ArcticMax

Ok guys, we haven't discussed anything deeply technical lately, so here it is:

if there are 2 engines that are identical in displacement and # of cylinders, then it is my hypothesis that the one with the longer stroke is faster.

I may be totally wrong.

But then, I may be right. The reason is, I thought about why bimmers are so fast with only 2.8 and 3.0 liters, and I think it is because those engines are under squared.

If an engine has a longer stroke, then the pistons and the rods have to travel longer distance in one revolution than the equal engine with shorter stroke and bigger bore.

One time, I rode in a friends Lumina, and when he punched it, it was fast as hell in first gear. I swear, that thing pulled harder than any automatic Maxima I've been in.

Good old American pushrods have excellent low end torque, and they are undersquare.

I think it makes sense; if the cylinders travel more linear distance in one revolution due to long stroke, then is it not producing more "pushing" power than a shorter stroked version?

Any physicists? real smart people? can someone answer this?

The_quorum

I'm not exactly sure on specific weights, but the 3-series BMW's are also a LOT smaller (I read somewhere it was classified as a sub-compact) than the Maxima and put out a comprable amount of HP. Maybe this would explain the quickness?

mzmtg

iTrader: (11)

OK...here goes.

You are right...in theory. However, its not the fact that the cylinder travels farther that makes it faster. Since the stroke is longer, that means that the distance from the bottom of the connecting rod to the the crank center is longer. Since the con rod is pushing from farther out...its like pushing on a longer lever. Pushing with the same force on a longer lever makes more...torque.

As usual, this is a doulbe edged sword. This longer lever means that for a given RPM, the engine with the longer stroke will have a hihger maximum piston speed. Think about it, at a constant RPM, an engine with twice the stroke will have twice the piston speed. Now remember, the piston must accelerate from a stop at top dead center to its maximum speed at mid stroke and back to a stop at bottom dead center. As you can imagine, all this acceleration and deceleration puts lots of strain on the parts. Heavier duty parts weigh more...putting more stress on other parts...etc...etc...

Once again, finding the balance is why automotive engineers get paid the big bucks.

(I apologize to any engineers who may have been offended by my interchanging the words stress and strain)

brubenstein

All else being equal, and few things are, an engine with a longer stroke will produce more torque and less HP. The increased lever arm of the longer crank throw gives more torque. The increased piston speed of the long stroke gives a lower limit to the maximum RPM of the engine. (It's the acceleration forces and peak velocity on the compression rings that limits and engine's max RPM.) Remember that HP = (torque x RPM)/5252, so lowering the maximum RPMs lowers HP.
Purpose built racing engines are very over square, high reving engines.

Shingles

As brubenstein said, it will have more torque. BTW, the lumina "feels" faster becaues it's got more torque.. it might not actually be faster. Step in a turbo diesel truck and you'll swear it's faster than a maxima, only because it's got 350+ lb-ft down low. Don't be fooled by how fast the car feels.

Also, the bimmers are faster probably due to weight. If you just look at engine itself it's one thing, but to look at the whole car, other things such as gear ratio, weight, wheel drive, tires, etc all have an effect. So one can not really say that the bimmer is fast because it's under squared.

-Shing

Black VQ

Of the BMWs. The 2.5, 2.8, 3.0, and 3.3 six cylinders are all inline six cylinder engines. Now in general, I6s make more torque than a V6. There might be a few exceptions to the rule, though. That's another thing to consider.

Interestingly enough, the new M3's 3.3L I6 makes 269lb-ft of torque, while the Mitsubishi Lancer EvoVII pumps out 282 from a 2L I4! Like I said, there's exceptions.

Albertt

You're forgiven.

Jeff92se

iTrader: (82)

Since when has a short block made any power? All the engine's power(or power potential) is located in the heads. No matter what the bore/stroke design is, the head design is #1 most important factor. Contrary to popular belief, long stroke engines WILL rev. But proper breathing and proper blueprinting/balancing is paramount.