Overtightening the ratchet...
Overtightening the ratchet straps to keep the car in place is something that Casey Lawson of C&C Motorsports tries to avoid. He’s seen that cause a significant loss in measured horsepower. Under no load, the tires look totally normal on the Dynojet’s big 24-inch single roller.
Unlike eddy current dynos, inertia dynos use some cheap, basic physics to determine horsepower. The car rotates a drum that has been precision machined and measured to determine the Polar Moment of Inertia (J) of the drum. That Polar Moment is then multiplied by however many rpm/second the drum is accelerating as measured by an encoder on the end of the drum and divided by a constant to give a torque number. Horsepower is then easily derived by multiplying torque by rpm and dividing by another constant, 5,252. No complicated electronics other than the encoder is required, which is what makes the price of inertia dynos so much more reasonable than their eddy current cousins.
No tuning changes were made and fortunately the environmental conditions, as with the remaining tests, were almost identical to the original test, so repeatability should be guaranteed. Surprisingly, the results were anything but repeated. The car made almost 100 more horses on the Dynojet! Peak power came at 6,200 rpm, this time at an SAE-corrected 564 rear-wheel horsepower with torque reaching 522 lb-ft at 5,400 rpm. We’d seen some small differences in dyno numbers in the past but nothing this big, so we were a little confused and wondered if there was a way they could have had a proverbial thumb on the scale. Lawson showed us the uncorrected numbers though and sure enough, it corrected down again. He assured us that with the Dynojet, the weather conditions were measured internally, and they didn’t have a way to fudge those numbers.
Shacklett Automotive Machine in Nashville is where we’ve done several dyno tests and felt very comfortable trusting the accuracy and reliability of their SuperFlow SF901 engine dyno.
Even at full-tilt boogie,...
Even at full-tilt boogie, there was no visible deflection of the tires on the Dynojet inertial dyno. Lawson pointed out that though these are technically radial street tires, they are sticky enough to rob some power. He’s seen soft-wall drag slicks drop the measured output over 50 rear-wheel horsepower and suggests a “normal” street radial for dyno testing.
Very similar to a torque converter, a water brake dyno like the SF901 uses a viscous coupling of water to the housing to measure work. A driveshaft hooked to the engine’s crank spins an impeller inside the brake housing as water is fed into it. The water couples with fins on the inside of the brake housing and tries to rotate the housing. The force of that rotation is measured by a strain gauge, which records the torque produced. Horsepower is, again, a simple math equation away.
Normally, an engine dyno test would be done at the beginning of an engine’s lifespan, but since we had plans to freshen up the old girl we decided to do this test last since we had to yank the engine anyhow. Making every effort to be sure test conditions were as close as possible to the chassis dyno tests, we installed the same full exhaust and mufflers as run on the car as well as the alternator and vacuum pump. Again the tune-up was unaltered and aside from a few hours of setup time, the engine fired right up.
The temp was a few degrees cooler, but the relative humidity and, more importantly, water grains were a little higher balancing out any advantage. The results were once again surprising. It made 675 horses at the same 6,200 rpm and 653 lb-ft at 5,000 using an SAE correction factor that bumped power up this time. The rpm range looked the same but the power numbers were far from close.
Regardless of whatever number the dynos spit out, none of them would be relevant if we didn’t have a timeslip to know how they relate. It wasn’t easy finding a dragstrip open in the middle of January, but after several calls we discovered that Montgomery Motorsports Park had an eighth-mile race the weekend after we finished all the chassis dyno pulls, so we got up early and headed south for warmer weather. It turns out that the track temp was hovering around 48 degrees. Lousy for traction-starved bracket racers, but good for us since that was right in line with the conditions we had on all of our dyno pulls, and it would show real-world equivalencies.