_Mark_· Premium Member
Actually, the dyno *doesn't* factor it out. If there was a way to input your car's drive ratio, then I'd say yes, but there isn't. The reason we use 4th gear is because the effect of gearing is lessened due to the 1:1 gear - but you're still going through the final drive ratio of 3.07-1. This is why folks who do gear swaps see different dyno results.SeriousEric said:Good question. The # that the dyno reports should actually be called flywheel torque as measured at the rear wheels. You are correct in that a stock viper making 500 lb.ft torque (at the flywheel) is actually putting down 1500 lb.ft. at the rear wheels (minus losses).Clint Sever said:By the way - what does the dyno measure then? I know Vipers dyno in fourth cause the ratio is 1:1 - does that cancel the torque multiplication factor? In what gear would the RWTQ be 2456?
In my mind, if the ratio is 1:1 and the rear is a 3.07 then the dyno is measuring power at the 3.07 ratio - why then is the RWTQ 1000 ft/lbs less when measured on a dyno?
So how does the dyno *know* how to factor out such things as gearing (trans & diff) and wheel/tire size? It does that by a ratio of drum speed to known engine speed. They're measuring your rpm, right? And the dyno knows at any instant how fast the drum is turning and that allows them to factor out the mechanical advantage (torque multiplier) due to gearing etc.
I've never understood why dyno software didn't include an entry for final drive ratio - because then it would seem it should be able to tell you your driveline loss, measured values, and engine values. As I think about it, you may need the flywheel weight as well, so maybe this is a pipe dream.
The bottom line is that an engine makes torque on an output shaft to make you go. HP is a mathematical derivation of that force and is pretty meaningless to the overall task at hand. TorqueM is right in that its the final RW measure that is most important - and he's right - because that includes the gearing.
Small liter motors have to spin at ultra-high revs in order to make... TORQUE! They then translate those high output shaft rpms to the wheels through gearing, which translates a relatively small amount of motor torque into RW torque (with the help of a flywheel).
The question is flawed, and Tito had it right. Moving something down the track as fast as possible is about Torque - and the area under the curve is where its at. In fact, moving a locomotive and a train full of cars is about torque too, same with a Semi, or MarkO's tractor.
The reason we still talk about Horsepower at all - is because as TorqueM pointed out, it is a measurement over time. A Semi has 600+ lb/ft of torque, but my lowly stock Viper with its 500 lb/ft will beat it in the 1/4 every time. This is not exactly a HP issue - though it looks that way because my Viper has twice as much horsepower as the Semi. It is largely a gearing issue - which translates my torque over time much better than the semi's.
And finally, there are other factors, such as vehicle weight and flywheel mass. In my Semi example - imagine gearing it such that the semi was designed only to move its own weight (without any trailer). You'd then have a pretty speedy 1/4 vehicle (and I think I've seen this on Neckvision). The problem is that it is not ideal, because the low motor RPM is not ideally suited to drive the gear ratio.
I hope I've made some sense. I believe we'll start to see this play out with more events like the V10 Nats. We're getting to the point where we have several cars making huge power and torque, but I expect them to have interesting 1/4 results. As John pointed out- turbo cars have lag, blower cars have power right off the line. The torque exerted over the duration of the run looks very different for both cars - but the overall "area under the curve" will be similar.