Rules for 2013

[joncowen]

I really like using the car's curb weight as the base weight, as Nick mentions. This allows people to remove some weight which is essentially free (if you don't need those points back) or add a few modifications to make up for it. It allows removal of spare tires, tool kits, back seats etc, which can be put back very easily after the event. It also leaves room for safety items like seats/harnesses.
I think it also promotes making cars faster on the cheap. Removing interior parts is a cheap way to reduce the weight of the car.
I think these base weights are fairly arbitrary, since the rule applies to everyone, so i don't believe it hurts anyone.

[brucesallen]

So, in fact, torque shown on a dyno sheet is not the wheel measured torque but a calculated shaft torque, right??

The torque is reverse calculated from wheel HP using (5252 X hp)/rpm. So basically it is crank torque with drive-train loss subtracted. I suppose you could call it shaft-torque but I've only every heard that expression when talking about electric motors.

That is 5250 X wheel Hp/shaft rpm?? That sort of makes sense but that means the chassis dyno is producing wheel Hp and shaft torque. So If I all I have is manufacturer Hp and torque, I should use the torque but multiply the Hp by .80? Sure needs clarification in the rules and spreadsheet.

[Chrispy]

That is 5250 X wheel Hp/shaft rpm?? That sort of makes sense but that means the chassis dyno is producing wheel Hp and shaft torque. So If I all I have is manufacturer Hp and torque, I should use the torque but multiply the Hp by .80? Sure needs clarification in the rules and spreadsheet.

No it's giving you an expression of torque at the wheels, just like the HP. And yes, you can get an estimate of wheel HP/torque from crank shaft figures by using a multiplier of 0.85 (15% loss) for dynojet like figures or 0.8 (20% loss) for dyno dynamics/mustang.

I think the rules document or spreadsheet goes by 15% but I will have to double check.

[dradernh]

you can get an estimate of wheel HP/torque from crank shaft figures by using a multiplier of 0.85 (15% loss) for dynojet like figures or 0.8 (20% loss) for dyno dynamics/mustang.

I've had BMWs on Dynapaks at Turner & Dinan, and both were using 18% dl loss.

[chaos4NH]

I remember Nick saying 10% for 2 wheel drive and 15% for all wheel drive when we use manuf crank figures vs. dyno figures, for the COM calculations

[blindsidefive0]

I remember Nick saying 10% for 2 wheel drive and 15% for all wheel drive when we use manuf crank figures vs. dyno figures, for the COM calculations

Hi Sam, close... the rules actually outline 15% for 2wd, 20% for awd

[agrabau]

Hi guys, I wanted to mention a couple of things regarding our Dyno Dynamics LB450 here at http://www.dentsport.com and the measurement process if people choose to use us as a dyno shop.

Gearing doesn't matter. The gear change changes the relationship between the motor and the gearbox but it does not change the power put to the ground. The car feels like it has more torque because there is a time component in the way the car can use the gear up.

In other words, we could dyno a car in 1st gear or 6th gear and the power would be the same. The run would be shorter and the car would buck violently in 1st so we don't.

This means-

Torque as a measurement to determine a class is therefore not out of the question if people should choose to use it.

Second,

Our dyno has what's called "Shootout mode" which is a standardized mode, set by Dyno Dynamics and absolutely not adjustable by the user (all other modes are). We could run the cars in shootout mode to get a standardized, non-adjustable classification. We don't use this mode for tuning because having no control on the ramp rate and load is not good for tuning but would be fine for pulls to determine power.

Alex

[Chrispy]

Thanks for the info Alex!

[brucesallen]

Alex,
Please educate me on the torque that comes with your dyno sheets. It must be a representation of crankshaft torque calculated from Hp, not wheel torque. Even if the transmission is in 1:1, the actual wheel torque is crank torque times the rear end ratio (usually around 3:1). We need to decide what to put into the COM spreadsheets and also how to calculate that number if we only have manufacturer shaft Hp and torque.

[agrabau]

Bruce, there's a lever arm with a load cell on the dyno that measures newtons of force which is converted into ft lbs.

Hp is a function of that measurement and RPM.

[Shawn624]

So you are actually only measuring torque with the physical device and then calculating hp?

[agrabau]

Correctamundo

[Chrispy]

So you are actually only measuring torque with the physical device and then calculating hp?

This is for power absorbing dyno's (Dynodynamics, mustang etc). Inertia dyno's measure HP by calculating time to accelerate a fixed mass, so for those dyno's gear does matter and torque is calculated from the HP. Either way you measure torque and HP have a fixed relationship by the formula given earlier.

Now I just need Dent to tune my car

[nateh]

Chris, I don't think gear matters (at least to the first order, neglecting sensing errors) in an inertial type device.

Torque is just power divided by RPM, just as force is just power (energy per unit time) divided by velocity. If you know one, and you know the RPM, you know the other. No ambiguity about this.

The error you're likely to get are in losses and resolution of the instrumentation. If the dynos are decent, these should be small compared to what we care about - though I can believe that the two types might have known biases, and here I yield to people who actually work with them. (In this context, I am just a generic mechanical engineer.)

There's no reliable way to know the ratio between crank and wheel HP. In principle, it varies among cars, and should also vary some with RPM. But it sounds like there is some industry know-how that we can use, and if we agree on a set of standards (i.e. embody the standards in the rules), then we are doing all we can reasonably be expected to do - and it is far less crude than our (as Nick says ) legacy classification system.

People with dynos pay good money for them, and they charge for their time accordingly. This is a significant cost built into the proposed new classing system. But I think it's a cost worth bearing. Also: if you modify your car for more HP, don't you want to know what you got for your money?

[Chrispy]

Dynojet always requests that the operator use the gear that is as close as possible to the 1:1 gear, but this is probably just to improve accuracy or reduce drivetrain loss. I suppose all it really needs to know is the relationship between drum speed and engine rpm to calculate. Mustang Dyno's read about 10% higher if you drop down a gear, so if the street guys get their heart's broken by a low reading in 4th, they run it again in 3rd and they go home with a number they can brag to their friends about

These days, just about everyone with a modified engine has had a dyno tune, it's the only way to know that you are running safe and getting the most out of the engine. So generally speaking requesting plots from these folk shouldn't be a problem and they can be approximately normalized based on type. I don't think we're looking for super high accuracy here, so as long as it looks reasonable it should be fine. Nick also found a way to calculate power fairly accurately from a GPS data logger so that is an option too. If we're all on the honor system I have no issue with stated figures either, but it would probably make your fellow competitors happier if you showed your real numbers when doing the math.