FWIW - Oversquare = Bore greater than stroke.

Otherwise, you're right. The F20C's short stroke is one of the major factors in its high-rpm capabilities, along with the excellently flowing head that allows power to keep building at such high RPM. The increased stroke of the F22C (North American market AP2 motor) is the main reason for the lowered redline in USDM AP2s.

Per b.r.i.a.n. the constantly-variable timing in the k20's vtec system is responsible for its better mid-range power. The argument can also be made for its "square" (bore = stroke) design resulting in a strong useable powerband. So, to answer your question:

The F20C makes power over the K20 as a result of its superior flow and high RPM. Though, it should be said that stock F20Cs don't make much more power after 8200 RPM, so don't let people fool you by saying "they just make more power stock because they rev higher". It's the overall design that allows high-RPM operation AND produces more power.

Arguments can be made to the overall superiority of both. The F20C's design and engineering just allow it to make more power out of the box.

- James

 

Just to add a little bit, an oversquare engine is better at making more power up top and is easier to push to higher rpms than a square/undersquare engine of equal displacement. Bigger bore means a larger valve surface area:cylinder volume ratio. What that means is that air can fill the cylinder faster, which is a great advantage at high RPM. At the same time, if you increase bore and keep displacement equal, you have to decrease stroke, and that's where the easier to rev higher comes in. How high you can rev something is dependent upon materials and engine design, but how easy something is to rev relatively speaking is very dependent on bore:stroke ratio.

 

The things I've really noticed are the F22C's lack of balance shafts and VTC vs a K20A2.

 

I often hear people say that the short stroke is why it revs so high, and while the stroke is indeed the limiting factor for many engines, it shouldn't be thought that the stroke is short for how high it revs. In fact the stroke is rather long for how high it revs. The F20 holds/held (IDK if it still holds the record) for highest average piston speed for a production engine. As charlie said, its a very special engine.

 

In the US, that's true, I think there was a European Type R version that was 200hp. Maybe OP is from somewhere else?

 

Yeah Ep3 only has 160bhp and revs to 7000, I should know I have one. They must be talking about the Type R..

 

I think you guys forget that we are talking BHP not CHP (brake horsepower and crank horsepower). Differences in power can not be entirely attributed to engine, because it also includes things like intake, header and exhaust. IE Honda gave the s2000 a ginormous cone filter stock, which also has access to fresh hair and has a guide for it. A pretty big exhaust for a N/A car (mind you it also complies with noise regulations) Header is fairly free-flowing. I mean for a guy driving an EP3 to get to 240hp its not as difficult as if we were to jump to 260-280. I believe the S2000 is also a certified ULEV vehicle. Which i find is the most spectacular part.

 

While the constantly-variable timing of the K20 provides benefits, it's not the sole reason for that engine having a better low- and midrange than the F20. As I mentioned, it's all about design intent. Honda designed the F20C with a high-revving, motorcycle-like powerband in mind, which consequently gives up some lower end in order to be more efficient at higher rpm. This is partially covered up by VTEC, since you now have separate valve timing for low and high rpm. However, there are myriad other aspects making up an engine and its powerband, including intake/exhaust geometry, crank stroke, etc. In the case of the F20, those are designed for high rpm efficiency.

Having said that, the K20 was designed with a more flexible torque curve in mind. When one compares the K20 and F20, I can see how you can draw the conclusion that being more square makes for a "strong useable powerband" but correlation does not imply causation. Just because the bore/stroke is different between the two engines, does not imply that it is the sole reason for an engine having a more useable powerband. The longer stroke of the K20 allows it to develop more torque (given equal cylinder fill between the engines), which translates into a "happier" low end, but limits how fast the engine can spin. Thus, it's camshafts, valve timing and intake/exhaust tracts are designed to be most efficient at lower speeds.

My point is this: I completely agree that, when holding displacement per cylinder constant, an "oversquare" engine will be able to rev higher than a square or "undersquare" engine (given the rest of the engine is designed to do so). However, this is because if you hold displacement constant, an oversquare engine must mathematically have a shorter stroke. It is the shorter stroke which keeps mean piston speeds down which then keeps piston/rod accelerations and forces down. It is the force that a piston/rod can take that dictate maximum engine speed, and those forces are directly related to stroke! Notice that there is no mention of bore there.

To conclude my point, do not attribute an engine's high rpm capacity to bore-stroke ratios, but to simply their stroke! A square 5.0L V8 will not be able to rev as high as a square 3.0L V8.


Just to add a little bit, an oversquare engine is better at making more power up top and is easier to push to higher rpms than a square/undersquare engine of equal displacement. Bigger bore means a larger valve surface area:cylinder volume ratio. What that means is that air can fill the cylinder faster, which is a great advantage at high RPM. At the same time, if you increase bore and keep displacement equal, you have to decrease stroke, and that's where the easier to rev higher comes in. How high you can rev something is dependent upon materials and engine design, but how easy something is to rev relatively speaking is very dependent on bore:stroke ratio.
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Not really. The B/S (bore/stroke) ratio actually has nothing to do with "how easy" a typical production engine is to rev. "How easy" implies fast engine acceleration (quick changes in engine speed). In order for something (like an engine) to accelerate quickly, it should be as light as possible. Therefore, a quick revving engine is very dependent upon the weight of the reciprocating parts (crank, rods, pistons, flywheel, etc), and not really at all on B/S ratio.


I definitely agree that the F20 (and F22 for that matter) has very high mean piston speeds for a production engine, I do not dispute that fact!