Bore vs Stroke - What Makes More Power?
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Published 2020-01-08
Bore and stroke cause differences in both power and efficiency.
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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
www.sae.org/publications/technical-papers/content/…
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All Comments (21)
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Paid 30k for school on cars and this guy explains it better than almost any teacher
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One thing for sure. There was nothing boring about this video. I was so mesmerized, I nearly had a stroke.
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Simply put: Short stroke creates power at high RPM. Long stroke creates more power at low RPM.
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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.
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Extremely complicated concept meets extremely simplified answer...this is why I love this EE!
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"If you do some math..." This is Engineering Explained. There's no "if" about it; it's going to happen.
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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
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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.
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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.
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I love these videos with a host who is actually educates on the subject and can illustrate the information. Thank you Engineering Explained
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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.
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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 😀
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honestly engines with high stroke just bore me at this point
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i like big bores and i cannot lie, you other brothas cant deny
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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.
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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.
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Holy crap I’ve always wondered about this thank you
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My 1986 Corvette uses a 1973 400 SBC block, its bored .030 to 4.155 and the engine is stroked to 3.875... 420 cubic inches. 6" Rods and a very short skirt forged piston from JE. 11.28:1 compression
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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.
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I have learned more in this 15 minute video than my 2 years of college. :P
