Why is it so hard to measure HP?

Because not every dyno shop is concerned with using their tools properly? As I mentioned earlier, I have personal experience with two very different dyno types and they ended up with peak numbers that were very, very close. A slight difference in shape of the curves on turbo cars due to the way the Rototest does a sweep, but that's about it.

It's hard because there are a lot of variables, and it takes a good operator to minimize them. You're doing science. Science is hard.

Keith Tanner wrote:

bigbens6 wrote: A dyno can read differently on a cold run vs. the 30th run in a row as well. To be as accurate as possible you would have to state a given HP, list ambient conditions, and all other variable factors, and that would be honestly a little absurd.

That's effectively what the correction factors are intended to do, take a number of variables into account (temperature, pressure, humidity). It's up to the operator to minimize the others. Of course, some operators are better than others and some dynos might suffer more from run-to-run variation than others. For example, one that uses hydraulic pumps to provide the resistance will be sensitive to fluid temperature unless something's done to compensate.

Or, way back when I got my old 2000 Mustang GT dyno'ed, I learned folks can use the correction factors kind of dishonestly.

I think my car with a midpipe and catback was making about 240 to the wheels on this shop's dynojet, and when I asked about the correction factors noted on the sheet, (which were accurate) the operator told me that he tried to be as diligent as possible to maintain accuracy, even as it cost him business.

This was just when the new 2005 Mustang GT's were coming out, and some shops would make ridiculous correction numbers to put higher numbers on a customer's printout. He showed me one dyno sheet of a guy who went to a competitor's shop who believed his 05 was a factory freak, and putting down over 300 hp to the wheels bone stock.

I don't think you could recreate those weather conditions on Earth, but sadly I can't recall what they were.

Keith Tanner wrote: Because not every dyno shop is concerned with using their tools properly? As I mentioned earlier, I have personal experience with two very different dyno types and they ended up with peak numbers that were very, very close. A slight difference in shape of the curves on turbo cars due to the way the Rototest does a sweep, but that's about it. It's hard because there are a lot of variables, and it takes a good operator to minimize them. You're doing science. Science is hard.

Fair enough.

So out of curiousity... what sort of percentage of dynos being operated out there would you think would be somewhat accurate?

Keith Tanner wrote: If I take the windshield off my Locost, it has a faster ET. Does that mean it has more power?

that means that it's using more power to push the car forward instead of fighting resistance, so yes, you have more power being utilized to do what it is intended to do. same thing as removing weight or moving it around to where it can be better utilized. dynos are for tuning and comparing changes- the track is where you put the power to use.

I've seen a Subaru tuner claim a flywheel horsepower number that was approximately double the measured wheel horsepower after he was done applying all of his correction factors. And this was a guy who claimed a high level of expertise and was featured in a lot of magazines. Yeesh.

I have no idea what percentage of dynos are run properly. That's a pretty tough thing to try to figure.

the key to it all is this though, use the same dyno, same operator, and work with him/her to get useful numbers.... that simple.... if you want a dyno queen then find the shop in town putting out the highest numbers, put ice on your intake, change in warm oil right before a run on a motor just warm enough to get the rings seated well, and have that 1 money run on race gas.... there are plenty of ways to get 1 good run on a dyno.... but it seems to me you want useful numbers...

or if you have a dyno dynamics you can reduce or increase the load to read where the operator wants it to.......(why the berkeley do we not have a "rolling eyes" smiley?)

Paul, get back in the shoutbox.

Wyn Dyn chassis dyno - 138whp 108 #/ft - dyno run - http://s79.photobucket.com/albums/j143/oldeskewltoy/ae71/?action=view&current=...

Dyno Dynamic chassis dyno -145whp 112#/ft

Dynojet chassis dyno - 142whp 110#/ft - dyno run - http://vimeo.com/37280876

based on atmospheric conditions/differences... please tell me the difference. I see 3 different dynos... and I see 3 VERY similar sets of numbers.

On a different engine... my little All-Trac Corolla planted 52hp and 60#/ft.... untill we raised air pressure in the front tires from 24 psi to 28psi and then she made 73hp and 77#/ft

Fianlly when trying to compare whp and crank hp.... I recommend reading this - http://www.modified.com/tech/modp-1005-drivetrain-power-loss/index.html

I was victim of a shop cranking the DD rolling road resistance......poor car smoked like a diesel the whole run and took forever to rev, yet still netted 152fwhp/195fwtq......car had previously dyno'd on a dynojet at 225fwhp/207fwtq. Car in question was a 15.5psi '05 Saturn Ion Redline(stock at 12.5psi gm claimed 205/200). I view this as proof a dynometer is only as honest as its operator. Which is why I laugh at people saying, "I have more power according to the dyno, so you don't stand a chance" when I'm at the dragstrip......I normally laugh at those people these days.

Dyno's are tuning tool, does the final number really matter? What does matter is you test...you tune..you test was it an increase? Good or bad? most shops couldn't care less how accurate they are they just want to see change. Now whats not right is them cheating or changing the set up so you show bigger changes.

44Dwarf wrote: Dyno's are tuning tool, does the final number really matter? What does matter is you test...you tune..you test was it an increase? Good or bad? most shops couldn't care less how accurate they are they just want to see change. Now whats not right is them cheating or changing the set up so you show bigger changes.

I agree... the only time it gets really annoying is if you're not wanting to exceed a certain power number due to motor strength or some other restriction.

Inflated numbers will cause you to leave some on the table.

Keith Tanner wrote: There is a fairly good reference point: factory numbers. A stock car should read pretty much the same on any chassis dyno and their power ratings are set under much more controlled conditions.

You're assuming that the HP numbers are accurately rated. For some manufacturers, it's more of a minimum guarantee, For others, it's a pipe dream.

IIRC, more often than not, the LS1 engined F-bodies were making 5hp shy of rated power... at the wheels.

Yes, there are a few exceptions. I know about the LS1 F-bodies as well as certain other muscle cars that were underrated for political reasons. But most are going to be legit.

Not reading through all this stuff.

My school teacher explained it all well. Torque is a real number. HP is a made up number to sell performance parts. You guys have all seen a torque wrench Right? Ever seen a HP wrench? Nope. HP is a mathmatical equation that is based off of torque and rpm. The way a dyno READS this information can vary wildly. I can type in random numbers and make my 100hp fit read 300 and print you a graph to prove it. That's why to me, dyno numbers are are like guys arguing about your dick size, Yeayeayeayea, until I ask your wife it's all talk

Knurled wrote:

Keith Tanner wrote: There is a fairly good reference point: factory numbers. A stock car should read pretty much the same on any chassis dyno and their power ratings are set under much more controlled conditions.

You're assuming that the HP numbers are accurately rated. For some manufacturers, it's more of a minimum guarantee, For others, it's a pipe dream.

Not as much as it could have been in the past- the current SAE standard that is agreed upon for advertising is witnessed, and is to have a full dress kit as installed onto the car.

Way better than the old days of gross HP.

In reply to alfadriver:

The lsj supercharged gm's were all underrated.....whp was approximately what gm claimed as bhp. It still happens now, contrary to popular belief.

Gross HP was fun. You'd have 200hp engines rated at 250-300, and then you'd have 425hp engines rated at 425hp, and then you'd have 550hp engines rated at 430hp so that most people wouldn't buy them because who'd spend an additional 25% for only 5hp? Cut down on warranty claims that way.

I think "gross" allowed for no accessories, carburetor and ignition timing optimized at every RPM point, and then the marketing department would apply a fudge factor.

Eurobeaner wrote: Not reading through all this stuff. My school teacher explained it all well. Torque is a real number. HP is a made up number to sell performance parts. You guys have all seen a torque wrench Right? Ever seen a HP wrench? Nope. HP is a mathmatical equation that is based off of torque and rpm. The way a dyno READS this information can vary wildly. I can type in random numbers and make my 100hp fit read 300 and print you a graph to prove it. That's why to me, dyno numbers are are like guys arguing about your dick size, Yeayeayeayea, until I ask your wife it's all talk

Maybe you should do a bit of reading. HP is a legitimate measurement of work. Get your torque wrench and a stopwatch.

Eurobeaner, you and your school teacher must have missed physics class. Do you also think watts are "a made up number to charge people for electricity"?

Knurled wrote: One guy who recalibrated a dyno noted that the unit in question used an 8-bit ADC on the load cell. In theory, then, if the dyno could meaure up to 1000ft-lb of torque, then the smallest change it could measure would be about 4 ft-lb, and THAT assumes no in-use sensor drift, resistance, noise, etc.

Small clarification... Systems which use low bit-count ADCs usually have switchable gain stages which increase precision for different levels.. e.g. the same dyno could have 4 ft-lb accuracy from 500 to 1000 ft-lb, but 2 ft-lb accuracy from 250-500 ft-lbs, and 1 ft-lb accuracy from 0-250 ft-lbs.

Corrections can be applied to the analog signal before the analog-to-digital conversion, which may keep the accuracy within the quantization error.

Keith Tanner wrote:

Eurobeaner wrote: Not reading through all this stuff. My school teacher explained it all well. Torque is a real number. HP is a made up number to sell performance parts. You guys have all seen a torque wrench Right? Ever seen a HP wrench? Nope. HP is a mathmatical equation that is based off of torque and rpm. The way a dyno READS this information can vary wildly. I can type in random numbers and make my 100hp fit read 300 and print you a graph to prove it. That's why to me, dyno numbers are are like guys arguing about your dick size, Yeayeayeayea, until I ask your wife it's all talk

Maybe you should do a bit of reading. HP is a legitimate measurement of work. Get your torque wrench and a stopwatch.

and if you can get your torque wrench up to (i think) 5250 rpm, the number of horse powers and torques will be the same...

yamaha wrote: In reply to alfadriver: The lsj supercharged gm's were all underrated.....whp was approximately what gm claimed as bhp. It still happens now, contrary to popular belief.

Different kind of measurement- which is exactly the problem with chassis dynos if you don't know what you speak.

the standard test is a long one, with everything up to temp.

The typical chassis test is dynamic, and many boosted engines allow for "overboost"- which is not part of the SAE procedure. contrary to popular belief.

That, and what's the calibration procedure of the wheel power measurement devices... just wondering. Since engine dynos are calibrated every day as part of the warm up procedure.

I'm going to volunteer my car to see what sort of variance we see across at least two dynos with no other changes to the car at all.

This particular discussion has been hot in the turbo Miata community for years.

HOPEFULLY i will have the second set of numbers within a month, but if not, i promise i'll bump the thread in spring.

Will wrote: This is a serious question. A horsepower is a unit of measurement. No different from an inch or a pound or a liter. Yet we get vastly different power measurements using a Mustang Dyno, Dynojet, or Dyno Dynamics setup. People never say "Well, my car has a 100-inch wheelbase using a Mustang ruler, but a 105-inch wheelbase using a Rulerjet." But that's EXACTLY what you hear regarding power numbers. How has this not been resolved? Even if the different dynos use different methods of calculating power, a horsepower is still a standardized unit.

2 more factoids to throw in the fire:

Horsepower is a standard unit, but WHEEL (or hub) horsepower (what chassis dynos measure) is not.

Also, not all dynos measure torque and calculate HP. Inertial dynos do the opposite.

alfadriver wrote:

yamaha wrote: In reply to alfadriver: The lsj supercharged gm's were all underrated.....whp was approximately what gm claimed as bhp. It still happens now, contrary to popular belief.

Different kind of measurement- which is exactly the problem with chassis dynos if you don't know what you speak. the standard test is a long one, with everything up to temp. The typical chassis test is dynamic, and many boosted engines allow for "overboost"- which is not part of the SAE procedure. contrary to popular belief. That, and what's the calibration procedure of the wheel power measurement devices... just wondering. Since engine dynos are calibrated every day as part of the warm up procedure.

In the case of the Rototest, it's done using a beam and a weight at multiple positions. That particular dyno uses a strain gauge to measure torque directly. I don't know if you can calibrate a Dynojet.

The "take the same car to several chassis dynos" test has been done over and over. I think either Car Craft or Hot Rod did it in the past few months. Those magazines usually do dyno tests on an engine dyno and measured at various steps instead of a sweep, which eliminates the inertial losses that can wreak so much havoc with a sweep test. The Miata turbo community will never be happy, it's the nature of the beast.