If you don't like them, go read another Primedia magazine like Super Street; it's written by ignorant apes like yourself. Coleman and Jacquot are two of the most intelligent writers and car guys in the magazine industry. So I leave you with this David Baker: eat a *ick.Cliff SpurlockCincinnati, Ohio
Dear DaveDirect "Dear Dave" tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations and technical nerdisms in this space each month.Can you stump the geek?
Dave Coleman is not wrong!I was reading Dear Dave in the September issue when I noticed a slight error in your explanation of how a dyno works. In "Inanimate Mathematicians," you write "A Dynojet, which measures horsepower and then calculates torque based on rpm ..." I believe that it is actually the other way around.
A dyno works by appling load to the engine. This load is measured in lb-ft and is then calculated into power with this formula:
Hp = rpm x lb-ft/5252
As far as I know (and I am often wrong) there is no way of directly measuring horsepower.
Chris Glencross
Toronto, Ontario, Canada
Almost everything you said is correct, but you can still file this letter in your personal collection of wrongness. There is, indeed, an easy way to measure power. Power is just a measure of work done over time. Work, in a scientific sense, is force applied over a distance.
A horse lifting a 33,000-pound bucket of coal is exerting 33,000 pounds of force. That same horse lifting that bucket one foot is doing 33,000 lb-ft of work. And if it takes a minute to lift that one foot, the horse made one horsepower. If the horse can do it in 30 seconds, it just made 2 hp. And if a monkey lifts the bucket in 15 seconds, it just made 4 hp. (The bucket can't see who's lifting.) A Dynojet roller is a lot like a bucket of coal: big, heavy, and too dumb to know what's pushing on it. Your typical Dynojet roller weighs 2,600 pounds. (Well, actually it has enough rotational inertia that accelerating it with your tires takes the same amount of work as accelerating something that weighs 2,600 pounds.) Accelerating this roller requires a known amount of work, and the more quickly you can accelerate it, the more power you're making.
Blammo! Direct measurement of horsepower.
You can also measure horsepower directly by simply weighing your car and then measuring how hard it accelerates on a perfectly level surface with no air resistance. It's that air resistance thing that makes measuring power on the road so difficult.
Now, for the torque part, your formula is absolutely correct. If you use the rpm of the rear wheels, you'll measure torque at the axle. If you use engine rpm, you'll get torque at the engine (but with the parasitic losses of the drivetrain).
Your theory that dynos measure torque with a big, well-calibrated brake and then calculate horsepower is correct for almost every dyno that isn't a Dynojet. Given the same test conditions, the results will be the same either way.
Better?I'm currently building a project I call "Project Better-Than-Dave." Basically, I've read everything you've written about Project Silvia a number of times and want to make a few of my own improvements. Whether they are valid improvements or not is yet to be decided.
I'm a little more into the drag racing than you, so making a considerable sum of horsepower is high on my list. That, and my dad and uncles have all assured me that an import can't run faster than my uncle's 12.7-second Camaro.