Where's HP Measured?

Factory is at the crank.

 

the factory measures the HP off the flywheel. there's about a 15-20% drivetrain loss from the flywheel to the wheels themselves, so the hp at the wheels is a bit lower than the printed 172 you read on the papers. i dont know much about why there's a drive train loss, but i do know it exists in every car. do the math, and see what 20% off the printed 172 hp will give you, and that number is just about the estimated amount of hp you have at the wheels.

 

Factory advertised HP and Torque is measured at the flywheel.

 

It's much more complex and ranges depending on RPM, Load and Gear Selection. Losses come form many places. In the transmission as the gear starts to rotate it picks up oil, has a large wetted area and the loss follows a normal V^3 drag power law. As it picks up speed it tends to fling the oil and carve a groove in the oil bath reducing drag by entraining air. As it flings oil the oil depth reduces, again reducing drag. It moves to a loss approx proportional to speed regime.

Losses in hookes joints and cv joints tend to be small, and pretty much proportional to power.

Losses in gearboxes tend to be rather larger, and tend to be worse in percentage terms at both extremes of the power range.

Losses in diffs can be quite spectacularly large, and again tend to be least at mid load.

I shouldn't read all these darn SAE papers...

 

my bad. flywheel. i got confused :blush2:

 

20% is a very large nuber

for the Mini's drivetrain loss. Think more like 12% or so...

Matt

 

well, i say 20% because it's an average. ive known cars to have an average of 10-20% drivetrain loss. i guess it really depends on the car lol.

 

In England!

You forgot losses due to friction between the tires and rollers.

In the R53s I know a lot of people felt MINI under-rated the actual HP and there was a big variance in actual outputs, I don't know if this was resolved with the R56.

RWD and AWD cars had much larger losses like that but in a transverse engine layout efficiency is higher since the rotational force doesn't have to be turned 90 degrees at the differential, nor does it need to spin a large, long driveshaft, to turn the drive wheels.

 

20% was from autobox muscle cars

and 15% for manuals as a rule of thumb. But we're all forgetting that friction and drivetrain loss reduction has been engineered over the last 30+ years cause it's basically free gas mileage and emissions reduction for the very same power plant. If you look at the WHP numbers many are getting, the engine numbers would just be too effin high with a 20% correction.

Also, if you look at stock wheel HP numbers vs quoted, it seem that a lot of cars would have to have crank numbers much, much higher than quoted for a 20% loss to make sense.

But in a way this is a pointless discussion. I haven't seen a single Mini engine put on an engine dyno so this is bench racing at it's worst. All one ends up with by claiming high drivetrain losses is artificially high estimated crank numbers. They sound good and look good, but at the end of the day don't mean squat.

Matt

 

Matt -
And the Brits always quote the higher BHP and never WHP when they dyno their cars on sites like MiniTorque.com, so what % correction factors are they using? :confused5:

 

BHP

is Brake Horse Power.... This site says that BHP is what we would call gross HP. That is engine HP without accessories (I actually didn't know anyone really used this anymore, who runs an engine without the accessories?)

What is quoted in the US is net HP, or HP of the engine at the crank with all accessories mounted and running. I don't know what load requirement is there (like the A/C compressor is present, but not engaged, or the alternator is on, but what load it's pumping out I have no clue, maybe Nathen read that in one of the SAE papers...) But whatever it is, it includes the frictional losses via belt flex and turning whatever pullies and pumps that are present.

But Net HP and BHP are both crank HP, not wheel HP.

As far as what correction factor other use, you'd have to ask them. But I'm guessing that HP is quoted in whatever local units are dominant without regard to real definition. Without a ton of work to mount up to an engine dyno, everyone is reading wheel HP (or hub HP if useing a dynopack). Really, that's what gets the car going. It would be nice if everyone just used raw numbers as measured at the wheel or hub and left it at that.

Matt

 

I used a correction factor of 50% which gave me BHP of 327 on a stock JCW with an RMW tune. Woot@! I win.

 

And this is why...

working backwards to crank HP is a waste of time!

Matt

 

Not if you win.

 

I refuse to accept your standards of measurement and substitute my own.

My car produces 2785 babypower at the wheels.

Also, it seems that over a short distance the average horse can produce about 20 horsepower so maybe the horse isn't the best standard to go by.

 

And over the long run...

all a horse can make is dogfood and glue! So I think you're on to something there. Anyone for kilowatts?

Matt

 

Babypower? I LOVE it:devil: