I ran this dude from a 50 roll and beat him by two car lengths, and last week he barely lost against this guy in a Camaro that runs low 11s and makes like 500 hp, so I figure I’m making 600 hp.
Right. Horsepower by proxy.
This kind of guesstimation is rampant in the hot rodding community and, truth be told, is the basis for most of the bench racing that goes on. Comparing an unknown or untested car against one with a real history of track or dyno time is the only way most of us can relate one combination to another. The question then becomes, how much power do you really have? How are you going to check it? Are you sure you trust the numbers?
We posed this question to several racers and their answers provided the impetus for this comparison. You see, some gave answers in terms of flywheel horsepower, some with wheel horsepower using an eddy current dyno, and some using an inertia dyno. So which one is right? Searching for the answer put us on a rocky path to enlightenment.
We began our journey by having a straight-up talk with chassis dyno owner/operator Erin Carpenter of Carma Performance Engineering in Nashville. In between blasts from a tunnel-rammed 362-cube LS-powered S-10, he explained that most dyno customers show up with a posse of their buddies wanting to puff their chests and get a sheet they can proudly display on the Internet. Forums and dyno sheets have become the new dragstrips, where e.t.’s have been replaced by peak horsepower numbers, and actual performance be damned. Unfortunately for some of Carma’s customers, their bubble gets burst when they see the true power their car makes. We almost never use a correction factor when we dyno, because the numbers that it puts out are actually what it is doing on that day, with that weather, on that dyno.
Before running the “real”...
Before running the “real” tests, we ran the engine with air running through the old intercooler and bonnet and discovered that it robbed 93 rear-wheel horsepower! We ended up running the rest of the tests with the bonnet removed and a fan blowing fresh air toward the front of the car.
So how is it that the car that is strapped to the dyno doesn’t make nearly the power as that other one on the forum with the same setup? Carpenter says a shop looking to boost their business could easily boost power numbers and egos by fudging correction factors. You could change the correction factor if you wanted to. You can also go in and reset barometric conditions or temperature. Another thing on some dynos is that they’ve got a place to set the drivetrain inertia. So on a motorcycle you’d put the inertia really low, but you could jack it way up, and it would correct and show huge numbers.
OK, so what are these correction factors? Basically, they just take the current weather conditions and modify your dyno curve so it reads as if you were at a barometric pressure and temperature standardized by the Society of Automotive Engineers (SAE). Ideally, any given engine in any weather condition on any dyno should give the same power curve, but is that really the case?
We’ve made tons of dyno pulls on engine and chassis dynos, but we’ve never really taken the same engine and run it on an eddy current chassis dyno, an inertia-style chassis dyno, and a water brake engine dyno to see how they all compare back to back to back. We also wanted to see what those numbers added up to in terms of real performance at the strip. Fortunately, there happened to be a big-blockpowered Nova ripe for the test.
Our test mule was a ’70 Chevy Nova that has proven reliable and repeatable as a race car, paramount in the comparison. Equipped with stock-style suspension, the car has been as quick as 8.67 at 158 mph using a twin-turbo setup. The hair dryers were pulled off and regular Hooker Super Comp headers were installed to bring the car into a more real-world status.
With a 9-inch rearend housing, 3.73 gears, a Powerglide trans, and a 3,500-stall converter spinning BFGoodrich drag radials set at 30 psi, the driveline was as common as anything else.