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View Full Version : Lightweight flywheel vs top speed



ace33joe
09-25-2006, 04:36 PM
First of all, I am not interested in increasing top speed or even trying to reach top speed of my EVO, but I read some interesting claim in another car forum, so I would like to listen some info from mechanical experts here.

The claim: "Light-weight flywheel improves engine response, but reduces top speed noticeably"

I thought if you reduce rotational mass of flywheel, you can actually transfer more energy to tire/wheels (ground)?

Then others tried to explain why top speed reduces as fly wheel lightens up as follows:

"Inertia is very important near top speed, so if you have a lighter flywheel, you have less inertia. This is the same as rotating something, heavy one is actually easier to rotate at high rotational speed."

Some people say "Reduced top speed is because of poorly balanced aftermarket product, so it may increase friction due to vibration at high speed"

I have considered light weight flywheel question as "compromise between drivability vs. response", but some one suggested another aspect, so I just got curious. :)

vtluu
09-25-2006, 04:59 PM
The Evo is rev-limited, not drag-limited, even in stock form. Add more power and you'll likely still be rev-limited even after lightening the flywheel, regardless of the effects (or lack thereof) of the latter.

Inertia, rotational or otherwise, would affect how quickly a car decelerates from a given speed after you lift your foot off the pedal. Otherwise Newton's 1st law applies. Think of a drag-limited situation. Basically your car is rolling along at a certain speed, and without the effects of air resistance (drag) and driveline friction, you could coast at that speed forever. Air resistance applies a force opposite the direction of motion, trying to slow the car down. To counteract that force and maintain that speed, your engine applies torque to the wheels which translates into directional force (in the forward direction). In a drag-limited situation those two forces balance each other exactly--your engine can only supply so much torque. Inertia doesn't really enter the picture anywhere in this situation.

But maybe I have the physics all wrong. ;)

evo_dadi
09-25-2006, 07:12 PM
bah!!mr.know it all :P :lol: j/k tam good explanation tho.

MarkSAE
09-26-2006, 07:07 AM
We should all bolt on heavy wheels to get a higher top speed!

ace33joe
09-26-2006, 12:41 PM
Thanks for the explanation, Tam.

I still can't understand why light weight flywheel can hurt top speed.

MarkSAE
09-26-2006, 12:54 PM
Who wrote the statement "Light-weight flywheel improves engine response, but reduces top speed noticeably" anyway? Does that guy have any credibility?

Having lighter parts certainly will not hurt top speed. Lighter rotating components means that less energy is required to spin it. With less energy being wasted to spin the part, more energy will get transferred into more power to the wheels.

ace33joe
09-27-2006, 10:22 PM
I just read it from some forum, and I don't know those guys. I originally though exactly same as you MarkSAE. Less energy to spin the flywheel = more energy to spin tires/wheels.

But some guys were supporting that strange claim saying that they experience the same thing. :?

BTW, didn't some guys say lightweight flywheel increased vibrational noise, and knock sensor picked them up as knock, and the ECU threw SES light and pulled timing? If it is similar to this case, it may explain why they got less peak power.

Unowned
09-27-2006, 10:31 PM
The reason why it will eventually hurt top speed is because when you have a heavy object rotating at a high rate of speed, the energy it already contains in it's spin is more than a lighter object at the same speed, meaning the lighter object would need more energy to keep it spinning at the same rate as the heavier object.

KareBearPowa
09-28-2006, 10:32 AM
We all drive 160mph so this is very relevant. :lol: