http://forums.toyspeed.org.nz/viewtopic.php?t=58377
Lightening a flywheel does not change your power or torque at all.
Torque & power come from the combustion of Fuel/air mixture. Taking metal off the flywheel or adding more
on will not get more air/fuel into the combustion chamber.
Think about.. how do turbos work... they force more air/fuel into the cylinder..
same with big cams, head flow work etc etc.
Put a car on a dyno, take it to any point in the rev range (peak torque, peak power, whatever)
hold it there using the dyno brake and get a horsepower figure. Change flywheel put it back on the dyno,
take it to same revs, hold it there and get the horsepower figure again.
The horsepower figure will be exactly the same. (if nothing else has changed).
But wait...
If you put it on a dyno and do a dynamic run (a typical rolling road "dyno pull") where the car
is not being held at a set number of revs then a lighter flywheel will show an increase in torque and power.
Why? Because the engine is making the same amount of torque & power, but now instead of having to
accelerate a big heavy flywheel to 8000rpm it is accelerating a much lighter flywheel.
Therefore more of the engines torque is going to the wheels where the dyno is reading it. (dynos dont read power, they read torque and rpm, then calculate power)
Remember high school physics? Force = Mass x Acceleration.
So if the engine produces the same amount of force (torque) but has less mass (weight*)
to push around it will push it faster.
When you hold the engine at a set number of revs on the dyno the flywheel speed is constant,
so no force (torque) is being used to accelerate it so all the torque goes to the wheels.
Part three:
"everyone tells me when you put a lighter flywheel on you lose torque.. but you just said it doesn't change.."
Yep. That bit above where i said on a dynamic run you'll accelerate faster works the other way too.
If your engine isn't making enough torque to pull you up a hill because you are in too high gear
you dont have a big heavy flywheel to keep you going.. therefore your entire car slows down faster.
The actual torque hasn't changed, just the amount of acceleration/deceleration due to less weight (mass).
And a few more things that all relate to this:
Light flywheels make a huge difference in lower gears, and not so much in higher gears.
Why? think about how long it take to rev your car from 3000rpm to 7000rpm in first gear.. not long.
But in 5th gear it takes a lot longer.
But the change in flywheel speed is the same. So the amount of energy it takes to accelerate the
flywheel is the same. but if you take that energy off the engine at a slower rate (ie. over more time)
you are taking less energy per second from the engine.
*Weight isn't actually the whole story with flywheels.. what really matters is Mass Moment of Inertia.
A bit of a mouthful ain't it.
You could have two flywheels that weigh the same on a set of scales, but one could
be a lot "heavier" in terms of the flywheel effect. Why?
Basically because the further out from the center the weight (mass) is the faster it is moving.
The faster it has to move the more energy it takes to get it there.
So a flywheel with less weight on the outside edge will spin up easier than one with all the weight on the outside edge.
credit to Grrrrrrr!
