bhp at the wheels

[Mike1405]

Hi all,

Just picked up my LCR today, an onvi yellow 53 plate and i love it! The previous owner supplied a print out after a remap of 229bhp at the wheels, what would that be at the flywheel do you think? 250bhp+?

Thanks.

 

[adam cupra 20vt]

Thats 265.5 mate

 

[Mike1405]

Cheers mate much appreciated, bit more than I thought! Is there a formula to work it out or something?

 

[adam cupra 20vt]

For front wheel drive cars you add 10 bhp to the wheel figure and divide the result by 0.9

 

[rickyjones1209]

not sure about that....

mine is 238 at the wheels and 265 at the fly. i lose 27bhp through transmission loss. i'd expect each car to be different but not by too much surely??

 

[adam cupra 20vt]

Thats the way I was told to work it out but if I'm wrong someone please correct me

 

[AndyHazza]

Oh dear... i can see this opening up a huge can of worms lol good old power figures hey!

Form our experience with most VAG 1.8T's etc the front wheel drive cars never tend to loose more than 15-20bhp through the transmission. So (as a very rough guess) i'd say its probably 245 to 250 (ish). The only way to tell truely what it is to have it on a rolling road that physically measures the flywheel figure by measuring the resistance of the drive train on the coast down.

 

[rickyjones1209]

not sure about that....

mine is 238 at the wheels and 265 at the fly. i lose 27bhp through transmission loss. i'd expect each car to be different but not by too much surely??

 

[adam cupra 20vt]

How do you get a true transmission loss on run down because obviously the cars going to be in neutral or the clutch is down so the drive train is not even connected at that point?

There's going to be friction obviously but not as much as when it was under load in gear?

I guess the only way to work out the losses properly is to do a run on a normal engine dyno and another run on a chassis dyno then take one away from the other?

Its been years since I went to college learning about this stuff. lol

 

[lordgibbness]

How do you get a true transmission loss on run down because obviously the cars going to be in neutral or the clutch is down so the drive train is not even connected at that point?

You would usually leave it in gear for the run down to be calculated, but I don't believe this is entirely accurate either.

The calculation for losses in the drivetrain is usually expressed as a percentage - not as a fixed figure, which is why it would be different for every car. For FWD cars the losses are usually said to be about 15%.

E.g.
If you have 260 at the flywheel this would be 221 at the wheels ( flywheel * 0.85 )
If you have 200 at the wheels this would be 236 at the flywheel ( wheels * 1.18)

This is the generally accepted method for calculating losses although some rolling roads are setup in all sorts of ways. For example my car recorded 211 at the wheels and calculated 281 at the flywheel (which would be a 25% loss). It was probably setup for a 4WD drivetrain.

 

[MarkE]

This thread is going to deteriorate rapidly.

 

[lordgibbness]

This thread is going to deteriorate rapidly.

Perhaps, but just thought I'd add the common way that losses are calculated for FWD cars. The main issue is actually how accurate and how recently calibrated the dyno is. In the case of the one I used I don't think it had been...

 

[adam cupra 20vt]

You would usually leave it in gear for the run down to be calculated, but I don't believe this is entirely accurate either.

No your right it can't be accurate and it wouldn't matter if it was left in gear or not if the clutch was down as it would not be connected....although there obviously would still be the resistance of the tyres and transmission.

Tyre pressures also make a difference.

And don't forget most places can input there own percentages for losses so they can basically give you what they want you to have [not saying they do]

 

[lordgibbness]

I think I'm missing something here - why must the clutch be disengaged?

 

[adam cupra 20vt]

Yeah silly me....Your right it can be left in gear on the rollers I guess?

The places that used to roll my older cars always used to put it in neutral....I think a lot of places do this? its always been done on my older cars.

 

[Rocky Rocastle]

This thread is going to deteriorate rapidly.

 

[ibizacupra]

Oh dear... i can see this opening up a huge can of worms lol good old power figures hey!

Form our experience with most VAG 1.8T's etc the front wheel drive cars never tend to loose more than 15-20bhp through the transmission. So (as a very rough guess) i'd say its probably 245 to 250 (ish). The only way to tell truely what it is to have it on a rolling road that physically measures the flywheel figure by measuring the resistance of the drive train on the coast down.

except coastdown is not measuring losses of a transmission underload tho as it was during the power run

it is again another aproximation and another factor

 

[dirk1978]

More reasons why RR figures can never be relied upon.

Best way to get an accurate RR figure is to use somewhere that has a good reputation for accuracy.

 

[cocadave]

More reasons why RR figures can never be relied upon.

Best way to get an accurate RR figure is to use somewhere that has a good reputation for accuracy.

How does somewhere get a good reputation for accuracy if there isn't anywhere to get it clarified

 

[lordgibbness]

It's a little like the wheel balancing machines - they need to be accurate systems to start with, but then calibrated often.