Gears? How do they effect dyno numbers?
09-11-2007, 01:26 PM
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Gears? How do they effect dyno numbers? How do different gear ratios effect dyno numbers? I know 12 bolts and 9 inchs take more power to turn, but what about the ratios themselfs?
How much would a 2.73 ratio differ from a 4.10 ratio in terms of power? Both cars are six speeds.
Thanks guys.
How much would a 2.73 ratio differ from a 4.10 ratio in terms of power? Both cars are six speeds.
Thanks guys.
09-11-2007, 08:21 PM
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4:10's will show a loss on the dyno compared to 2:73's. I would quess in the neighborhood of 10/15rwhp (torque numbers will drop also).
Try searching this topic in this section and in the "axle and gearing" section. There were graphs that were posted that summed it all up.
Try searching this topic in this section and in the "axle and gearing" section. There were graphs that were posted that summed it all up.
id have over 460 easy if i had 3:42's 
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09-14-2007, 12:32 PM
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Average is 10-12 for same rear with different gear. Gear and rear changes are more like 12-15. Most of the loss is in the ratio as stated. But this can be reduced with proper install and micronited gears. I have seen 8-10rwhp with just micronite finish. So that would pretty much eliminate all loss from gear swap.
09-20-2007, 01:47 AM
#9
FormerVendor
Originally Posted by V8bamf
i find this kinda weird. why is this?
For any given change in the inertia dyno drum speed the engine has to also gain speed based on of course the gearing.
When you gear down, the engine now has to speed up much more vs the drum so for the same change in drum speed the engine speed changes more so more power much be diverted into the engine's rotating inertia for the same increase in dyno drum speed.
You still have more total TQ at the rear wheels so you accelerate faster in each gear but you do lose a little power to having to accelerate the engine faster too. You will also usually have less power when you dyno in the lower transmission gears.
The lighter the rotating assembly is the less you will lose when you run lower gears but you will always lose more power the lower you gear when you are doing an acceleration and inertial dyno run.
At steady state the gears will make no real difference unless they cause lower or higher frictional losses but if you aren't accelerating the inertial part doesn't matter.
Of course we are always worried about acceleration and we want light rotating parts AND the correct gearing for our intended use whether circle track, road course or drag race.
09-20-2007, 02:54 AM
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Originally Posted by racer7088
For any given change in the inertia dyno drum speed the engine has to also gain speed based on of course the gearing.
When you gear down, the engine now has to speed up much more vs the drum so for the same change in drum speed the engine speed changes more so more power much be diverted into the engine's rotating inertia for the same increase in dyno drum speed.
You still have more total TQ at the rear wheels so you accelerate faster in each gear but you do lose a little power to having to accelerate the engine faster too. You will also usually have less power when you dyno in the lower transmission gears.
The lighter the rotating assembly is the less you will lose when you run lower gears but you will always lose more power the lower you gear when you are doing an acceleration and inertial dyno run.
At steady state the gears will make no real difference unless they cause lower or higher frictional losses but if you aren't accelerating the inertial part doesn't matter.
Of course we are always worried about acceleration and we want light rotating parts AND the correct gearing for our intended use whether circle track, road course or drag race.
When you gear down, the engine now has to speed up much more vs the drum so for the same change in drum speed the engine speed changes more so more power much be diverted into the engine's rotating inertia for the same increase in dyno drum speed.
You still have more total TQ at the rear wheels so you accelerate faster in each gear but you do lose a little power to having to accelerate the engine faster too. You will also usually have less power when you dyno in the lower transmission gears.
The lighter the rotating assembly is the less you will lose when you run lower gears but you will always lose more power the lower you gear when you are doing an acceleration and inertial dyno run.
At steady state the gears will make no real difference unless they cause lower or higher frictional losses but if you aren't accelerating the inertial part doesn't matter.
Of course we are always worried about acceleration and we want light rotating parts AND the correct gearing for our intended use whether circle track, road course or drag race.
Very good wording Erik. Here is something I wrote on LS1tech in 2001 trying to explain this to myself. After seeing this thread I did a search to find it and reread it. Seems like I was in the ballpark and seems you somewhat corroborate the same concept.
383LQ4SS11-24-2001, 07:58 PM
No theres alot more to it than that. It has to do with motor spin up rates, 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.
Al
No theres alot more to it than that. It has to do with motor spin up rates, 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.
Al
Last edited by 383LQ4SS; 09-20-2007 at 03:14 AM.
09-20-2007, 03:31 AM
#11
FormerVendor
Damn I am still up but yes you have a great explanation. Some people just see this and some don't but it is somewhat hard to put into words. With a good explanation like this then I think almost everyone can see why the power changes like it does.
