azaz

I have been entertaining the thought of adding a JWT flywheel, and was wondering if anyone with a lightweight flywheel can give me some answers to a question or two.

1. Has your mpg increased or decreased with the addition of the
flywheel?

It seems to me that it would cause the mpg to decrease, but I would like for the one's that have the flywheels to speak up and let me know either way. If so how much has it changed?

2.How does if affect torque? Negatively or positively?


Please give me as much info as you can, as to how and why it affects it either way.

azaz

Does no one like me LOL I didn't think it was that hard of a question.

How about it?

Everyone with a lightweight flywheel check your mileage and let's see what you are getting!

jreiter

iTrader: (6)

No change in my mileage. I still get about 25-26 mpg on the freeway cruising at about 75 mph.

Also, slightly quicker acceleration in the lower gears, but only a little. Maybe.

People often say that a lighter weight flywheel can reduce your torque, but that's just not right. Your engine is still making the *exact* same power and torque regardless of flywheel weight. With a lighter flywheel, though, you're simply removing some of the restriction caused by the inertia of the heavier stock flywheel. Due to the removal of that extra rotational intertia, my car accelerates a bit quicker and engine brakes a bit harder with the JWT flywheel.

The single best advantage of the JWT is, if you ask me, the ability to rev match faster for faster shifting. If you're looking to get a whole lot more out of it than that, you might be disappointed.

All this being said, I love mine. I think a 14 pound flywheel should've been stock on the Z. It's not much more difficult to drive in stop and go traffic, by any means. The slight gravel sound below 2k rpms under load is a bit weird, but not a big deal.

azaz

Thanks for your reply Jreiter. I really do appreciate your time to explain things so well.

Anyone else have .02 to add, or did Jreiter sum it all up in one post?

TIA,
Tony

Jason@Performance

iTrader: (11)

If you only change the flywheel and keep the stock clutch you arent doing a huge weight reduction so you dont loose alot of MPG... You will gain acceleration horse power. not peak horse power...

If you change your clutch to a lighter clutch like the NISMO you will have even more of a weight reduction so you will loose more MPG but have better acclereation...

If you go to the extreme and get the TILTON Flywheel and clutch set up (Flywheel and clutch weighing 16lbs combined!) then yuou will expereince a large drop in MPG on the freeway but acclereation horse power will be increased 30RWHP...

http://members.aol.com/performancenismo/tilton.avi

azaz

Cool, thanks Jason! I am looking into changing both flywheel and clutch. Most likely it will be the JWT with the Nismo clutch, at least that is what I am leaning towards right now.

Would love to do the Tilton setup, but I do drive in traffic a bit and the price for the tilton isn't in my budget (man these house payments take my play money away)

Thanks for the replies so far, anyone else care to gimme their .02?

eazy

Jason - What exactly is the reason for the significant drop in MPG on the highway? I would have thought it would be the opposite. The lighter weight is a twofold advantage. There is less energy being tied up in the drivetrain, and there is less overall weight to move forward. How would that negatively affect fuel consumption on the highway? In city driving I could see MPG getting hurt from over-revving and revving the engine much more than you would normally. I guess I just don't understand how a weight reduction is supposed to cause a significant drop in MPG. Can you clear things up for me? TIA!

jpc350

I have seen this same claim made before (By Jason)? . The thought is since the car slows down faster with the lightened flywheel that additional fuel is needed to get the accel back up. Which is partially correct. The car does decelerate faster but it also takes less fuel (not more ) to get the flywheel up to rev. A lighter rotating mass requires less energy (fuel) to add initial angular momentum keeping it there as well as restoring any loss of angular momentum when slowing down..

jreiter

iTrader: (6)

jpc350 brought up the same issues I'm pondering. Yes, a lighter weight flywheel has increased engine braking so it slows down quicker when you take your foot off the gas. Thus, it seems like it would become harder to smoothly maintain your speed on the freeway. (You'd be on and off the gas all the time.) With a heavy flywheel, the massive weight of the flywheel helps smooth things out.

However, I'm still not sure why this would affect freeway mileage. There must be some aspect I'm not considering. Even though a heavy flywheel can help smoothly maintain your speed, it takes more gas to get it up to speed. (A lighter flywheel takes less gas to get it up to the same speed.) Either way, once you are up to speed, it should take the same amount of fuel to maintain the speed of both flywheels. This is an important thing to remember: *maintaining* the speed of a light vs heavy flywheel should take the same work. It's when you *change* their speed that the weight difference comes into play. A lighter flywheel's speed will more easily be changed.

But anyway, I'm guessing there is some other factor here that we're not considering that is affecting gas mileage with a lighter flywheel. It's probably something to do with the inaccuracy of the human foot on the throttle pedal. In order to maintain our speed on the freeway, we might be letting off too much, then back on too much, back and forth the whole trip. That could have adverse mileage affects. Also, I've found the cruise control in the Z to be a bit hamfisted, too, especially after putting in a lightweight flywheel. It overcompensates a bit much when letting off the gas. Slows down too quick, then has to catch back up.

And I'd also have to disagree with Jason's view on the flywheel weight differences. He says only going down to a JWT isn't much change, but geez. That's going from a 28-30 pound flywheel to a 14! A 50% flywheel weight reduction is a big difference in my book! (Although I do agree that going from stock to the Tilton is way, way more of a change... obviously.) That would just exacerbate these theoretical throttle overcompensations even more, I suppose.

azaz

My theory on the flywheel mpg issue is kinda like spinning a bicycle tire at 30 mph and spinning a car tire at 30 mph. The one with more inertia will be the harder one to stop. So the heavier flywheel keeps the engine spinning easier useing less power from the engine to maintain motion. Thus less engine braking. So even though it takes less power to move the lighter flywheel initially, it takes more power to keep it moving because it has less inertia. Thus the increased engine braking.

I could be wrong on this I'm no engineer, and I don't have any proof to back this up. It just seems like it would work that way to me. I could be wrong so if there are any engineers that care to explain this please feel free to.

Jason@Performance

iTrader: (11)

if the flywheel doesnt hold as much inertia, then it will slow down faster... when you are crusing on the freeway at say 70mph in 6th gear... you are appling some throttle and the flywheel keeps momentom going... if that flywheel will not keep the same momentom you have to apply more throttle there for using more gas...

sorry about the spelling... staurday night

Murry

That sounds close but not entirely right to me. According to your theory if I reduce the mass of my car will MPG decrease too.

jpc350

A simple analogy to verify a lighter flywheel takes less fuel (energy) than a heavier one..Imagine a bicycle that has a light rear wheel. Try imagining going on a long trip. Then imagine taking that same trip on a bike that has a heavy rear wheel.. At the end of the trips which bike will have you dissipate more energy?

Jason, while your argument seems intuitive (loosing more momentum) the lighter fly wheel has less mass (less inertia) and requires less energy(fuel ) to get it up to rpm's initially as well as restoring it when slowing down than a heavier one

(H=IW).. H=Momentum, I = Inertia, W=Angular velocity ( convert to rpm)
A smaller I results in a lower H to maintain the same W

azaz

According to Jason mpg will be reduced. I was just interested in how much. That seems to be directly related to how much weight reduction you have. I guess the how's and why's this occurs needs to be researched, but it has been recorded by those that have the lighter flywheels that mpg will drop with a lighter flywheel.

Murry

I'm sure it has more to do with how they drive than the energy stored in the flywheel. Use cruise control on a car with a stock flywheel and then on a car with a lightened flywheel to eliminate the human factor. There is no reason for lower mpg in a car with a lighter flywheel. As jpc350 pointed out, flywheels with lower moments of inertia require less torque produce the same amount of acceleration T=I*alpha (form of F=m*a)

John@Victory

iTrader: (36)

jason is speaking from experiance not thearetics
he has the clutch in his car
if he says you lose mpg then he is qualified to say so as the tilton is in his car

Murry

Science and Scientific method don't take anyones word as proof-no offense. There could be another explanation for his observed decrease in MPG.

alininger2001

iTrader: (1)

The flywheel is lighter, this reduces rotational mass, which would require less energy to accelerate. This is good for a performance standpoint. The problem is the lighter flywheel will store less kinetic energy than a heavier flywheel when they spin at the same rpm. Even if you did have cruise control on the engine will have to speed up as the drive train slows down. If the flywheel causes the drivetrain to slow down faster, which it doesthe engine will have to work harder to maintain its speed, thus decreasing you MPG on the highway. There is a possisbilty it will improve your MPG in the city, which would help us, but not on the highway.

John@Victory

iTrader: (36)

forget it...you alread have the answers..
not going to debate it

Murry

The energy stored in a flywheel is =1/2Iw^2 or the rotational moment of inertia-which is analogous to mass in a linear system-multiplied by the square of its angular velocity. Take two flywheels made out of the same material and with the same outer diameter but one is twice as thick as the other and you can see that the thicker one will have twice the energy stored in it than the thinner version if they are spinning at the same speed. It is true that it takes more energy to slow the thick flywheel but it takes the same amount of energy to get it back up to speed. The net change in energy to the system is zero-assuming that you didn't disengage the clutch or hit the brakes.