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I have heard from several sources (NOT ENGINEERS) that if the rotating mass in a vehicle is reduced then overall HP and speed is increased. As in take off speed and topend speed. Is that referring to the:
1. Flywheel?
2. Driveshaft?
3. Wheel / Rotor?
4. Crankshaft / valvetrain components?

Would reduction in weight for these components result in increased RWHP AND Torque? If it does, is it noticeable for street cars or only race vehicles? My friends tell me the difference in weights from stock is significant. I say it is only beneficial in track / race cars. Who wins the bet? Thanks.
 

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Hi Miles,
I suppose it depends on how you drive your car on the street. I don't think lighter rotational mass will help mileage much.

But lighter rotational weight does improve performance issues, especially unsprung weight in wheels and rotors as well as flywheel and drive shaft weight.
 

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Your vehicle is drag limited. Even if your powertrain and vehicle was weightless the top speed would be the same as long as you are driving though the air.

As far as rotational weight, the less you have the more energy your car can put towards going quicker. If you do lots of stop and go traffic, less rotational mass will help MPG.
 

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wmiles said:
I have heard from several sources (NOT ENGINEERS) that if the rotating mass in a vehicle is reduced then overall HP and speed is increased. As in take off speed and topend speed. Is that referring to the:
1. Flywheel?
2. Driveshaft?
3. Wheel / Rotor?
4. Crankshaft / valvetrain components?

Would reduction in weight for these components result in increased RWHP AND Torque? If it does, is it noticeable for street cars or only race vehicles? My friends tell me the difference in weights from stock is significant. I say it is only beneficial in track / race cars. Who wins the bet? Thanks.
all of the above could be considered rotational mass, depending on the context you are speaking of.

Generally, lightening any of the above should increase acceleration. drivetrain related components will help the engine rev faster, and therefore will tend to help accelerate faster. Decreasing wheel/tire weight will improve handling, as the wheels will change direction; i.e. easier turning.

None of the above will have any meaningful direct effect on engine power output (as measured at the flywheel), but they will have the effect on reducing driveline losses, so may show up as an increase in power as measured at the rear wheel.



of the above, lightweight driveshafts would likely make the biggest difference, and one that you can feel - even on the street.

Next would be lighter wheels/brakes, this would likely more be felt as being able to turn the steering wheel easier. Issue here being that if you go too far then you may compromise the ability of the suspension to 'keep a line', it will be so easy for the wheels to turn (side to side) that you may need to constantly correct your direction - kind of like trying to follow a set of ruts that you don't quite fit in. Find the right balance, and you will have a handling improvement that you can use on the street.

Lighter flywheels, a fairly common upgrade, even for street cars. Engine will rev faster. Be careful though, flywheels are often designed to be part 2 of engine dampening and are made to work in conjunction with the front crank damper. Lightening one could cause the engine to be thrown slightly out of balance and suffer wierd harmonics and possibly shorter service life.

Lightened crankshafts, leave to dedicated race cars. They will let the engine rev more freely (FUN!), but tend to be weaker. Tends to kill the fun when the crank snaps...

lightened valvetrain, also leave to race cars. the gained benefits tend to be so small that it's not worth the greatly increased cost and finicky life...





So, short answer: They will all benefit a street car to various degrees, but may not always be worth the potential headaches. But, carefully selected lightweight components will increase a street cars performance noticably. IMHO, your friend wins that bet, sorry.



Personally, I plan to go to a lightweight (aluminum) driveshaft on my Bullitt sometime this summer Several other members here have already, and love they them.

On my '89 Taurus SHO (3.0 Yamaha DOHC FTW!!!) I will be be changing to lighter wheels and low mass flywheel. Unfortunately, I will also be increasing brake rotor size and therefor increase wieght, as the stocker 10" front discs are far too small to be really useful - especially if I decide to throw a Turbo on it, like I am contemplating.
 

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Titanium valves, rods and crankshaft----need very deep pockets for these
 

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rblack4405 said:
Titanium valves, rods and crankshaft----need very deep pockets for these

OUCH!

You are very correct about that sir.
 

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As Mike says, be careful lightening things like the flywheel. You can actually hurt performance, and in a street car especially.
 

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wmiles said:
I have heard from several sources (NOT ENGINEERS) that if the rotating mass in a vehicle is reduced then overall HP and speed is increased. As in take off speed and topend speed. Is that referring to the:
1. Flywheel?
2. Driveshaft?
3. Wheel / Rotor?
4. Crankshaft / valvetrain components?

Would reduction in weight for these components result in increased RWHP AND Torque? If it does, is it noticeable for street cars or only race vehicles? My friends tell me the difference in weights from stock is significant. I say it is only beneficial in track / race cars. Who wins the bet? Thanks.
Eliminating weight from a vehicle will aid its acceleration, but eliminating weight from the powertrain will provide greater benefit for your efforts. You will not only gain acceleration, but increase horsepower.

You don't have to stop at the aforementioned items. Eliminating weight from your hubs, rotors, wheels, tires etc. far outweighs the benefits of reducing weight from non-rotating parts.

I have heard others say that the advantage of rotational mass reduction over non-rotational is 1:7.

I personally cannot prescribe to this, because rotational speed and location of reduction has to be factored in, but I guess we could use the 1:7 ratio loosely ;)
 

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Ya ya ya, it helps.

But it's much easier just to add more power! :D

More fun too. ;)
 

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As others have pointed out:

Reducing drive line rotationg mass will improve acceleration and reducing unsprung weight will improve handling. Any weight loss will improve mileage and acceleration performance.

Your top speed is limited by horsepower and wind resistance (drag). But reducing rotating mass will get you to your top speed quicker.

From aircraft.....increasing hp does little for making a plane or helicopter go faster. It can increase useful load and make a difference in climb performance, but it takes a lot of hp to go just a little faster. Above about 120 mph or so, it's all about the drag/wind resistance.

As also pointed out....one downside to a TOO lightened driveline is the the rapid decay of rpms (between shifts for example). In some cases this can actually make the car harder to drive as well as the reliability issues already pointed out.

As an aside....in helicopters this rotating mass are the rotor blades. In the case of a small, light helicopter (ie Robinson R22), you get a cool 1.5 seconds or less to lower the collective pitch control in the event of an engine failure. After that the blades get so aerodynamically distorted that they basically just stop spinning and fold up. And the fuesalage takes on the flight path of a flung steel I beam.

So, extra rotating mass isn't always a bad thing.
 

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one thing not mentioned is the effects on torque...lightened rotating mass will equate to faster spool ups and increased HP....but....rotating mass is also important when one takes torque into consideration. It may not necessarily be that much of an issue in high output engines, but anyone who has ever worked with low horsepower engines knows that a heavy flywheel is the most critical component towards useable output. What this equates to is the mindset that any changes you make to an engine, while having a positive change in one area may have a negative change in another....it's called balance....the question is always 'what do you want?'
 
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