Power losses through transmission and tyres.

[danny69]

When reading through power outputs and then comparing the "at the wheels" figure to the flywheel figure it seems there can be large variations between different cars.

For instance, mine is losing almost 55 bhp thrugh the transmission whereas as higher spec race car with 230 BHP was only losing 25 bhp.

 

Seeing as every horse power becomes increasingly more expensive to find it would seem freeing up what is there and delivering to the road with less tranmission losses would be worthwhile.

 

Now the race car had a straight cut gear box, but is that the whole story or are there other ways of seeing more power at the wheels?

 

Interestingly the guy who tested mine on the rollers thought that a large amount of the losses come through the tyres, but I can`t get my head around that really, as surley that is where you would want there to be friction.

 

Also I think I may have one of the rear brakes adjusted out a click too far so this would lose a bit if the drums were rubbing.

 

So where else can I look, one piece props? friction reducing additives in the gearbox?

 

Your thoughts please.

[Vista]

55bhp sounds like quite a lot but it does vary from car to car (and roller to roller)

 

Straight cut gearing is more efficient than helical gearing as there is no end thrust to deal with. End thrust necessitates thrust pads / washers and these generate heat, hence the lesser efficiency than straight cut boxes.

 

Yes friction reducing additives will help, and I guess what he means by loss through tyres is that lower quality tyres will slip rather than grip, this will also lose power between the flywheel and the road.

[Wessie]

like vista says a straight cut box helps, but the racecars probably got a higher quality clutch ? better quality diff? diff bearings? all those will lose power if there not in good conditon, also if the racecars running light wheels then thats a fair mass that it doesnt have turn like you say running a one piece properly balanced prop would probably help. alos making sure the tyre pressures are correct will help

every little helps - good luck on the quest for power

[danny69]

Cheers for the input so far

I have got new rear wheel axle bearings, fresh correct grade oils, light alloy wheels (revos) , tyres pressures were 28 lb in each rear.

 

The clutch shouldn`t come into it should it? Unless it was slipping?

 

But I will make sure the brakes aren`t dragging.

 

Surely a grippier tyre would show more losses as there would be more friction?

 

My propshaft is slightly longer than it need be, it is quite close to the rear gearbox oil seal, hopefully it isn`t binding up.

I`ll have to loosen the centre bolts and make sure there is still some free play on the gearbox splines.

[Wessie]

im usnure on the tyre situation, someone with more knowledge would be best to advise you on that we've run most of our cars on allsorts of tyres slicks, road tyres etc when we RR them, but we've never really done power runs, its only for setting the engines up.

The better clutch in race/rally cars is mainly for piece of mind, when launching hard and dragging the clutch when pulling away (clutching round corners aswel) all helps to kill clutches on rally cars !

i dont know what the average transmission losses are on road cars so im probably not the best person to advise ! but once the brakes, (possibly the prop as you say) are sorted you should see an improvement

also when was the gearbox last rebuilt?

and remember every rolling road is different

[danny69]

Yeah, the gear box is an unknown quantity. I have refreshed the oil but beyond that I don`t know, so it is likely that that is the main loss.

But an expensive one to improve and I wouldn`t want a straight cut gear box on the road anyway.

Your right about all rollers being different, but I wonder if the losses when shown as a percentage would also vary.

In theory they shouldn`t but I guess that depends on how they are calculated.

[bortaf]

Power loss is a percentage of the power made so the higher the power the more you loose, 55 BHP is a lot if your running a pinto but not if your running a 250 BHP V8.

Lasst RR run i had i lost 25 ish BHP on a pinto and then got it down to 20 by fixing a rear wheel bearing, flat/low pressure or high sidewall tyres will loose power as they deform and absorb some of the power, thats why over pumping the rear tyres can give a higher reading

[danny69]

I lost 55 bhp out 181 bhp , my tyres are 205 60`s.

Interesting what you say about higher pressures giving a higer figure on the rollers, but I guess the corrected figure would be the same, it is just that there is less friction on the harder tyre.

[Toby]

Power loss is a percentage of the power made so the higher the power the more you loose, 55 BHP is a lot if your running a pinto but not if your running a 250 BHP V8.

Lasst RR run i had i lost 25 ish BHP on a pinto and then got it down to 20 by fixing a rear wheel bearing, flat/low pressure or high sidewall tyres will loose power as they deform and absorb some of the power, thats why over pumping the rear tyres can give a higher reading

 

This is a sound answer - although I'm not sure about losing 55hp out of 250

 

Your brakes must be rubbing or something else is wrong or the RR is giving a "generous" tranmission loss.

 

I would have thought your car would pull up pretty fast in neutral if it had 30hp losses in the brakes (guessing there's 25 in the transmission.) That's like a 850c mini pulling in the opposite direction!!

 

You do want reasonable tyre pressure (and the same every visit) as the distorting side wall of the tyre will absorb power. (28 psi should be ample but the Dyno Dynamics recommend 50psi)

 

If it's highlighted a problem then that's good - IMO rolling roads are mainly for comparing modifications and cant really be used universally around the country to compare engines as they all seem to read a bit different. Keep going back to the same one if you're happy with the service.

[RUSTYPUP]

That's like a 850c mini pulling in the opposite direction!!

 

:

 

I'm not sure you'd notice that

[Toby]

That's like a 850c mini pulling in the opposite direction!!

 

I'm not sure you'd notice that

 

Don't be nasty to the little ones!!