Max power vs gear

[sloth]

This is probably a dumb question, but why do they typically dyno a car in 4th gear? Will the maximum power be progressively less, the lower the gear, or does it stay about the same in each gear?

Thanks,

Greg.

[franks]

This is probably a dumb question, but why do they typically dyno a car in 4th gear? Will the maximum power be progressively less, the lower the gear, or does it stay about the same in each gear?  

Thanks,

Greg.

isnt it because 4th gear is a 1:1 ratio???

[Leewah]

It is because the standard Skyline gearbox is a 1:1 ratio in 4th.

Dynoing in 3rd will give a higher, hence false reading.

[cul202]

power is a measure of energy derived from torque combined with speed (rpm)

thats why if you raise the rpm where an engine makes max torque (even if the torque stays at the same amount) the power will go up.

the lumpier cam, carb, etc i put in my red motor raised torque only by 10-15 Nm but increased power by close to 30kw as it moved the torque curve further up the rev range.

so theoretically (note, i said theoretically) the power change in different gears should not be that great. in a lower gear torque will increase but wheel speed will (read should) decrease by the same percentage. in practise there are many variables that make the power level change in each gear but it shouldn't be a massive difference.

[sloth]

Thanks! I'm *beginning* to understand this, I think. I read somewhere else that the frictional losses in the transmission are greater as the gear is lowered, and this is one reason they use 4th gear - it represents the truest measure of power at the wheels which the car is capable of.

I did read elsewhere about the acceleration being greater in lower gears (e.g 3rd) - i.e - the dyno will spin up faster, giving a falsely high reading. This didn't make sense to me at first, because I thought that the dyno should still be able to measure the true power (and get the same result), because the dyno will be spinning slower in 3rd gear - i.e - the car is travelling slower over the (false) road. I think the problem is that we are trying to measure *dynamic* power, not static power. So, whatever gear we do decide to dyno in, the rate of rpm rise should represent what the car would actually experience when driving.

For example, if we were to dyno a car on a standard dyno, in 1st gear, the load presented to the engine would be huge compared to what it would experience on road. So, the resulting power figure would not reflect the *dynamic* power which is available by using things like lightened flyhweels and smaller pulleys etc etc. Another issue I read about is the risk of wheel spin in lower gears.

Up until now, I thought a chassis dyno was a very sophisticated thing, which actually tried to mimick the car being on the road. However, it seems that it's just a big heavy wheel, and thus, it's "tuned" for a fairly typical load in 4th gear.

I think.

Greg.

[Duncan]

actually, as I understand it a dyno measures torque, not power, which is why lower gears give overstated figures, and you try and use 4th (as franks said, closest gear to 1:1) the problem is that if you have the 180km/h speed cut you can't take it over about 6200 in 4th so people have to use 3rd instead.

[sloth]

actually, as I understand it a dyno measures torque, not power, which is why lower gears give overstated figures, and you try and use 4th (as franks said, closest gear to 1:1)  the problem is that if you have the 180km/h speed cut you can't take it over about 6200 in 4th so people have to use 3rd instead.

This is exactly what I have read elsewhere, but I'm not sure it's correct. When a lower gear is used, the dyno will be spinning slower, so the power will be proportionally less, cancelling out that increase in torque.

This link is pretty good: http://www.superflow.com/support/cycledyn-theory.html

What I said in an earlier reply about the mass of the dyno being tuned to be about right for 4th gear (or a particular vehicle speed range) is definitely wrong - according to that link the mass of the dyno actually simulates a particular vehicle weight, so if the weight is a good compromise, meaningful runs in *any* gear should be possible, although the acceleration rates won't match on-road rates exactly, even if the dyno mass produced an exact match to the vehicle weight, due to traction and drag etc, I guess.

Anyway, next time I have a dyno done I'm going to ask them to do a quick run in 3rd gear. I bet the resulting power will be *less* than 4th gear, not more, because of the increased transmission losses in the lower gear. In 2nd gear, my AP22 accelerometer reports a maximum power at the wheels of 138kW, despite a very recent 4th gear dyno run reporting 168kW.

Greg.

[JimX]

To everyone that thinks that 4th gives you a 1:1 ratio - is your diff a 1:1 ratio diff as well?

There is actually little point in dyno'ing in 4th gear other than to show how well it pulls in...well, 4th gear. First and second usually aren't very useful because you can't even get full boost in 1st, and 2nd is still making the power curve a little too short to be useful. Third gear is most popular for a lot of Jap imports with speed cut, because in 4th it can run into the speed cut.

Peak power should be roughly the same in any gear. It is fatter in lower gears though, which can be a reason to do it in lower gears, ie to impress people. But for a real purpose of dyno'ing a car, ie to tune it for maximum power/efficiency, it doesn't really matter if it's in 3rd or 4th.

If you've got no speed cut and a LOT of power, 4th can also be better because it'll be easier to keep traction.

Edit: Everytime I see this thread title I get reminded of Homer Simpson!