Transmission Losses

[Guest mcramsay]

I recently had the zero on a rolling road and the figures were as follows

Peak bhp 168.8

Wheel bhp 125

Max torque 145.5 lbf/ft

 

It seems I'm loosing 44 bhp through the transmission It seems higher than it should be? Is there any thing that could cause this? Or is it normal to loose that much in the transmission?

[Longboarder]

Coastdown figures represent something, not quite sure what. They are made up of losses in box, diff, prop joints, driveshaft joints, wheel bearings and tyres. Largest factor is probably tyres, (compound, pressures and slip). They vary in different gears and wether the operator coasts in neutral or still ingear but clutch down. They also measure the losses when you drive the tyres and transmission components from the wrong end. (Anyone believe the losses in a gearbox or diff are the same whether you drive the input or the output shaft?) Coast down figures represent something and it may or may not be related to the transmission losses when the engine is doing the driving. The flywheel figure is often farcical with all sorts of corrections made to improve it. To start believing the greater difference is likely to mean mechanical problems is a few steps too far. It's more likely to represent the difference between the wheels measurement and the enthusiasm of the operator.

As an example at the rolling road session at MB Motorsports a few months ago the following coastdown percentages were recorded. 34%, 32%, 41%, 40%(bike engine single wheel drive), 36%, 25%, 26%, 19%, 30%(bike engine), 25%(mine), 17.5%(MB car). As most of the cars were pinto/zetec T9/ MT75 rwd a spread of 17.5% to 41% seems unlikely. There is a tendency for higher power cars to have lower percentage coastdown losses which seems correct. All the different tyres and pressures must be in there somewhere but I feel unconvinced about the whole concept of flywheel figures except when measured on a dyno.

My conclusion is that the figure 168.8 is suspect. I doubt you are missing 44bhp.

 

Nigel

[Snapperpaul]

Tend to agree with Nigel

168 is a projection 125 is an actual

We all like the bragging rights of flywheel BHP

But in reality if you car weighs the same as your mates then wheel bhp is the figure you can trust

 

And don't forget if your mate is 6 foot 10 and 150Kg and you 5 foot 4 and 50Kg that's the difference between a CEC & gearbox and a BEC

[knights_templar]

must agree with above.

 

RR compare run up to run down times at a known load to calculate BHP.

 

if you wan't to know a true engine figure you need a dino.

 

RR is OK for camparrison of vehicles on the same RR and reasonably accutate between RR, unless there is some fiddling, eg applying handbrake on run down.

 

As Nigel says better bragging rights, who can say different, you have a fancy, unintelagable, graph to prove it

[richardm6994]

IMHO flywheel bhp being calculated from wheel bhp is about as accurate and helpful as Mystic Meg!!! It's just something that the rolling road company gives you to make you feel better after hearing your wheel bhp!

 

As what Nigel has pointed out, the only flywheel bhp figures to believe are those measured on a dyno....

 

And at the end of the day, wheel BHP is the only thing that's going to matter in the real world!

[Guest Rallyreason]

The WHP does look to be low for what you have.

The best why is for you to compare the cars performance with a similar car. Obviously the track would be ideal due to nothing coming the opposite way etc.

 

My Zetec powered Zero running ZX9 carbs and mega jolt shown 164WHP on a RR session. These figures seemed a little high, but i know that it holds its own with other kits running throttle bodies and higher BHP. Whats most important is it makes me smile as much as anyone else.

[kevin the chicken]

I did quite a bit of reading after I had my Redtop Series 7 put on the rolling road and it does seem that horsepower figures can vary quite a lot depending on various factors. I wasn't really too bothered about horsepower readout, being more concerned that the damn thing held together but I thought it might have been a bit higher than 151bhp. I did notice the chap readjusting the straps a couple of times and spraying some jollop on the rollers because of wheel slip/spin. Also I hadn't thought to inflate the tyres up to what they would run at on a normal car, I have been running them at 18psi and this can apparently make the difference of several horsepower due to the flex of the rubber. So next time I'll put some more air in and see what difference it makes. As the previous post says smiles are more important than horsepower and it gives me plenty of those.

[Guest mcramsay]

It does 0-60 in around 4.5 seconds so wether it's got 120 bhp at the wheels or 170 it's still bloody quick, and I'm guessing if I had some major transmission issue then it wouldn't be as quick as it is

[agent_zed]

well assuming it is around 500kg you have around (2x 125bhp) 250bhp/ton which is more than a Porsche Boxster S 3.4L - [2012] for example, so can't be bad. And that is going off your WHP which i am assuming the porsche is going off the higher BHP figure. 340bhp/ton (2 x 168bhp).Then you give Porsche 911 GT3 4.0 997 - [2011] a run for its money. assuming i did my maths right

 

 

http://www.autosnout.com/Cars-Bhp-Per-Ton-List.php

 

either way 4.5 secs says something is working.