Hor-Opp B/S vs Inline B/S

[Bud Tierney]

While I'm not really into the old horizontal-opposed (flat) 2 and 4 Cyl auto etc engines, I 've noted that, generally, their bore is larger than their stroke...while almost all inlines are tho opposite: their bore is smaller than their stroke..(a few "square", and even fewer "oversquare" inlines are out there, but they're unusual and rare)...

Is/Was there some technological reason/s for this difference??

[padgett]

For some it depends on the tax where they were built (UK had some odd rated HP laws). Others depend on torque (longer stroke generally means more torque at a lower RPM, still others depend on the gas they were intended to run on (knock is easier to control with smaller bore), and other try to get more power from a smaller engine by increasing either or both.

 

If really interested I'd suggest a copy of Sir Harry's "The High Speed Internal Combustion Engine. 

 

For an engine designed for today's pump gas my nut theory is that you want a 3.75"-4" bore and a 3.25" to 3.50" stroke. Adjust the number of cylinders to suit.  With VVT you can make 90% of peak torque from 2000-7000 rpm. With DI you can control detonation under boost.

[Spinneyhill]

The longer the stroke, the lower the maximum revs possible in an engine. There is a limit to maximum piston speed. Oversquare engines also put less force on the crankshaft and connecting rods. They are used where more horsepower than torque is required and then use gearing for top end performance.

 

For the same engine capacity, longer stroke means smaller bore = smaller valves = less efficient engine and less horsepower.

Edited July 14, 2019 by Spinneyhill (see edit history)