Nissan 200SX Club - South Africa

The answer to this month's question may be a no-brainer to some, but since it remains a common debating point, and since we get emails like this one all the time, we've deemed it worthy of exposure. Plus, having recently installed and tested the effects of a lightweight ACT Prolite flywheel with our DC2 as part of the "Ultimate Efficiency" tech series featured last month, now might be the best time to clear the lightweight flywheel air once and for all.

The first part of understanding horsepower/torque relationshi ps is to realize they are both measuring the energy output of an engine. Torque, as it applies to the reciprocating combustion engine, is the amount of rotational force it puts out, usually measured at its crankshaft or its car's wheels, quantified by distance (ft) and mass (lb). Imagine tightening a bolt to 72 lb-ft of torque with a two-foot-long torque wrench (the correct amount needed to tighten those Honda B-series flywheel bolts). If you're holding the wrench at its end when it clicks, you're pushing with 36 lbs of force (72 lbs/2 ft). If you're holding it halfway toward the head, you're pushing with 72 lbs of force (72 lbs/1 ft)-either way, 72 lb-ft of torque is exerted on the bolt.

Now, picture tightening those bolts in one second as opposed to one minute and you're on your way to understanding horsepower, which adds time and a constant multiplier to the equation. One horsepower is defined as the energy needed to lift 550 pounds one foot in one second, and an engine's horsepower rating is simply it's torque multiplied by rpm, divided by 5,252 to convert to revolution per minute (rpm). Therefore, any change in torque results in a proportionate change in hp. No matter how fast you tighten that bolt, it's still experiencing the same amount of force.

Lightened flywheels reduce the amount of energy produced by an engine that's spent in moving its components (parasitic loss). Since flywheels are solid, unsprung components of a vehicle's driveline, a lightweight flywheel will decrease parasitic loss at a constant rate, improving a vehicle's horsepower and torque output (measured after the flywheel) throughout it's entire rev range.


FICTION
At a constant rpm, horsepower and torque both change proportionately. If installing an aftermarket header, for example, seems to decrease torque but increase horsepower, the fact is that at the rpm where torque is decreased, so is horsepower (and vise versa)-changes are simply more apparent to horsepower at higher engine speeds, and torque at lower engine speeds, due to the function of calculating horsepower from torque. Installing a lightweight flywheel doesn't decrease torque, but it does decrease inertia, meaning more torque will be needed to break traction with a clutch-kick (drifting or burning out), or to accelerate a vehicle off the line during a launch (drag racing, daily driving).

I just going to add to this based on my own experience and a few myths in my eyes that I have heard.

A light weight sub assembly, (flywheel,Pulleys,Crank) will not cause your car to have a more aggressive idle regardless of cams based on my experience the idle has gotten better with my lighter subassembly.

Increasing rotational weight on your subassembly will not increase torque just potential energy stored for use later the above dyno proves this point also please keep in mind that if this myth was true race teams wouldn't be using aluminium flywheels




How a lightweight flywheel works

How does a lightweight flywheel work? Amongst the majority, there are two schools of thought concerning light flywheels. The first is that they do not contribute to power output. The second is that they do. Which thought is correct? In fact both, in a way, are correct.

If we measured the power output of an engine first with light flywheel and then again with the standard part on an engine dyno, no change in power will be seen to occur. At first it appears that the light flywheel has done nothing and was a total waste of cash. This is not the case. A dyno that shows max power at constant revs does not demonstrate what happens to an engine's power output in real life situations - like acceleration. If an engine is accelerated on a dyno (we are talking about a rate of around 2000rpm a second ) it would show a power output of around 20%-25% less than at the constant rev state.

The reason for this is that when accelerating a vehicle the engine not only has to push the total mass of the car but the internal components of the engine need to be accelerated also. This tends to absorb more power as the extra power is used accelerating the internal mass of the engine components and is why a motor accelerating on a dyno will produce less power than at constant revs. Also it must be remembered that the rate of acceleration on the engine internals is much greater that the rest of the car. This would then suggest that by lightening the flywheel, less power would be required to accelerate it and therefore more power would be available to push the car along.

Now, it may seen unbelievable that by removing a few pounds from the flywheel a noticeable difference to a 3000lb+ car’s acceleration will be made. In fact the difference is quite noticeable and the secret behind this is hidden within the gearbox. Everyone knows that cars accelerate at a greater rate in low gears, this is because a car’s gear box basically a mechanical lever and just like when using a leaver to lift a heavy object, the gearbox reduces the mass of the car that the engine sees. For example, in first gear an engine will see the car's mass as only around say 250lbs but the engine internal mass would still remain around 45lbs.

As for the "virtual" weight loss of a typical lightweight flywheel in the 3-series or M3, we've prepared the full mathematical analysis:

Calculations for UUC flywheels showing exact "virtual" weight loss in each gear -

M3 / 3-series (E36 and E46, 1992-2004) click here to download Acrobat .pdf file

M5 / Z8 / 540i (1996-2003) click here to download Acrobat .pdf file

It is now easier to see were the extra performance comes from when you lighten a flywheel. You effectively "lighten" a car by more than 10% in first gear just by removing mass from the flywheel. As the gear used increases this "lightening" effect is reduced. This is why car’s acceleration improvement reduces in higher gears, to very effect in top gear. Great for drags and tight race tracks but will not increase a car's top speed.

You will see the calculations include the diameter of the flywheel, weight lost (same overall rotating mass difference in UUC Stage1 or Stage2 due to pressure plate weight differences), gear ratios including 6-speed application, and typical diff ratio.

The effective "virtual" weight losses are:

GEAR M3 and 3-series "virtual"
weight lost: M5/Z8/540i "virtual"
weight lost:
1st gear 346.5 lbs. 394.4 lbs.
2nd gear 133.15 lbs. 151.7 lbs.
3rd gear 68.9 lbs. 75.4 lbs.
4th gear 46.18 lbs. 48.5 lbs.
5th gear 36.15 lbs. 37.6 lbs.
6th gear 30.04 lbs. 31.0 lbs.


A general rule of thumb for weight loss equivalence to "gained" power is approximately 10lbs/hp. That is for every 10lbs lost, the car gains the effective performance increase of 1hp.

With that in mind, the effective performance increase expressed in gained power can be expected to be the same as the "virtual" weight lost due to the flywheel in each gear divided by 10:

GEAR M3 and 3-series "virtual"
performance gain: M5/Z8/540i "virtual"
performance gain:
1st gear 34.6 hp 39.4 hp
2nd gear 13.3 hp 15.2 hp
3rd gear 6.9 hp 7.5 hp
4th gear 4.6 hp 4.9 hp
5th gear 3.6 hp 3.8 hp
6th gear 3 hp. 3.1 hp



This gear-dependent gain is also another reason why a typical 4th-gear dyno pull may not show a significant difference - the calculations show that little more than 4hp would be detected, yet a 4th-gear dyno run shows nothing of real-world acceleration through the gears. Improvements in rev-matching and upgraded clutch clamping power remain regardless of gear.

Due to the nature of the "virtual" weight loss, typical 4th-gear dyno runs may show miniscule differences. Real-world acceleration runs will show improvement equivalent to the "virtual" weight loss.

A real world example:

Hold a broomstick horizontally across your chest, now spin around quickly a couple of times and stop quickly.

Now tape a brick or weight on both ends of the broomstick and repeat the spinning.

You are effectively the engine, the broomstick the flywheel.
Heavy flywheel with the brick, light flywheel without.

Most of you may realise, without even having to try this, that with the bricks it will be more difficult to start spinning and stop spinning, but that it requires the same effort to spin at a constant speed with or without the bricks.

Its the same with flywheels, a heavier flywheel will have a greater inertia and so will require more energy from the engine to be accelerated but will also resist deceleration more than a lightweight flywheel (making taking off and hill starts easier).

A heavier flywheel will supply more torque when the motor is deaccelerating, simple physics, but this same heavier flywheel will require more energy to get spinning.

A heavier flywheel will supply more torque when the motor is deaccelerating, simple physics, but this same heavier flywheel will require more energy to get spinning.

Please correct your statement on the below URL as a heavier flywheel will not result in more torque on dyno (Possibly on deceleration, That servers almost no purpose), The statement might cause some confusion to learning members

http://200sx.za.net/phpBB2/viewtopic.php?t=5522

Thanks

Nope because when the motor starts to loose torque (torque curve starts to drop off) the heavier flywheel will release its inertia and the torque will be higher than if it had a light flywheel.

So if the torque curve would normally drop off at 4500rpm and you fit a heavier flywheel the torque curve will only drop off at say 5000rpm, at 4800rpm the heavier flywheel has produced more torque. (just using arb numbers to get my point across)

Gary, you like heavy flywheels? I've got one on my desk for you.

24.5kgs, and that is a normal 30cm ruler in the first pic.





I think we've got some heavier ones coming off engines in the factory. The supplier sent the wrong ones on the engines and had to airfreight 3 of the ones in the picture. Ouch.

hehe, Guy no thanks. I like them light, I think mine is 7.8kg or something.

Gary57 wrote:Nope because when the motor starts to loose torque (torque curve starts to drop off) the heavier flywheel will release its inertia and the torque will be higher than if it had a light flywheel.

So if the torque curve would normally drop off at 4500rpm and you fit a heavier flywheel the torque curve will only drop off at say 5000rpm, at 4800rpm the heavier flywheel has produced more torque. (just using arb numbers to get my point across)

Nope, sorry to say it but you are wrong here.

Because the engine is still ACCELERATING between 4500 and 5000 the flywheel requires an energy INPUT to accelerate it, and a lighter flywheel will require less energy to accelerate it between 4500 and 5000rpm, even if the torque curve is dropping off.

Now if you were towing a big load at full throttle at 4500rpm and the speed started to drop to 4000rpm a bigger flywheel would help as it would resist the change more.

Nope, sorry to say it but you are wrong here.

Because the engine is still ACCELERATING between 4500 and 5000 the flywheel requires an energy INPUT to accelerate it, and a lighter flywheel will require less energy to accelerate it between 4500 and 5000rpm, even if the torque curve is dropping off.

Now if you were towing a big load at full throttle at 4500rpm and the speed started to drop to 4000rpm a bigger flywheel would help as it would resist the change more.

Agreed, sorry blonde monent.

Where can i get a light flywheel?
And what do they cost?

OK cool, So you will fix the statment.

Where can i get a light flywheel?
And what do they cost?

Import, Currently paying R3300 for a lightweight clutch and flywheel for the SR20DE

Have a look at .'s post in the articles section. He got a Driftworks lightweight flywheel, 5.5kg, for about R3400. excl shipping from the uk.

But I hear Gary may have a couple of lightened flywheels floating about.

Dont use ally, if your going to change fly look at the ACT street or ultra, dont go lighter, you loose to much pull away momentum