Transmission losses - Mk1 Gearbox?

[crazylegs]

you have lost me a bit here.can someone tell me how they calculate the flywheel figure.it must be based on some sort of fact it cant just be a guess can it?

[Icsunonove]

Crazylegs,

I was kind of hoping that this thread would die (since I appear to have upset some people) but since you asked I will answer the question. I simply don't want anyone to post a reply that is completely wrong.

The flywheel figure is guestimated by using what is known as a "coast down" test. Unfortunately this "coast down" test has been proved by a number of sources to be wildly inaccurate. To be honest with you it is utterly worthless.

To give you an idea, total transfer losses (gearbox, tyres, everything) for an average transverse gearbox is around 15%. Another very rough way of calculating this is (for engines producing up to about 200bhp): Flywheel horsepower = Rear wheel horsepower +10 bhp / 0.9.

I've found a great example of what I am talking about...

..........The XR2i is FWD of course so if we apply those two rules to 108.5 flywheel bhp we get either 92 or 88 bhp at the wheels respectively. So that's the sort of level of wheel bhp that one would be expecting if the quoted flywheel bhp is correct.

To run the test, the car was warmed up and given a couple of runs on the rollers to stabilize the temperature of the tyres, gearbox oil and engine. A power run and a coastdown were then done in each of 3rd, 4th and 5th gear with a few minutes for the car to cool down between each run to keep the figures consistent. So first let's look at the how the wheel bhp changed in each gear. The figures are as follows:

3rd gear - 95 bhp at the wheels
4th gear - 92 bhp at the wheels
5th gear - 88 bhp at the wheels

So why do the figures show a drop in power as a higher gear is used? The engine of course is producing exactly the same flywheel power regardless of which gear the car is in - what is changing here is the real transmission and tyre losses. A higher gear means that the tyre speed on the rollers goes up too - this leads to more power being absorbed as heat and friction - the measured wheel bhp therefore goes down a bit. There are other factors at work here too but it is not the aim of this particular article to go into all of these in depth. The key thing is that the figures show a reasonable and predictable trend and are in the estimated bhp range calculated above.

It makes the point though that there is no such thing as just one true wheel bhp for a given car on a given set of rollers - it depends on tyre pressure, gear ratio and a host of other things that have already been covered in previous articles. Now devotees of the "coastdown loss" system would say that it should compensate for this - it should reflect the larger losses in higher gears by showing a larger coastdown loss which when added back to the wheel bhp ought to give a flywheel bhp that stays the same in each gear. So let's now look at the coastdown losses that were measured on each of those runs and see if they actually do what is claimed. The coastdown losses were as follows:

3rd gear - 17 bhp coastdown loss
4th gear - 27 bhp coastdown loss
5th gear - 44 bhp coastdown loss

We can add those losses back to the wheel bhp to get the estimated flywheel bhp that so many rolling roads these days quote you.

3rd gear - 95 + 17 = 112 bhp
4th gear - 92 + 27 = 119 bhp
5th gear - 88 + 44 = 132 bhp

Well clearly something isn't working here. The coastdown losses (whatever it is that they are actually measuring) are rising much more in a higher gear than the actual transmission losses are, leading to larger "flywheel" bhp figures in the higher gears. The engine is producing the same power all the time and.........

Hope this thread dies now......

[shish]

i think the point lauren was making here is that a rwhp figure is not accurate because it not the same as the rwhp that you will see ON THE ROAD, due to the fact that a rolling road will have 4 contact areas on the two tyres where the road will have only two. Given you might also need a couple of people sat in the boot to stop wheel slip you also have extra weight/friction that you would not normally have on the road. Tyre pressures will also affect it. The gear the run is done in will add another vairable, a run in 3rd will have less losses than a run in 4th for example.

this extra friction is born out in the transmission loss / coast down figure seeming higher than we'd expect, by working back and calculating a flywheel figure your throwing away that error to see a somewhat accurate figure of flywheel hp.

[superchargedsam]

from reading all these answers its quite obvious the onlt real figure that means anything is the flywheel figure that is calculated with the engine on a test bench as that will give you a very accurate reading for what power the engine is producing and not what the rest of the car is wasting ! With power figurs or RRs there are just far to many variables for accurate and real world figures to be used but its fine for pub braging I guess, at the end of the day take it to a track thrash it and enjoy yourself as thats what its all about ! At 17 I had a 60bhp bike that ate all hot hatches off the line, in a straight line until about 130 and had virtually all 2 wheel drive cars on the twisties (apart from an amziningly driven R5GTT) so its horses for courses, the MK1 NA is about 120-130bhp but overweight by about 150kilos so lose the wieght free up the power, dont waste money on engine mods when you can spend it on brakes and save yourself a few seconds per lap by outbraking everyone else ! I rekon I lose as much bhp though the transmission on my celica GT4 as some MR2s put out at the wheels so imagine freeing up that power mmm mmm mmm, aint gonna happen though !

Tom I also did the calorific fuel values experiment for my physics A level using a calorimeter and that gave some interesting results (especially trying to get diesel to combust in the classroom environment!).