[QUOTE]Originally posted by 3ngin33r1
[B]42.

 

Interesting discussions. I see we have the usual mix of those that understand hp/tq and what accellerates a car (i.e. FCGuy) and those that don't, but it's refreshing to see many of those in the latter group so willing to be educated.

One note on stroke and engine stress. Think about how the crank/rod/piston connect and move together, and you'll see why an increase in stroke increases engine stress. It's not the length of the stroke that matters (as much), it's the width of the stroke.

Stroke width? Yes. The crank/rod bearing has to travel a larger circle, so that means more distance side-to-side (as well as up & down) in the stroked engine, and because of this larger circle, the angle between the rod and crank increases. So now on every revolution the rod swings back and forth a few more degrees than it did before. This increased angle causes more friction, and more heat. Also, the angle between the rod and piston increases, so the friction goes up at the piston pin too. And there's more - because of the larger angle between the rod and piston, more force is "wasted" pushing the piston against the cylinder walls, so yup, more friction there.

As one President would say "Friction - bad".

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I agree, few engines fail from thermal problems, compared to the typical over-rev and bounce-the-valves-off-the-piston failure. The latter is not due to increased stress, but simply the inability of the valve springs to control valve motion at the higher rpm's.

I also agree that piston speed is linear with respect to rpm, but is piston friction linear? I have no information to support it, but I would bet not.

Damn, here I am agreeing again. I can't believe those numbers either. Sounds to me like someone was trying to make an impression, rather than give factual information.

If a 12.5% rpm increase (9k/8k) really did double engine stress, then an engine that revs to 13k would experience more than 16 times the stress, which sounds totally absurd to me.