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It is the total gear ratio squared, where the gear ratio also includes the ratio of the effective radius of the flywheel to the radius of the tire. One factor goes into the angular acceleration of the flywheel, and one factor goes into the torque at the axles. So this can be as much as 100 in 1st gear, and that would really affect the launch that you can get. In 2nd gear, about half as much effect, and going down from there.
No differences there @Meller. We are on the same page. I don´t have to explain yoú why I am also striving to reduce mass of the other things hooked up to the crankshaft thus gearbox thus rear wheels.

Problem with first gear is that unless it is a first gear hairpin, you lose the bulk of the advantage in feel difference on take off with the clutch. Once going with the clutch fully home you indeed do have the full advantage but I mentioned féél and the short time left in first does not give much info to notice the difference. Second and third, although less pronounced in difference, give more féél.
Nevertheless, your numbers totally underline the HUGE effect.

What is crystal clear is that the rear axle is a whole lot more lively witch less rotating engine mass. Mine has lsd and it is an almost sinful joy to light up the rear. It is só connected to the loud pedal.

All in all, though it does not give you any more hp, those availeble hps have a less heavy task so have more effect.

Thanks @Funnyman
 

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One thing nevvah evvah mentioned it that is not only about the weight on the scale but more importantly the distribution of that weight in the part. In this case vary aptly it is all about the MR2; the distance from the mass to the centre of rotation. A lighter one may very well have móre rotational inertia than a different design slightly heavier one.
Do compare the Fidanza aluminium and several steel ones. The CC one is a real good example of mass reduced where it counts most; at the outside.



I have little doubt (no proof so still room for doubt) that the CC steel one has less MR2 than the Fidanza flywheel making the safer/cheaper choice look even better.
It's definitely a night-and-day difference compared to the stock flywheel.

Interesting that you brought up the alternator in another thread because I've always thought we can potentially improve engine response further between shifts with a lighter alternator. My only question is whether or not it will be sufficient for charging.

I look forward to your findings because I'm constantly looking for ways to improve the engine response. Because everything is mostly light (comparatively speaking) to begin with, it becomes harder and harder to find areas that can be improved.
 

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@Celia

As to charging it should be well enough for my use.
Would fit an alu waterpump pulley if it where available. Will probably have one made when I find a 1zz water pump at the breakers. there is some 200 grams there to be gained relatively easy and it all adds up. The idler wheel on the tensioner is not worth the hassle.
Counting the 1.5 kg off the alternator rotor and not counting the airco delete I will be at over 6 kg less rotating engine mass.
Your numbers illustrate how noticeable it is on my sub 900 kg car with Enkei RPF1s, even not taking the 100x in first ;)

Concerning all being relatively light, that cuts both ways since all lightness added weighs relatively a lot.
I mean 10 kgs off a 1500 kg car are less noticeable than off a 750 kg one.
I have clearly felt this when starting off with the 1060 kg (topped up). Every weighty delete was increasingly noticeable.
Talking about the engine only it is not all thát lightweight because the engine is one from the generic parts shelves used in various models. The alternator being a good example of being óverdimensioned for the MR2.
Same thing the harmonic damper on the crankshaft: If you delete the airco, lighten the flywheel, lighten the alternator and pulley, the whole harmonic thing is wáy off so you decide about the ´science´.
 

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@Celia

As to charging it should be well enough for my use.
Would fit an alu waterpump pulley if it where available. Will probably have one made when I find a 1zz water pump at the breakers. there is some 200 grams there to be gained relatively easy and it all adds up. The idler wheel on the tensioner is not worth the hassle.
Counting the 1.5 kg off the alternator rotor and not counting the airco delete I will be at over 6 kg less rotating engine mass.
Your numbers illustrate how noticeable it is on my sub 900 kg car with Enkei RPF1s, even not taking the 100x in first ;)

Concerning all being relatively light, that cuts both ways since all lightness added weighs relatively a lot.
I mean 10 kgs off a 1500 kg car are less noticeable than off a 750 kg one.
I have clearly felt this when starting off with the 1060 kg (topped up). Every weighty delete was increasingly noticeable.
Talking about the engine only it is not all thát lightweight because the engine is one from the generic parts shelves used in various models. The alternator being a good example of being óverdimensioned for the MR2.
Same thing the harmonic damper on the crankshaft: If you delete the airco, lighten the flywheel, lighten the alternator and pulley, the whole harmonic thing is wáy off so you decide about the ´science´.
What did you do with the harness connector that leads to the compressor?

Did you just cut it off and cap that portion of the harness to prevent it from hanging around in the engine bay?
 

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For those not getting i or doubting the numbers like the 100, 50 times, it is perhaps useful to understand that it; 3 kilos off the rotating engine mass times 100 = 300 kilos is NOT taking off 300 kg off of the car. It is taking it off the moment of inertia of the engine which is through the gearbox directly connected to the rear wheels.
When adding lightness by taking is off the car it is simple but only part of the masses involved. The rotating mass of the engine in effect on acceleration ´weighs´ more than it´s weight % of the car.

The 100, 50 or whichever gear multiplication applies in OEM guise too. Decreasing the rotating mass must be seen against the OEM moment of inertia of the drive train.
Reducing that in effect with 6 kilo times the multiplication factor equals the power produced by the engine liberated to do work on the mass of the car.

In other words; the engine does twó jobs; accelerating the rotating mass of itself ánd propel the car. Reducing the former means more engine power to accelerate the cár and the effect is consíderable.

A good example illustrating it is a supercharger. You can see it as an engine propelled by gasoline; a pump propelled by an external power source.

A supercharger ´generating´ 50 hp. has another job; that of propelling/accelerating itself and it´s own mechanical losses. The 50 hp generated are in fact 75 generated.
Reducing the friction, reducing the weight of the rotors etc. means reducing the internal job, making it say 15 hp meaning that exact same supercharger now can generate 65 hp with the same driving force.

Moral of the story is that in this case less líberates more.
 

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I'm in the market for a new clutch, and I'm seriously thinking about getting a lighter flywheel. It's for a stock 2000 Spyder that I drive about 1,000 miles a year, so I'm leaning towards the Exedy ST1 clutch. The difference in weight between Fidanza (alum) or steel (MWR) is 0.3 lbs. Everything I can find on the differences refers to rotating mass and drag vs daily drivers.

As you all know, the MR2 is a go kart of grown men, so I'm looking for ways to liven it up. Are there any advantages/disadvantages to aluminum vs steel lightweight flywheels?
Grown Women and men!
 

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So, I have a lightened flywheel in my ALFA Spider… Did it during a gear box rebuild because i felt like the stock one was holding up the gear box revs and putting excessive wear on the synchros. So, not a daily driver. I probably stall it proportionally more than I do the MR2 or my daily WRX. But humiliation brings learning so I don’t do it that often. FWIW, the only real downside is when I run the A/C. The 88 ALFA is Bosch Jetronic (analog ) injection control so idle control is primitive and she’ll stall at traffic lights if you dont watch it and hold the revs up when the compressor kicks in.
 

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Was researching the moment of inertia of the ice engine and tripped over a representative value.
The 2006 Honda 1.8 i-VTEC engine including the drive train has an MI = 0.1782 kg/m2

This seems low but it is the weight of the rotating parts in relation to the radius. For a disc that is the weight times half the diameter. For say a camshaft or a crankshaft it is less than half the diameter of the rotating circle of course.

With the value for the Honda engine we have a nice benchmark reference for the 1ZZ-FE. With this benchmark it is now easy to see the effect of a lighter flywheel, crank pully etc.
Assuming that the flywheel is a disc with the weight evenly distributed, with half the radius being 7.5 cm. = 0.075 m. then a 3.5 kg reduction is 0.005625 kg.m2 amounting to 3% total of the engine´s MI.
Do note the assumption about the flywheel whereas in reality the weight reduction is mostly outsíde of half the radius. A q&d gives me close to 4%.

Maybe 4% does not look impressive but it is. Just by fitting a 3.5 kg lighter flywheel, the WHOLE rotating inertia of the entire drive thing has been reduced by 3% which can directly be related to the power needed = 4% less power loss.

You can do the same for a lighter crank pully, ditto alternator pulley and slowing the rotor down by say 30% for the MI equals reducing the weight by 30%.

For the SMT the effect is larger still because that is with the engine uncoupled from the gearbox.

This proza is not about the 3 or 4 or however much %.
It does go to show that it is quantifiable as well as the order of effect.

Taking this to driving reality the effect on acceleration of the cár is obviously reduced by the mass of the whole thing and depends on the reduction, the gear you are in.
The effect on lighting up the rubber though is not diluted by the car mass and in the lowest gears the effect is massive fun 🥰
 
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