incubuseva

iTrader: (13)

So I had a thought, (search resulted nothing) - if I have a heavier rotating assembly, and move my power band up a few k, would I get more torque? I know that it would accelerate slower, but at a certain point would it give me more torque because of a flywheel effect? Or would this just give me more inertia? I'm just talking about if the only differences between the two were the mass of the rotating assembly.

Thoughts? Experiences?

Mazderati

iTrader: (3)

I've seen guys do it in drag racing. Works because the mass is already spinning before the clutch is dropped, providing a harder launch. You don't typically want static parts to be any heavier; i.e., driveshaft, axles, wheels, tires.

t_g_farrell

iTrader: (1)

This is why the rotary flywheels are so heavy in the first place.

peejay

Rotating mass has absolutely zero to do with engine output.

RotaryEvolution

iTrader: (30)

it just gives a perceived increase in torque since the inertia of the engine carries through slower. heavier rotating assemblies are also bad for higher revs, since their balance issues will carry through more at higher revs.

peejay

I felt in increase in acceleration from any engine speed including idle when I put the 9lb flywheel on.

It depends on how you drive. If you let off the clutch and then apply throttle then yes, it will feel like you "reduced torque" because your driving technique is horrid, and you are bad person and should feel bad. That's not an engine output issue. All the flywheel does is store energy, sapping energy on acceleration and giving it back on deceleration. A heavy flywheel is no different than heavy wheels or heavy anything else in this regard. It reduces acceleration.

Don't forget that torque and horsepower are two different ways of calculating the same thing. Ask yourself if a heavier rotating assembly will give you more power.

Jeff20B

iTrader: (13)

I tested an R5 assembly (heavy) and then an S4 assembly in the same vehicle with the same aftermarket flywheel (16 pound custom machined VW cast iron to fit a rotary counterweight in a baja). The S4 NA assembly launched better and seemed to have more get up and go. So this kinda flies in the face of conventional wisdom where they say heavier = better launches. Of course there were other factors too such as the S4 NA was a 6 port FC engine with a Nikki carb on it, while the R5 was a typical 4 port 13B with a hitachi carb.

I've since gotten a light steel (not iron) flywheel from KEP through Mazdatrix since then. It weighs 12 pounds. Car is still coming together so haven't driven it yet. The engine is already built with R5 rotors so they're staying.

The weights break down into this grouping:
rotor+rotor+rear cw+flywheel=result
11.5 + 11.5 + 4 + 16 = 43 baja with R5 and iron flywheel
10 + 10 + 4 + 16 = 40 baja with S4 NA and iron flywheel
11.5 + 11.5 + 4 + 12 = 39 baja with R5 and light steel flywheel

So it comes down to 43 pounds vs 40 pounds vs 39 pounds. Which do you suppose will launch the best?

peejay

This whole conversation is ridiculous. Maybe next we'll get some potatoes and cork up the exhaust systems to see if backpressure increases power too.

incubuseva

iTrader: (13)

Peejay don't be a dick.

Does anyone have dyno graphs that will prove anything?

C. Ludwig

iTrader: (27)

Peejay isn't being a dick, he's being pragmatic. There is a confusion here between inertia and torque. An engine creates torque. A dyno measures torque. A dyno will not directly measure inertia. Inertia is the energy you're referring to that will help accelerate the car, car not engine, from a stop. Torque, or engine power, is used to build the inertia. There is no free lunch. An engine will not make more torque with a heavier rotating assembly. All other things being equal, an engine will produce the same amount of torque at the crank shaft regardless of rotating assembly weight. It, the engine, will accelerate faster with a light assembly. That doesn't mean it makes more power, it means it has less inertia to overcome. It WILL require more torque to accelerate a heavier rotating assembly and more torque to build up the inertia to help launch the car. Now there may be a benefit to the trade off of using engine torque to create inertia for a drag type launch. Those benefits go out the window and become liabilities when the same car gets repurposed as a road racer. There is a balance for everything. But, the bottom line is, a heavier rotating assembly will NOT magically produce more torque. Period.

Mazderati

iTrader: (3)

Weight of the flywheel has a direct impact on the force produced; not the power. From a standing start, sidestepping the clutch on a heavier flywheel is going to produce more force than will a lighter flywheel, all else equal. From a rolling start where the engine and drivetrain are already mated, a lighter flywheel should accelerate faster, all else equal.

Force = Mass * Acceleration
Acceleration = Force / Mass

For drag racing, the question on whether to use a heavier flywheel seems to revolve around the priority of a hard launch or faster acceleration during the rest of the run.

Weight distribution of the flywheel also matters, so flywheels of a given weight will not necessarily produce the same force or acceleration.

Jeff20B

iTrader: (13)

turbo anyone?

RotaryEvolution

iTrader: (30)

a drag racer who relies on more inertia to launch is simply overcoming the shortcomings of the car and their own techniques to produce better launch times. the downside is the lacking acceleration will hurt their trap speed and end gate times. lowering their 60' times from 1.6 to 1.55 might give the impression that they are moving in the right direction but ultimately that is lost once rolling.

inertia is simply the engines resistance to change in RPMs, this DOES help take off from a stop but once you're rolling it hurts performance. it might also benefit a car set up for hillclimbs solely but on flat ground it wouldn't.

factory cars have heavier flywheels mainly for navigating easily in heavy traffic, a light flywheel will require more effort to get the car moving from stationary. they also reduce the jerkiness of the car from acceleration to deceleration and vice versa, and smooth out the idle, etc, etc.

Jeff20B

iTrader: (13)

I have another short story to add to the mix. REPU. Heavy R5 rotors. Heavy stock 30 pound flywheel. This could launch really well, but once you're rolling, it would take forever to reach redline. What's worse is because the rear diff gearing in these is so short, the vacuum secondaries in the stock hitachi carb would never open in 1st gear. It was literally the same as free reving the engine. You should know that carbs with vacuum secondaries will never open when free reving, or at least they wouldn't for me. I think they were set up like this from the factory. However you'd think the higher load from the 30 pound flywheel would make them open, but it didn't. I think it has something to do with the speed at which the engine freely accelerates, and the heavy flywheel obviously slows things way down so the secondaries can never open like this. The only reliable way to open them while free reving is to go mechanical secondaries. This is for you EFI-only guys in case you didn't know this stuff about carbs.

What was the fix? Swap to a light steel flywheel from Racing Beat? The secondaries open every time. You do lose a small amount of launch ability, but it is more than made up for once rolling.

This is basically a real world example of what Ben just said.

Now let's fast forward some. The REPU has an S4 NA rotating assembly and a stock S4 NA flywheel which weighs 24 pounds but has noticeably less mass at the outer edge than the 30 pounder and less than a GSL-SE flywheel which was an option on the older (R5) engine if I had kept it. Still a 4 port 13B so I can use the exact same carb for testing. As a fresh rebuild I haven't gotten on it yet to test how well the secondaries open, but the launch is better than the light steel and is about the same once rolling. It is also much better than the 30 pound stock flywheel in every way.

Again here is a breakdown of weights if it means anything to you guys.
11.5 + 11.5 + 30 = 53 R5 rotors and REPU flywheel
11.5 + 11.5 + 4 + 13 = 40 R5 rotors and light steel flywheel
10 + 10 + 24 = 44 S4 NA rotors and S4 NA flywheel

And the important numbers are 53 vs 40 vs 44 pounds. This is a truck so 44 it is. Notice it's a lot closer to 40 pounds than it is to 53. No wonder it launches better and accelerates better once rolling. The slightly higher compression lighter NA rotors help with bottom end as well. I like it so much the other REPU is getting the same treatment.

sevens4me

iTrader: (1)

I enjoyed this conversation...No, not being sarcastic...But how does inertia work? How does the inertia feedback to the engine and tell it how much to resist a change in rpm? How does any object "know" how to react to an outside force applied to it? If there were more matter in the universe, would there be MORE or LESS inertia for any given object? Just curious, sum of yous sound smart. Prolly wrong place, though.

RotaryEvolution

iTrader: (30)

there's too many variables to list, what i do is take extreme examples and use those for a basis of comparison.

take a stock 25lb flywheel and replace it with a 200lb flywheel. the engine will struggle to change in RPMs but the stored energy stationary will transfer instantly to the drivetrain. a launch would be instantaneous with very little RPM deviation but once rolling you also now lose your ability to accelerate rapidly, the car would also decelerate very slowly via engine braking.

now switch the 200lb flywheel with a 5 lb flywheel, the inertia is now mainly the engine itself, the result will be bogging if not using the potential in the clutch to slip on takeoff, once rolling your acceleration is maximized by the ability of the engine to speed up as quickly as it is able to.

there's a reason why you don't hear racecars that sound like tractors when free revving, because they use the latter example. how much inertia you need for your launches depends on you, your car and what you are doing with both. track cars never launch, in fact most F1/indy/stockcars need a push to get going. drag cars need some inertia but how much is up to your own trial and error, because launching is a critical part of what they must do.

more inertia also can have a major detriment with high revving engines, fragile heavy masses like cast/heavy stock flywheels become very unstable once crossing beyond 10,000RPMs and are prone to exploding once they develop hairline fractures in the webbing material. the heavier the mass is the more balanced it must be, lighter masses already have this mainly built in.

Jeff20B

iTrader: (13)

Notice this GReddy flywheel is very thin at its outer edge to help prevent flywheel explosions. It's all steel with the ring gear cut directly.



It's not like Racing Beat's aluminum which uses a failure prone (at high RPM) OEM ring gear bolted on.

RotaryEvolution

iTrader: (30)

ACT's chromoly flywheels are built in a similar way with the ring gear being part of the base material and machined out of it. but i've still seen even the chromoly wheels develop hairline cracks.

Jeff20B

iTrader: (13)

I found a better picture of the aluminum flywheel. Here you can see how there's nothing keeping the ring gear from breaking off in pieces at high RPM. It just has three locking screws that prevent it from spining by itself when the starter is engaged. Otherwise no welds, no nothing. Just the thickness of the gear itself to keep it together at high RPM.

peejay

It's normal for the ring gear to be an interference fit, no welding at all. If you have a stock flywheel with some bad teeth, usually the bad section is only on half of the tooth, and you can get some more life out of it by knocking the ring gear off with a hammer and punch, flipping it over, and heating it with a torch until it slips back on. (Heating with torch is the recommended method of ring gear installation, incidentally)

One could argue that the grubscrews make a stress riser. One could also argue that welding also creates a stress riser.

One could further argue that ring gear failure can be considered to be a never-happens event in theory and in practice. The main failure item seems to be the pressure plate, which is lots heavier, gets thermally shocked on a regular basis, has nothing supporting it on its outer radius (unlike, say, the friction surface of an aluminum flywheel), and tends to get surface cracks over time. And it sees side loads through three discrete locations (the torque straps).

Clutch disks fail, too, but fiber material just explodes into fluff and pucks tend to not die very easily if at all.

incubuseva

iTrader: (13)

Ah thanks for all the informative posts here. I've learned a lot.


I actually did have a flywheel fail on my A4. Was drag racing - 1.8t GTRS on a stock clutch. It cracked all the way through in 3 places. Was like driving with the clutch slipping in the pp and flywheel all the time. I had to drive like that for a week too before I could replace it.