Strapping my classic to a dyno might have been a bad idea
I have always been curious to put one of my vintage cars on a dynamometer to test how much power it makes. However, such a test never made sense: Most of my cars are stock, with known factory outputs, and the few that are modified aren’t tailored for true performance. Currently, the most promising candidate is my 1965 Corvair: The flat-six under the trunklid sports a mild cam, high-flow air cleaners, upgraded electronic ignition, and a custom exhaust using era-correct headers. On the right day, it really sounds the business.
But … I know where the restrictions are in this engine and what it needs to make big power. The modifications on my car likely haven’t increased horsepower by a significant amount. The epitome of all bark, no bite. So when an acquaintance purchased a house that included a garage with a chassis dynamometer, I wasn’t scrambling for a session. Bad influences—nay, friends—wore me down, and before I could make any excuses about “fragile old car,” the Corvair had a date with a psychic.
That is the role of a dynamometer. It asks your car a few questions, tests its reaction to a challenge, and reveals something about that car based on the test.
First, the challenge had to be set up properly: The Corvair’s drive tires sat between a set of rollers, and straps from ground anchors cinched down the rear from a couple of carefully selected points on the suspension. The front end was doubly secured: a strap and a chock for each wheel.
Then came the hard part: the math.
Before we had started the engine, we had a little problem. The Mustang-brand dyno installed in the my friend’s two-car shop space uses a factor to interpret the data from the load cell. That factor stems from an older metric of “horsepower at 50 mph.” The nerds call it road load coefficient: essentially, it is how much horsepower a vehicle needs to drive down a flat road at 50 mph. Mustang Dynamometer has a handy chart for most models going all the way back to … 1971. No Corvairs built after 1969.
The Corvair’s silhouette—grille-less front end, flat tail—is unlike that of most vehicles in the 1970s. Substitute models weren’t obvious. We looked at the numbers for some cars, averaged a few, and … guessed.
Why sugarcoat it? For me and the Corvair, the numbers really don’t matter. Never did, never will. I was there to learn something new and have a fun afternoon.
We did, of course, have a side bet on whether my little white coupe would crack triple digits to the tire. The over/under figure stemmed from my memory of an old forum post claiming that the Corvair’s factory rating of 140 hp typically came out to around 100 hp at the tire. I was working with a … sort-of known quantity: The engine has been upgraded, but I don’t know its mileage and have no records of who built the engine, or when, or what parts they might have used. I’ve been inside the six enough to trust that it has been rebuilt and that the person who did the job was competent. That said, it’s still a little bit of a shrug as to exactly what is inside.
We entered our factor. Patrick, the owner of the dyno, talked me through the process: Drive up to the chosen gear and hold engine rpm at 3000. Give him a thumbs up, let off the throttle, and allow the engine to coast down to the desired start rpm. Give another thumbs up and, once Patrick returned the signal, mat the throttle.
The noise. The vibration. This subtle sensation of speed—burying my right foot, hearing the engine on song—sat right next to a feeling of calm: The only thing moving was the tachometer needle, which rose with all the speed of an octogenarian filling a buffet plate.
Right as the tachometer needle touched 5000 it was time to lift, put in the clutch, and let the dyno spin down a bit before lightly helping with the final slowdown using the Corvair’s brakes. Letting off the throttle was met with a big pop as flame shot out of the exhaust.
The cacophony of noise, vibration, and anticipation began to mellow. I was immediately proud of the car—nothing had exited the engine except a massive backfire when I closed the throttle. After letting the engine idle for a minute, we shut the car off. I got out, and we surrounded the dusty computer screen to see what the psychic had to say.
96 horsepower and 120 foot-pounds of torque. Not too shabby. 96 was in the ballpark, given that the original, 140-hp rating for this engine was measured in gross horsepower: The tests were done utilizing an engine, not a chassis, dyno and an engine from which most of the accessory drive had been removed. For the Corvair, the accessory drive is literally just the alternator, but the crank-horsepower figure is optimistic considering that many more components are needed to turn crankshaft rotation into forward motion. The transmission, differential, and other necessary accouterment all soak up a little power. Over the years I have heard drivetrain loss is in the 15 percent range for manual-transmission cars, but that “rule” is misleading at best. It’s hardly a rule, more of a generalization if anything. Being “down” 32 percent compared to the factory rating is actually not bad considering everything that can and should be factored in.
The peanut gallery checked my initial feeling of smug: At higher rpm, a screen of black smoke had poured out of the exhaust pipe—unburned fuel. Confusing, because when I went through and cleaned the carburetors years ago, I could tell there was no funny business inside. Jets were stock sizes, no evidence of machining. While their lack of modifications didn’t explain the smoke, it likely explained why the torque levels off and begins to drive down as rpm increased. What a lovely, flat torque curve.
Since we knew the car wouldn’t explode under the stress of the dyno, we decided to have some fun and take off the fan belt. The Corvair’s is an air-cooled engine: A bi-planar fan belt turns a magnesium fan that forces air down through the finned cylinders and cylinder heads. The system is critical to keeping a Corvair engine alive during regular use, but driving the fan sucks up a ton of horsepower. Your engine doesn’t even have to be air-cooled for you to observe the output difference with and without the fan: Plenty of people with access to dynos have tested flex fans versus electric fans on small-block V-8s.
After letting the big air-mover fans blow down the fan intake for a bit to cool things off as best we could, I popped the spring-loaded belt tensioner in and zip-tied the belt to the oil filter adapter to keep it from catching on the crank pulley during the dyno run. A clean pull felt a good bit spicier and the results revealed all those forum posts from armchair engineers (and real engineers) were pretty well spot-on.
Did I need to know these numbers? Of course not. And if I did, our method would have been a terrible way to discover them. Still, I got the opportunity to test my car in a safe, controlled environment. In just a few hours I learned the Corvair was running rich in the higher rpm range, something I likely would have never learned just driving around town. As I drove home, I thought about how to fix the fueling issues and debated with myself about plopping down the money for a modern cooling fan. Do I need it? No, but I now know what could be, and the prospect sure felt fun.
That psychic inside the dyno computer says the same thing to everyone and is also always right: You can make more power if you want. How much do you want to spend?