BHP figures, wheels or flywheel??
Discussion
G20RG B said:
When manufacturers quote BHP figures do they take the reading from the rear/front wheels or does it come from the flywheel as there is quiet a difference between the 2 measurements.
depends entirely on manufacturer, I witnessed an XFR on the Dyno at Surrey Rolling Road recently and that made 30bhp over the book figure 
and then a VXR8 N/a made 4bhp under book figure so it depends what they issue in the books, Italians and TVR use to be optimistic 
..... and before the Dyno comes into question it happens to highly accurate as i'm sure a few othe PHers can classify. Germans seem to be pretty spot on with power figures tho an E60 M5 made 505bhp I think and book is 507 ?????
jagracer said:
lankybob said:
Flywheel I believe. I also think that the transmissions in most cars do have quite an effect on the reading. However I could just be making this up.
You're not making it up, the drive train on my car saps around 45-50bhp.Jagracer you can't really put a standardised figure on all drivetrains but the thoery is correct
wackojacko said:
jagracer said:
lankybob said:
Flywheel I believe. I also think that the transmissions in most cars do have quite an effect on the reading. However I could just be making this up.
You're not making it up, the drive train on my car saps around 45-50bhp.Jagracer you can't really put a standardised figure on all drivetrains but the thoery is correct
jagracer said:
EDLT said:
Why are drivetrain losses (generally) measured as a percentage and not an absolute figure?
I've not heard of them being measured as a percentage, they always give me a specific figure.kambites said:
It always amazes me that a 40-50bhp (roughly about 30-40kw) power sap can be cooled effectively. That's like having 15 fan heaters in your gearbox. 
you should try fitting temp senders to your gearbox then...also, remember that 'peak' losses may be 30Kw, but how often is the gearbox at peak load?
jagracer said:
EDLT said:
Why are drivetrain losses (generally) measured as a percentage and not an absolute figure?
I've not heard of them being measured as a percentage, they always give me a specific figure.gearbox looses are never going to be X Bhp, it will always be closer to a % figure.
think about it, if it was X BHP, then you would never be able to turn a gearbox by hand!
other point is that rolling roads have to account for total drive-train looses, so that's gearbox/final drive/CV's/wheel bearings/etc and worst of all, tyre losses.
Realistically, tyres alone can exceed the total losses of the rest of the drivetain.
No manufacture (as in mainstream stuff) will ever use 'at the wheels' figures as it's totally meaningless.
jsg612 said:
The figure given is what the engine produces and not what the car is rated at.
But....do all manufactures use the same formula, I guess they would use engine BHP as this will produce a higher figure than at the wheels,Is it because you have a better chance of getting an accurate figure at the engine or is it the wheels?? still confused.If someone tells me there car is producing 300BHP can I believe him?? Surely the wheels is where it should be measured.
G20RG B said:
jsg612 said:
The figure given is what the engine produces and not what the car is rated at.
But....do all manufactures use the same formula, I guess they would use engine BHP as this will produce a higher figure than at the wheels,Is it because you have a better chance of getting an accurate figure at the engine or is it the wheels?? still confused.If someone tells me there car is producing 300BHP can I believe him?? Surely the wheels is where it should be measured.
there are ISO standards for how BHP should be measured and presented, and it does not involve nipping down to your local rolling road!
manufactures spend millions on engine design/build, part of this will be setting performance targets (of which power will be one) and then testing them to death to verify that they are achieving the targets.
apart from that, it would be illegal for them to put out BHP figures that were bulls
t.Power at the wheels is only ever used by after-market tuners, it's come from Americans, and it's invariably total bulls
t.Next time you go to a rolling road, as to see their dyno's calibration certificate....
Scuffers said:
kambites said:
It always amazes me that a 40-50bhp (roughly about 30-40kw) power sap can be cooled effectively. That's like having 15 fan heaters in your gearbox.
you should try fitting temp senders to your gearbox then...also, remember that 'peak' losses may be 30Kw, but how often is the gearbox at peak load?
jagracer said:
EDLT said:
Why are drivetrain losses (generally) measured as a percentage and not an absolute figure?
I've not heard of them being measured as a percentage, they always give me a specific figure.gearbox looses are never going to be X Bhp, it will always be closer to a % figure.
think about it, if it was X BHP, then you would never be able to turn a gearbox by hand!
other point is that rolling roads have to account for total drive-train looses, so that's gearbox/final drive/CV's/wheel bearings/etc and worst of all, tyre losses.
Realistically, tyres alone can exceed the total losses of the rest of the drivetain.
No manufacture (as in mainstream stuff) will ever use 'at the wheels' figures as it's totally meaningless.
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