Gears effect Dyno results??
02-16-2009, 09:07 PM
#1
Gears effect Dyno results?? I've been wondering about this for a while. I've heard that going from say a 3.42 to a 4.11 gear will negatively affect dyno results. I don't know if this is true, but I think if anything it would increase dyno numbers. I know if you switch from a 10 bolt to a 12 bolt or 9" dyno #s will decrease, but what will just a gear change do to dyno numbers?
02-16-2009, 10:26 PM
#2
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Here is a quote from ALQ4 posted on need2speed.com
gearing and internal friction of the motor, MOI(moment of inertia) of the rotating componets being spun up faster due to higher numerical gearing. The explanation is very involved. Alot more so than youd expect. Its not as simple as "the gears are heavier", or" The gears are not cut the same so theres more friction". Yes those come in to play but are minimal.
Ill attempt at a feeble explanation.
to make it easier to understand I use a trick. To explain some thing like HP and where its going its easiest to solve the problem in the simplest terms. Energy(in this case HP) and where its going. What is using and consuming the "energy" the motor is putting out?
Ok..say you floor your car in nuetral with no load. It takes 1.5 seconds for the motor to go from 800 rpm to 6200 rpm. Say we already know the motor puts out 320 flywheel hp from an engine dyno. So you can say it takes 100% or all of the 320 hp to accelerate the engines own components from 800 to 6200 rpm in that very short time of 1.5 seconds. Without doing more mods or lightening up the internals or reducing friction this is the maximum the engine can accelerate and it requires 100% of the power to do so. I call this the "Minumim no load spin up rate". Its an odd concept. We always think of the motor as in terms of power output. But fact is under certain curcumsatnces the motor uses much of its own HP(energy) to sustain or accelerate itself.
So lets go to the oppisite end of the spectrum to give a comparison. Now the car is on a dyno and in 4th gear. You spin it up from (for arguement sake)800rpm to 6200 again. Now it takes say 15 seconds to acclerate the exact same rpm range. Now where is the energy going? since the engine rpm is increasing at a much slower rate much less of the energy(HP) is being consumed to accelerate the engines own components. It is being transmitted to the wheels through gearing.
Ok now lets split some hairs an put in 4.10s in place of out 3.42s and if you timed the runs you would see that with 4.10s due to mechanical advantage would spin up from 800 to 6200 rpm at a slightly faster rate say 14.5 seconds. Since the time is closer(although slightly) to the "minimum no load spin up rate" there is more energy being consumed by the motor/drivetrain to spin up its own components.
Now to test this theory. Put a car on the dyno and dyno in 4th. See what you get. Then dyno in third and see what you get. Then second, then first. You would see that 4th gear dyno would be for arguement sake say 300 hp. In third you would see maybe 275. in second maybe 220 and in first it would be probably less than 200 hp to the wheels. And you would see that the acceleration times (800-6200rpm) would be less and less as you got into the lower gears. Again...the closer you get to the "Minimum no load spin up rate" the less HP youll see at the wheels and the more HP it takes for the engines own components to accelerate itself.
This is the simplified version. We could get into it further and compare acceleration times of RPMs VS mph in different gears. And see what work is being done at the wheels. The faster the motor spin up rate the less work at the wheels. Hence the lower dyno #s
None of these #s have been checked and they are just generalizations. So dont pick them apart please.
gearing and internal friction of the motor, MOI(moment of inertia) of the rotating componets being spun up faster due to higher numerical gearing. The explanation is very involved. Alot more so than youd expect. Its not as simple as "the gears are heavier", or" The gears are not cut the same so theres more friction". Yes those come in to play but are minimal.
Ill attempt at a feeble explanation.
to make it easier to understand I use a trick. To explain some thing like HP and where its going its easiest to solve the problem in the simplest terms. Energy(in this case HP) and where its going. What is using and consuming the "energy" the motor is putting out?
Ok..say you floor your car in nuetral with no load. It takes 1.5 seconds for the motor to go from 800 rpm to 6200 rpm. Say we already know the motor puts out 320 flywheel hp from an engine dyno. So you can say it takes 100% or all of the 320 hp to accelerate the engines own components from 800 to 6200 rpm in that very short time of 1.5 seconds. Without doing more mods or lightening up the internals or reducing friction this is the maximum the engine can accelerate and it requires 100% of the power to do so. I call this the "Minumim no load spin up rate". Its an odd concept. We always think of the motor as in terms of power output. But fact is under certain curcumsatnces the motor uses much of its own HP(energy) to sustain or accelerate itself.
So lets go to the oppisite end of the spectrum to give a comparison. Now the car is on a dyno and in 4th gear. You spin it up from (for arguement sake)800rpm to 6200 again. Now it takes say 15 seconds to acclerate the exact same rpm range. Now where is the energy going? since the engine rpm is increasing at a much slower rate much less of the energy(HP) is being consumed to accelerate the engines own components. It is being transmitted to the wheels through gearing.
Ok now lets split some hairs an put in 4.10s in place of out 3.42s and if you timed the runs you would see that with 4.10s due to mechanical advantage would spin up from 800 to 6200 rpm at a slightly faster rate say 14.5 seconds. Since the time is closer(although slightly) to the "minimum no load spin up rate" there is more energy being consumed by the motor/drivetrain to spin up its own components.
Now to test this theory. Put a car on the dyno and dyno in 4th. See what you get. Then dyno in third and see what you get. Then second, then first. You would see that 4th gear dyno would be for arguement sake say 300 hp. In third you would see maybe 275. in second maybe 220 and in first it would be probably less than 200 hp to the wheels. And you would see that the acceleration times (800-6200rpm) would be less and less as you got into the lower gears. Again...the closer you get to the "Minimum no load spin up rate" the less HP youll see at the wheels and the more HP it takes for the engines own components to accelerate itself.
This is the simplified version. We could get into it further and compare acceleration times of RPMs VS mph in different gears. And see what work is being done at the wheels. The faster the motor spin up rate the less work at the wheels. Hence the lower dyno #s
None of these #s have been checked and they are just generalizations. So dont pick them apart please.
02-17-2009, 08:03 AM
#3
Hmm, that's a good example but I'm still a little confused. So the closer to the "min no load spin up rate" the less hp to the wheels. Wouldn't any mod that makes the engine rev faster decrease HP? A light weight flywheel will make an engine rev faster decreasing "min no load spin up rate" but won't necessarily make the car faster under load, right?
Sorry, getting off topic a little. Has anyone done a gear change and had either a decrease or increase in dyno results. I know dynos are just a tuning tool but I'm kind of interested to know the effects.
Sorry, getting off topic a little. Has anyone done a gear change and had either a decrease or increase in dyno results. I know dynos are just a tuning tool but I'm kind of interested to know the effects.
02-17-2009, 05:29 PM
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