Flywheel power rant
Discussion
I work for a Dyno (rolling road) manufacturer.
90% of Automotive dynos are chassis dynos - the car's tyres spin on rollers in the dyno.
The dyno measures the car's power at the wheels.
What pushes you down the road is the car's motive force at the wheels.
So why do so many people want "flywheel power", which can never be more than an estimate?
We do our best, but the dyno doesn't know if you are running super slick oils in the diff / gearbox, or have a very inefficient driveline, or any number of other variables.
Why not stick with the more accurate figure of what we measure directly?
Is it as daft as that flywheel power is a bigger number?
Rant over. That feels better.
90% of Automotive dynos are chassis dynos - the car's tyres spin on rollers in the dyno.
The dyno measures the car's power at the wheels.
What pushes you down the road is the car's motive force at the wheels.
So why do so many people want "flywheel power", which can never be more than an estimate?
We do our best, but the dyno doesn't know if you are running super slick oils in the diff / gearbox, or have a very inefficient driveline, or any number of other variables.
Why not stick with the more accurate figure of what we measure directly?
Is it as daft as that flywheel power is a bigger number?
Rant over. That feels better.
doogz said:
Don't decent dynamometers measure mechanical drag on rundown? So they can provide a decent estimate yeah, still an estimate, but you talk about gear oils, the drivetrain losses can be roughly measured, can't they?
They make a rough estimate at best, and they can be grossly inaccurate. How can you possibly expect to accurately measure the transmission losses of a fully loaded drivetrain, by very lightly driving it from the wrong end?doogz said:
Don't decent dynamometers measure mechanical drag on rundown? So they can provide a decent estimate yeah, still an estimate, but you talk about gear oils, the drivetrain losses can be roughly measured, can't they?
<snip>
We (and I assume the other manufacturers) have put a lot of time into it, and the key word is "roughly".<snip>
One of the problems is that geartrain losses are asymmetrical, ie the power lost driving gear 'a' with gear 'b' is not the same as when the drive gear is driven. Losses also vary with load - the side thrust on heavily loaded gears causes losses in thrust races, etc. etc.,
We do the rundowns and so on, and a lot of comparison testing, but at the end of the day, there are a lot of variables involved.
The oil issue that i mentioned earlier is real - you will get a higher power reading with low-viscosity gear oil. Not huge, but measurable. Without the actual flywheel power changing at all.
AW111 said:
I work for a Dyno (rolling road) manufacturer.
90% of Automotive dynos are chassis dynos - the car's tyres spin on rollers in the dyno.
The dyno measures the car's power at the wheels.
What pushes you down the road is the car's motive force at the wheels.
So why do so many people want "flywheel power", which can never be more than an estimate?
We do our best, but the dyno doesn't know if you are running super slick oils in the diff / gearbox, or have a very inefficient driveline, or any number of other variables.
Why not stick with the more accurate figure of what we measure directly?
Is it as daft as that flywheel power is a bigger number?
Rant over. That feels better.
If you work for a dyno manufacturer, you really ought to know that they don't measure power, they measure torque and then derive power via a calculation 90% of Automotive dynos are chassis dynos - the car's tyres spin on rollers in the dyno.
The dyno measures the car's power at the wheels.
What pushes you down the road is the car's motive force at the wheels.
So why do so many people want "flywheel power", which can never be more than an estimate?
We do our best, but the dyno doesn't know if you are running super slick oils in the diff / gearbox, or have a very inefficient driveline, or any number of other variables.
Why not stick with the more accurate figure of what we measure directly?
Is it as daft as that flywheel power is a bigger number?
Rant over. That feels better.
Anyhow, to answer you question. In the UK we are dumb. Personally I always request RWHP figures when my car has been on a dyno, for the single purpose that any 'guessed' flywheel figures will be less accurate.
In my experience, wheel power is also inaccurate, or at least has the scope to be. I have dynoed my car in 3rd and 4th gear, different power in each gear. The increased tyre drag in the higher gear gave a lower result as the tyre was turning at 135mph vs 100mph. Interestingly, the calculated flywheel figures were similar.
No rolling road gives me the same confidence as a measuring device as a micrometer. Rolling roads are the only measuring device where they all read different. If I buy three micrometers from three different manufacturers I can measure something with them and they would all read the same within fractions of a percent of full scale deviation. I've used several dynos and none have been within 5% of each other in wheel horse power figures.
Which dynos do you make?
No rolling road gives me the same confidence as a measuring device as a micrometer. Rolling roads are the only measuring device where they all read different. If I buy three micrometers from three different manufacturers I can measure something with them and they would all read the same within fractions of a percent of full scale deviation. I've used several dynos and none have been within 5% of each other in wheel horse power figures.
Which dynos do you make?
300bhp/ton said:
If you work for a dyno manufacturer, you really ought to know that they don't measure power, they measure torque and then derive power via a calculation
Pedantry at it's finest, though I expect no less. If you connect a digital volt meter across a battery, what do you think it's measuring?Mr2Mike said:
Pedantry at it's finest, though I expect no less. If you connect a digital volt meter across a battery, what do you think it's measuring?
It's not pedantry, it's a rather large fundamental of what these machines are and do. They are a 'brake' device, which loads an engine, measures torque and calculates horse power, thus Bhp.Something like Dhp as used for tractors is very different an is actually a measured horse power figure rather than a calculated one and isn't done on a dynometer.
300bhp/ton said:
HustleRussell said:
Surely wheel bhp measured on chassis dynos is inferior in accuracy to a hub dyno?
Yes and no. hub mounted systems remove the tyre and wheel, but the rest of the drivetrain is still there.HustleRussell said:
What you meant to say is "yes, a chassis dyno is inferior in accuracy to a hub dyno"
It depends what you are wanting to know. If you want to know the actual power you are putting to the ground, then a hub mounted one doesn't show this. Also as there are far fewer hub mounted dyno's about, it may make such numbers less comparable.If you want to more accurately guess flywheel figures, then hub mounted systems will remove some variables. But there are still plenty of variables to still make it a guess.
I must stress that if the customer wants flywheel power, we give them the best we can. The question is more why people request it so often.
Having said that, I have my own car pretty accurately measured
. One of the perks.
I will stay coy about my employer for now, but if any of you remember a DOS based system with yellow bars from about 20 years ago - that was mine.
Having said that, I have my own car pretty accurately measured
. One of the perks.I will stay coy about my employer for now, but if any of you remember a DOS based system with yellow bars from about 20 years ago - that was mine.
People want flywheel figures because it's bigger, and it's nearer to what the manufacturer told them it would be.
I'm glad you give at the wheels figures, the last dyno I used it was a real challenege to get the wheel figures out of them, and even then I think they'd massaged them to appear higher, presuming that'd make me a happy customer...
I'm glad you give at the wheels figures, the last dyno I used it was a real challenege to get the wheel figures out of them, and even then I think they'd massaged them to appear higher, presuming that'd make me a happy customer...
AW111 said:
I must stress that if the customer wants flywheel power, we give them the best we can. The question is more why people request it so often.
Having said that, I have my own car pretty accurately measured. One of the perks.
I will stay coy about my employer for now, but if any of you remember a DOS based system with yellow bars from about 20 years ago - that was mine.
Quick question for you as you work in the industry,Having said that, I have my own car pretty accurately measured
. One of the perks.I will stay coy about my employer for now, but if any of you remember a DOS based system with yellow bars from about 20 years ago - that was mine.
Can you quote flywheel horsepower for a car with an automatic transmission given that they have a flex plate attached to the torque converter instead?
Does a torque converters ability to multiply torque ahead of engine speed affect your results?
Just curious and hoping you would know?
This has puzzeled me for a while...especially when people compare dyno figures+flywheel estimation to manufacturers numbers. The two ways of measuring are not even closely related... If you dont run before and after (of course on the same tyres for example) and then comparing your number to another setup on another dyno/car is not a good idea.
I myself will run on a hub dyno for my baseline run before I start tuning, just because I want to get rid of tyres (which you always want to when measuring stuf), to get better repeatability. And of course run same dyno before and after.
/engine simulation and testing engineer...
I myself will run on a hub dyno for my baseline run before I start tuning, just because I want to get rid of tyres (which you always want to when measuring stuf
), to get better repeatability. And of course run same dyno before and after./engine simulation and testing engineer...
hedges88 said:
Quick question for you as you work in the industry,
Can you quote flywheel horsepower for a car with an automatic transmission given that they have a flex plate attached to the torque converter instead?
Does a torque converters ability to multiply torque ahead of engine speed affect your results?
Just curious and hoping you would know?
I'm certain AW111 can give a better answer. But yes you can still estimate flywheel figures in the same way, but yes the stall can cause more error.Can you quote flywheel horsepower for a car with an automatic transmission given that they have a flex plate attached to the torque converter instead?
Does a torque converters ability to multiply torque ahead of engine speed affect your results?
Just curious and hoping you would know?
Many modern stalls will have a lock up mode, in this guise it should be more accurate, but you may not be dynoing with it locked up. Older and many tuned cars don't have lock up, so there will be a discrepancy. Normally accounted for by making the assumption of higher drivetrain loses.
High stalls on high power motors can give vary misleading results due to the torque multiplication. Indeed when I've dyno'd my Z28, the is a huge torque spike when it shifts gear, all due to the torque converter.
Gassing Station | General Gassing | Top of Page | What's New | My Stuff