Why Flywheel Rotating Mass Is Crucial For Best Sports Car Acceleration
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- Опубликовано: 8 сен 2023
- This week I compare and test three Porsche 911 flywheels. A stock Porsche G50 flywheel, an aftermarket performance G50 flywheel from Patrick Motorsports, and a factory light weight flywheel and pressure plate combo from a Porsche 915. All designed to work with the 3.2L Carrera engines.
Moment of intertia or rotating mass is very important for best acceleration because heavy flywheels can rob energy and torque from your engine.
I measured the moment of inertia of three flywheels by spinning them in my garage and analyzing the results so you can pick the best option for your Porsche 911!
If you like this techy content, then let me know: I've done a few similar engineering videos here:
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You took me back to physics class - cool.
Ha ha, this is a very common textbook problem.
Bravo Tom! Though this isn’t lab data, it’s data and in the world of Porsche pontification and posturing, having real data is hugely beneficial. Tires and wheels would be fantastic!!
Thank you! My hope is to offer food for thought with just enough data to be useful.
One thing I don't like about the 911 world is everyone copies each other. (Mistakes included, ha ha).
Wheel (rim&tire) weights and diameter also have dramatic effects on the spin up to red line in first gear. Good stuff....
Totally agree. I hope to compare wheels and tires next as they are the next big item to optimize/minimize.
Momentun matters! Why did Mr Porsche make the G50 flywheel 13 #'s heavier than the 915? Because of the way it feels as you shift the gears... smooth. He could have stuck the 915 in there. But everyone raved about the G50 for that feel, and I love it, too! If you make the flywheel lighter, it will spin up faster but will also slow down faster.
G50's do drive nice and smooth, just not as lively. Part of that is due to the engine management too.
Very good description and love the quick and dirty test rig.
Thanks 👍 I wanted data before installing one of these in MAC prior to Rennsport. I'll do another one with wheels and tires.
Very good video and demonstration of first principles testing, do more please!
Great, thanks for the encouragement. Once my 911 is driving again, I'd love to do more engineering vids.
Cool stuff. Another Master Class. Yes, it would be interesting to do wheels and or tires.
Ok cool, maybe after Rennsport😀
Great stuff Tom! That's quite a difference in torque values between the stock and lightweight components. I would never have guessed it would have been that much.
Thanks and glad you liked it. My numbers are most likely not that accurate, but it does help put things into perspective. It takes a lot of torque and energy to spin a 40lb clutch package. A Carrera engine only makes 100 ft-lbs of torque at 1500 RPM if that.
Interesting tests Tom. I've always thought of total flywheel weight, but I haven't thought about the distribution of that weight. Great summary of the actual torque gains at the end!
The Patrick motorsports combo has an inertia of 5.6 per pound whereas the stock flywheel is 6.1/lb. So the distribution of weight does matter. PM did a good job on their product.
That was really fascinating. I knew the concept but I never tried to do the math.
I didn't bore everyone with equations, but I'll send anyone who asks my notes and spreadsheet.
I think this is interesting content. I’d like to see some of the math and a quick brief on the physics software would be interesting too. I appreciate that you give fun info and are quick to the point.
The physics software is called tracker and it's free video analysis software. It turns out that cameras are pretty good timing devices. This video wasn't very popular, but I explained the tracking software more.
ruclips.net/video/8M2uUN5Psw4/видео.htmlsi=4BwOROneWaNMBpXz
I'll show more math on a wheel and tire video, but it's Newtons second law of motion. T=I*w
Rotation acceleration is change in RPM/change in time. That data came from the camera.
Kinetic energy is .5I * w^2
It's always a tough balance on how much to include, ha ha. I can see where people drop off when watching my videos and math is a sure way to lose people!
Moment of inertia is easy to find out. Impact on acceleration is also easy to calculate.
Yes, but there's also a lot of bad information out there too. Where are you getting published moments of inertia for old cars?
Hello bro good video
good info! No need to upgrade my 915 box then.
Not if you like the way it shifts. G50's do shift nice.
Good video, tires video will be helpful
Ok, it will be coming soon.
Just a couple of days ago Richard Holdener uploaded a video of a dyno test of a lightweight vs heavyweight clutch-flywheel setups. On LS motor yet it is relevant
Do you have a link? I'd like the details on how the dyno test was run.
easy enough @@GarageTimeAutoResto ruclips.net/video/aiCY5-5vizg/видео.html
Interesting data points. That's a lightweight 915 assembly right? Wonder how the stock one goes in comparison. Definitely interested in wheel data, as I've been eyeing off 17" rims to get better tyre choices.
The 915 was a stock flywheel with aluminum pressure plate, so nothing crazy.
Stay tuned for a wheel and tire video!
Friends that raced professionally always were trying to lighten rotation mass. They would say for every reduced pound of rotating weight would equal X amount of HP gain. FWIW thanks for the video
No sorry reducing rotating mass does NOT "increase hp". It cannot by definition of how hp is measured. To measure hp on a dyno you are supposed to do it at CONSTANT RPM. This means the rotating mass plays NO PART in the actual hp. Can a car with less rotating mass accelerate faster? YES but only barely faster than lightening the vehicle anywhere else.
@@stuartgray5877 so less rotating mass does not benefit to quicker acceleration?
@@johnvaluk1401 It DOES make a difference but only about the same as losing mass from anywhere else on the car. WHEELS and tires are part of the rotating mass so reduce it there first.
@@johnvaluk1401 Reducing rotating mass of the engine is also the most expensive weight loss.
Good discussion here guys. Reduced rotating mass can certainly make a car feel faster, but won't show up on a Dyno sheet. Dynos don't tell the whole story and racers know this to be true!
As far as removing weight from a vehicle, engine speed rotating mass is the most important especially in lower gears. The energy is proportional to RPM squared and since the gear ratios force more RPM's versus travel speed the rotating mass dominates the regular mass. Like 15 times more.
Second reduce mass from things that travel at wheel speed, then reduce regular mass. This information can be found in books such as Tune To Win.
Cool, want to put a lighter flywheel on my 124. Makes the difference I see.
Can you put the 915 clutch in a g50 gearbox? Something to think about for when I have to replace the clutch, which will be soon. Understand it also needs hidráulic pedal? Thanks, good stuff, as always
Yes the G50 cars use a hydraulic clutch. See this video where I bleed the slave cylinder. ruclips.net/video/2gLHQukzR7Y/видео.htmlsi=KdDlaZWUKdWjPWYb
A 915 clutch won't fit, but Patrick motorsports makes a short bellhousing system which should be as good as the 915 stuff. Check them out.
@@GarageTimeAutoResto thanks Tom. I’ll have a look at it but I doubt it makes sense to buy one and bring it to Spain, you know, shipping, taxes…
Bro I have also done first time Porch 911 CSI engine repair
Hi, I have always found that a heavy flywheel will not pick up the revs as quick as a light flywheel, but the light flywheel will not carry the revs going up a hill, easy, press the pedal harder... have fun
Makes sense to me. Some drag racers like a heavy flywheel too so they can launch harder when dumping the clutch. These flywheels can store a lot of energy.
This is a VERY timely video as I am trying to decide this weekend whether to replace my stock 915 clutch + flywheel with lightened unit. My understanding, which you confirm, is the 915 flywheel is already very light (albeit with less centered than an aftermarket flywheel). The benefits, using an RSR clutch and flywheel, would come from the much lighter clutch assembly. The engine is out so it's free other than the parts.
Did you say in the video that the pressure plate in the 915 combo was lighter than a stock 915 pressure plate? Also, is the flywheel in the 915 the stock one, or a replacement lighter flywheel from the factory?
I watched the video again. In your test, the 915 was a stock unit other than the lighter pressure plate. That sounds like a great, and cheaper, solution for the 915, given its light stock flywheel. Am I understanding that correctly?
Yes, the pressure plate is a Sachs aluminum one. It's lighter than the steel PP for sure. A good balance or machine shop can also remove a little bit of weight from your stock flywheel if you don't want to go full RSR. The 915 combo in this video is what I will use in my 911.
@@GarageTimeAutoResto Thank you! Very useful information
Fantasitic. But I must ask, why not use a motor, measure the amperage and spin-up to speed?
I like your idea of using a motor too. Inertia is Torque * delta T / delta RPM.
A tachometer can measure delta RPM, but getting the Torque value seems hard to quantify based on electrical current.
The string wrapped around an axle is a very easy to get the Torque number.
Couldn't you get close to the same results with your measurements mathematically using a yo-yo
No but that sounds fun!