VAP

there seems to be ample before/after wheel and brake rotor dyno tests using heavy/light wheels & rotors.

Almost all of these tests reveal a +/- 1WHP equivalency net increase per rotational mass pound removed. However with regard to rotors the lighter versions were never better in road course applications as lap times slowed due to smaller, less efficient brakes in spite of better/faster straight-line acceleration. Of course these tests are rife with different engine displacements and I'll be the first to admit thats a determining factor. A 5.9 litre engine will reap less benefit than a 2 litre engine.

While I'm not ready to embrace these numbers as gospel I do accept they're probably close to actual. Least closer than any guess I might come up with. And one thing I know to be empirical is that a lighter driveline rotational mass doesnt really "make" HP per se it does emulate it. A dyno will show no HP increase for driveline it will show it for axles. Because a dyno is static and only hooked up at the wheels it cant postuate driveline results. Conversely something like DynoRad or G-Tech Pro will show the benefit as HP increase due to time/distance parameters it uses that a chassis dyno cant.

If I factor in a conservative 25% reduction the numbers are still impressive. If I use a 50% reduction the benefit vs cost ratios are still fiscally justifiable.

Given the above info I've researched that would mean my flywheel/driveshaft modification are worth 37, 28 or 17.5WHP depending on which numbers we believe. If only driveshaft is considered that alone is a 15, 11.25 or 7.5WHP increase for a cost of only $100. Wholly under-priced/CHEAP modification for HP gain on a normally aspirated engine! Add $50 for a used 200TQ flywheel and a $100 flywheel lightening charge and its still a very cost effective approach to power. In fact we may have all wasted a little too much time trying to find additional ponies in the engine. There is a huge untapped gold mine of power available to us just in the drive line. If the above numbers are true there could be 61WHP untapped just in lightened flywheel, axle CVs, drive shaft/drive shaft CVs. Its certainly worth exploring!

If anyone can find me a rotational mass vs HP calculator or site that delves into the subject I'd love to reasearch this further.

Herb Adams, in his book CHASSIS ENGINEERING, says that "..the effect of reducing rotational inertia on driveline parts has 15 times the benefit of just reducing the weight of the car." If that statement is true and a lightened driveline doesnt even make 1WHP difference and you did as I'm doing by making the driveline 61lbs lighter it would still be the par equivalent of driving a 915lb lighter car with the same HP you have now! In my case a 172HP stock 12V engine in a 2,105lb (equivalent weight) car is almost more than I can bear to imagine. It would be a screamer on par with almost any normally aspirated and many turbo'd cars currently sold!

virtuapete

can we explore doing this for the coupe & other owners?

ill work on finding that calculator for you

Merkin

flywheel nets most gain due to highest rotation speed while drive shafts and CVs don't spin nearly as fast and totally gear dependent.

2nd gear @6k the flywheel nets a lot of gain but tranny is still going at 60mph.

Just Paul

Even if it doesn't take that much energy to keep everything turning, as your speed increases, work required to sustain that speed goes up exponentially. For example, to keep the driveshaft spinning at 4K rpm it takes X, and at 5K it takes x^3. If you can reduce x by the slightest amount, you've accomplished plenty at x^3.

I'm sure we can both agree as your speed increases you are going to "feel" that extra HP that is now available to accelerate the car further, say as the driveshaft RPM passes 5K rpm. Agree?

So the power gain would be noticeable but not necessarily at slower speeds (unless you're talking about the flywheel mods). Think of it as a shallower "power required" curve and a stationary "power available" curve.

Merkin

to factor in drive shafts and such.

now how fast is the car traveling when the drive shaft is spinning at 5k rpm.

only reason cars need exponentially more power to travel at 150 compared to 70 is because of wind drag.

also driveshaft turns much slower than the engine at lower speeds in low gears.

lightened parts surely will make a car accelerate faster, but once you get past 70mph drag factor is greater than weight factor.

Just Paul

First: Regardless if "drag is greater than weight", if you reduce weight you decrease the power required to maintain any speed. Furthermore, you reduce that power required to a greater degree if that weight is taken from the drivetrain (as stated by Herb Adams).

but back to aerodynics:

If you take into account aerodynamic drag, the power required rises exponentially with speed. If not it rises in a linear fasion. Even by multiplying the ft/lbs required to sustain such a situation, over a wide range I could see quite clearly how great of an effect weight savings would have on an automobile, especially if that weight has been taken from rotating components, regardless of speed.

VAP

no matter how light you make a flywheel, drive shaft, driveline, axles, CVs or wheels. Nothing in this "makes" engine HP. What it does is reduce driveline loss. Frees up power to the wheels that were lost before due to heavier rotational mass aft of the crank. There is nothing in the driveline that makes more power in the engine. Any/all increases in power are as a result of reduced driveline losses. While engine HP remains constant wheel HP is increased. Run a 172HP engine in a 3,300lb car and we all know what we get. Run that same engine in a 2,000lb car and the engine still has 172HP but the car now performs like our 3,300lb car with 400HP. And "that" aspect is not due to any transmission ratio but altogether in spite of it. Tho a larger displacement engine will enjoy less gain than a smaller one a transmissions ratios wont affect the % WHP increase as the physical law holds true at any/all ratios.

Regardless of ratios, or if its a one-speed transmission or a seven speed transmission the physical law is unchanged as will be any/all results relating to percentage increases on like-sized displacement engines with same engine HP/TQ numbers.

It's the law!

GregPac

<center><img src="http://pictureposter.audiworld.com/48348/flywheel.jpg"></center><p>There are two ways lightening a rotating driveline component increases power. First, and most obvious, is simply decreasing the weight of the car. I haven't done the calculation of that effect, but I've seen estimates of about a 1hp increase for every 10lb removed (though I think that's a bit generous).

The second effect is reducing the amount of power required to accelerate the rotating mass ( flywheel, driveshaft, rotor, etc). If air drag on the rotating part and friction in bearings are neglected, it take no power to maintain a constant rotational speed of a part. The power consumption is in the acceleration.

The power needed to accelerate a rotating mass is linearly proportional to it's weight. Therefore cutting the weight in half cuts the power needed to accelerate it in half. The key is calculating what the absolute values are. If it only takes 1 hp to accelerate a flywheel, it's not cost effective to spend alot of money to cut it's weight in half.

I started with an assumption that the car will accelerate from 0-60 mph in a fixed time. From there I calculated the average angular acceleration of the flywheel, taking into account the tire diameter, final drive ratio, and transmission gear ratio. Next, the rotational moment of inertia of the flywheel was calculated using it's weight and diameter. I assumed the weight was evenly distributed along the diameter. The torque is then the moment of inertia times the rotational acceleration. Horsepower is finally calculated from torque and RPM.

In the example above the power consumed by a 30lb flywheel in accelerating the car from 0-60 in 5 seconds was 16.2hp. A 15lb flywheel would therefore only consume 8hp, freeing more power to accelerating the mass of the car.

Note that the effect will be greatest in first gear, where the acceleration of the flywheel is greatest compared to the wheels.

Running the calculations again for a driveshaft prove interesting. Since it is downstream of the transmission, I set the gear ration to 1. The results show loss in a 4" diameter 15lb driveshaft is less than 1hp when accelerating the car from 0-60 in 5 seconds.

The calculator can also be used for wheels, tires, and brake rotors. They are on the downstream side of the final drive, so that must be set to 1. It take about 3/4 of a HP to accelerate each 40lb wheel/tire under the same conditions. There doesn't appear to be a significant gain available from lightweight wheel or tires.


Mance, reply with the actual numbers for your setup and I'll rerun the calculations. I can also email you the spreadsheet if you'd like to play with it yourself.

If anyone sees an error in my calculations let me know and I'll try to resolve it.

I hope this helps clear things up.

GregPac

Check out the technical article under the lightweight flywheel section. They take a different approach than I did, which ends up with a much more agressive prediction.

I'm not sure I agree with the basic premis of their calculation, which sets the rotational kinetic energy equal to the translational kinetic energy. I'll have to think about that a bit more.<ul><li><a href="http://www.uucmotorwerks.com">UUC Motorwerks</a></li></ul>