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Discussion Starter #1
Can anyone tell me if the transmission losses are more less for an auto box car.Wifes small Vitz should be around 160bhp but runs an auto box would this really sap power to the wheels or should the at the wheels figure be slightly more than a similat manual car.
 

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Phew! I read the title and thought, "Not that can of worms again!" :popcorn: :chuckle:
 

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from general info on various sites it says standard auto boxes do take more power and so would reduce power at the wheels. if u look at most specs for the same cars eg a bmw325i the auto car is a half second slower time 0-60.

maybe mechanics on this web site can expand on that......
 

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we spent some time messing around with my fto on the rollers to try and gauge what sort of losses the auto box had compared to a manual fto. Figures gained were 216-220 at the wheels with a calculated 245 - 252 at the flywheel depending on which run. Apparently it is very difficult to gauge accurately the losses due to the way an auto disengauges for coast down.

After much searching around the internet and reading up on various dynos i tend to use a 15% loss for fwd manual and 20% for fwd auto at around these power levels for a rough flywheel estimate. The wheel power is always the best to go by anyhow as thats where the power matters most.

Also worth saying that at a run I did perviously the torque convertor caused an interesting torque curve at the start of the run. I had an initial peak in torque of 205lbft at 2100rpm :)
 

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Discussion Starter #5
Vitz

Yeah thats what i thought but was reading an article recently which looked like the losses were less for an auto--might have to re-read the article to see if i picked it up correctly or not. So we should be seeing around 130bhp at the wheels for the little car then...
 

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I agree that in the lower gears the losses are higher due to the torque convertor, but in top, if you have a torque convertor lock up (basically a clutch that engages) then the losses will be similar to a manual...

Measurement of losses using coast-down will conversely give a lower loss figure due to "losses" in the drag reading due to the convertor...

You also have to be careful when putting you car into neutral at max rpm and letting engine rpm drop too quickly - this has melted a few BMW auto boxes I hear...

Cheers :)

Rog

p.s. some of the detriment in performance on autos is due to them often having fewer gears than their manual equivalent...
 
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