May rev quicker but Im thinking not based on this...power levels were cleverly taken at different rpm with the lighter one. Youd think it would though rotating less weight?
A gain in 1/4 mi time could be due to moving less drivetrain weight around kind of like lighter wheels or rotors, tires etc. Opinions?

http://www.superchevy.com/how-to/eng...eel-why-weight

 

So my theory is this... lighter flywheel is less inertial loss. So depending how the utilized dynamometer is calibrated, there can be a perceived increase in power because less is needed because of inertial calculations on the flywheel.

Option 2: the kept real loss on an engine dyno is the flywheel. In the chassis? Way more losses in the trans, wheels, axles, rear, and driveshafts... in the broad scope? Its a gain but will it really amount to a difference in et and mph... I'm not sure.

 

If the moment of inertia decreases compared to a heavier flywheel, then yes it will allow the engine to make more power.

Inertia ~ Mass * Radius^2 in general, so if mass goes down with the same size, it lowers the rotational inertia and helps the entire drivetrain move the car. Similar effect to removing something from the belt drive in the front of the engine.

 

The answer is no, a lighter flywheel will not increase engine power. If you hold the engine at steady state on the dyno the flywheem mass will have no effect on power. That being said if you are trying to accelerate the car as fast as you can the lighter flywheel will show improvements because it has less roataing mass so less power will be used to speed up the flywheel and more power will be available to accelerate the car.

You will see a power increase on an inertia chassis dyno because you are measuring acceleration over time but if you use a power absorbtion dyno that can hold steady state then the power difference will be zero.

 

As stated it does not affect engine power.
It does affect the power needed to rev the engine up.

It is also the other way around. A heavier flywheel will store more energy at the same rpm.

So if you dump the clutch from 5000 rpm the rear wheels will see more power if you have a heavy flywheel.
The engine will rev up slightley slower with the heavy flywheel but each time you shift up you can gain some back from the stored energy in the flywheel.

If you dump the clutch from the same rpm as you have on the finish line it could all even out and the gain is from the actual weight loss.

 

Technically speaking the engine makes the same amount of power, but less of it is used to accelerate a lighter flywheel. The difference in the car's acceleration is actually significant in 1st and 2nd gears because the rate of the engine's acceleration is pretty high. In higher gears it become insignificant, since the rate of crankshaft acceleration is much lower as each mph is gained.

 

The question you have to ask yourself is, can you live with the engine behavior with a lighter flywheel? It's going to idle rougher. If I had a stock cam, I'd probably be OK with it, but with the cam I have in my car, I need the stock weight flywheel. Giving up 5-10 peak hp that my butt-o-meter can't even feel is worth not having to constantly jog the gas pedal to keep it from dying at stop lights.

 

Ive driven stock and modded C4s with light FW and hated every second. They chatter, bog the engine down, pain to get rolling...ooh but it revs faster!
Big CI with lots of torque, autocross, road course maybe.

 

They don't all chatter and behave badly. That depends far more on the disc material you choose. A light flywheel with a stock organic disk requires a little extra extra attention when you're getting rolling, but is very easy to modulate and doesn't chatter at all. If you go with a puck-style metallic disk, you might as well call it an on/off switch. A dual-friction disk or Kevlar disk is somewhat in between. I've had three of those four (not the puck style) on a 13lb Fidanza flywheel so I'm speaking from experience.

The advantage of the light flywheel isn't just that it revs faster when not engaged (or in neutral). Nobody really cars about that (it sounds cool, I guess). The real advantage is that especially in the lower gears, the car actually accelerates faster. And that, after all, is the whole point, isn't it? It also will decelerate faster when you're braking into a corner, because you've reduced the total inertia of the car. Also, a light flywheel makes shifting smoother, especially at higher rpms. You have to keep in mind that a stock ZF flywheel was about 40lbs, so we are talking about a really heavy flywheel with loads of inertia. For comparison, most V8 flywheels through the 90s weighed in the range of 17-26lbs, stock, and nobody ever complained about them being hard to get rolling on the street. So a 13lb, full-diameter flywheel isn't that crazy. Don't forget that the disk and pressure plate assembly also add considerable inertia (I actually had a lightened pressure plate too), as does the crank and harmonic balancer. There are some crazy-light 7" diameter triple-disk clutch setups that are no good for street use, but I don't think the OP is asking about those.

I should add that the notion of stored energy is true, but really only from a standing start (i.e. drag launch). That's why drag racers generally don't use light flywheels. But for most of us, that's not important because on street tires you can already spin the tires in 1st gear anyway, especially if you dump the clutch. You don't need lots of flywheel mass to launch as hard as possible on street tires. And when shifting, the stored energy is irrelevant because unless you're powershifting (don't do that), you will be trying to match revs for a smooth shift anyway. Plus, any kinetic energy you stored will have had to come from somewhere: it comes from reducing your acceleration the same amount while you're on the gas with the clutch engaged. So any kinetic energy you feed back to the tires by powershifting is just energy you already lost to the wheels earlier: you're just recovering some of it while at the same time doing your best to blow up U-joints. But if you didn't lose it in the first place, it would be that much better!

 

I mean my stock flywheel weighs about 20lbs iirc. Its not light by anymeans but is comparatively light I suppose.

 

Maybe Engine Masters is wrong but they just did an episode on this. They had an engine that made 500 hp and 466 tq, and then took off 44lbs by swapping the harmonic balancer and the clutch/flywheel for small hubs. Then the engine made 516 hp and 478 tq with power being up throughout the entire range. They also tested each setup with a 600 rpm per second sweep rate versus the standard 300 rpm per second rate and found that the lighter setup only lost 5 hp where as the stock setup lost 12. So the results with the different setups was 511 hp versus 488 hp at the 600 rpm per second acceleration rate. Meaning that the heavier components eat up much more power in drag race type or other short gear ratio setups.

 

I though we had beaten this subject to death.

The original question of does a lightweight flywheel add power, the answer is 100% No. The flywheel adds nothing but an inertial load to help keep the rotating assembly rotating in the correct direction as firing is occurring and smooth the transition between firing events. As has been hit on, this consumes power, but in the same light as internal shafts in the transmission, clutch, driveshaft, pinion, ring, axle shaft, hub, and wheel/tire. Any weigh reduction in the drive train will result in additional power (and torque) from the engine reaching the outer diameter of the wheel to move the car. Not only do we have light weigh flywheels, but also aluminum (and for the High Rollers - Carbon fiber) driveshafts, and ultra light weight wheels.

It has been discussed about the benefits for acceleration, but... even steady state there will be an effect. Parasitic losses are real too. The more mass in the drive train, the more power consumption it takes just to keep it spinning. It's the same thing as a 3000lb car and having 4 people sit in the car and have it tip the scales at 3600lbs. It will require less energy (Power) to keep a lighter mass moving, or if you want to look at it this way, it requires more torque to keep spinning at the same speed. This is far less noticeable than the effect on acceleration and deceleration, but it is there.

Mass is usually the enemy, more mass means it is harder to accelerate, harder to stop, and harder to change direction. However, when employed properly it can help smooth operation, dampen vibration, control the center of gravity, and other applications depending.

 

No, engine masters got it right. And the faster the sweep rate you program in, the more loss a heavy flywheel creates (or the more power a lighter flywheel frees up). There's a semantic aspect here: the engine isn't creating more power, it's just having less of the power it creates sapped by the inertia of the flywheel. It's no different than reducing the car's chassis mass.

This is all true, but the reason people correctly focus on the flywheel is that it, the crankshaft, and the balancer are the only parts being accelerated at a 1:1 ratio with the engine RPM. The driveshaft's inertia is really negligible because its diameter is so small, and it is gear-reduced in all but 4th gear (in a C4 manual) and up, where the load of the car's mass and drag swamps all the rotating mass. Everything else (wheels, halfshafts, diff parts) is spun a much-reduced rate. The whole wheel/tire thing is way overblown. Many moons ago I ran some numbers on this and showed how even a 10lb difference in wheel/tire weight just doesn't measurably affect the car's acceleration. Their "sweep rate" (I like that term!) is just way too low.


To be clear, this is only an effect of mechanical friction losses in the bearings. But the difference created in that due to just the rotating parts is unmeasurably small. That is, there won't be a measurable difference in rear crank bearing drag (the bearing that will support the flywheel) with different flywheel weights). Ditto the drag on wheel bearings or pinion bearings with different weights of driveshafts or halfshafts. And wheels are supported by the ground rather than the bearings, so their weight makes zero difference in wheel bearing friction. But load 600lbs of people and crap into a car and you'll surely get some increase in frictional losses. I agree with that.

 

Yes, it is an increase in Normal force at any loading point, but as I was discussing the overall drive train, it all adds up. To be honest, at speed in a car, aerodynamics are a bigger load against steady state than mass. However, on a chassis dyno, all components from the crank to the wheel have to be accelerated and any reduction in mass will help. If you can get some mass off each component, not only do you reduce the overall wight of the car that has to be accelerated, you double dip by reducing the inertial loads that have to also be rotationally accelerated.

I would say that these lightweight components can sometimes be very cost prohibitive. If you estimate what a lightweight aluminum flywheel, titanium U-joints and yokes, carbon fiber driveshafts and half shafts, and lightweight racing wheels will cost, you could get a better return on investment of dollar/WHP from other sources. Doesn't mean the science doesn't work, but I bet you get a better hp/weight ratio from forced induction. The trade off is the additional mass is now harder to brake and corner. Where to spend the money comes down to intended use of the car.

 

I have personally witnessed tires and wheels make huge differences at the drag strip and on the dyno. Some of these cars have bettered their ets by more than 2 tenths in the quarter or gain as much as 20 rwhp. The larger gains stemming from cars and trucks swapping from large and heavy aftermarket wheels/tires. A guy I know with a 10 speed Mustang GT did a swap test just a few weeks ago while dyno testing. Took his stock wheels off and put on his new VMS brand drag pack and gained 9whp. Then he put it back on the lift and swapped in a carbon fiber driveshaft and gained another 3whp for a total of 12whp. He hasn't tracked it yet but is expecting about 2 tenths as his front runners have taken 36 lbs of the front as well.

 

People, let's do a thought exercise here. A typical driveshaft has a radius of maybe 4" (0.1m) and weighs maybe 15kg. Thus it has 0.15 kgm^2 of inertia (15 * 0.1 * 0.1). If the engine produces 400lb/ft of torque and the 1st-gear reduction is 2.7:1, then there is 1463Nm of torque at the tailshaft. The engine could therefore accelerate the driveshaft alone to 93163rpm in one second. At 50mph (getting near the top of 1st gear in a lot of cars) with a 25" tire and 3.42 rear axle ratio, the driveshaft turns at 2318rpm. The engine can accelerate the driveshaft alone to the top of 1st gear in about 0.024 seconds, but accelerating the whole car to that speed takes several seconds. Does anybody really the think there is a measurable difference if you shave a few kg off the driveshaft weight? Doing so means you reduce the time to accelerate the driveshaft alone by maybe 0.004s. That's your gain for spending big bucks on a fancy carbon fiber driveshaft. Yay.

OTOH a 40lb flywheel has around 650kgm^2 of inertia (4333x more than the driveshaft) and has to be accelerated to ~6000rpm (2.7x faster than the driveshaft) in the same few seconds and the torque available to do so is cut by 2.7 compared to the driveshaft. Cutting off 2/3 of its inertia actually does affect the car's acceleration in 1st gear. The improvement in acceleration will diminish in higher gears as the RPM sweep time reduces due to both taller gear ratios and increased drag.

Wheels and tires have lots of inertia and there are four of them. However, they only have to spin to 672rpm in the same few seconds, and there is now 5047Nm of torque available to get them there in a few seconds. Even if you shave 20kg from the four wheel/tires, that isn't going materially affect the car's acceleration because it's still a teeny fraction of the total load on the engine as compared to the mass of the car and (as speeds climb) the aero drag. The improvement would increase in the higher gears (typical dyno runs are done in 4th or 1:1), but that is also more than countered by the exponential increases in drag when the car is actually rolling. That drag eventually becomes more of a load than the car's mass and becomes the dominant force against which the engine is working. A chassis dyno doesn't account for that drag at all, and therefore will way overstate the improvement offered by lighter rolling stock.

ETA: I would agree that cutting 36lb of mass from the front wheels by adding "skinnies" probably helps improve ETs and trap speeds. However, keep in mind that drag "skinnies" not only reduce rotating and linear mass, but also rolling friction (due tot he very small contact patch) and aerodynamic drag (front tires are one of the biggest drag producers on any car).

 

36lbs is barely going to show in the 1/4mile of just general mass. Less mass plus less inertial losses will result in better times. My question is, for 12whp, or about 15 crank hp, for the price of a set of wheels, tires, and carbon driveshaft it is hardly worth it for the average street hobby car. Though, the science works. There are just a lot of other avenues to get an extra 15hp cheaper.

 

36lbs of unsprung rotational mass is a huge difference. So taking that off the nose with the lightweight wheels and skinny tires can easily equate to a tenth and has for countless numbers of drag racing enthusiasts.

No doubt it is a lot of money for 12whp but they are necessary modifications for his power goals in the future. Plus he loves the way the wheels look, had fun pulling on some cars at the roll races that he couldn't pull before and he has already done most if not all the easy and cheaper bolt-ons. The car makes just over 500whp. Pretty damn stout for a little N/A 5.0L that has never been opened up.

 

Please see my "edit to add" comment from above:
I can add that "unsprung" has nothing to do with acceleration, only handling. But if you're adding drag skinnies then I doubt you care about handling.

I think any gains in ETs are due as much more more to the reduction of contact patch and aero drag than to the weight reduction. And while that's an interesting case study, it doesn't apply to any setting other than pure drag racing. Those things aren't even safe for street use on a typical car.

In a real-world scenario it is sometimes possible to remove 36lb of wheel/tire mass from the whole car (9lb of each of the four), while keeping the same width and contact patch (or even increasing it). However, the problem then becomes that you can't separate any ET improvements due to inertial reduction vs improved traction from different tires. If you use the same tires but somehow remove 36lb of mass from the wheels, that's not going to result in a tenth reduction in ETs or any other form of extra acceleration. Your engine only has to accelerate the rolling stock to around 1500rpm in a 1/4 run (unless your car is really, really fast!) and it is trivial work for it to do in 12 seconds.

EDIT: We can do the math on this, too. The rotational inertia of a 50lb wheel with 26" diameter is 9.9kgm^2, and a set of four is 39.6. It's going to take 124lb/ft of torque at the axle to accelerate all four wheels to a trap speed of 115mph in 12 seconds (typical numbers for a modern fast production car). But the car with the drivetrain numbers I used above has 3726lb/ft in first gear at the axle. The inertia of the four wheels/tires only uses 3.3% of the total torque/power to accelerate them. Even if we removed 10lbs from each wheel, that only means we free up 0.6% of the torque, or 2.6lb/ft. That's nothing. It will not cause a measurable increase in the car's performance.

 

Man, I just can't see a .1second being described a huge. So many other factors can affect a time more than a .1sec. Faster is faster, and I am not going to discredit it from being an improvement, but hell for most of the folks on this board a .1 would be hardly noticeable. Now, to a seasoned driver who has his car dialed in, finding another .1 could be great, but huge? I don't know. I just can't get on board with that. Seems a pretty steep price to pay for .1sec. Maybe we are just talking a different class of car.