[_Mark_]

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?

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).

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.

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.

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.

 

[_Mark_]

Holy crap, in the time it took me to type my post the levee broke on this thread.

To highlight:

A dyno measures TORQUE, not hp -

Changing drive ratio, overall tire diameter, etc, changes the Torque reading at the wheels, and thus the dyno gives a different reading.

Its simple physics people.

 

[_Mark_]

As Sun Ra said, the sidewall flex is for contact patch, not mysterious stored energy.

 

[_Mark_]

You can beg to differ all you want!

There is no dispute that energy is stored in the sidewall flex - but it is dissipated in about .5 seconds on that dragster, and probably accounts for about MAYBE 6" of forward movement.

The tires wrinkle for the same reason that your clutch plate has springs in it.

 

[_Mark_]

Fair enough Mark - but by your figures - how many hp would it take to shoot a 2200lb dragster 6" from a stop in 0.5 seconds running the gearing they run?

I submit it is measureable - and that maintaining that deflection on the tire on a rolling dyno will pick it up...

Wouldn't prima face' evidence be that others have done just that?

No. In fact, in the other Dyno thread you'll see that David Weaver states to increase pressure to 35 lbs to get rid of any sidewall flex. Flexing that sidewall is sacrificing power to increase traction and reduce the moment arm of the axle around the tire. As the tire's RPM increases, the moment arm can increase.

My point above was half the answer, Dean said the other half.

 

[_Mark_]

Ah but yes it does.

Remember we are not talking race slicks so the amount of wind-up will be very small, but enough to have an effect.

God I wish I could explain this. :bonk:

You can explain it fine. What are you talking about, a Viper on a dyno or what?

 

[_Mark_]

Same OD (and weight) = same reading by laws of physics.

However, I think your point was that due to the greater sidewall flex of the 16" combo, the reading will be different. Assuming you pump up the 16" combo adequately, the flex will be neglibible and the difference will be lost in the noise of any series of dyno pulls. And if you did not pump up the 16" combo, the reading would be less, because energy is wasted in the tire flex instead of being transferred to the drum (there is no traction issue here like with the dragster from rest).

 

[_Mark_]

Holy crap, in the time it took me to type my post the levee broke on this thread.

To highlight:

A dyno measures TORQUE, not hp -

Changing drive ratio, overall tire diameter, etc, changes the Torque reading at the wheels, and thus the dyno gives a different reading.

Its simple physics people.

Mark, hate to disagree with you but you're wrong on a number of counts here. JPH was correct that the inertial dyno (like a Dynojet) measures the rate of acceleration (change in drum speed between discreet time samplings) of the drum and back-calculates torque from it. Torque is a static property that cannot be measured directly from a rotating device like a dyno drum. So although Hp seems unreal to many of us and we dismiss it as nothing more than a derived entity, in actual fact the dyno only measures power and reports torque as the *mathematical* afterthought.

Eric is right. I was under the impression that the dyno drum worked like a torque wrench does, but it does not. It does in fact measure horespower and extrapolate torque using the rpm sensor of the engine.

I stand humbly corrected.

Changing the drive ratio and tire diameter does have an affect on the force that your tire applies to the ground and hence to the dyno drum but I will reiterate that the dyno does factor this out in it's calculations of flywheel Hp and Tq. You said earlier that it couldn't be doing this because there's no way to enter drive ratio and tire diameter into the dyno program. My response is that you don't need to enter that crap because the dyno has a much more simple and elegant way of factoring out the torque multiplication due to gearing and tire and it does so using the ratio of engine rpm to drum rpm. If your crank were hooked directly to the drum axle with no gearing induced mechanical advantage they would rotate at the same speed, right? If your motor rotates the drum at half the engine's rpm then there's a mechanical advantage (torque multiplier) of 2:1 at work somewhere in the driveline, correct? The dyno doesn't give a shit whether that multiplier is due to tranny, diff and/or tire size. It simply knows to divide it's measured power/Tq numbers by 1/2 to get the flywheel values ... it's really a beautifully elegant system when you realize how it works.

As far as why we put the tranny in 4th (1:1) to run dyno pulls? It's not because 1:1 is necessary for the calculation to work, it's because in direct drive (1:1) you get less power loss through the clusters than you do when it's in some other gear. You could perfectly well do a pull in first gear or in 6th but the "rule of thumb" loss figures that we all apply would be wrong.

Changing the ratios of the above (gear ratio, tire size, etc) affects the curve for sure, but I can see that it should not affect the peak readings. However, gear swaps rarely result in similar dyno measurements, but I'm happy to chalk this up to different driveline loss and/or just typical dyno differences between trips.

So I humbly drop trou and offer up my lilly white ass for a public VA caning, and can only add this link with a good writeup of how the whole thing works:

http://www.bristoldyno.com/info/whatis.htm

(I'm stickin to my guns on the drag tires though!)

 

[_Mark_]

So in the interest of taking this baby to a 4 pager...

I have no doubt Jerome that you have witnessed it (I was careful to say ("by laws of physics"). Rarely are two different tire sizes on two different rim sizes the same OD. Even the same size tire in two different brands isn't even the same OD.

And rotational mass could certainly affect the reading, as it will affect the change in rotational speed over time.

So yeah, measure with stock gts rims you'd get one reading, measure with lightweight bbs rims, you should get a different reading.

(Whoa, I gotta get to the airport...)