Lightening the Flywheel

In his "Easy Modifications" advice, John Connolly (Aircooled.Net) wrote the following regarding lightening the flywheel -

Note: Engine removal is required for this modification.

If you are uncoordinated at driving a stick, or have a HEAVY car (Baja, Bus or Type 3 or 4), this modification is NOT for you. The lightened flywheel will allow the engine to accelerate (resppond) faster. Do NOT add a heavy duty clutch unless you get an 8-doweled crank and flywheel to keep the flywheel on tight! Don't forget that changing flywheels requires you to re-set the endplay, and it's strongly recommended (by me) that you change your flywheel seal (main seal) and o-ring. You are in there, so you may as well do it. Check your clutch while you're at it, too!

Here's the theory -

Lightening the flywheel cannot result in increased steady state horsepower as measured by a dynamometer (dyno). After all, the flywheel is not an energy creating device. However, since a flywheel does absorb and store some of the energy generated by the engine during acceleration, a lighter flywheel does result in increased transient state energy delivered to the rear wheel, and some therefore measurable acceleration improvement.

A lightened flywheel will not increase horsepower, but it sure does decrease acceleration times. It has been said that "..the effect of reducing rotational inertia on driveline parts has 15 times the benefit of just reducing the weight of the car." Supposedly the difference in how the engine spools up is not subtle, but positively sensational. Some have said that lightening the flywheel "is the single most important modification that I have made in quite a while, and I have made quite a few."

As far as perceived acceleration goes, your engine sees the mass of your car as a point stuck way out on some lever arm that it has to twist. If your engine is direct drive (i.e. no gear reduction), you'd need quite a bit of torque to get that all that weight moving faster. So somebody invented gears, which has the effect of changing the length of the lever, as far as the engine is concerned.

Suppose you had a magic flywheel with all the mass concentrated at the outer edge. Now the flywheel is stuck directly to the engine, so you can't reduce its effective moment via gearing. The only way you can reduce the moment is by lightening it and/or changing its mass distribution. If you could somehow remove 10 lbs from the rim of the flywheel, and the flywheel's radius was also one foot, then that would have the same effect on acceleration in first gear as reducing the mass of the car by 10x13.57 or 135.7 pounds. Now I am guessing the flywheel's radius is more like six to eight inches or so, so 10 pounds off it's outer edge would have the same effect as reducing the car's mass by more like 70-100 pounds (in first gear). Only you can't take that much weight off the edge, and moments of disks look more like 1/2mr², etc. etc. The point is that in first gear, the mass of your car appears to be only 20-30 times that of 10 pounds out at the edge of your flywheel, as far as the engine can tell. So the reduction of weight of the flywheel begins to be pretty significant. Expect bigtime effective acceleration improvements in first gear for proper flywheel lightening, similar to what you'd expect from reducing the weight of the car by anywhere from 70 to 100 lbs or more. The benefits decrease in higher gears in proportion to whatever the ratio is.

Obviously, the lighter your car is to begin with, the bigger an acceleration improvement you'll see since the flywheel mass represents a larger portion of the perceived mass of the car.

Rob's comment: Whilst all the above is true, it's important to note that the VW engineers did not put ANYTHING on the car they did not think was needed - it was built to a price like every other car on the market - and at the CHEAP end of the car market at that. So whilst lightening the flywheel will benefit you in one area (engine response time and improved acceleration in the lower gears) there IS a price to pay. The flywheel is there to do one thing which has two distinct benefits - it adds momentum (stored energy) to the spinning engine. Firstly, it smooths out the power pulses between the firings. Your 4 cylinder engine only fires twice in each revolution, so it needs to STORE some of that power between firing pulses so you get a smooth running engine. That's especially so at idle and low RPMs where the power pulses are small. The second thing is that to get the car moving, you need to apply power from the spinning engine to the stationary vehicle via the clutch. The energy stored in the flywheel is very important in getting a smooth transition from stationary to moving. So, if you lighten your flywheel, the price you will pay is a "lumpy" idle and it might need to idle at higher rpms, which means changes to the timing and the carburettor; and it also means you'll have to rev the engine more when moving off or risk stalling. This increases the wear on your clutch plate at a miminum. Don't underestimate these effects either, continually having to goose your throttle at each set of traffic lights WILL get wearing after a while, and if your partner drives your car, there will be a LOT of bunny hops and stalling before they get used to the different way of driving. Your car, your choice.