Rolling road used in my case was a Bosch model which indeed gives rwhp also but the flywheel figures are the ones printed out.In my case the rear wheel figures are unfortunately out of memory as it hold only 10 runs and these are since long passed since my dyno.

 

But some interesting facts anyway:

-the transmission losses, measured after the run ,with the clutch depressed are adapted if the power is increased.

 

Indeed , as a stock car with say 100 hp is tuned to 150 hp some 15% or 20 hp are said to be transmission losses.

If that car is tuned again to 200 hp then it would be wrong to assume the losses still to be 20 hp ; BUT this is what is measured as the clutch is depressed , always this same value regardless if the car has 100 or 300 hp.

So, an advanced rolling road adapts these figures to the increasing power output so that the initial 20 hp loss will be increased by a factor so to still be proportional with the increased output, and this figure is added to the rwhp to give the flywheel-hp on the printout sheet .

 

 

Some general noteworthy facts

-always measure in the same gear

Most boxes have a ratio in fourth at say 1.3 to 1 and 0.95 to 1 in fifth.

The power output will be overstated in 4th and understated in 5th gear.

eg 100hp measured will be 100 x 1.3= 130 hp in fourth gear and 100 x 095 = 95 in fifth.

-The friction in the gearbox is greater as you go further away from the 1/1 ratio

-the buggest source of transmission loss are the tires; about 50%!

-each driven gear absorbs 1 to 2% engine power.

-measured rwhp can depend on tire inflation , tire size and suspension angles (!!)

 

I agree that what counts is, what you measure at the rear wheels.

 

So I will insit on having the raw rwhp number next time and knowing in which gear it is obtained.

 

 

cheers

rik

I would just like to comment on one thing you say.

 

I think you'll find that most 5sp manual gearboxes, have 4th as direct i.e. 1:1 and 5th is invariably an overdrive (which you correctly state) 0.752 in a Zed btw.

 

In the auto, top is a direct ratio, whilst O/D is 0.694.

 

I think that some of the Autos on our RR day were tested in direct top.

 

Tim

I thought that (probably wrongly) that the reason you had higher transmission losses recorded on the hotter but otherwise similar car was because the higher output usually comes at higher rpm and the higher you go in rpm the more the transmission losses pile up. When you declutch after the rpm where the power begins to fall off the rr then measures the rate of deceleration from that rpm and so figures the higher transmission losses.

 

So I reckon you still need to know the rpm the losses are at for comparisons of transmissions.

Cheers Rik. Interesting reading.

 

Especially the bit about tyres giving 50% of the loss. My head is playing with that one - I'm sure there's an idea hiding in there somewhere !

 

Dave

Originally posted by Hairsy

Cheers Rik. Interesting reading.

 

Especially the bit about tyres giving 50% of the loss. My head is playing with that one - I'm sure there's an idea hiding in there somewhere !

 

Dave

 

Nah Dave. It'll handle crap when you take your tyres off it!

But I drive a 4x4 which doesn't go round corners anyway so nothing to lose.