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      06-05-2011, 09:19 PM   #1
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c63 and M3 displacement question..

I will preface this by saying, this is NOT a thread about the m3 vs the c63. We all know the pro's/cons of each car.

I had a few technical questions though, the m3's displacement is 4.0.. and the c63 is at 6.2

Now what I wanted to know was which motor is technically under more stress? (As in power wise) Does the displacement have anything to do with it?

I do understand the c63 is easier to extract more power from because of the higher displacement, but why?

So basically, i need a crash course on displacement.. So you motor heads can enlighten me...
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      06-05-2011, 10:01 PM   #2
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We would have to know the rotating mass and stroke length of each reciprocating assembly but if I had to venture a guess I am pretty sure the rod & pistons of the m3 will endure more force @ redline due to their higher acceleration.

Displacement has a little to do with it but it's mainly the RPM, the displacement plays into how massive each reciprocating assembly is (bore although the materials & manufacturing techniques play a larger role) and how fast the piston is being accelerated and decelerated each revolution.

All the info you could ever want right here: http://www.epi-eng.com/piston_engine...celeration.htm

You will just have a tough time getting values for some of the dimensions like rod length & piston mass.
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      06-06-2011, 10:09 AM   #3
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Quote:
Originally Posted by IFX View Post
...Now what I wanted to know was which motor is technically under more stress? (As in power wise) Does the displacement have anything to do with it?
As you may be aware, this can get pretty damned complicated, but since the M3 is making more power per unit of displacement, it is technically under more stress. It has to dissipate more heat from a given area, turns a higher rpm to generate that extra power per unit of displacement, etc.

Of course, to generate a more formal answer to this question, one would have to document all the stress factors, and that in itself gets pretty damned complicated. Then you'd have to weight those factors. As an example, is the centripedal force generated by a specific stroke length more or less important than the instantaneous G forces acting on a piston and rod assembly as they reverse their direction twice each revolution. Those G forces, in turn, are influenced by both rpm and the rod-length to stroke ratio.

And blah blah blah with a bunch of other stuff.

Quote:
Originally Posted by IFX View Post
I do understand the c63 is easier to extract more power from because of the higher displacement, but why?

So basically, i need a crash course on displacement.. So you motor heads can enlighten me...
So to get back to basics, think of an engine as an air pump. It pulls in air, mixes it with gasoline, blows up the mixture, and pumps out the exhaust.

You can flow more air in and out of the engine in a given time slice by adding cylinders, making each cylinder "bigger" via an increased bore and or longer stroke, or turning the engine faster.

Or, you can add a turbocharger or supercharger, compressing the air into the engine so you get more air in and out at a given displacement.

Or, you can do all of the above.

The more air you can pump per minute (or second, or whatever), the more power you can make. There are complicating factors, but the previous sentence is dead flat true.

Bruce
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      06-06-2011, 12:41 PM   #4
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thanks serious, and bruce! Will review that link when i get home.9
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