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 Post subject: Re: Transmission Losses
PostPosted: Fri Mar 15, 2013 5:22 pm 
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IMO the most meaningful figure is wheel horsepower. This is what's put down onto the tarmac and should correlate directly with accelleration.

The main factor which will affect WHP relative to 'hubs' is the tyre losses. This is where running prius tyres pumped up to 80PSI will gain a good few BHP. Again, doing ths would also make the car faster on the road (but not round corners :lol: ) so is a true comparison.

A dyno should be calibrated to read correctly the power being fed into the rollers. This IS the WHP so should be accurate.

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 Post subject: Re: Transmission Losses
PostPosted: Fri Mar 15, 2013 5:56 pm 
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Treacle wrote:
Darryl - read Diceman's post above and see if you still think that. The at the wheels figures will be different on different dynos as the losses that are calculated also include the losses of the dyno itself etc. so at the wheels figures are also meaningless for comparison unless try are done on the same dyno.



Edd, my engineering science project was on power lost in various stages of gearboxes, drive shafts and chain & sprockets.

Go read some simple physics stuff, and then you will realise all these dynos are using ... F=ma namely the drum / rollers the car is resting on. And a simple counter of its rpm. Throw in time and you've got acceleration. The drum is a known weight. The rest is easy, use a few know sae correction factors and you are good to go.

The dyno run the other day ( somewhere near bristol wasn't it ? ) first thing wrong with the runs that day...... Phil's chart showed 20 degree C temp. Wasn't everyone saying how cold it was in that room that day ?

Irrespective of whatever tyre pressure was used in each car that day, or how heavy the car was, the drum was measured nothing else.... The force to speed up that drum at the actual rate achieved on each dyno run for each car gives you the force available at the wheels. Ie. The last (lowest ) value left of what the engine produced from its crankshaft is what was used to turn the drum. Assuming the drums bearing system didn't degrade over the course of the day. The wheel figures are fact. Now maybe the dyno software "adjusted" the measured power shown on the screen or prinout, but from one car to another it shouldn't of changed.

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 Post subject: Re: Transmission Losses
PostPosted: Fri Mar 15, 2013 6:10 pm 
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You're at the mercy of dyno operators with all of them though as most kit has correction factors for temp, pressure, owners desire to hit x bhp, likelihood of an axe wielding incident is said target isn't reached etc

I agree with nibbles that wheel HP is ultimately the most useful even if it's prone to inaccuracy
In some ways the same is true of 1/4 mile times. They are inherently free from fiddle factors so a competent driver should see decreasing e.t with increasing HP. In fact over the years e.t -> bhp calculators have proved remarkably accurate due to the fact that torque curves are by and large flatish and it's torque that creates acceleration


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 Post subject: Re: Transmission Losses
PostPosted: Sat Mar 16, 2013 1:48 am 
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The strange one with mine was that there was no driver or owner interference, I rang saying my cars done 134,000 miles ( at the time) can you stick in on the rr and see what its doing at the fueling at full boost...

So there was no intention of a full our power run, more a hows my car doing as I wasnt actually even in the cell the first runs where it hit that power they claimed...

But the wheel figure was as above... I don't take any of the numbers as gospel but it was a nice figure to see and it certainly feels fast to me ( and smokes BMW m3's with ease) so I just live happily knowing i'm happy with the old girl


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 Post subject: Re: Transmission Losses
PostPosted: Sat Mar 16, 2013 8:36 am 
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the idea behind the dyno dynamics broad brush 25% correction form drive train loss is primarily to speed up measurements I believe but it should also mean that all will read the same regardless of whose dyno you use.


the operator can however fudge the ambient correction factors.


as a guide however when my car was mapped onndyno dynamics rollers I made 269bhp. at wheels at around 1.25 bar iirc, the dyno owner suggested the 25% flywheel correction is too high and estimated actual fly power at 335-345 bhp

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 Post subject: Re: Transmission Losses
PostPosted: Sat Mar 16, 2013 1:50 pm 
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or, lets look at this from the other side ( laws of physics way )

lets say Toyota really know what the engine produces in power at the crank ( I think this is a fairly safe bet )
and lets take a standard uk ST185 ( with about 204bhp ) and a known top speed of just a fraction over 140mph
('ve seen 140 to 142mph quoted)

now i'm going to suggest rolling resistance is proportional to speed, and i'm going to give it a figure of :-
0.015 * vehicle weight (lbs) * mph. so that just increases with speed, and for a ST185 at approx 3500 lbs, means we loose nearly 10bhp at 70mph.
( 0.015*3500*70 = 3675, now that is lbs of force, so divide by 375 to get HP, 3675 / 375 = 9.8 bhp really, lets call that 10bhp )


now the other loss is aero drag, and we have some known values, namely the frontal area of the vehicle 21.sq.ft
and we will go with a published drag factor of 0.35
now drag HP is expressed as ( Cd * area * speed^3 ) / 150000
which at 70mph comes out as 16.8 HP


lets do the sums at 140mph now.
(0.015 * 3500 * 140) / 375 = 19.6bhp wow, rolling losses of sub 20HP

for aero,
(0.35 * 21 * 140^3 ) / 150000 = 134.456bhp, lost to aero drag.

add the two figures together and we get a smidgen over 154bhp.

so the starting 204bhp ( toyota figure ) take our of laws of physics power lost ( I know i've thrown in a 1.5% of vehicle weight, but its close, really close to what you will measure by using four tyres on a road surface for a road tyre, use solid wheels and things change a bit ) that 154bhp lost, means we have somewhere lost 50bhp.
Or in fact we haven't used all the power that is at the wheels, and hence the vehicle can use the spare power to accelerate to a higher speed. Eventually, however the spare power wont be enough to reach a higher speed, and then the car wont accelerate any more.
in the case of a ST185, its 142mph, where the rolling & aero drage amount to 160bhp or so, thus the power lost in the journey from crank to the wheels is 44bhp.

ie. the gt4 transmission ( from crank end ( dyno-ed by toyota ) to the power at the wheels shows 44bhp lost somewhere...
now it (the power lost) will vary slightly from the input power to the tranmission to the power at the wheels, but only some is sideway loading on gears lost to side thrust, the support bearings on the gearbox shafts, the bearings in the diffs, the prop supports, they are supporting the shafts at 90 degrees to the movemnt, and thus dont loose power based on power put throught them, but loose power based on rotational speed.
and that rotational speed losses dont change between a car making 200bhp at the crank and one making 300bhp at the crank, if they both run in the rev range lets say from 3000rpm to 7000rpm.

now dyno runs are typically done in fourth gear ( sometimes 3rd ), and thus the speeds are not in 140 to 160 top speed range of ST185 or ST205, they are around 120 to 130mph depending on the gearing in the gearbox and tyre size. so we should be looking at less lost power than the 44bhp on the ST185 at 142mph


people who have done emperical study of cars crank power, and dyno-d power at the wheels, and known top speeds, show quite a good set of equations showing averaged power lost from crank to wheel

and its often quoted as
4WD ( crank bhp * 0.84 -10 ) ie 16% lost + 10 for the parts not affected by power ( ie wheel bearsing, support bearings )
RWD ( bhp * 0.88 -10 ) this is more lost power than fwd, as its got an extra gear set turning rotation speed 90degrees
FWD ( bhp * 0.90 -10 )


so we dont loose 100+ hp in the transmission, until we are in the 550bhp crank range.

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 Post subject: Re: Transmission Losses
PostPosted: Sat Mar 16, 2013 1:58 pm 
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Dale, taking your 250 wheel HP, and westons 320bhp at the crank, if the 250 wasnt' fiddled too much by the dyno operator having wrong temps, or other correction factors applied,

then 250 * 1.19 + 10 = 307bhp. in other woords, i'd reckon westons didnt have the temp correction or the barometric pressure set right fo the day, or prehaps, the altitude programmed right.

but 250 whhel, is a shade over 300 at the crank.

another way to prove thats right, is take a flat out run, and see how fast you can reach.... if you had the gearing right ( ie, for where your engine is producing max power ) you should reach just over 165mph.

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