Undersquare engines are more suited for low end performance, while oversquare engines are more suited for high end performance. Square engines are something of a compromise between the two. In terms of performance (torque and horsepower output), I was wondering if there is a specific cam profile setting and/or RPM at which it is best to choose one approach over the others. Also, do turbochargers and other forced induction technologies change the general undersquare low/oversquare high pattern, or does it remain largely the same?
It really depends on how you’re tuning, turbos seem to like a slightly oversquare design in game but their cam profile is dependent more on turbo size and spool speed then the bore and stroke. The limiting factor of undersquare is piston speed, you start to stress your internals at lower rpms the longer your stroke gets so an aggressive cam might be reaching peak performance beyond the rpm capacity of your rods or pistons. over square on the other hand is limited by valve float more often then pistons or rods, more bore means bigger valves and so more valvetrain weight, a higher cam can actually improve reliability by stiffening the springs. So there isn’t really one ideal size for any one application, but here’s a link that explores a possible real world standard comprise. roadandtrack.com/new-cars/car-technology/a24855/the-500-cc-cylinder-is-the-way-of-the-future/
similar bore/stroke ratios as my HSB engine
some remarks, but not general rules:
bigger bore = bigger valves, bigger flow, but heavier valves tend to float
smaller bore = better efficiency due to smaller piston crown and better use of gas expansion
a good engine should be a balance of both, for a given cylinder capacity, if you have a small stroke, and smaller piston travel, you can make use of higher rpms, but the bigger valves will start to float, an then is the opposite way around, small bore, long stroke, the pistons suffering while valves are ok
so pushing the envelopes of you parts at the same time is key for a reliable and powerful engine.
then there’s the latest industry tendency of 500cc cylinders with the golden proportions of 82/94.6 bore x stroke, which gives great efficiency, usually mated to turbochargers and spinning no faster than 6-6,5krpm
i usually keep bore and stroke equal
My preferred method:
- Rough out the engine with the general configuration you want. If planning a turbo motor, don’t install it yet.
- Select your desired redline and make sure the engine’s not knocking.
- Increase the stroke until it starts to impact reliability, then drop it back one click.
- Increase the the bore until either a) you reach your desired displacement or b) valve float starts to affect power. Light green valves are okay, yellow or red are bad.
- Install the turbo if required, then play with all the sliders until everything is how you want it.
- If you change your redline, go back to step 3. If you start to get valve float, go back to step 4.
Steps 1-3 are my way as well, maximum safe stroke for a given redline is the best way to make a lighter and more efficient engine.