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Dyno Horse Power VS GPS Horse Power???


olepokey

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This has been clipped from a post on the American Iron forum from Todd Covini about the Nationals, Dyno and GPS (Trackmate) Horse power.

What formula do you use for frontal drag resistance for each specific model of auto to figure GPS Horse power? I would ASSume the weight of car, frontal drag resistance and the amount of time to cover a known distance? Right or wrong, I for one would like to know how the GPS HP is figured…

 

Robert Gray

 

 

Quote from AI forum

 

As an added level of verification, seven of the top AI racers were carrying GPS on board for the championship race, including these 3 cars. Dyno results were compared to GPS results and horsepower as passed on the dyno was found to be consistent with HP calculated using GPS equipment.

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Weight and time to cover a certain amount of distance. The frontal drag resistance will effect the time it takes you to cover that distance so this would not be needed as a seperate parameter. In other words, if you've got a slippery front end, you're gonna be able to go faster and cover more distance quicker which plays into the calculation.

 

And BTW, I'm not an expert but I do use a Traqmate and this is what I've gathered from talking to several others who are...

 

Ed

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The above is basically true, but for purposes of verifying to the AI hp/wt rules, an inertia dyno does not care how aerodynamically efficient your car is (there are eddy current chassis dynos that can be programmed to compensate for drag coefficient). The rules require you to be at a given weight for your horsepower, regardless of how much more (or less) efficient your body may actually be in putting that power to the track. So if the GPS is being used in lieu of a dyno verification, drag coefficient does need to be taken into consideration. Given two cars of equal weight and HP, the more aerodynamically efficient car will accelerate faster and show more "GPS HP" than the other car, even though it is still perfectly legal based on a static dyno test.

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The above is basically true, but for purposes of verifying to the AI hp/wt rules, an inertia dyno does not care how aerodynamically efficient your car is (there are eddy current chassis dynos that can be programmed to compensate for drag coefficient). The rules require you to be at a given weight for your horsepower, regardless of how much more (or less) efficient your body may actually be in putting that power to the track. So if the GPS is being used in lieu of a dyno verification, drag coefficient does need to be taken into consideration. Given two cars of equal weight and HP, the more aerodynamically efficient car will accelerate faster and show more "GPS HP" than the other car, even though it is still perfectly legal based on a static dyno test.

You are exactly right. I think the two methods should be used in conjunction with each other in order to keep guys legal. Continue to use the dyno as we do now and use the GPS as a monitor when a dyno can't be at the track with the idea that some variance has to be realized...

 

Ed

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Ok alot more to it when Greg G and I used to do it for TT enforcement.

It is a great directional tool but usually it can vary by 10rwhp either side due to following.

 

- Flat surface or hill?

- Weather adjustments must be made and timed to track session.

- Ram air highly effective could show 10-15rwhp more than sitting dyno doesn't (my old vette with a vararam had this effect).

- aero if way different than standard assumption (but our AI cars are mostly brinks compared to a vette or others so really not an issue).

- did you read the right speed for that car's gearing (meaning 4th gear pull).

 

The traqmate can do it and the key is you need to correct for weather like a dyno correction factor. At road atlanta one year I was curious and Dec to Aug was almost a 40rwhp change but when adjusted was pretty consistent and is clearly enough to catch someone out of bounds enough and you seal and dyno those guys.

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THANKS for all the replies...

 

Let me break out my calculator and see if I can figure this out...

IMG_4107.jpg

I have been hearing SO many conflicting stories about the HP numbers on the Trackmates and the real world hp numbers involved. I can believe the average racer can install a unit in their car and can see HP numbers that would be within 5% of a wheel dyno IF both were being done at the same time...

I realize that a Tri-axis accelerometers and today's computers can do amazing things, and I know the program has a generic drag coefficient figured in and I didn't know how it applied to different cars.

Is there any data from the trackmates from the Nationals that is available for viewing, figured just after the Nationals would be a good time to ask questions.

 

Robert Gray

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How does a GPS derived HP take into account headwind/tailwind that can/will sway the HP readings?

 

 

There was no wind at the track this past weekend...

 

 

Richard P.

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I spent 2 hours looking at 944 Spec TM data from Nationals. Many things affect the number - drafting, how level the track is at any one point, how much fuel you've burned off, etc. We saw HP #'s vary more than 13 HP lap to lap in the same car (nearly 10% for us). The track is not a lab, lots of variables out there...

 

You guys may have a better chance at making it work, because your HP numbers are bigger, and things like drafting have less of an impact.

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How does a GPS derived HP take into account headwind/tailwind that can/will sway the HP readings?

 

 

There was no wind at the track this past weekend...

 

 

Richard P.

 

I'm going to pretend that was sarcasm.....

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How does a GPS derived HP take into account headwind/tailwind that can/will sway the HP readings?

 

 

There was no wind at the track this past weekend...

 

 

Richard P.

 

I'm going to pretend that was sarcasm.....

 

 

Yea, probably only funny to those at the track. The wind just wouldn't stop. It wasn't a storm that came through and blew a bunch of stuff over. It was just constant.

 

The conditions at one event also aren't that relevant to the real issue of trying to account for all the variables that have a very real impact on the accuracy of calculating HP from GPS or accelerometer data.

 

 

Richard P.

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It can be made to work, you just cant use one data point. Statistical analysis comes into play as well to eliminate some of those variables you mentioned.

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It can be made to work, you just cant use one data point. Statistical analysis comes into play as well to eliminate some of those variables you mentioned.
I disagree. GPS data from the cars will never be a substitute for the dyno. At best it can be used to monitor the performance of the cars and indicate when a car needs closer scrutiny. Variables such as aerodynamic drag, frictional losses (from tire rolling resistance, alignment, bearing condition), head/tail winds, etc. cannot be accurately accounted for, nor should they. This class is about power to the rear wheels, and if you happen to build a car that has better aero or less rolling resistance, that's all fair.
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I totally agree it is not to replace the dyno just complement it. If someone reads 20 over on GPS then you send them to the dyno. Reality is only top 2-3 car ever get dyno'd and everyone else has nothing.

In fact in some regions there is never a dyno available so we need some general checking device. Its better than nothing is my point and is directionally close at least

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How does a GPS derived HP take into account headwind/tailwind that can/will sway the HP readings?

 

The same wind affects two cars on the track at the same time. Compare both cars in the same sections of track at the same time and if the acceleration rates are the same during the headwind/tailwind sections, probably consistent results as compared to the dyno. Especially when all of them are the same body style. <>

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Especially when all of them are the same body style. <>

 

 

Huh? What if everyone running the same body style is in collusion to be the same amount over???

 

Richard P.

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My concern with it would be where is the hard line drawn using GPS data that could get someone DQ'd. Use it all you want to figure out who to keep an eye on is great but if we're going to be using it as a decision maker thats a whole new ball game. In that case, EVERY factor including processicing data statistically has to be disclosed in full in the rulebook. No "the GPS said it made XXX hp - DQ" hankey pankey.

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