Bore vs Stroke - What Makes More Power?

What Makes An Engine Better? Big Bore vs Long Stroke
Bore and stroke cause differences in both power and efficiency.

All piston-cylinder internal combustion engines have a choice to make with regards to bore and stroke. Should an engine have a large bore (the width of the cylinder)? Or would it be beneficial to instead have a small bore and a long stroke? Or should the bore and stroke dimensions of the engine be identical? We'll discuss how bore & stroke affect engine RPM, engine airflow, heat losses with regards to surface area, and flame duration. What makes the most power may not always be the most efficient.

With regards to bore and stroke, an engine is called "oversquare" if the bore is longer than the stroke, "square" if the bore and stroke are identical, and "undersquare" if the stroke is longer than the bore.

SAE - A Study Isolating the Effect of Bore-to-Stroke Ratio on Gasoline Engine Combustion Chamber Development

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[jameslawn]

Paid 30k for school on cars and this guy explains it better than almost any teacher

[byteme]

One thing for sure. There was nothing boring about this video. I was so mesmerized, I nearly had a stroke.

[baronlowe]

Extremely complicated concept meets extremely simplified answer...this is why I love this EE!

[nsdm]

My step father worked at SWRI in the engine department. He tried explaining this to me when I was a kid when he ran the engines for this series of tests. RiP C. Wilke. 2019

[johnbecay]

a note on increased piston speed mentioned at the end of the video--increased piston speed "pulls" on the intake charge harder. this puts a little more fuel charge in the cylinder and boosts power.

[findtherapists]

Long stroke is very common with large ship engines. I've regularly worked with 600mm bore but a 2400mm stroke, though the max rpm is only about 100rpm.

[twowheellifemagazine]

I love these videos with a host who is actually educates on the subject and can illustrate the information. Thank you Engineering Explained

[danielchin]

"If you do some math..."
This is Engineering Explained. There's no "if" about it; it's going to happen.

[oobatz]

Bravo! I worked hands on a NHRA ProStock engine, on and off the track and you got it. There’s also more positive snowball effects of large bore performance engine such as raised camshaft and shorter pushrods to name just a few that all stack up to max hp & efficiency at a targeted rpm range. Great video.

[shadowboy]

For a given displacement, a bigger bore will exert a stronger force on the crank throw, which compensates for a shorter stroke. The reason is because as the bore gets bigger, the piston top has a larger area for combustion pressure to work on. So from a torque perspective (F x L), bore increases F, but stroke increases L. The bigger bore allows for higher revving though.

[qxtmdfj]

honestly engines with high stroke just bore me at this point

[ForTehNguyen]

i like big bores and i cannot lie, you other brothas cant deny

[wrx]

Simply put: Short stroke creates power at high RPM. Long stroke creates more power at low RPM.

[joshualong]

This is the explanation I've always wanted on creating power based on cylinder geometry. Seriously!
Thanks so much for filling in all the gaps in what I put together on my own.

[mueesli]

I would have really appreciated being taught some of my engineering classes in this style of teaching. It seems so much easier and I am not even a native speaker in english, yet I still think I am understanding everything 😀

[crowvelle]

I feel really good when someone walks in while I'm watching one if EE's videos. Don't get me wrong, I've learned a lot, but this is like getting caught reading about quantum mechanics, or reading the dictionary.

[machetekid]

Holy crap I’ve always wondered about this thank you

[Stuff]

Great video indeed. Impressive to fit such amount of details in just 15 minutes.
I used to work as an engineer on a container vessel / car carrier (MS Tampa) equipped with a MAN B&W 8 L90GB Main engine, utilizing a 900/2180 mm bore/stroke. That would be just above 7 feet stroke, providing 36 000hp @ 94 rpm. Quite the opposite of an F1-engines bore/stroke ratio.

[clipperwing]

Thank you, that is an awesome explanation of the relationship of bore and stroke ratios, something I pondered for many years, keep up the great work, I enjoy your channel.

[raybrensike]

Something to remember about this is that a longer stroke means the throws on the crankshaft of a longer stroke motor results in both a higher piston speed as well as more torque, which is why longer stroke motors tend to produce more torque but can't be revved as high rpm.