gab

I wish my C Coupe could rev as fast as TT, I experienced fuel cut off couple times because I didnt realize how fast it revs up. The C Coupe doesnt even has that kind of throttle response as the turbo car.. hmm

maybe we can:

(1) get a factory flywheel and lighten it by a few lbs

(2) cast the same flywheel with lightweight alumnium

WyattEarp

A lightweight flywheel would likey be a terrific upgrade combined with the 3.69 ratio rear end and while your at it lets add a carbon fiber drive shaft for the ultimate in spin up time as well as conversion to ground.

gab

anyone knows if someone has a lightened flywheel upgrade for the MB 6sp transmission? or how much work is to get it ligtened at a shop?

mdp c230k

Remember, if you lighten the flywheel you are going to loose torque. The beauty of this engine is torque. You will rev quicker but have much less grunt of the line. Its fine in a high revving rice burner but not in a lower rpm grunt like ours. you will be displeased if you do it!

WyattEarp

Torque is measured in ft-lbs.

According to my Physics text the Torque <I>t</I> about some reference axis (the driveshaft) is proportional to the Angular acceleration <I>a</I> (as angular acceleration increases so does Torque).

<i>t</i>=<I>Ia</I>

where <i>I</i> is equal to the moment of inertia. This formula is the rotational analogue to Newton's

<i>F</i>=<I>ma</I>


<i>I</i>=<I>mr^2</I>

Since the flywheel's mass makes up part of the moment of inertia if thats all you took into account it would seem that reducing the mass of the flywheel would reduce the torque; however you fail to take into account that the reduction in mass allows the engine to accelerate the flywheel faster. Worst case scenario you break even if the resulting net gain in acceleration is proportional to the loss in moment of inertia. My guess however is that the gains in acceleration are greater than the losses in moment of inertia resulting in more Torque. I will sit down and derive all the equations and post them for those who are interested.

Of course your welcome to disagree but I'd hope you could back it up with some sort of analytical proof.

mdp c230k

This is good for a high revving engine where the increased rpm can compensate for the loss of rotational mass. In a street car like ours with a relitively low max rpm and the extra drag created by the supercharger you need to low end torque provided by the heavy flywheel. You will see that there is a fair amount of torque eaten up by the supercharger, without this torque you engine might actually rev slower because it is struggling to pull the compressor!

WyattEarp

The limitation of the engines ability to rev has to do with the internal and external masses or / forces that the engine must overcome (the force required to turn the supercharger can tell us in turn the equivalent differential mass it adds to the moment of inertia).
The net effect is that all differential masses are integrated to calculate the moment of inertia. As revolutions per minute increase our angular acceleration is increasing. Hence if the engine can rev (accelerate angularly) for a given moment of inertia and the relationship between moment of inertia and angular acceleration is inverse then as one increases the other decreases.

The result is a net reduction to the moment of inertia so angular acceleration is going to increase you will not have a slower revving engine it is mathematically impossible. The only unknown here is what is the equation that relates the angular acceleration to the moment of inertia (constant, linear [y=mx+b], quadratic, logarithmic, exponential). Your assumption seems acceptable on the surface but it is wrong because you treat the flywheel as it where a separate component but it just part of the moment of inertia which is the integral of the radius squared of the differential masses. I will sit down tonight and derive all this.

Just think of it this way the rear wheel torque is always smaller than the torque at the flywheel, Why?

1) The transmission driveshaft and components add mass thereby increasing the moment of inertia.

2) The mechanical efficiency of the differential is not 1.0.

By your analogy making the transmission parts lighter would make us lose torque.

mdp c230k

take a look at this article http://www.mustangsandmore.com/ubb/D...orqueVsHP.html
it goes into detail on this subject with all the eqs worked out

WyattEarp

Now I know you don't understand a word or equation I wrote.

The link you posted has absolutely nothing to do with what I am talking about.

Seeing as how Mercedes Benz is touted as the engineers car I think its sad that no one on this forum could actively engage or was even interested in this thread.

Oh well I guess thats life

Happy Couping

PS Please take the title Engineer away from Video Engineer. That title is saved for those who can actually integrate, differentiate and have actually passed a course in dynamics. In Texas it is actually illegal to attach Engineer to your name or title unless you are a certified PE.

Spyke

Well Wyatt, I'm an engineer who makes a pretty decent living developing exhaust systems for OEMs. My degree is in aerospace engineering and mechanics, with an emphasis on propulsion, specifically internal combustion engines. Yeah, you could say I've got an interest in this stuff.

The reason I didn't comment up 'til now on this thread is because there has been so much double talk and mis-application of formulae that I didn't feel like even trying to figure out what the heck people were trying to say. And certain people are beating others over the head with so much tech jargon I can hardly stand it. (In fact I can't hence my post now)

Bottom line is, it really doesn't matter what your flywheel weighs, your cylinder is going to produce the same BMEP regardless, which will translate to the same torque. How the heck people have twisted numbers around to make it look like a spinning mass creates torque I don't know... (And please note I'm not directing this comment to anyone in particular. I've seen so much half baked theory here I gave up trying to remember who said what)

A heavier flywheel will change the driveability of the vehicle due to the flywheel's STORAGE of energy. It doesn't CREATE squat. Nothing. It resists acceleration. That's it. It's harder to bog down an engine that has a heavier flywheel. That also means it's harder to rev it up.

But to say a heavier flywheel adds torque? So if I put a 5000lb flywheel on my car it'll have tons of torque?

c'mon, no one really thinks that, do they?


And back to wyatt... not to nitpick, but you'd best take another look at what you've written. Item one: Just because rpm is increasing does NOT mean the angular acceleration is increasing! rpm will increase with a CONSTANT acceleration. Acceleration is a rate of change. Someone who throws out as many tech terms as you should know that... my god man, it's a very basic concept of high school physics.

I'm also guessing that in a couple places you've used the "=" sign to represent the term "proportional", which is misleading at best.

Then theres the part where you relate the force required to turn the blower with the moment of inertia, when earlier you related the moment of inertia to the square of the radius of an object, when in fact, if i remember right, the force required to turn a blower goes up with the cube of it's rpm. I coul dbe wrong on that, I'd have to check.

Anyway, 'll leave it at that. We can nit pick and throw a bunch of academic terms around all day and not get anywhere, other than boring the hell out of the others.

I guess my main point is, lighten up on the guy. From what I've read of your stuff you know just enough to be dangerous... I applaud your interest and enthusiasm, but I'm guesing you're not an engineer either...

I don't mean to flame, I just get PO'd when people throw out a bunch of tech terms and incorrect information AND get on someone else because they don't understand... all that being said, Wyatt is absolutely right, there is no way removing mass from a flywheel will make an engine rev slower!

WyattEarp

1) Your absolutely right having a heavier flywheel will not produce more torque since the engine will not be able to spin as fast.

The formula states that t = I * a

where t = torque, I = moment of inertia, a = angular acceleration

2) You are absolutely correct rev / min is a velocity. that is a technical screw up on my part.

3) As far as acceleration being a rate of change you are correct but so is velocity just one derivative lower. One is the rate of change of velocity the other is the rate of change of position.

4) Wherever the = sign was used to my knowledge it was used correctly. Torque is proportional to the product of the moment of inertia and angular acceleration

5) I said: (the force required to turn the supercharger can tell us in turn the equivalent differential mass it adds to the moment of inertia). Perhaps it sounds convoluted (I was trying to use an analogy) but all it means is that since the engine has to turn the supercharger via the belts in order to calculate the torque from the angular acceleration the force required to turn the supercharger can be equated to a mass so that we can add this mass and all masses to calculate the total moment of inertia.

6) Spyke wrote: (but I'm guesing you're not an engineer either... ) I guess not up to your standards but I am an engineer.

7) I admit it is a rather sloppy presentation of my thoughts. I intended to clean it up and post it. I guess I kinda went off the handle when I asked for him to back up his statements with physics and he sent me off to a page that had absolutely nothing to do with it. maybe I'll write an FAQ on engines, torque and unsprung masses. Wanna help?

8) I highly appreciate your flame. Don't get PO'd if you don't beat people over the head with technical terms then maybe everyone in the forum would have gone and gotten themselves a 5K lb flywheel and then off to the tracks.

Regards
WyattEarp PE

WyattEarp

Since I have your ear maybe you can answer me a question.

Is the ideal exhaust sytem one that creates no back pressure on the cylinder or is some back pressure required or wanted in the design of an engine?

How is this different for normally aspirated, supercharged and tubocharged engines?

Am I dangerous?

Cryptnotic

I think the problem here is that the flywheel does not generate the torque. In the above equation, t = I * a, the torque, t is generated by the engine.

Lowering the moment of intertia by reducing the weight and/or size of the flywheel would definately increase acceleration. The downside of this is that the engine would feel more "rough". In the case of the c-coupe, it's a 4 cylinder engine which has its power stroke twice per revolution of the cam. The flywheel "smooths out" this power by transferring rotational energy into a spinning metal plate (the flywheel). It also increases the inertia (rotational momentum) of the engine.

Anyway, if you make the flywheel too light or get rid of it almost entirely by using a custom clutch, you can cause the engine to stall at idle because of insufficient inertia to keep the engine running. For that you just increase the idle speed.

Another thing you should note is that the engine will slow down faster when you take your foot off the throttle because of the decreased inertia.


Cryptnotic

vadim

Wyatt,

What Spyke tried to communicate was that the whole discussion was flawed from the very start, since it was based on the assumption that it's the flywheel that is a source of energy for the rest of the drivetrain.

Flywheel should only be regarded as a mechanical capacitor, that's it. And as such, it acts as a damper. Lighter flywheel means faster engine spin-up, but more torque spikes from the engine to the rest of the drivetrain and vice versa. (e.g. if you abruptly drop the clutch, lighter flywheel won't have enough energy to release to counter such impact, and more of it will hit the crankshaft, instead; conversly, lighter flywheel may pass more of combustion-related vibration to the clutch, gearbox and differential, thus increasing wear of these components).

Reverse situation with a heavier flywheel.

I'm not an automotive engineer, but common sense tells me that the weight/size of the flywheel is optimized for each particular engine and therefore shouldn't be changed based on some vague ideas. Just think of it this way: MB engineers had spent years developing and testing this engine - do you think they wouldn't have optimized it in the process?

gab

I have seen dynos with lightened flywheel on and it did "free" up some horsepower and put more horespower to the wheels.
lighted flywheel will affect driverbility in the sense its easier to stall on a hilly start and i think 20%-30% lightened is very good for street setup

mdp c230k

Spyke,

tommy

The "problem", especially with luxury car makers, is that the weight of the flywheel tends to be optimized for smooth delivery, not speed.

I've got a lightened flywheel on my crx, and the difference was very noticeable; the car revved much quicker. Which is a good thing on that car, since I've got a 4-1 header, and things don't get really interesting until after 4k. But, I'm not sure that I'd want a lightened flywheel on my daily driver. I dealt with that car for about 18 mos. on my n. Jersey commute, and wouldn't want to do it again.

mdp c230k

Spyke,
Thanks for the great posting! Wyatt, the reason 'I didnt understand' what you wrote was because it was so wrong! As Spyke stated acceleration is not as you have defined it. All I was trying to say was, a car with a light flywheel is not as tractable in the stop and go traffic of the city. The heavier flywheel stores more energy at the same rpm than does a light one. Please, you dont know who you are talking to most of the time, dont think that you are ANY more qualified than the next guy if you dont know him. Spyke has proven that!

mdp c230k

Wyatt,
I think it a shame you think so lowly of a very well educated VIDEO Engineer. That is, being a SOFTware Engineer! Please note that insults and degrading remarks have no place here. You might think you know more than I, you are most likely wrong. The diffence is that I dont care to insult you when you post errant or just stupid remarks. Grow up a little, you need to realize that there is always someone smarter than you even wish you were. By insulting me and my qualifications you only made Spyke show you how wrong you explaination was. Feel smart now? By the way, the link I sent you to explained the importance of torque and HP. All the eq's were correct too!

Spyke

D'oh! Sorry about that, I work with a lot of semi-rebel, hands-on guys, and when we see a lot of formulas being tossed around it raises flags... sounds silly but most of the guys I've met (sorry gals, I don't meet many female motorheads) that really know their stuff don't talk like that...


Regarding 7) I've spent a lot of years and wasted hours on these boards trying to educate people. It's a losing battle. If somebody has heard from their uncle Cooter (who grew up next to Billy Joe Bob's cousin who owned a gas station and raced on the weekends) that higher octane gas controls knock by burning slower, they will fight to the death that it's true, logic be damned.

What I'm trying to say is, you're on your own with the FAQ page!

More later (on exhaust systems) I'm in training all day and don't have much time right now...

WyattEarp

Lively Discussion, how nice.

WyattEarp

I learned my lesson I'm not touching this one anymore.

Spyke

Dangerous? We'll see I guess!

Ok, we could write pages on this one too. First of all, define "ideal". I'm not trying to be a jerk, it's just that, like everything else, it all involves compromise. What's ideal for Lincoln probably sucks for Ferrari... and vice versa! I'll throw out a few generalities as I haven't got time to write a thorough paper right now...

In general terms, less restriction is better. NO restriction is somewhat moot in that if you're not running drag pipes you probably have some restriction in the system. Also note that drag pipes work great, at ONE rpm. they suck at others. It's that compromise thing again.

If someone is designing a system to shape a torque curve, it is the dynamics of the flow they take advantage of, not the restriction level.

The engine is a system: lowering the backpressure of the exhaust without making any changes can hurt your 0-60 times. Also, even if you compensate for it to take advantage of the lower restriction, the fact is you'll get better gains by improving your intake. This is from memory here so I should really check it to be sure, but I seem to remember that the intake is a primary factor in the power output of the cylinder, the exhaust is a secondary factor. (Moral of the story, Spend your money on head work! That's where the big power is hiding...)

The design of the intake is more important on a naturally aspirated engine than a blown engine. Turbos provide restriction bit are still considered to be "free" horsepower. Blowers add no restriction but consume a lot of the power they add. (To address another post: the engine is not "struggling" to turn the blower! It's struggling to accelerate that heavy flywheel!) The exhaust system on a turbo can usually be less involved than on the other cases since the turbo itself quiets the exhaust. (Many turbo bikes don't even use a muffler)

I'm really rushing through all this so I'm sure there are some misleading statements... sorry about that!

Bottom line: In the real world, some backpressure is needed for the engine to run right. In my opinion, the impact of a free-flowing exhaust is typically over-rated. (Bikes again: Back in the day, installing an exhaust with 1/10 the restriction of the stock pipe on a Honda CBR900RR, then rejetting, netted MAYBE 5% more HP, typically less than that. 2-3% was normal, IIRC)

Oh yeah, one last thing: it is VERY rare for an OEM to design an exhaust with the torque curve in mind. It's NVH all the way. The only requirement we have is a maximum backpressure... and that can be on the order of 12" Hg for a typical program.

WyattEarp

I'll tread lightly on this one. You're absolutely correct all the equations were correct; however, the page has nothing to do with nor does it address the subject of this discussion. Your original statement is:

My whole attempt was to explain to you via physics equations why you cannot loose torque. I don't think Spyke proved me wrong he did find a mistake in my use of acceleration analogous to velocity.

If I insulted you I apologize I really don't know how to play nice.