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diff gears in relation to dyno output result

ari666

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hi all. i remember seeing a dyno reading once where the torque curve was something spastic like 755NM, but the HP curved topped out at 200 is HP. some members said it was the result of the car being tested in 3rd gear instead of 4th and was wondering if final drive would have an effect on the figure read off a dyno.

discuss:
 

HARVZ-6

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Lower the gear = more torque

basic principle, hp= torque/time

torque gets you moving, hp keeps you there

if that makes sense

faster it spins, less torque
 

old mate

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my physics knowledge is limited but i'll give it a go.
horsepower is calculated like this.

hp=(torque x rpm)/5252.

gear choice for a dyno run has a huge effect on the torque reading that is given, because of how how the ratio acts as a torque multiplier. as does wheel size. so does the diff ratio. so does a torque converter.
so when you're getting a torque reading at the wheels, its showing the force that is acting on the dyno rollers themselves, not the force that the engine is emitting. its usually called tractive force.
i think you can calculate engine torque on a chassis dyno, but its super hard. heaps of ratios and stuff like bearing drag etc.

ps. im sure i could answer it better but its bed time.
 

klampy

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im not sure your horsepower would change, as horsepower is defined as the "rate at which work is done" and torque is related to the turning force of the crank, or something along the lines of the amount of "work" that is available to be accelerated by the "horsepower". if my memory serves me, i could be wrong. stranger things have happened.
 

old mate

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i don't reckon the horsepower would be any different, but there would be more torque.
 

VNexecutive

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how can the horsepower be the same with the torque reading being different? a car with lower ratio diff gears is gunna give a higher hp reading off the dyno than the same car with standard diff ratios. if i understand the way a dyno works properly.

in theory, if the fact he put in 3.08 diff ratios instead of 3.55 does make a difference, then you would have got a higher reading than if he put in 3.55 instead of 3.08

thats the way i look at it anyways. im sure if im wrong, someone clever will come and correct me.
 

Brados

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From my Mechanical Engineering knowledge, Power (KW or HP) is power the engine can create and is usualy constant. Changing dif gears would increase torque but lower the maximum RPM that can be acheive thus the overall power of Engine (Torque*RPM) is still constant. So would have no effect on final maximum HP/KW.

I dont have any Direct experience with Dyno's and testing though so whether this translates to real world experimental data as there are other factors to consider that effect power output such as friction losses etc that vary with RPM. Need a dyno guy to give us the answer to this one. But i would expect it to be a small variation.
 

Not_An_Abba_Fan

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Ahhhhh.....the old "pretend" HP/kW figure.

A dyno measures torque, nothing else. It can't even tell what RPM the engine is doing without some input from the user. The dyno then uses the input data to give a torque reading. It then calculates the "power" figure from that using a formula.

By changing diff gears you are changing torque so the "power" figure has to change. But then it also depends on where that peak torque is achieved in the rev range and that will have an effect on the end result as well.

Example, say you pulled 300ft lb of torque at 5000rpm, that calculates to 285hp, but if you went a bit shorter in your diff gears and got 332ft lb of torque at 4500rpm, it still equates to 285hp. But....say it pulled the same 300ft lb at 4500, you would get a drop down to 257hp, even though you haven't done anything to the engine.
 
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