Measuring horsepower with a chassis dyno

What are the limits of a chassis dyno for measuring wheel horsepower?

It seems like the interface between a rubber tire and a smooth steel drum would have traction limits. I used one once for driveability tuning and recall the upper limit being pretty low.

 

Do you mean when a car has A LOT of horsepower and it loses traction on the dyno?

 

The cars are "ratcheted" down on all four corners, thus providing sufficent downforce on the dyno wheel to provide the necessary traction for engine testing. Perfect example being the Veyron dynoed at over 1,000 horsepower......Mark.

 

I have friends with cars nearing and over the 1000HP. Rear wheel drive cars that get rachet strapped down, have cement bags placed in the boot and people sit in the back seats to keep as much weight over the rear wheels as possible to avoid wheelspin..............yet they still wheel spin on the dyno.

Here are pics of an Aussie ute with 1895HP at the rear wheels.

I would LOVE to know how they did that without smoking the tyres on the rollers!

Here is an early vid of that engine. They are making over 2000HP at the wheels now apparently.

http://www.youtube.com/watch?v=3UJQPttNBzw
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What's interesting about the dyno printout is that the tyre pressure is 50lbs. Those turbos look big enough to shallow a small kangeroo.....Mark.

 

The reason I brought this up is that I'm suspicious of the horsepower claims.

Not that the car won't make that much power (a whole different subject worth pursuing separately) just that once you get to a certain point (I'll bet way before 1000 HP) you can't apply enough downforce with straps to keep the wheels from spinning.

Just because the machine says a billion horsepower doesn't make it so.

 

The thing that gets me about chassis and even engine dynos, is that people (owners basically) believe them. A person my dyno his car and the walk around sayinig it makes 436.7 HP.

A dyno is at best a 1% accurate device. Add to this the error of correcting that ~1% device based on humidity, atmospheric pressure, and temperature; and add another good fraction of a percent or more because the dyno cannot provide enough cooling for the radiators or even cool air for the engine, not to mention slipage between the tire and the drum,.....

And we have, at best a 3%-ish accurate number. Thus, for a vehicle wth a motor producing exactly 400 HP at STP that day, might read as low as 388 or as high at 412.

And then there is the motor, ECU, and gasoline. If one HAD a perfectly accurate dyno, and started with a full tank of gas with enough cooling for the radiators (and gearboxes) and cool air to the motor; take this car on a dyno and make pulls until the gas tank is empty. The motor would display 2%-3% variation all by itself. Even with a perfectly accurate dyno.

Therefore, there are only about 2 digits of precision in a corrected result; and sometimes not even 2 good digits to boot.

 

Good points but I think you are being generous.

 

That is generous
my Eddie current Dyno has 21% drivetrain loss on most every car run
however, it has been absolutely spot on simulating real world loading when tuning ECUs with data aq on Dyno vs street or track virtually identical

 

Tuning under moderate loading that simulates a "real world" driving situation is what these machines are for.

 

Lol!

 

Dynojet dynos are more generous than Mustang dynos. The % difference of course depends on the car, but you can guarantee at least a 2% loss with a Mustang. Our local Mustang dyno is known as the heartbreaker dyno. Very useful tool regardless.

 

Whoa hang on there dude. Not in my experience. In fact I found the Mustangs to be a whole lot more generous than the Dynojets. Overall, I think that chassis dynos are unreliable in terms of providing an absolute power output value. Too many variables to hang your hat on. The only venue for this is a lab where you can fix your variables and not have to make that wank adjustment for drive line loss or parasitic drag. Even then, these rigs post results within a range and users generally average a set of tests.

At best a chassis dyno gives you a measurement tool for demonstrating differences between runs and modifications. That's their forte; not providing absolute output numbers. Heck, HP is a calculated value resulting from torque and rpm, so only torque really gets measured. RPM measurement is problematic too. Like when we tried to measure gothspeed's car off the plug wires and got corrupted signals with an induction sensor. I attributed this to the fact that the 355 runs all their plug wires down a channel on top of the cam cover and this makes them prone to interruptions in the signal. In fact, I'd like to see a modification for this back to the 348 style of keeping the wires seperate, but I digress.

Chassis dynos, both Mustang and Dynojet also use software to interpolate the measurements based on atmospheric variables, the so called SAE smoothing. Heck, how does that work?! Then they estimate gross power using a linear relationship to account for drive line loss. You think a Ferrari has the same loss ratio as a Corvette or an Integra?

Still, regardless of the deficiencies, these are the only tools most of us have access to so use them with confidence. Just be realistic about what you are reading. BTW, I have heard of dynos with power ratings of under 400 HP and over 400HP to 1000HP theoretical, but not over this, not that they don't exist but they must be pretty expensive. Consider with a 21% gross up, measuring 800 HP gets you 1012 HP gross. You really think it takes 212 HP to move that drive train? In fact, I believe that the loss is a close range from a fixed value, not a straight line ratio.

 

Absolutely not.

An Integra only has four gears in use in the tranny+differential (dyno use), the input shaft gear, the mainshaft gear, and the crown gear and the output gear. The first two are the chosen gear ratio, the second two are the differential ratio.

A Corvette also has 4 gears--except in 4th gear where the input shaft is coupled to the mainshaft directly (no gears involved). Thus, when dynoing in 4th gear, one only has the differential gearing loss (and the half shaft loses wich even a solid axle car does not have).

A F355 has 8 gears between the engine and the tires! The drop gears (25:29) the 90 degree gears (19:20) the selected gear (1st-6th), and the output gears (16:57). The only thing moderating the loss in the Ferrari gearbox is the heavy use of ball bearings and close tolerance machining. Finally, the Ferrari transmissions actually have oil pumps. And of course, the half shafts with their constant velocity joints.

These are out best tools.

The only real way to determine the actual loss in the transmission is to dyno an engine on an engine dynamometer (say via the missing clutch at the back of an F355), then slap on the clutch and dyno again within 1/2 hour or so on a chassis dyno. Would make for some very interesting comparisons.

By comparing the engine TQ curve to the chassis TQ curve, one could determine the friction losses in the transmisison at various loads and various RPMs and in various gears. And settle this debate once and for all (at least on that car that day).

 

I've done that jillion times over the past 20 years. On Freud, Land & See, Superflow, DTS Dynojet and Mustang. Dynojet was about 15% drivetrain loss on every car. Mustang, my mustang, is 21% from a 2 liter 911 all the way to brutous 599 as compared to "stock" numbers. Also comparison from race engine we built and tested on 5 different engine dynos to at least 5 different chassis dyno's shows about 15 to 20% drivetrain loss. However not all engine dynos read the same either. The Freud water brake I had years ago read low, but my engine always seem to be at the front at the finish line due to good "realistic" loading for tuning as is the Mustang now.

Don't forget about gear fluid....there is a difference that can be seen on the dyno but longevity vs peak power is a compromise I'm not willing to take.

 

Sorry I meant numerous engine(s) not one

 

Dynapacs solution to the wheel slippage problem where the vehicle's tires are removed and axles are then bolted to the engine dyno style load cells. Load cells have wheels/rollers which make them portable and easy to store out of the way when not in use. ​

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Too many horror stories from reputable shop owners I know and too much dinero.....

 

Our local Mustang dyno guy sets up his pulls inputing 15% drivetrain loss for every vehicle just so he knows his base. Everyone locally knows our Mustang dyno guy as the heartbreaker place and to NOT go there if one is going for internet bragging rights lol. He even has a calculation for about what a Dynojet would show for that particular vehicle according to previous testing on both styles of dynos. Do a search on dynojet vs mustang dyno and you will find some interesting information regarding both and how different they calculate bhp. Most I have ever found find dynojets to be all too generous. Doesnt raelly matter to me. I could care less about their individual bhp findings so long as the results are somewhere in the ballpark of what the engine is rated at. I just find them useful for knowing a/f ratios at WOT etc.

 

With SEMA coming up, I am definitely going to visit both the Dyno and Mustang booths (and anyone else) and put this question to them. Should make for some interesting arguments, eh? I show Mustang giving my 348 SS (312 gross HP rating from the factory) 265 RWHP and the Dyno giving 246 RWHP in stock trim. Personally, I don't believe either one of them, or the factory for that matter, but as I said before, you don't trust the absolute numbers.

 

You guys are talking about the driveline loss and how it skews the calculation.

No it doesn't. The dyno does not pretend to measure BHP, but rather RWHP. Every time I have had my car dynoed, it was the RWHP that was printed on the sheet, not BHP.

BHP is largely irrelevant. Only the power that is making it to the wheels is what accelerates the car. If an engine makes 600hp but only puts 350 to the wheels, it's going to lose out to a 500hp car that puts 400 to the wheels (all else being equal).

From all that I have heard over the years and every test I have seen, Ferrari is "beyond optimistic" (to put it mildly) with their claimed horsepower figures.

Regarding driveline loss, it is not a static number, but a %. There have been many many tests by many many companies to substantiate this. I've heard of data from numerous tuners as well as OEM's and race teams. Manual transmissions are generally 10-15%, mostly around 12%. Automatic and 4WD are generally more around the 20% mark.

 

To revisit my original question, I'm calling BS on claims of more than 1000 horsepower (and probably considerably less) measured on rollers.

 

Not necessarily. We are talking about roller dynos that measure RWHP then make adjustments for atmospheric conditions (smoothing) and drive train loss to render a gross HP estimate. The RWHP measurement is just that, however, I found that different machines (only Mustang vrs. Dyno in my case) measure the same car differently. Of course, this makes the adjustments goofy too.

MY main point is that we should use the tools we have - dynos measuring RWHP - as yard sticks to determine modification, upgrade and maintenance impacts on power output. Using them for their absolute values is meaningless except in situations where all variables are controlled (like in a lab) or everyone in a competitive population (say a racing class) is using the same machine in the same way.

I am still going to ask the guys at SEMA about their machines. I did it last year and it was fun to hear their explanations of how one might measure drive line loss. Some of the proposals were pretty involved.

 

Can we at least agree on several key points? ::

1: it is the RWTQ that creates longitudinal acceleration-- f=m*a (at least until aero effects slow acceleration down)
2: it is the RWHP that govern overall performance-- p=f*v
3: The RWHP measured on one dyno should be equal to the RWHP measured on another dyno (within twice the measurement precision on each dyno--say 5%)
4: RWHP to BHP conversion is mearly about bragging rights--that is who cars if the tranny consumes 15% or 35% if the RWTQ is 3200 lb-ft/s**2 in 1st gear a 3200 pound car will accelerate at 1.0 Gs in 1st gear.

 

I don't think they are meaningless as measures of absolute value... there certainly is a margin for error, but guys who are seriously into the power their cars make can generally get results within a few hp of each other from different dynos on different days provided the car is in the same shape (not heat soaked one time and cool the next). That has been my experience.

I have no doubt that a person who dynoes 400hp is making less than another guy who dynoes 450hp on the same kind of dyno, even if one is in Maine and the other is in California.

I definitely do think too many people try to work back to BHP calculations just for bragging rights - everyone loves to believe their RWD car has 22% driveline loss