Transmission Loss

[SPGTi 2]

JUst to add my bit. I always use to be a purveyor of the only true reading is at the wheels, but then I got involved with some fairly powerful RWD machinery. Running them up on the dyno was always interesting and made me see that ATW figures could be more easily manipulated than ATF figures.

With the RWD cars we always did a std power run and then get as many people as possible to sit in the boot to try and prevent any loss of traction. As soon as the boot was loaded the ATW figure always dropped but the ATF figure remained pretty constant unless there had been a traction problem and then the ATF figure increased. To me this showed that the method of measuring / estimating the losses was pretty good and could provide repeatable ATF figures whereas the ATW figures never gave the full story.

I also saw a yank tank with a 3 speed auto box on the dyno, this showed massive losses between wheel and fly figures. The engine came back in a different car with a manual gearbox (no changes to engine spec) and showed increased ATW figures but the ATF were within a few %.

 

Steve

[Batfink 201]

I always just took the wheels figure then applied DB's formula for transmission loss. Put my mi16 at 173bhp at the fly (146bhp at wheels) which I think is a fair estimate for a standard engine on TB's. I'd be embarassed if I was getting over 20% losses on fwd.

To be honest power didnt matter when it flew past virtually everything on the road at the time - which was nice.

[kaich 6 1 Cars]

Generally speaking, a 205 GTI will lose 21% through its transmission on stanard gearbox oil, and 16% if it is using synthetic oil.

 

so wich kind of oil do you use for gearbox ?

[snillet 1]

so wich kind of oil do you use for gearbox ?

 

I was wondering that to. If the gearbox is nice and warm ,does the synthetic oil really give you that much less friction ?

[taffycrook 2]

I always just took the wheels figure then applied DB's formula for transmission loss. Put my mi16 at 173bhp at the fly (146bhp at wheels) which I think is a fair estimate for a standard engine on TB's. I'd be embarassed if I was getting over 20% losses on fwd.

To be honest power didnt matter when it flew past virtually everything on the road at the time - which was nice.

 

DB's article has miss informed so many people about rolling roads, some of what is said is good solid advice but some of it is plain wrong.

I would agree that a mi16 on TB's is going to make around 175-185 bhp.

But it is overly simplistic to use the formula in the article, what gear was the 146 bhp produced done in, was any weight added with straps/bodies what size wheels and tyre pressures etc etc.

 

Over 20% seems alot when you first look at it, but how can it be expressed as a %?

If a 8v engine ran on the same rollers under the same conditions the losses would be similar, a figure of around 30 bhp @ 120 mph flat out in 4th.

But the % would be 23% and yours 17% 30/130 x 100 = 23 against 173/30 x 100 = 17.

yet both cars have to compress the tyres the same amount with similar gearboxes and in this case the same wheels and tyres. Same weight within reason (mi16 is heaiver).

 

Quite a few of the mi16 conversions i have been involved with have had before and after power runs.

The losses are pretty close as long as the max speeds are within a few mph of each other.

 

Look forward to someone squaring this circle.

[Sandy 191]

And here it comes

 

I don't think a percentage covers it accurately, it's an estimate as I've said all along and I'm inclined to agree with Dave Baker's thumb rule formula, that there's a constant element and a variable element to the loss. What you appear to be saying Taffycrook (if that is your real name) is that you get around 30bhp loss at a given speed (120mph in your example) regardless of the total power output. By doing so, you are collectively estimating the sum fixed and variable losses as a fixed loss, which is no more right or wrong than using a percentage, is still an estimate and not a true measurement as I said before.

 

The logic for this is that if you spin the drivetrain up 120mph unloaded, it will produce a given drag - fixed loss. If you then apply a torque load at the same speed, the drag will be increased - variable loss. The transmission itself produces a fairly consistent loss under load, since it's suffers little flex and most of the bearings and thrust surfaces will only see a relatively minor increase in drag under load. The tyres however will roll happily along under little load without getting hot; but under load, the distortion they suffer produces a great deal of heat, which is power being converted to heat as a loss. That distortion and hence loss varies significantly with load.

[John_B 11]

Taffycrook (if that is your real name)

 

Cruel parents!

[saveloy 8]

so wich kind of oil do you use for gearbox ?

 

I use Morris fully synthetic 75/80 oil. Although something similar from Mobil, Redline, Amsoil, Silkolene would be fine. Try to keep to the standard numbers. Most companies will do only a 75/90 grade though.

I used the same rolling road before and after. The 5% was my experience.

[taffycrook 2]

And here it comes

 

I don't think a percentage covers it accurately, it's an estimate as I've said all along and I'm inclined to agree with Dave Baker's thumb rule formula, that there's a constant element and a variable element to the loss. What you appear to be saying Taffycrook (if that is your real name) is that you get around 30bhp loss at a given speed (120mph in your example) regardless of the total power output. By doing so, you are collectively estimating the sum fixed and variable losses as a fixed loss, which is no more right or wrong than using a percentage, is still an estimate and not a true measurement as I said before.

 

The logic for this is that if you spin the drivetrain up 120mph unloaded, it will produce a given drag - fixed loss. If you then apply a torque load at the same speed, the drag will be increased - variable loss. The transmission itself produces a fairly consistent loss under load, since it's suffers little flex and most of the bearings and thrust surfaces will only see a relatively minor increase in drag under load. The tyres however will roll happily along under little load without getting hot; but under load, the distortion they suffer produces a great deal of heat, which is power being converted to heat as a loss. That distortion and hence loss varies significantly with load.

 

No its not my real name.

I have not claimed that the flywheel power is what is measured on the chassis dyno, I am saying the losses are measured as best they can with the equipment available.

We now agree that the loss in the transmission is fairly consistant, the increase due to load is not enough to effect the figures too much.

The load on the tyres is where it all gets very complex. This is why tyre pressures have such a big effect.

Once again it is subjective, but at 120 mph its not heat that is the worry its the force required to deform the tyre many times a second as it comes into contact with not one but two surfaces. (in the case of a dyno with two rollers) This is where the effect of strapping the car down reduces the tendancy of the car to climb out of the rollers on and off load, this reduces the effects of the torque reaction which varies the load on the tyres from the shift in the centre of the mass.

So this tyre distortion varies most with speed.

 

This is why if you only look at the figure at the wheels and apply any sort of formula or % you could be fooled into seeing something that is just not there.

 

As an example a tuner has promised a gain in power output, he runs the car up in 4th gear and has 100 bhp ATW.

After his fettling he runs the car up again but has 100 bhp ATW, now he can make this look better than it is by

I, increasing the tyre pressures by 20psi (less deforming of the tyres) [105 bhp]

II, running in a lower gear say 3rd (less deforming of the tyres) [110 bhp]

 

In both cases the ATW figure will be slightly higher as the losses will be lower. If these losses were measured the power which for the want of a better expression is called the flywheel figure will remain constant. [130 bhp]