Bore vs stroke long rod vs short

[DAM shop]

-- ​I would like to start a conversation about Bore vs Stroke. Long rod vs short rod. I would like to hear your well thought out answers as to why you think one is better than another. Is Big displacement and a small stroke better for producing bigger horsepower? Is the bigger stroke and small bore only good for producing more torque at the lower end? i know there are plenty of scientific theory's on how this all matters but I want to hear from you builders. I would like to keep this topic to the 4 strokes being that is what we are familiar with.
Do you sacrifice reliability and longevity by going big bore and big stroke? what are the Benny's to a long rod vs a short one. Does piston skirting really harm a motor of this size? We here at the Dam shop build mostly 90/110 motors but obviously any 4 stroke would be the same concept. ​

[Gabriel]

My limited understanding: Long stroke makes torque. Think, diesel and tractor engines. Typically they are slower to rev.
Short stroke makes horsepower. Think Formula 1 and 4 cylinder japanese sport bikes. Far north of 10K RPM. Can't really get the 20K rpm of an F1 engine with a long stroke because piston speed would rival the Starship Enterprise.
For drag engines, there's no replacement for displacement BUT and large bore and a short stroke is preferable. Chevy's 427 comes to mind. All other things being equal the 454 might make more torque but the 427 will come up much faster.

My truck in high school was a good example. My old man built it. It was a Ford FE390. Stock stroke but was bored to use a stock 428 piston. That gave 406 CID. If you so much as gave the pedal a dirty look that thing would jump like a bullfrog.


Rod length, I have yet to spend the time to really understand it BUT a longer rod will increase the dwell time at TDC. This lets you get a better burn on your mix. That's always a good thing.

As with ANYTHING on an internal combustion engine, there are trade offs for every perceived advantage.


As far as reliability, Problems you might encounter would include thinner cylinders which make things run hotter. Higher compression ratios CAN make for harder starts. Not sure it's really an issue but the bigger the engine, the more fuel it will eat......but the 90/110 engines you're talking about, how much can they REALLY use? LOL!!



Great topic. Maybe someone who actually knows something can chime in and I'll get to learn something today.