Dynos and gearboxes
pekwah1 - 31/1/11 at 10:04 PM
Hi guys,
When having a dyno done on a rolling road, what kind of effect does the gearbox have?
I.e. say i had a 150hp engine on two identical cars, but they had different gear ratios, would one seem to produce more power than the other?
obviously a shorter ratio should give better acceleration, so does this equate to the dyno thinking it has more power?
discuss below:
prawnabie - 31/1/11 at 10:13 PM
A Dyno is fixed to the crank/flywheel, there is no gearbox/final drive involved.
[Edited on 31/1/11 by prawnabie]
pekwah1 - 31/1/11 at 10:14 PM
ok, so i mean a rolling road power test.....
prawnabie - 31/1/11 at 10:15 PM
I would imagine on a rolling road you would used the gear closest to 1:1 to give a true figure, although I have no idea really!
liam.mccaffrey - 31/1/11 at 10:15 PM
no it wouldn't, because chassis dynos measure torque with respect to rpm (power is calculated) . With identical engines and different gearboxes
(assuming identical frictional losses) the relationship between torque and rpm is proportional.
The overall gear ratio acts as a torque multiplier, go from a 1:1 to 0.5:1 will halve the output speed but double the measured torque
[Edited on 31/1/11 by liam.mccaffrey]
pekwah1 - 31/1/11 at 10:16 PM
but power is different to torque, or do they work out the bhp from the torque?
franky - 31/1/11 at 10:16 PM
it will still generate the same about of turning force on the drum, its that ability that is translated to torque/bhp.
liam.mccaffrey - 31/1/11 at 10:24 PM
Thats not right sorry if you change the gear ratio the measured torque will definitely change
quote:
Originally posted by franky
it will still generate the same about of turning force on the drum, its that ability that is translated to torque/bhp.
torque is rotational force (moment)
power is a rate at which work is done due to force
Dynamometer work by measuring the rate of change of angular acceleration and using that to calculate the torque against time (Newton second Law)
daviep - 31/1/11 at 10:36 PM
Horsepower is a function of torque and RPM, so changing gear or diff ratios only trades one against the other. Lower gear = higher torque but lower
RPM.
Davie
pekwah1 - 31/1/11 at 10:37 PM
so back to original question, how would it effect the results of a rolling road?
daviep - 31/1/11 at 10:42 PM
quote:
Originally posted by pekwah1
so back to original question, how would it effect the results of a rolling road?
It won't 
liam.mccaffrey - 31/1/11 at 10:42 PM
Long story short, you would get a similarly shaped torque map but with different limits, but your overall power output would not change. Thats the
theory anyway, its seldom that simple.
quote:
Originally posted by pekwah1
so back to original question, how would it effect the results of a rolling road?
pekwah1 - 31/1/11 at 10:52 PM
interesting.... i'm guessing that's because as already stated they take into account drive ratios etc?
Well moving the conversation on, how about lighter flywheels and alloys?
jeffw - 31/1/11 at 10:56 PM
If you run cars in 3rd gear you will get one set of figures while doing the same car in 4th gear will give slightly different figures so expect slight
differences between gearboxes. This is real life on a RR rather than theoretical as I have tested this on a variety of cars on a variety of different
rolling roads and there is always a difference.
liam.mccaffrey - 31/1/11 at 11:01 PM
quote:
Originally posted by pekwah1
interesting.... i'm guessing that's because as already stated they take into account drive ratios etc?
Well moving the conversation on, how about lighter flywheels and alloys?
If you reduce the rotational inertia, the same torque output will have a greater effect in line with the following relationship.
angular acceleration= Torque/Rotational inertia
You can see that if you reduce the rotational inertia (lighter flywheel and alloys etc) for the same static torque output you will have a
proportionally higher acceleration force.
MakeEverything - 1/2/11 at 06:35 AM
quote:
Originally posted by liam.mccaffrey
quote:
Originally posted by pekwah1
interesting.... i'm guessing that's because as already stated they take into account drive ratios etc?
Well moving the conversation on, how about lighter flywheels and alloys?
If you reduce the rotational inertia, the same torque output will have a greater effect in line with the following relationship.
angular acceleration= Torque/Rotational inertia
You can see that if you reduce the rotational inertia (lighter flywheel and alloys etc) for the same static torque output you will have a
proportionally higher acceleration force.
Actually, its Rotational Mass. 
liam.mccaffrey - 1/2/11 at 09:20 AM
quote:
Originally posted by MakeEverything
quote:
Originally posted by liam.mccaffrey
quote:
Originally posted by pekwah1
interesting.... i'm guessing that's because as already stated they take into account drive ratios etc?
Well moving the conversation on, how about lighter flywheels and alloys?
If you reduce the rotational inertia, the same torque output will have a greater effect in line with the following relationship.
angular acceleration= Torque/Rotational inertia
You can see that if you reduce the rotational inertia (lighter flywheel and alloys etc) for the same static torque output you will have a
proportionally higher acceleration force.
Actually, its Rotational Mass.
Actually, its moment of intertia.
atspeed racing - 1/2/11 at 09:28 AM
Without getting picky...
It makes little difference what gear you are in when doing a power run. You do end up with a different reading if you run a high powered car in say
2nd gear and then do the same run in 4th gear, but this is due to the PC's ability to record accurate information fast enough. Some of the small
race classes we have to run in 2nd gear other wise the run takes to long and the reading is wrong due to heat build up and on the high power cars we
use 6th gear to try and make the run longer. We like to see a wide open throttle power run taking about 8 to 10 seconds.
If you run a 150/200bhp car in 3rd or 5th you will see little change.
Alan