trackdaycareracer

What's the point of lightweigth flywheels? Please bear with me as I try to explain what I don't understand.

A flywheel will store energy to get a more even output from the engine despite the various strokes.
A flywheel will resist change in rotational speed. Thus, in the three strokes after the powerstroke, the flywheel will help overcome friction and other loads that will try to slow down the vehicle. During the powerstroke, the flywheel will resist change in its rotational speed and thus reduce the acceleration in engine speed more than if the engine had no flywheel.

So, would a heavier flywheel be an advantage at lower RPMs and a lighter flywheel be an advantage at higher RPMs? Therefore, a 4-stroke engine with a 5.500 RPM rev limiter would benefit from a heavier flywheel and a 4-stroke engine with a 9.500 RPM rev limiter would benefit from a lighter flywheel?

Thanks for reading.

wildcat077

From what i understand there is a great benefit to replacing the heavy 3.6 993 flywheel with a lighter
unit in order to lose a good 20 or so lbs of rotating mass off the crankshaft.

In my case,i'll be installing a lightweight KEP pressure plate over the Winter in my 89 3.2 in order to save 15lbs off the end of the crankshaft with the hopes of having my engine spool up quicker at lower rpms ... not as much gain as the 993 as to replacing the flywheel though ,only 4 lbs or so ...

Surely there are more knowledgeable members who will chime in with a somewhat more detailed scientific explanation

Cheers !
Phil

spare tire

YOU GET MORE HORSEPOWER AND TORQUE The numbers that are quoted for horspower and torque for a motor are usually with the motor out of the car and on a dyno that gives you a printout of what power your engine makes. These numbers are from your crankshaft and there is no flywheel or clutch or gearing to alter the horespower number. When you put the engine back in the car and try to accelerate, you have all the above moving parts that have to be accelerated along with wheels and rotors. When you measure the horspower and torque with all that stuff, It is called horspower AT THE WHEELS. This number is 10-15% lower than engine horsepower as quoted by engine manufacturers. A LWFW will NOT change the engines horspower or increase it, But it WILL increase the wheel horspower because you will now lose 7-12% of the engines power instead of 10-15%. Another big bonus of a LWFW is if your car runs great before the LWFW, then it will rungreat after with more power and torque. If you install a new exhaust and/or intake or change camshafts, you can get more power But Murphy's Law says You will have some problems to solve as well as more power.

spare tire

A horse race, the horses that are racing are twins and exactly equal. Jockey one is an Offensive lineman for the Chicago Bears and he weighs 320 lbs. Jockey two is my little sister and she weights 120 lbs. who will be faster?

trackdaycareracer

Thanks for the replies.

Spare Tire: Will an engine on a test stand at a steady 4000 RPM produce more torque (and thus horsepower) with a LWFW?

The flywheel will cause no frictional losses as it is fixed to the crankshaft. The driveline has frictional losses and will cause the power at the rear wheels to be less than at the crankshaft. How would the driveline losses be larger with a heavier flywheel?

At a steady RPM, when the vehicle encounters an incline, wouldn't the heavier flywheel have stored more energy and thus get the vehicle up the hill faster?

Just asking, not arguing, as I would like to understand why. I'm fully aware that most race vehicles have LWFW to get them faster around the track. At the same time, those engines are more difficult to drive with higher risk of stalling and a higher drop in RPMs during gear shifts.

P.S. it appears that engine dynos are connected to either the crankshaft or the flywheel D.S.

Thanks again.

spare tire

An engine on a test stand is without a; flywheel, clutch or transmission. The dyno attaches to the crank. There are several dynos that measure power and torque at the wheels. They usually strap the car in a steady position and the rear wheels sit on a big roller or drum. The drum is 360 degrees around with about 60 degrees of roller exposed above the ground. The other style is a dyno -pac where the car is jacked up and the wheels and tires are removed and a big hub is bolted to the drive wheels. The bolted hubs stick out from the car like the handles on a rolling pin. (as in pie crust) The Dyno-pacs are then pushed towards the car and swallow the splined hubs. The roller style is not so good for 600 or 1000 horsepower turbo cars because you can get wheel spin where the tires sit on the rollers.

spare tire

[QUOTE=trackdaycareracer;10014923]Thanks for the replies.

Spare Tire: Will an engine on a test stand at a steady 4000 RPM produce more torque (and thus horsepower) with a LWFW?

The flywheel will cause no frictional losses as it is fixed to the crankshaft. The driveline has frictional losses and will cause the power at the rear wheels to be less than at the crankshaft. How would the driveline losses be larger with a heavier flywheel?

No frictional loss but you have more mass to accelerate. The electric starter on your lawn mower is small because the piston and crank and rod is small. 6 pistons and six rods and a longer heavier crank need a larger starter motor even if the compression ratio is the same for both engines.

At a steady RPM, when the vehicle encounters an incline, wouldn't the heavier flywheel have stored more energy and thus get the vehicle up the hill faster?

No, It will have more stored energy and will resist slowing more. It will not climb the hill faster.

Just asking, not arguing, as I would like to understand why. I'm fully aware that most race vehicles have LWFW to get them faster around the track. At the same time, those engines are more difficult to drive with higher risk of stalling and a higher drop in RPMs during gear shifts.


LWFW will accelerate faster BOTH up in RPMs and Decelerate down faster in RPMs. Less Mass = quicker responce by the forces acting upon the object.

mark kibort

Think of this way. the flyweel is a rotating mass that does require HP to accelerate, true. But, that power will depend on the rate of change of acceleration. by definition, that is HP.
so, the HP gain or loss depends on the rate of acceleration.
the net net is this: in 1st gear the HP losses might equate to 20hp, 8Hp in 2nd, 3hp in 3rd, 1hp in 4th and .4Hp in 5th.


if the rotating mass is on the wheel and tire, it acts as if it was 1.5 or 2x as if it was in the car. (not talking about effects of sprung mass on handling)

Hope that helps

Mark

chuck911

Additionally, and especially important for daily driving, a massive flywheel helps smooth out engine response resulting in improved idle and especially driving smoothness and ease in pulling away from a stop, or when idling along in slow or stop and go traffic. A lighter flywheel feels great every time you blip the throttle shifting gears, and yes you will notice better acceleration at least in 1st and 2nd. But read up on all the comments about launching the Carrera GT for how easy it is to stall a car with a very small light flywheel. So there is a tradeoff, so think about how this will affect your daily use of the car in deciding if its worth doing.

Another thing, its not just the flywheel. Everything above is also true for the pressure plate.

mark kibort

there is really not much savings of weight, even with a lighter flywheel, on the rotating mass. think about it, you still have about 25 lbs of clutch hanging on the flywheell AND, dont forget about the driveline, trans gears as well as the rear rotors, hubs, wheels and tires! a 5 lb lighter flywheel will save almost nothing you can feel. However, what it is good for is the bliping the throttle for shifts. you still have the 50lb crank, flywheel and pressure plate spinning , not to mention the pistons , rods and the accessories on front of the engine, including the harmonic balancer. So, the real deal here is that the reason that lightweight clutches can feel so fickle on the street, is because they usually have a racing clutch assocated with it.

In the end, the "lighter flywheel" generally is more hype than anything else.

spare tire

RIDICULOUS -2

jhg41977

Think of a stationary bicycle. One has the heavy wheel found on a spinning bike, the other has a standard bike wheel which is just suspended off the ground. Assume there is no 'free-wheel' and the pedals spin as the wheel does. How do you feel trying to increase the rpms of each?

trackdaycareracer

Thanks for taking the time to give me your input guys

To further muddy the topic, the reason behind my questions is that a racer told me that he just wasn't good enough to drive with an engine with a very lightweigth flywheel. Once he got an engine with an original flywheel he did a lot better, competing on a national level alongside a racer that later became world champion. Perhaps by lightweigth flywheel he meant all the moving parts in/on the engine, I don't know. This led to some considerations regarding my son's race engine where I have a choice between a heavier flywheel with a larger diameter and a smaller, lighter flywheel. As he just completed his first season in racing, he is still in the learning phase.

Any thoughts on this is greatly appreciated.

spare tire

The reason you asked the question should have been in your first post. Some people jump into racing with very limited experience. Many times I have seen a newby Cupcar owner kill the motor in the pits or paddock or on the grid in an attempt to get rolling. A full blown race car is loaded with quirks that improve maximum performance but hurt the everyday drivability. If you let the clutch out and do not do it correctly, (smooth, progressivly and add throttle at the right time) You can stall the motor. This leads to nervousness as you watch your friends drive around you and you screw up again. Restart the car again and go. The quick fix is a standard flywheel. (but you will be SLOWER) less horsepower to the wheels. Left foot braking is also difficult to learn because it is usually on the clutch. They are all skills you must master is you want progress, lower lap times.
Competing alongside a Champion is not a not the issue. Does the National level Champion have a LWFW? You can bet yer *** he does.

Byprodriver

Lightweight flywheel helps you when you want to change speed.
If you want to increase speed a LWFW has less resistance to increasing the speed of the engine & thus the speed you are going.
If you want to decrease speed a LWFW has less resistance to slowing down also.
These situations come up often when racing.