Rolling road

[Dave Eastwood (Gadgetman) - Club Chairman]

Fourth is often used - normally it's "straight through" (1:1 gearing)

[Steve_Farrimond]

Fourth is often used - normally it's "straight through" (1:1 gearing)

sounds obvious now you say it Dave, why is it that the bhp is taken in 3rd? surely this would read different dependant of gear. it would make sense that all figures were tested in 4th, as you say 1:1

[Dave Eastwood (Gadgetman) - Club Chairman]

Often they are, but other gears are used sometimes, not sure why they've used third in your case, or may be that's how they prefer to do it? (Engine tuners are notorious for being er, quite focused individuals, who will often have their way of doing things that get results for them!)

 

Then again, it more common for some guys that specialise in turbo cars to use third for various reasons.

[Hammy]

Any kind of diff only has an effect if the wheels are turning at different rates - If this doesn't happen there is no mechanism to absorb power as both sides of the diff are rotating at the same speed and there is no "slip"

If the diff absorbed 30% of the power , on say a 100 bhp engine that would be 25kw - imagine that heat being produced from the diff housing - it would need to be red hot to give off that heat

If it absorbed that I want it dumped pdq !

[Hammy]

The power reading will not be affected by which gear is used

Run down losses includes driving the ancillaries , gearbox, back axle losses and the tyre/ dyno interface and I'd guess dyno losses - so could appear high .

[Steve_Farrimond]

just the torque then? any simple explaination?

[fatbaldbloke]

Power is torque times speed, so if the gearing "exaggerates" the torque the speed is reduced accordingly.  It sort of cancels itself out.

[dombanks]

fb that would make sense.

 

so using 3rd would mess up the torque but allow a power run that was over quicker with less speed being needed at the wheels to reach max rpm IYSWIM....

[Hammy]

It shouldn't mess up the torque either ! As the engine rpm is measured and the power output by the dyno , then engine torque is calculated by dividing one by the other .- in just the same way as the diff ratio doesn't affect the result

The dyno only measures power , torque is calculated .

[robo1968]

Here's a useful site that explains things:-

http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

[Dave Eastwood (Gadgetman) - Club Chairman]

Oh good grief. I give up.

[Dave Eastwood (Gadgetman) - Club Chairman]

Here's a useful site that explains things:-

http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

 

A quick scan of that link shows what looks like some nice simple explanations.

[robo1968]

Anyone know how much power is sapped by an ATB diff?

[Hammy]

None unless you go around a corner

[robo1968]

None unless you go around a corner