of around 50 percent. We ran five series of three dyno runs, just like the first test with the stand quick-change, and averaged the numbers. The maximum horsepower was 298.13 and the coast-down numbers were 8.4 hp at 85 mph and 11.42 hp at 100 mph.

We gained an average of 14.25 hp overall and reduced the parasitic losses through the rear end by 50 percent at 85 mph and 52 percent at 100 mph. We ran this test with as much control over the environment as possible. With the right resources, we could have monitored the various temperatures more closely to better substantiate the results. Chris felt we did all we could given the time constraints and equipment available to short-track guys like us, so the numbers were well within a believable range. He felt that we had indeed recorded some meaningful data.

At the end of the last set of runs, we experimented with pinion angles and actually learned some interesting facts. Our Winston Cup consulting engineer, Terry Satchell, told us the most important factor in pinion/driveshaft/transmission angles was to have equal angles at each end of the driveshaft. This would be the most efficient design, yielding less loss of horsepower from bearing bind at the chassis attitude that is the result of the car accelerating. What we discovered was not what we had expected.

We did experience a slight gain in horsepower due to changes in the pinion angle. We could not change the transmission angle, so all of the changes were to the pinion. We recorded a run at zero pinion angle and then at 10 degrees of pinion angle. We expected a loss in horsepower at the greater pinion angle, but actually experienced a net gain in horsepower. This was very confusing until we thought about what Terry had said. When the pinion was at 10 degrees, the angles at the ends of the driveshaft were opposite and closer to equal. This was a better situation providing less resistance than when the pinion was at zero degrees with the transmission angle at a much different angle.

Most racers only measure the pinion angle relative to the ground when they should measure both the pinion and transmission angles relative to the driveshaft. The angles should be equal and either opposite or in the same direction to help eliminate vibration and drag through the U-joints.

This yields more rear wheel horsepower as well as longer component life due to vibration-free movement as the bearings rotate. You should always maintain some amount of pinion (driveshaft to pinion and transmission shaft) angle so that the needle bearings in the U-joints will be loaded. The slight angle forces them to rotate, which helps keep them lubricated. Running zero angle in the U-joints will ruin the needle bearings in a hurry. Maintain two to four degrees of angle (transmission to driveshaft and driveshaft to pinion) at each of the U-joints.

If all of your handling problems have been sorted out and the engine is up to horsepower with all of your competitors, it may well be time to take a look at horsepower losses that may be robbing you of power at the rear wheels. Do some dyno time once in a while to make sure your driveline system is up to par. Reducing power loss through the driveline is all part of putting together that total winning package.

(Special thanks to Travis Carter for use of the dyno and Robert Yates Racing for the services of Chris Robinson).

Dynojet Dynamometers Tiger Rear Ends