Does Stroke Matter?

[Barnsnake]

I have a bit of a problem with the author's example. The model airplane engine is probably a cross flow two-stroke and the marine engine is a four-stroke deisel.
The airplane engine might be good for 100 hours, while the marine deisel is designed to run 50 years. Comparing apples to oranges (or is that peas to watermelons?).

Torque is force times lever arm. A longer stroke results in more leverage and therefore more torque at equal bore and cylinder pressure. I don't want to rekindle the torque-vs-horsepower argument, but horsepower is a function of torque.

Whether you want a long stroke or short stroke motor is a matter of application.
Personally, I think a big, broad stroker powerband is nice in a road car. For the drag racer that only runs below 6000 rpm in the staging lanes, the broad powerband is useless.

[olddog]

Originally Posted by Barnsnake

I have a bit of a problem with the author's example. The model airplane engine is probably a cross flow two-stroke and the marine engine is a four-stroke deisel.
The airplane engine might be good for 100 hours, while the marine deisel is designed to run 50 years. Comparing apples to oranges (or is that peas to watermelons?).

Both engines were two strokes. He discussed that bore limits port size in 2 strokes, as well as valve size in four strokes. He gave examples of large 4 stroke comparisons, too. About a half dozen examples in all. I just happened to remember the one mentioned, because the huge size difference was so striking.

[blykins]

Originally Posted by Barnsnake

Personally, I think a big, broad stroker powerband is nice in a road car. For the drag racer that only runs below 6000 rpm in the staging lanes, the broad powerband is useless.

You wouldn't believe what the stroke is on an 800ci Mountain Motor Pro Stock engine....

The cam (duration and LSA) determines the powerband.

For a street engine, my motto is go with as many cubes as you can get.

To get a 427, it's easier to go about it with an aftermarket block and crank, otherwise, you have to do a lot of machining to get exactly 427 with a production block. You either have to use an offset ground 400M crankshaft, or you have to bore the block out past .060" and use a 4.100" stroke. Even then you're only at 425ci. It's not "better" to have a bigger bore/shorter stroke, that's just the way it comes out with the aftermarket block combos. You guys shooting for the "magic number" have no clue on what you're missing out on with the larger displacements.

Think about it in terms of power per cubic inch. Builders often use that as a "benchmark" for determining how strong an engine is. A lot of my engines are in the 1.4hp/ci range.

If you have a 331 that makes 1.4hp/ci and a 347 that makes 1.4hp/ci, which is going to have the most power?

Keeping all other items the same, increasing the stroke will generally increase torque and broaden the curves.