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Gearbox & Transmission Question


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It may have been asked before, but i was unssuccessful on a search of Topics.

It is commonly accepted that the transmission of a 2wd car absorbs 20% of the available power from the engine( as a ball park figure)

So does anybody know how much power is absorbed by a transmission and transfer box for something like a rangey/ disco?

Thanks in advance for any help.

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It may have been asked before, but i was unssuccessful on a search of Topics.

It is commonly accepted that the transmission of a 2wd car absorbs 20% of the available power from the engine( as a ball park figure)

So does anybody know how much power is absorbed by a transmission and transfer box for something like a rangey/ disco?

Thanks in advance for any help.

According to my "Race and rally car source book" about 25%.

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If only Nige had got some numbers from the nice Dyno man :(

I know Jen dynoed Bob (300TDi Disco) for a bit of fun at a Scooby dyno day and it came in at a whacking 26bhp :lol: , but then so many factors affect dynos that if you're not prepared you can get very wrong readings.

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Umm, so reading the thoughts so far it could be 25 - 40 %, fair spread of possibilities?

Or according to fridge's reply, it could be 75%????

Based on a disco doing 110bhp and recording 26bhp at the wheels???

Surely it can not be that much??

Any gearbox guru's out there who may know?

Thanks again.

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You're assuming the Disco ever made 110bhp, that may be the factory figure but who knows what they really are when they roll off the line, or after a few years use? You're also assuming that when Jen did the dyno she'd had time to sort out all the bits and bobs you should do before a dyno session, even the wrong tyre pressures can give low readings.

Has no-one else dyno'd a landy? :huh:

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Yeah, i appreciate that there are factors that could affect the dyno read out. My comments were assuming facts, as i was trying to get a feel for what the spread of %ages was.

Equally all cars perform differently and the 20% is just a 'say' base line.

My instinct suggests 40% purely because you are running through two transmissions, therefore twice the power loss, but i hoped someone could confirm this from any data taken, or recorded from testing.

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Umm, so reading the thoughts so far it could be 25 - 40 %, fair spread of possibilities?

Or according to fridge's reply, it could be 75%????

Based on a disco doing 110bhp and recording 26bhp at the wheels???

Surely it can not be that much??

Any gearbox guru's out there who may know?

Thanks again.

I'm no expert, so I'll just pass on the info I've read.

Rear engine single seaters with cold narrow tyres 9% eg. Small hillclimb or F/Frd.

Rear engined s/seaters with wide and cool tyres 11% eg. Big hillclimb.

Circuit single seaters with hot wide tyres 12.5% eg. Formula 1,2 etc

Fullrace saloon/sports with engine over driven wheels 15% eg. Le Mans,Silhouette saloons, Mini, Imp etc.

Road going saloons/sports 'as above' and FE/RWD Full race 18% eg. Fiesta, U2, Clubmans, mini, X19

Road going saloons/ sports FE/RWD 21% eg. chevette, MGB, Rover, Jaguar XK

Road going saloons/sports with 4WD 25% eg. Quattro, Range Rover.

It states" should you lack a rolling road figure for power at the driven wheels the flywheel bhp, generally given by tuners, racing engine builders and the major manufacturers should be adjusted as follows."

Or look at it logically,is a 4WD cossie that much slower than a 2WD cossie? The answer is no.

40% is way out, Tyres play abig part as can be seen in the above, Measuring through 4 sets of tyres is always be worse than through 2.

I would of expected the losses through transmission and tyres to be more like 26hp rather than the output fot that Disco.

FF is right about power though, Roland Marlow (?) reckons on getting about 90hp from a good TDI on an engine dyno and as for 350hp TVR engines..... :lol:

Tyre pressure plays a big part in the numbers on a rolling road as FF has already said.

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SOA 93, many thanks for that.

Cool, that just about answers all the questions and queries i had.

I also appreciate that the tyres used in a rolling road situation effect the performance, muds for example create huge drag compared to a road pattern.

You have provided the information that i was looking for.

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