effects of engine rotating weight, flywheels, flexplates . . .

[mikedc]

Just got to wondering some things. I had posted this in the engine section but it's broader stuff than just the motor . . .


#1.

Flywheel lightening = faster throttle response, but not as smooth running motor as a whole.

engine rotating assy weight (crankshaft, rods, etc) reduction = same thing.

But if you can control the amount of rotating weight by just adding & subtracting weight at the flywheel in huge amounts, then why does anyone think about keeping weight off the crank/rods/etc for revving purposes?

(Yes, I know there are plenty of other good reasons to want a lighter mass assembly in the motor. But people often seem to talk about lightening the assembly specifically for the flywheel-type purposes. For example, you often hear people talk about how short-stroke motors love to scream and rev quickly compared to longer stroke motors.)

Is the engine's rotating weight just something that racers try to fight against (for flywheel-type purposes) only after they're already running the lightest flywheel that is practical?







#2.

What is the auto-tranny equivalent to lightening the flywheel/rotating weight?

A normal auto-tranny flexplate doesn't have any real excess weight on it in the first place. It's obviously not built with added weight for its own sake like a manual car's flywheel is. So if you wanna get that same trade-off effect (that a lightweight flywheel would have on a stick-shifted motor), then what do you do on an automatic?

Would you make that change with a different torque converter? I assume a much tighter one?


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[bijanmaleki]

#1 I would assume the addressing of the flywheel over the rotating assembly would be for cost effectiveness.

#2. Auto cars are generally cruisers or drag racers. Where the light rotating mass helps a road race car, a drag race setup can use the extra momentum off the line to get the car moving.

[Johnny Blaze]

Smaller converter has similer effect as well. Pick up a stock 12" converter and then pick up an 8" or 9". a lot of weight difference.

Stock and super stock guys also lighten up the internals on th350s a lot to make the car quicker, even if it only a few tenths, every bit counts in class racing.

[CarlC]

What requires more power to accelerate, a 50# crankshaft or a 50# flywheel? Why? That will answer the basics behind the question.

So, given that one of the above components is going to require a lot more power to acc/dec, how much mass would need to be removed from each component in order to have an equal effect? At what cost?

Weight reduction is significantly easier in one of the components.

[b-man]

Interesting post by Pontiac Super Stock racer Lynn McCarty concerning engine reciprocating weight reduction and the resulting performance gains:

http://forums.performanceyears.com/f...5&postcount=93

Same racer posting on drivetrain weight reduction:

http://forums.performanceyears.com/f...&postcount=168

[sik68]

Regarding #1, I would say that in a manual car, I think you're correct that it's a bit I'll-advised to spend a bunch of money on a light rotating assembly in order to say "it will rev faster" only to bolt on a heavy flywheel, where the majority of engine-speed rotating inertia is.

In an automatic car, I think inertia is a lot harder to analyze (for me at least) because of converter slip...there isn't as much of a correlation between engine speed and drivetrain speed. A smaller converter does help though as mentioned above.

If you want to get an idea of how quickly an engine will rev with a minimal amount of inertia bolted to it, just check out a Nelson Racing Engines youtube video...ridiculous rev rates because they are well built engines....but more importantly because there is just a flexplate bolted to 'em.

[Vegas69]

It all comes down to a trade off of drivability and race car stuff. I'll give up a little of the track for a nice drive. Flywheels are heavy for a reason, they store energy and make it mucyh easier to take off from a stop light. I was talking to Jason Pettis over at Pettis Performance who built my new engine. He said they have experimented with adding flywheel weight on the dyno. The numbers don't change it just happens faster.

[mikedc]

In an automatic car, I think inertia is a lot harder to analyze (for me at least) because of converter slip...there isn't as much of a correlation between engine speed and drivetrain speed. A smaller converter does help though as mentioned above.

That's the issue I don't get either.

I could see the automatic setup equating to a lightweight flywheel. But a stock heavy flywheel (which automatics typically compare to) is dozens of pounds. A flexplate + converter + some fluid inside the converter . . . I don't see how that would even come close to a stock heavy flywheel in inertia.

Of course the fluid resistance would lend some of the inertia too. But I don't see automatics revving very quickly even when the fluid isn't a big factor. (Imagine when the car & drivetrain is coasting faster than the motor, and you blip the throttle a bit to make the motor catch up. The motor still doesn't feel much like a lightweight flywheel setup even then.)


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[sik68]

A converter reduces engine response because of the drag that the fluid induces on the engine side of the converter. So it's not inertia as much as it is a resistance. It's not like a flywheel which can store and release energy...it's energy lost in the form of heat. I am not a converter expert, but I would imagine that the more quickly you try to accelerate through the fluid, the resistive load goes up exponentially.

[TnBlkC230WZ]

In a standard road going car, the wieght of the crank and flywheel are needed for inertia. You still want the lightest pistons and rods that are pratical. Anything you can do to reduce the wieght of the piston rapidly changing directions should help with efficiency, ecconomy, NVH, and the life of the engine. It would be really hard on the bearings, crank and main webbing to have a heavy piston changing directions at 7500 RPM.

[monteboy84]

In a standard road going car, the wieght of the crank and flywheel are needed for inertia. You still want the lightest pistons and rods that are pratical. Anything you can do to reduce the wieght of the piston rapidly changing directions should help with efficiency, ecconomy, NVH, and the life of the engine. It would be really hard on the bearings, crank and main webbing to have a heavy piston changing directions at 7500 RPM.

Bingo.

[Later-A-body]

I built my Pontiac 455 with all lightweight forged internals and reduced the flywheel weight to 23 lbs. I wanted a motor that was easy on parts, and would be capable of revving faster. I have had great results. The motor revs extremely fast, but I have not lost any driveability with the lighter flywheel.

[monteboy84]

I built my Pontiac 455 with all lightweight forged internals and reduced the flywheel weight to 23 lbs. I wanted a motor that was easy on parts, and would be capable of revving faster. I have had great results. The motor revs extremely fast, but I have not lost any driveability with the lighter flywheel.

Keep in mind, that's still a HEAVY flywheel in comparison to a flexplate with a triple disc mounted to it. I would fully expect a car with a 455 and a 23lb flywheel to come off the line at a stoplight without a problem. If the vehicle is light enough, even the triple disc setup would be sufficient.

[CurtiSS 69]

Another thing to think about is the distance from the crank centerline the mass is at. For example Newton's Law applied to rotation:

Torque = Inertia * Angular Acceleration

Rotational Inertia = Mass * (Radius)^2

The closer the mass is to the centerline the less torque will be required to accelerate it. Notice also that once you are at a steady speed the Angular Acceleration = 0, so steady speed torque is independant of the mass. Although during the acceleration phase it is not.

Regards

CurtiSS 69

[AlleyCat]

To further CurtiSS69's point, understand that the further the mass is from the crankshaft centerline the more torque that's required to move it. Which means long stroke engines have an inherent disadvantage to quickly accelerating the same mass as a short stroke engine would. However, a long stroke engine makes more torque, so the disadvantage somewhat cancels itself out. Then again, long stroke engines have higher piston acceleration and peak piston speed which both add to friction (mainly from the rings).

[Twentyover]

Think I remember an article in PHR a long time ago where Dave Vizard evaluated the effect of a light flywheel on a drag car. My memory of the numbers is that in first gear, the car accelerated at a rate that was equivalent to a car w/25 hp more, in secoond, maybe half that, and in direct about 3hp more. Long time ago and my adled memory may not have number correct.

[exwestracer]

I can put the advantage of low rotating inertia into practical terms. The cars I play with are direct drive, i.e. NO flywheel, flexplate or harmonic balancer. The driveshaft is literally bolted to the back of the crank. These are 468 or 480 injected big block Chevys with 830-870 hp, over 650 lb/ft of torque, and a powerband as wide as Texas...

There are some really sharp guys who have been playing this same game for 30+ years, and they are STILL trying to lighten up the rotating assembly. That should tell everyone something...

It's impossible to explain how fast these engines will rev. Years ago I was in the pits and watched some idiot "expert" whack the throttles on a super open like they do with drag engines...it pitched a rod out the pan while he was still going to WOT. Seriously.

With a race car, we accept the fact that there are things it won't do well...like roll out from a stop light at idle. If you need the car to be able to do that kind of thing, don't bother spending a bunch of money on lightweight components. The Can-Am car I used to maintain had an injected 509 BBC and a 7 1/2" Tilton triple disc clutch on a reduced diameter flywheel. Even at 2100lb wet, you had to slip the clutch and be real careful with the throttle to get it rolling without stalling it.

It's a great technical discussion, don't get me wrong...

[MonzaRacer]

OK here is the straight scoop. Rotating weight/inertia are required to impart some of the torque produced by the engine.
According to what research I did many years ago, its a delicate balance of inertial rotating weight and power production.
Lighten up the rotating assembly too much and your trade off is less launch capabilities, say in drag racing.
Many years ago, when small cubic engines were all the norm, throwing a 80lb truck flywheel on a screamer 302 or 327 helped store enough energy AND give a large enough clutch surface area for higher horsepower applications such as drag racing.
Now with the advent of performance cylinder heads and computer developed cams and such to increase power production its not unheard of for a smaller cubic inch V8 to make large hp numbers and NOT need the heavy ,energy storing device known as a heavy flywheel.
Now days we strive to lighten the rotating assembly as much as possible to let the engine rev faster. And with proper air/fuel flow the smae engines that could barely make 500-600hp back "in the day" can now develop many more hp and lb ft torque.
Think of the ability to use a LSx making 600hp and similar torque back in the 60s, you could have cleaned up but then back then clutch, tire and even driveline technology just could not have taken the abuse.
As we were allowed to apply more power to the ground, then others figured out how to produce more.
The way I was taught was t oget the best acceleration curve with the flattest torque curve possible. This with proper gearing would allow the car too accelerate quickest.
Also lends to the torque getting you off the line and horsepower gives you top end speed.

[Roadrage David]

Rezen why pontiac engines are ""known"" to have massif amounds of torque is because of its massif crank!!.
redusing the rotating weight in useing lighter componeds around the crank is good to have it rev quiker. the torque wil stay the same...

[MonzaRacer]

Rezen why pontiac engines are ""known"" to have massif amounds of torque is because of its massif crank!!.
redusing the rotating weight in useing lighter componeds around the crank is good to have it rev quiker. the torque wil stay the same...

UH no. Sorry because I have built Pontiacs and with light weight internals nd it makes little difference in the amount of torque produced. Torque production is relevant on air flow , air flow quality and chamber design affecting burning characteristics.
I can pull balancing sheets for Pontiacs with less rotating weight than small block Chevys, and all it really effects is the engines ability to accelerate faster, not produce more torque.
RPM ability is what lighter rotating weight gets you. Yes you get some free power from not turning heavy rotating assembly.