fuel consumption on lightened stock flywheel

for those how have lightened their stock flywheel, whats the fuel consumption like? does it consume more or stays the same

Mine is using loads more fuel purely because im thrashing it even more now! :-D

Otherwise im not sure the lightweight flywheel would effect the fuel economy but not 100% sure about that :-]

In theory it could improve fuel efficiency because there's less mass in the drivetrain.

imho i have 1.5 kg off my stock flywheel...and if I keep the revs low i'm about 1.5mpg worse than before, since i can never seem to keep the revs low throughout the complete fuel charge............ it is very good, mirca just keeps saying rev me baby! .... a comment from my workmate....more tow than a roman sandal!

Hmmm debatable as because it spins up more easily you tend to stick the boot in a bit harder.
But all things being equal take less energy to get going but then you loose a bit when it comes to hills as you have less energy stored as in the heavier flywheel but then again you have overall less mass to move.
To hard to call methinks.

Definately. Infact the more i think about it, the more the small motors overall lack of torque probably cancels any efficiency benifits from a lighter drivetrain.

Say if we were talking about something with more than 2.0L displacement, then yeh it might work out...

I just found this on Autospeed, not related to fuel consumption but may be of interest.

B) Weight loss = for every Kg taken off the crankshaft approximately 4.2Kw is gained in power. Average power gains that can be expected are up to 7Kw for normally aspirated engines and up to 9Kw for forced air systems.

http://www.autospeed.com/shop/category_639/browse.html

As I understand there will be NO top end increase in power. But reducing crankshaft weight will change the shape of the power curve giving more power mid range.

For a minute there I though that bullsh1t was an autospeed article. Pisses me off that people can publish lies with no check.

The best analogy for the effect of lightening a flywheel is like taking weight out of the car.
Various people say the payback is like 1kg off the flywheel is like 3 or 4 or 5 or 10 or 20kg off the car.
Obviously is depends on the car but you get the idea.
Also need to remember that it's all about inertia & weight on the outside of the flywheel counts much more than weight near the centre. Inertia is proportional to the square of the radius.
And so it follows that lightening the pulleys will probably have less than 1/4 of the effect of taking the same amount of weight off the flywheel, due to the diameter differences.
And there's far more weight to be removed from most flywheels.

And so it follows, if a car is driven at constant rpm's, lightening the flywheel will have no effect on fuel consumption.
But if you live in an area with hills and windy roads, where the engine will need to change rpm's quite often, then yes it should be possible to consume less fuel. (Just like you use less fuel without 4 mates stuffed into your car)

Yeah, it will NOT give you any more power, so top speed, will remain unchanged. And as you say above nz_aj, at constant rpm fuel consumption will remain the same.

A lightened flywheel DOES allow more power to reach the wheels under accelerating conditions because less energy is being wasted accelerating the engines rotating parts up to speed! So that should in theory result in less fuel consumption because the engine is not working as hard. I have no idea if that change would be measurable in a real life situation. I guess it would.

If anyone is thinking of fitting a lightened flywheel, any chance you could do before and after dyno tests to show us all the true difference in power distribution?

A lightweight flywheel gives you more torque at the wheels. Because for the same combustion, less of that energy is absorbed by the heavy flywheel, and gets to go directly to the wheels via the crank and gearbox etc.

More torque at a constant RPM means more power at that same RPM. More torque at an engine's highest RPM, means higher power. A lightweight flywheel does increase the engine's top power output.

Trev also proved this at the Melbourne dyno runs. He beat all the other NA micras that had the same mods, he was the only one with a lightweight flywheel with those same mods (extractors, intake etc).

If a lightweight flywheel on a given car somehow dictated that the torque at the peak RPM was the same, well then yes the peak power would remain the same. But this shouldn't ever be the case because whatever energy the motor output before, its now going to have more at any RPM because we've just removed a heap of rotating weight out of the system, allowing more of the engine's effort to go directly to the wheels.

So I'm backing the claim that lightweight flywheels do increase both torque and power. Power = torque x revs.

As for fuel economy, I agree with White Knight and aj somewhat, that its more complex than a simple function. I think it depends on a whole heap of variables, driving style, roads, conditions, car, engine capacity and percentage of power/torque increase that the flywheel brought compared to what power/torque the car had previously. Yeah less effort is required by the engine to make the same power, so for accelerating and getting up it, I would have thought you would save fuel as aj says, but then fuel consumption is also a function of power being made. So if you are making more peak power, depending on a car's ECU, its *possible* that it could feed in more fuel - maybe for safety margins or whatever its reason may be.
But when you're cruising, you're having to keep the foot further down so that car can hold the same momentum, so are you infact using more petrol just to sustain the same constant speed while cruising.. Or depending on gear selection, you might be able to hold the same speed in 5th gear now that you used to have to stay in 4th gear for. Which would indicate that you will be saving fuel just because you can stay in 5th gear instead of 4th.

Interesting!

Once I get my clutch fixed I'll go get another dyno at the same place and we'll have a solid comparison to have a look at.

also will head down the qtr mile again.

time to get practical

Damn.

Who wants to help me with deconstructing the physics behind why a lightened flywheel works, what effect it has being bolted onto the crankshaft, and how much power would the motor make if you measured the power directly from the crankshaft and not the flywheel and therefore would you get an increase in power if you lightened the engine's internals?

lol

Not me.

Reality.

equation 1: T = I * theta-double-dot

ie: the 2nd derivative of theta (where theta is the angular displacement of the flyhweel)

We're effectively using an equivilant of F=ma and an equivilant unit that is 'mass' for spinning things

So lightening ANY component of the engine's moving mechanical parts you will get an increase in response time of the rev changes - you will lose inertia. (same thing, you make something lighter it will be easier to move because it has less inertia)

In layman's terms, it won't take as much power to make the internals spin faster, and the internals will stop spinning faster as they can slow down more quickly.

Here's a diagram. T is torque the engine produces.

(Thanks to alex b for resizing pic and a mate of mine who is 3 years into doing some liquid/quantam mechanics course thing at uni)

And as for fuel consumption differences - came to a conclusion that "we'd be splitting hairs" to try and figure out how much difference it would make.

Although it is safe to say that you will use more fuel while you a) have fun thrashing your revvy engine) and b) while you adjust to driving the car effectively (as you have almost ttoally changed the power and torque characteristics/curve of the motor).

So yeh, the answer to my oriignal question is YES - if you remove weight from the flywheel (or engine's internals, if you were serious) you WILL get an increase in power output.

Its all well and good with numbers but here's some real world stuff for you

Reality is though, that when my ONLY mod was the lightened flywheel, cisco's White Super S had a bit of trouble trying to hold me off (mods were extractors, exhaust, fuel reg and intake)

Surely he was making more power, but I was making more torque at lower RPM's... He'd slowly start pulling away into the higher revs.

Here's the comparison I made after mine was installed...

Model - 1997 Nissan Micra LX
Passengers - Driver Only
Air Temperature - 10.5 Degrees Celsius
Surface - Dry Road (Bitumen)
Modifications - Removed: rear wiper arm assembly, parcel shelf, hubcaps, and mudguards. JUN Lightened Flywheel installed.

Rolling tests:
Before installation - After installation
of JUN Flywheel of JUN Flywheel

2nd gear 30kmh - 60kmh – 4.33 secs - 3.34 secs
3rd gear 60kmh - 90kmh – 5.02 secs - 4.42 secs
4th gear 50kmh - 80kmh – 10.43 secs - 7.40 secs
5th gear 60kmh - 90kmh – 18.20 secs - 14.34 secs

High RPM from dead stop:
1st gear 0kmh - 30kmh – 2.20 secs - 2.47 secs
0kmh - 100kmh – 13.155 secs - 11.97 secs

Time taken for revs to return to idle after 2 secs (full throttle) jab of accelerator:
2.19 secs - 1.43 secs

Time taken for engine braking to reduce speed from 80kmh – 60kmh in 5th gear:
8.20 secs - 12.03 secs

So as you can see, it made a substantial difference to a STOCK Micra.

I didn't record it, but the fuel economy only suffered around 30-50L per tank and I think this is only because I was driving the car with a bit more 'enthusiasm' than I used to

Sean

And to add onto Sean's very informative numbers there, would it be too brave to say that the overall improvement in his times listed were caused by an INCREASE in power?

You would have to stip quite alot of interior trim to get such changes in recorded times.

The flywheel will only give around 3-4kw@wheels gain max. Its in the extra torque that it unleashes where the real gain lies.

Removing non-reciprocating mass has basically no effect compared to reducing drivetrain mass.

If you read the post properly you'd see that cisco's White Super S would start creeping up on me again into the higher revs. As you know, power only really starts to matter in the higher rev range whereas torque is what determins acceleration speed.

Therefore the flywheel generates more torque gained than that of peak power

Sean

The conclusion you draw from your otherwise correct calculations is wrong.
If the flywheel was previously "absorbing" 4kW of power, why doesn't it get red hot. Think about how hot it would get in there running a couple of stove elements flat nacker??? You can say the flywheel is storing energy (mass at speed). If it has less inertia it will store less energy at the same rpm.
Inertia, nothing more. Nothing.
Any dyno figures you see "proving" more power only prove that reducing inertia enables more rapid acceleration. (See formulas above) Any dyno you people are likely to be using, the procedure goes something like stick it in gear & floor it. Power is calculated from how long it takes the dyno to accelerate. If you remove mass from your flywheel, it will appear to be making more power, but you're not. Your wheels are simply accelerating faster, with the same amount of power, because there is less inertia (see formula above).
Go back & reconsider my opening statement about "reducing inertia is like removing weight from the car".
It's the same.
Think.....
If I strip the interior of my car, do I suddenly make more power with the engine because I can now accelerate more quickly?
No, there is just less inertia.

Yep you're right.

I am still confusing the units. Ah well, what more can you do when you bite off more than you can chew other than chew like hell?

Should teach me a lesson for trying to understand things I havent touched on in over 16 months.