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post #1 of (permalink) Old 02-14-2002, 06:44 AM Thread Starter
 
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Engine configuration\'s effect on torque?

How does the way an engine is designed (Straight vs V) affect torque production? I read about this a long time ago, and seem to recall it has something to do with how the piston rods get leverage on the crankshaft. However, I can't remember exactly the physics involved.
Anyone have any specific answers for me???


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post #2 of (permalink) Old 02-14-2002, 07:21 AM
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Re: Engine configuration\'s effect on torque?

I don't think the arrangement of the cylinders has any big inherent significance. In the state of affairs today, most V6 engines are designed from existing V8s with two cylinders lopped off. The V8s they come from are short-stroke, high RPM, high horsepower engines, so the V6 derivative comes out the same way. The only inherent V6 advantage to high RPM is that the intake manifold is more compact, with shorter runners which tune to a higher frequency.

Contrary to popular opinion, long-stroke engines are not inherently good torque producers either. The relationship between long stroke and good torque is indirect; long stroke engines are necessarily low speed engines because piston speed is a limiting factor. Because of that limitation, long-stroke engines are designed from the beginning to be high torque, and the entire engine, primarily including the induction and exhaust systems, is designed accordingly.

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post #3 of (permalink) Old 02-14-2002, 10:37 AM
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Re: Engine configuration\'s effect on torque?

I would have to disagree, respectfully. And that doesn't mean that I am right. From my study of the phenomenon, I believe that long stroke motors can make more torque as a function of their 'slow' operating speeds. Along the lines that Jim_Lou mentioned, since the motors usually have lower operating speeds they tend to be volumetrically efficient (they usually breathe good... not necessarily by design - I mean if anyone trys to convince me that the BBD can flow 400 cfs then I'm going to croak) Anyways, so they are slow and efficent motors, but they still got lots of displacement with fuel and air... but with those long and heavy rods they aren't going to be accelerating that crankshaft like a Formula Car motor. So that force goes into drivetrain as the function of pull instead of acceleration. Afterall, torque is the ability to do work (pull) which my 258 has, as well as other inline6 motors, but what they don't have a lot of is the abiltiy to change the RATE at which work is done (better known as horsepower).
One last possibility is that if you draw a circle representing the end of a crank throw path, then draw a line representing the connecting rod working in an engine, you'll see that configurations with longer rods but with the crank remaining the same diameter, the force vectors going into the crank are better for torque than acceleration. My .02

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post #4 of (permalink) Old 02-14-2002, 10:46 AM
 
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Re: Engine configuration\'s effect on torque?

Sully hit it on the head...

For more information, go to http://www.howstuffworks and search for "engines". You will come up with articles on virtually every engine ever made - Stirlings, steam engines, rotary motors, two-stroke diesels and much more.

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post #5 of (permalink) Old 02-14-2002, 11:18 AM
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Re: Engine configuration\'s effect on torque?

Torque has nothing to do with slower vs. faster piston speed. Rather it is longer vs. shorter crank throw. Long stroke engines by definition have longer crank throws and thus more leverage on the crank.


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post #6 of (permalink) Old 02-14-2002, 11:38 AM
 
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Re: Engine configuration\'s effect on torque?

Longer stroke will produce more torque, as it's a function of leverage on the crank. That is independant of inline vs v configuration. Common take on rod length (correct or not?) is that it affects the torque slightly, by having the piston dwell at the top of the stroke longer. V motors are preferred by engineers where compactness is a virtue. As well, mentioned previously, the intake runners are closer together, more equal in length making for better tuning for high rpms (generally=horsepower). An advantage, but challenge, is the crankshaft design on v motors. Producing even firing intervals on a V6 requires separate crank throws, making for a challenge in crank strength. Those cranks don't look at all like V8 cranks, unless you are talking some of the older designs where the rods are side by side for a pairing of cylinders. Inline motors have more distance between the crank throws and are easier to manufacture with bulk between the throws. But their length produces other problems relating to torsional flex and vibration. Seems like everythign is a compromise.



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post #7 of (permalink) Old 02-14-2002, 12:21 PM
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Re: Engine configuration\'s effect on torque?

Long stroke engines have a longer crank throw, which means more leverage on the crank, which means more torque. Also, short stroke engines have bigger pistons (for the same displacement) which means more push on the crank, which means more torque. Theoretically it's a wash. While there are slight theoretical differences, the real difference is what an engine is designed for.

Anybody remember the big GMC V6 truck engine from the mid 60s? It was a roughly square design (bore approximately equal to stroke), but a huge torquer. They put it into lots of light and medium trucks. I think they also built a V12 Diesel version. They never put it into a passenger car because it was a truck engine; a heavy, hell for stout, slow-turning, slow-revving torque machine.

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post #8 of (permalink) Old 02-14-2002, 12:44 PM
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Re: Engine configuration\'s effect on torque?

Good stuff, but I'd like to refine it a bit from my perspective.

Long stroke engines will generate more low-end torque, which is in the usable range that most of us operate in like 2500 rpm.

Equal-sized, (displacement), engines with shorter stroke can produce more torque, as evidenced by newer engines of today, but the greater torque doesn't arrive until the upper mid-range of rpm like 4000. Not an area we operate in very much.

The key is not how much torque is produced by each engine, but how much is produced in a usable range, like 2500 rpm.

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post #9 of (permalink) Old 02-14-2002, 01:24 PM
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Re: Engine configuration\'s effect on torque?

Any coments on why some company's choose to go with the Flat configuration, Subaru, Porsche?

post #10 of (permalink) Old 02-14-2002, 01:24 PM
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Re: Engine configuration\'s effect on torque?

<blockquote><font size=1>In reply to:</font><hr>

The key is not how much torque is produced by each engine, but how much is produced in a usable range, like 2500 rpm.

<hr></blockquote>

That's the key to the whole thing. If you want to go to the drag strip, it's ok to shoot for the higher rpm. For most people it's the low rpm range that counts or you could use 8.xx:1 gears.

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