Bore vs Stroke - What Makes More Power?

A deep dive into the power and efficiency differences between undersquare and oversquare engine designs.

1y ago

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. Learn all about the differences below.

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Comments (16)

  • Wow, finally an answer to my sleepless nights.

      1 year ago
  • Good stuff.

    Reminds me of when I studied Automotive Engineering, 40 years ago.

      1 year ago
    • Was that particularly fun, I wonder 👍

        1 year ago
    • Yes, in a odd way. Had a good time at college, probably why I ended up spending about 10 years studying, then a bit more lecturing.

      Would go back as student tomorrow if the funding was there.

        1 year ago
  • I'm to lazy to listen to the video so can somebody just give me a quick summary

      1 year ago
  • I think (and I could be wrong) that another reason for big bores being better at power is the rate at which petrol ignites. It doesn't happen in the same instance, but travels at a set speed in the mixture.

    So having a large bore with a small stroke means the initial blast happens with the most fuel closer to the spark plug, but since the stroke is short, not all the fuel has time to burn. Then any un-burnt fuel is ejected, which is the nice blue flame out of the exhaust you get in high power engines.

    Small bores and long strokes lack the initial big blast, as most of the fuel is further from the spark plug, but gains in efficiency as the longer stroke means all the fuel is burnt before the stroke ends, so the maximum amount of energy is extracted.

      1 year ago
  • oh you are going 'there' Jason?

      1 year ago