Torque vs Crank Angle, Flywheel Weight, and Transmission Happiness (strength) - Miata Turbo Forum - Boost cars, acquire cats.

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Old 03-08-2015, 07:50 PM   #1
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Default Torque vs Crank Angle, Flywheel Weight, and Transmission Happiness (strength)

Always wondered about this, so thought I'd post up and see what you guys think.

So consider a 4 cylinder engine that makes say, 100 ft*lbs of torque, that's AVERAGE torque, as measured on a dyno. But in reality, gasoline 4 cylinder engines are not delivering "constant" torque vs crank angle, but rather it depends. Here's a graph for a 4 cylinder engine, showing torque vs crank angle.



According to that graph, if the average torque was 100 ft*lbs as we said, peak is over (edit 2 time) 350 ft*lbs! Looks like about 370 peak to me.
The thought process is, if the 4 cylinder's torque delivery is actually as bad as this graph shows, then it seems like more flywheel weight would help smooth it out, as in pull those peaks down! In theory those peaks are probably what's putting the highest load on the gear in the transmission, so reducing the peaks would make the transmission last longer. In theory. Or, running a lighter flywheel could be making this worse, increasing the peaks!

Thoughts? Data? Theory? Anything?

If this is true, one could make an argument for keeping the stock flywheel in the interest of keeping the transmission happy. Same for a sprung clutch disk, spring will compress and lower the peaks in this chart.

Kinda makes me wonder how many people who have broken a 6 speed run a stock flywheel now.
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Torque vs Crank Angle, Flywheel Weight, and Transmission Happiness (strength)-et-x-4%2520cyl.gif  

Last edited by patsmx5; 03-09-2015 at 06:12 PM.
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Old 03-08-2015, 10:53 PM   #2
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Old 03-08-2015, 11:04 PM   #3
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Thanks for the post with links, lots of actual info there!

From your 3rd link, a summary at the bottom says:

Quote:
The simple bottom line is:
Don't even think about using a flywheel with a low mass moment of inertia, and
Don't even think about using a torsionally-rigid connection between the engine and gearbox.
Interesting.... This agrees with what I was thinking, and they even mention in this article about how the peak torque pulses cause FATIGUE FAILURE OF GEARS.... Hmm..
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Old 03-08-2015, 11:17 PM   #4
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Based on your links, if your goal was to break transmissions as fast as possible, you'd want to run a 1.6L lightweight flywheel with a clutch disk that had no springs in it.

And if you wanted to maximize transmission life, you'd run a stock 1.8L flywheel and use a sprung-hub clutch disk.
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Old 03-08-2015, 11:27 PM   #5
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And a third thought, I currently have a 10lb flywheel in my car and experienced gear noise on decel. I'm now guessing the "noise" is actually the gears constantly getting positive and then negative torque applied to them, so they load up in one direction, then unload and the slack goes the other way till the gears "hit" and thus the noise. I bet that's it! And with the stock flywheel, the extra dampening from the higher mass stops this enough that the gears stay meshed at low speeds, thus no noise. Makes perfect sense!
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Old 03-09-2015, 12:26 AM   #6
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I was thinking gear whine was a misalignment in the transmission to PPF, not much to do with the flywheel.

Assuming most people take the transmission off to install a lightweight flywheel, maybe they are really easy to line up where they whine. I've heard of all different kinds of miatas whining, with OEM and aftermarket parts, and my (probably) stock/oem clutch/FW hums like crazy.

https://www.miataturbo.net/general-m...erating-46137/
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Old 03-09-2015, 07:52 AM   #7
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Most performance modifications done to a car reduce reliability and longetivity. That's just the game we play.
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Old 03-09-2015, 08:43 AM   #8
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I have wondered about the additional transmission noise I hear on decel after installing the light weight flywheel as well. Even thought about sticking the 1.8 factory flywheel back in with the next clutch, but I sure love the snappy heel toes. Pat, I never thought about it this way.

The decel noise seems to be worse right around 3600 rpm. Significantly above or below that range and it is quiet, but between 3800-3200 rpm it sounds like something is loose. I used to think it was a bad throwout bearing (new Mazda with less than 10k on it), but after replacing the clutch recently and still hearing the noise I am sure it's not a breaing issue. Also I had the car on the lift and had my brother rev the engine to the 3600 rpm sweet spot while I hit the diff, tranny, bellhousing and oil pan with a mechanics stethoscope. The noise was clearly coming from the center section of the tranny (gear section). No noises from oil pan, diff or bellhousing. What is strange is that I heard the noise while he was just free reving the engine at 3600 rpm, not just deccel.
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Old 03-09-2015, 09:38 AM   #9
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i think decel noise is a combination of "all of the above". think about it, if the torsional vibration can cause gears to clack back and forth, then i would think its safe to assume the whole drive line will also do the same to some degree. this is evidenced by dif bushings also contributing to decel noise among other things.

i think it would be really interesting if we could find some way to model or measure the resonant frequencies of each gear in the trans and the motor its self at various build levels, and see how they compare. the engine should be relatively easy to measure, not so sure about a multi speed trans. if the issue is "matched" resonance between these 2 components, then no reasonable amount of damping will fix it.
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Old 03-09-2015, 09:47 AM   #10
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Don't let Nitrodann read this thread, he might learn how an engine works.
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Old 03-10-2015, 02:33 PM   #11
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How many +275 whp cars are running a stock flywheel?
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Old 03-10-2015, 02:43 PM   #12
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Quote:
Originally Posted by ryansmoneypit View Post
How many +275 whp cars are running a stock flywheel?
Most of them, I would imagine.
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Old 03-10-2015, 02:53 PM   #13
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Quote:
Originally Posted by Joe Perez View Post

Most of them, I would imagine.
While you're not wrong, you're an *******. Lol. He was clearly talking about miatas specifically. Just a poor choice of words.
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Old 03-10-2015, 03:01 PM   #14
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Quote:
Originally Posted by Ryan_G View Post
While you're not wrong, you're an *******. Lol. He was clearly talking about miatas specifically. Just a poor choice of words.


Even if you narrow the question to just Miatas, it doesn't really make sense to me. Granted, the number of Miatas making upwards of 275 on the stock block is sufficiently small (and their owners sufficiently crazy) as to make broad generalizations difficult, but to me, the question seems to imply that running non-stock flywheels on >275 HP Miatas is the norm.

I'm having trouble understanding why that would be.

Granted, some folks will be running combination flywheel / clutch systems (like the 949 Racing dual-plate unit) which necessitate the use of a non-stock flywheel, but for those using stock-geometry clutch discs, I see no reason to assume that the production of large amounts of horsepower positively correlates with the desire to install an aftermarket (and presumably lighter-than-stock) flywheel.

Or, at least, what the justification for this correlation would be, if one did exist.
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Old 03-10-2015, 03:05 PM   #15
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Quote:
Originally Posted by Joe Perez View Post


Even if you narrow the question to just Miatas, it doesn't really make sense to me. Granted, the number of Miatas making upwards of 275 on the stock block is sufficiently small (and their owners sufficiently crazy) as to make broad generalizations difficult, but to me, the question seems to imply that running non-stock flywheels on >275 HP Miatas is the norm.

I'm having trouble understanding why that would be.

Granted, some folks will be running combination flywheel / clutch systems (like the 949 Racing dual-plate unit) which necessitate the use of a non-stock flywheel, but for those using stock-geometry clutch discs, I see no reason to assume that the production of large amounts of horsepower positively correlates with the desire to install an aftermarket (and presumably lighter-than-stock) flywheel.

Or, at least, what the justification for this correlation would be, if one did exist.
Nobody has that data. But when I built my last setup, i was chasing more power and upgraded the flywheel from stock to a 10lb FM one, as it's supposed to save about 8lbs, and make the engine rev faster, and it's new so I needed one anyways why not upgrade. That was my logic, seems to be common from the threads I read leading up to me purchasing it.
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Old 03-10-2015, 03:12 PM   #16
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What I am really wondering is, on my engine (hoping for 275 whp) on the super twin disc, am I putting my 6 speed at risk? Would the risk really be less with a heavy flywheel and pressure plate?
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Old 03-10-2015, 03:17 PM   #17
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We simply don't have the data. Saying it would or wouldn't is conjecture.
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Old 03-10-2015, 03:17 PM   #18
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the twin plate clutch I ran is probably one of the lowest moment of inertia setups ever run in a miata short of some crazy 5.5" twin plates that race guys run. Stock 5 speed. Survived for years and years and tens of thousands of miles of boost and light clutch / flywheel assembly. It was my daily driver.

I had ridiculous noise on decel because of all those metal-to-metal floater plate interfaces.

By the way, a small series of springs in a clutch disk do very little to reduce the shock load from a rapid clutch engagement. Maybe they do something to reduce vibration, but they are small springs and likely fully compress under most light loads.
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Old 03-10-2015, 03:31 PM   #19
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Yeah I'm thinking the springs are for torsional vibration, not for when you are taking off and launching the car. Like you said under a clutch dump the springs would be defeated, but I could see them being effective for torsional vibrations as the time frame is so small that they don't need much length as the time to accelerate everything enough to compress the springs fully is too small.

My guess is there purpose is to reduce the torsional rigidity between the crank and gearcase as Sean's links suggest is important for aircraft gearbox life. All car clutch disk I've ever seen that are stock have those springs on the clutch disk, not just miatas.
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Old 03-10-2015, 03:47 PM   #20
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This makes me feel a little better. I don't mind the noise, I just don't want to replace transmissions in the name of a few milliseconds better acceleration.
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