Do lightweight flywheels make much difference?

[jdgFirefly]

I don't think its this is something you can do an apples to apples comparison on. On one hand a heavier flywheel will always require more energy to accelerate to the same rpm, but on the other hand the flywheel is also needed for low rpm operation. and changing the flywheel changes how you drive the car. Its a trade off that depends on driving conditions, driving style, engine and gearing. For example if you are in stop and go traffic or you pulse and glide the lighter fly wheel will save you energy when accelerating. However I do a lot of low rpm cruising and never rev above 3000rpm, so if I had a lighter flywheel I wouldn't see the energy savings form accelerating but my little 3cyl wouldn't be able to chug along at 1000rpm and I would have to rev it higher to shift through the gears and also raise the idle speed, so in my case I wouldn't see the same benefits only the drawbacks.

Another thing to consider is that if you have a lighter flywheel and need to rev the engine higher and slip the cluch more jsut to launch you might be using the same energy as you would if you launch from a lower rpm with a heavy flywheel.

As for the dyno test showing hp gains. It should show hp gain's when doing a dyno pull because the engine is quickly accelerating to the redline, the heavier flywheel takes more energy to accelerate and hp is the rate of energy output. so the faster you rev up the engine, the more hp is required. If you could test hp at a constant rpm you would see no significant difference.

Sorry for not giving any verified yes or no answers, But I think theirs to many variables for that.

[steffen707]

Originally Posted by jdgFirefly

I don't think its this is something you can do an apples to apples comparison on.

As for the dyno test showing hp gains. It should show hp gain's when doing a dyno pull because the engine is quickly accelerating to the redline, the heavier flywheel takes more energy to accelerate and hp is the rate of energy output. so the faster you rev up the engine, the more hp is required. If you could test hp at a constant rpm you would see no significant difference.

I think maybe you're confused about my apples to apples definition. The perfect scenario would be to take a car and use scangauge to calculate what mpg you get an a specific X mile stretch, drive home, change flywheels with the same car, same tires, same exact speed, same weight (because you would top off the gastank) go to that same X mile stretch and have scangauge calculate mpg. In doing this you would eliminate as many variables to isolate the results to just the flywheel and weather (unfortunately I don't have an enclosed course to test on). this would be an apples to apples scenario where the only thing you changed as far as variables was just the flywheel.

Horsepower actually is "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)."

And dynos can test the torque and thus horsepower because horsepower comes from torque, at specific rpms, That's what the dyno plot is, its showing at all those rpms what the torque of the engine is. They do have dyno brakes that can allow you to tune hp for specific rpms. So there are actually VERY SIGNIFICANT gains made from this one particular car on importtuner.com with a lighter flywheel with nothing else changed at not just the top rpm but along every rpm of the usable rev range.

True most people wouldn't do my apples to apples "test", but i'm hoping somebody on ecomodder has tried it. Its valuable info for people that are going to extents of taking mirrors off cars and making underbody pans to know if it helps or hurts or doesn't help much at all, and therefor is more of a pain in the butt to drive and not worth it in their opinion.

metrompg showed 10% gain in mpg from alternator delete on his car, does everybody on this site now do that? No way, but for some that's their cup of tea.

[dcb]

especially when faced with replacing a dual mass flywheel

The trick to a really smooth idle is to turn off the engine.

[jdgFirefly]

Your apples to apples test wouldn't do the lighter flywheel any justice. It would be good to test the extra drag on the bearings due to the extra 10lb but the real energy savings will be seen during acceleration. Also if the flywheel changes your engines stall speed it may cause you to drive in a different gear at a given speed. If the test is at that given speed you will get different resulted. So what I'm trying to say is that results of a single set of tests may vary from what you get in day to day driving.

All I was trying to say about the dyno results, is that it takes a given amount of energy to accelerate a given object, but how fast you want to do it determines how much power you need. If you reved up the engine slowly the power needed for the flywheel will be less despite the fact that the work done on the flywheel is the same. Keep the engine at a constant rpm and no work will be done on the engine. All I wanted to point out is that the 8hp gain isn't constant but conditional.

I too hope to see some real test results, but it doesn't hurt to look at it from all angles first, so that one can understand how the test results may or may not be applicable. To all situations or specific situations.

[jdgFirefly]

Keep the engine at a constant rpm and no work will be done on the engine.

I wanted to say at a constant rpm no work will be done on the flywheel.

[autoteach]

As I am already pissing people off, I will continue giving my input as I still know that what I know. Last night I didn't have patience for this, but I gave a very correct answer. It had nothing to do with my patience with the OP, but the fact it was late and I just figured a correct short answer might be as well. As I have no interest in giving examples, as the examples already given are continuing to be question, I will give you some more food for thought. (by the by, in the last year I have machined considerable weight from 3 different flywheels, with 3 major different results. all were powersports related).

As shade tree was on to it, what is the difference between parasitic drag and an "unwillingness" to accelerate? A flywheel is not parasitic drag, period! Parasitic items use energy... alternator, ac, power steering, water pump, hydraulic brake pump, etc.
Flywheels dont use energy, they store it. (think regen braking)

What is the difference between the inertial effects of wheels, driveline (not flywheel), etc vs the flywheel?

Why does that transmission on the light flywheel vehicle chatter the gears?

As someone else mentioned, would you get a gain from stop and go driving if taking off took more throttle?

Why do drag racers decrease flyweight to increase traction when they are slipping the tires too much?

Would there be a greater gain from removing the parasites? What about increasing electrical demand while getting rid of pumps? Removing the water pump and power steering have shown significant gains in power (and it is at all times). Electric water pumps do something very unique. They pump at a constant rate, and the alternator is already producing excessive voltage (ask a motorcycle shop about Regulator/rectifiers)

[steffen707]

Originally Posted by jdgFirefly

Your apples to apples test wouldn't do the lighter flywheel any justice. It would be good to test the extra drag on the bearings due to the extra 10lb but the real energy savings will be seen during acceleration. Also if the flywheel changes your engines stall speed it may cause you to drive in a different gear at a given speed. If the test is at that given speed you will get different resulted. So what I'm trying to say is that results of a single set of tests may vary from what you get in day to day driving.

My purposes are strictly to find out if a lighter flywheel will make more mpg cruising at 70mph in my 97 honda civic dx with a vx motor and crx hf gearing.

this isn't my day to day driving but I take long cross country trips and want to get better gas mileage at 70mph while doing so.

I'm not looking for a 2 week test to compare day to day driving, i want an average mpg over a particular mile stretch of road at 70mph. LOL,

Also I thought with a manual car, engine rpm, gear and final drive as well as tire diameter determined speed?....anyways.

[steffen707]

So at first it was stated that heavier is better, then there must be a formula that calculates the proper weight, was the 'correct answer"

There must be a reason why auto makers don't use 100lb flywheels to gain mpg, because then the prius, insight, ect. ect. would have them.

I too believe there's some kind of formula based on the part's relationships to determine best weight. Again i asked earlier, is that 4, 7, 10, 17, 30 lbs? I don't know and neither do you, or you would have said what the formula is. Which is fine, i wouldn't expect somebody to know what this formula is. Maybe honda civics 16.6 lb flywheel is the optimal weight for mpg, or maybe it is lighter, but they felt that this gives the best overall driveability?

Your slew of questions are all very good ones and i'm sure some of their answers may help to learn the overall picture of flywheels relationship to mpg, but......

I'm gunna wait for apples to apples info or until i'm ready to swap the vx motor into my 97 civic hatch and i end up doing the test myself.

[dremd]

We discussed this some in my 6 speed conversion thread.

DMF's (which are typically very heavy) decrease fuel consumption by allowing operation in a taller gear than would be possible with a lighter/ less dampened flywheel.

However, unless you have a very tall top gear then I would assume that you are cruising at 70 in top gear regardless, so no gain to be had.

Now with heavy pulse + glide a lighter flywheel would certainly help (less mass to spin up/ down.

Hey auto tech, Where did you get info on lighter flywheels + traction? I find it more difficult to launch cars with lightweight flywheels, but that could be a function of what clutch is typically attached (double and triple disc for us Supra guys).

[dremd]

Oh, and a few notes from a guy who just (450 miles ago) went from a Single mass flywheel (19 lbs with clutch bolted on) to a dual mass flywheel(52 lbs with clutch bolted on).

1) I can run 100-150 rpms less with same load.
2) Starting is much smoother.
3) bump starting is now effortless vs cumbersome (could never do it with picky passengers)
4) idle is much more quiet
5) other drivers are much more apt to drive it at lower rpm's than they were before (small sample size of 2).
6) shut down is now smooth and silent.

Best sum it up is smooth, smooth, smooth.

EDIT: as an FYI: I thought I had a DMF to begin with . . . . previous owner swapped it (apparently)