The Dynojet basically acts...
The Dynojet basically acts like a dragstrip in a building, using only acceleration to determine horsepower. Stick shift cars typically show more power on eddy current dynos, but often high-stall automatic cars will show more on an inertia dyno since it doesn’t keep the converter from doing its job properly.
The car scaled in at 3,501 pounds with owner Shannon Carnathan at the wheel and a tank full of Shell 91-octane in back. We cycled the car down the strip several times to get a good read on what it would do. Sixty-foot times ranged from 1.58 to 1.61, and the car was consistent enough that it ran 6.6s at 106 mph. Good enough for us, so we loaded it up and made the five-hour drive home, digesting all of the information at hand and trying to make sense of it all.
This exercise raised as many questions as it provided answers and required further digging to get to the root of them. It seems the biggest question we had was how the numbers were corrected from the raw data. All the dynos we used had an SAE correction factor but it turns out that they all used different factors. On the eddy current dyno, they used a correction factor SAE J1995, the Dynojet used SAE J1349, and the SF901 used the SAE J607. A lot of letters and numbers that didn’t mean anything yet.
We were fortunate that the...
We were fortunate that the Hangover Nationals, usually raced New Year’s weekend, were moved back three weeks. Montgomery Motorsports Park had their track prepped well, and consistent e.t.’s were the payoff. The car normally runs 5-second e.t.’s with twin turbos, so slowing it down over a second made the passes uneventful and almost boring.
Basically, J1349 corrects the atmospheric conditions to that of 77 degrees F, zero percent humidity, and a barometric pressure of 29.234 inches Hg, allowing for accessories, full exhaust, and emissions equipment, and gives the net horsepower of the engine. Standard J1995 uses those same conditions, but excludes accessories, giving us the gross output of the engine, which tends to be about 20 percent higher than net. Standard J607 provides gross output as well, but corrects to a more lenient 60 degrees F, zero percent humidity, and 29.92 inches Hg. In the end, all this means is that these should give respectively higher horsepower numbers. This would all make sense if the results from our eddy current and inertia dyno numbers were switched, but they weren’t. So there must be something else amiss here.
After looking through pics of our dyno sessions, we noticed a significant amount of tire deflection on the Dyno Dynamics dyno, which had two smaller drums instead of the one large drum like the Dynojet. While the tires are basically centered during the strapped-down and unloaded-yet-rolling state, once the engine goes to WOT, the tire climbs forward on the drum. With the one large drum, deflection didn’t really change but on the twin roller setup, the tires were majorly out of shape and the horsepower-robbing deflection got worse as rpm increased. It looks like the design of the dyno itself might have been the source of our low numbers on Carma’s dyno. Perhaps bumping up tire pressure to 45 or more would change our readings. But maybe not. Altering the atmospheric conditions entered into the computer would also show a different power reading, but typically that would be skewed upward if we were dealing with an unreputable dyno operator, which we weren’t. Maybe, like flow benches, some dynos are just stingier than others. What we do know is that plugging 675 horses and a 3,500-pound race weight into various e.t. calculators shows that those numbers are pretty accurate.
It wasn’t easy but we squeezed...
It wasn’t easy but we squeezed the mufflers and all into the dyno cell and ran the engine exactly as equipped in the car. As a final measure, we even used the fuel pump and regulator off the car and ran the engine in open-loop feedback to be sure no tuning changes would show up.
There is no doubt that the vast majority of dyno operators are as straight up as those we frequent, but you can bet that if someone is bragging about dyno numbers that are too good to be true, they probably are. Ask to see the uncorrected numbers and the weather conditions. You can correct your own numbers using calculators online. If they don’t add up, then you know the dyno shop is cheating bigger than a slacker in finals week. As for us, we’ll just brag that we have a 675 hp car that can beat some guy’s car who barely lost against a Camaro that makes like 500 hp.