Confirm understanding of lightweight mods
 

I was reading a thread about lightweight drive shaft and flywheel, something I might be looking into either before or after going FI. Something, something "rotational mass"..

So the consensus I would like to confirm is:

If you put a lightweight flywheel and drive shaft the car does not move any faster, but the engine spins faster, which only makes the car feel faster and more responsive to input.

Can someone confirm my understanding? I understand that its not adding power, so can I basically describe it as either one of the following:

1. car makes the same power, but the wheels receive it sooner (less input lag?)
OR

2. car makes the same power, but less of the power is lost before reaching the wheels. (so 205bhp/175whp -> 205bhp/185whp after mods?)

Thanks

Well if you want to get technical you will see an decrease in weight which would make the car faster, how much faster is the real question. (not enough to notice)

OP: you have the right idea. The car should be faster, and feel more responsive, but all this would likely be difficult to measure as the driver adds in too much inconsistency.

Those mods are all part of the bigger picture. You car might be just a bit more responsive but this is not really the objective of these mods. Once you combine light driveshaft and flywheel with light wheels, light brakes, light batery and interior delete and basically any diet mod your car will loose a lot of weight and that is good. Nevertheless I still think it's a good mod if you have the money.

The car moves faster and with a lightweight flywheel it brakes also faster (less stored rotational energy).

Not all of the power is lost. Some is lost and is converted to temperature. Other as mentioned above is stored to the flywheel.

#2 is more accurate except the difference is probably half that or less. There's little to be gained beyond the static weight loss because the gains are also dependent on the acceleration rate, and the acceleration rate of the flywheel and driveshaft are limited by the acceleration of the car.

Braking is same no matter what flywheel, and how short braking distance can be depends almost only from tires. Be it engine braking (of different flywheel mass)+some amount of normal brake usage, or clutch out (and no flywheel impact at all) for very probable downshift and just brake usage, car will stop same, one just provides braking resistance/friction between car & tires using different means. Max grip from tires to road won't change, thus shortest possible braking distance too. If engine pumping losses + brakes will lock wheels, or just brake press will lock, braking won't change. And if one counts flywheel mass as static weight, then one kg less or more won't affect braking distance noticeably. Simplest to not count different flywheel impact in braking at all.
As for acceleration, it will improve with lighter flywheel. Flywheel needs energy to spin up or spin down, and if it turns at crank speed (rotational energy depends on rpms / mass / size-distance from rotational center) there might be some energy to be saved up for actually accelerating car. But: 1) worth to not go overboard, for example to not lighten both crankshaft pulley & flywheel, do only one, as some torsional vibrations dampening is needed even for boxer engines, for crankshaft bearings to last longer, 2) gains might be less then expected, possibly making other areas of improving performance as better as buck/performance, 3) too light flywheel may add some difficulties in rev matching/at daily driving, due very quick rpm fall-off. 4) remember that max speed or fuel economy is not impacted by different mass of rotational weights. Once it's something spun up at some speed and energy is stored in that spinning mass, keeping moving at constant speed will require same energy to overcome same friction at drivetrain/engine/tranny and air drag and tire rolling resistance losses (which are constant at constant speed).
I recall here seeing thread on how much someone saved on lightening miscellaneous components on twin. Check it, and go through that list, and note for each point, how much saved, and what (ill)side-effects or reducing/worsening something each point/upgrade does and then do choices according to what/if you are willing to sacrifice/spend on. For example, one can save a lot by simply removing stuff from rear trunk, installing lighter battery, installing smaller window washing bottle (or not filling it full), lighter wheels, with not much impact on practicality/functionality. Rear seat deletion/sport front bucket seat upgrade/AC deletion/lightweight BBK/noise insulation removal/interior ripping is at a bit harsher impact to practicality/impact to NVH/hit to budget. Lightweight flywheel/CF or alu driveshaft .. i'd probably leave at very end, if there is nothing else left and you go all the way for track-only heavily modified car.

Apparently factory had another opinion about the lightweight flywheel. They changed it in the 14R60 and it was not at the very end of changes. For example, the particular car didn't have a stripped interior and it was streetable and legal. A heavier flywheel is just a compromise for day to day driving under low speeds and heavy traffic. If the car is mainly a fun weekend car or if it is not used too much on traffic, then I would definitely suggest this change together with a 3 puck metallic clutch.

nikitopo: how many twins are tracked? I wouldn't bet even on every tenth. Probably one in 50 or 100. How much time/mileage is spend during daily drive during traffic with all it's caveats/specifics from those that are tracked? Everything is compromise. And what might be acceptable in very low number very limited series won't work for most, thus i'd be very careful with choices done at such or even more so, at race spec cars, where creators simply didn't care about daily driving at all.

This. Big picture. Add up all these mods.

May not feel a difference from one mod. But ALL other things being equal, there will be a performance difference that could be noticeable in a timeslip, track time, etc.

Technically, clutch engaged, change in rotating mass should impact both acceleration and braking. I.E. heavier flywheel, clutch engaged and in gear, hitting the brakes will be the same as a slightly heavier car. Lighter flywheel the opposite. But to your point, positive impact to acceleration and braking is fairly mild in the lower gears, and virtually no difference in upper gears. The bigger difference you'll probably notice is how the engine behaves with the clutch disengaged (throttle blip response, how quickly revs drop on disengagement).

Oh no

I have been researching this a lot as I have every other possible performance mod (besides FI) and was contemplating doing something like this. There are a lot of in depth articles discussing pulleys, flywheels, and driveshafts.
Long story short a fly wheel will increase performance about 4-6hp although you do lose inertial benefits in between shifts based on how quickly you can shift (be out of gear rpms drop quicker)

A driveshaft will add about a 1/8hp increase in performance from lessening rotational mass. Assuming a 15lb reduction and no increased diameter (like aluminum driveshafts have which make any gains from weight null)

Lightweight pulleys will have even less effect then driveshaft with most potential risk. Underdrive pulleys do work, how much depends on the percent the pulley is being underdriven. Typically you can gain .6hp from underdriving the alternator pulley, and 1hp from underdriving the water pump.

fluidampr crank pulley is interesting and some “theories” say the increased harmonic dampening especially in higher rpms reducing energy loss from reducing vibrations. Also it is supposed to be a better dampener then stock so should be safer.

Mostly the benefit is just overall static weight reduction. I see a good .1sec increase/decrease in 0-60 per every 1/4th tank gas I have meaning 100% full vs 1/4full from weight reduction is about .3sec quicker.

Yes something around 4-6hp on our cars is quite reasonable. Inertial loss between shifts might be there, but it is nice to try and shift a bit quicker :)