Given the same displacement, cam, compression, ignition and injection timing, bearing sizes, oiling, valve configuration, rod:sroke ratio, which engine is more fuel efficient in terms of mpg: oversquare(big bore) or undersquare(long stroke)?
Advantages of oversquare:
>less piston ring frictional losses
>less sidewall thrust
>less windage losses
>smaller valvetrain with less friction losses
Advantages of undersquare engines:
>more torque thanks to longer crank arm leverage
>less heat transfer into piston = more energy transfer into crankshaft
>less crankshaft bearing friction
There's arguments for either side. Which one would be best?
Also, less heat transfer into the piston would mean you could possible bump up compression ratio up a fraction of a point on an undersquare engine. Would this change anything?
>>13881240
no one here has any idea as to what any of these things are or mean.
>>13881240
sometimes i paint my calipers red to go faster.
>>13881294
It was posted in QTDDTOT but pretty much ignored so I thought I'd ask again.
>>13881240
Square Pistons ftw
>>13881240
A 125cc single cylinder gc engine wins all by default.
>>13881240
>>13881294
Pic related
I would love to hear a real list of pros and cons though. I'm assuming best balance is right around square for most engines unless it is for special applications (long stroke for diesels and torks, short for high revving engines like F1 and bikes).
Over-square seems like it would have lots of cool tricks because of what you could do with such big valves. But I'm just talking out my ass with anything like that.
Oversquare, less friction overall compared to undersquare.
>>13881294
basically when the Pistons are larger in diameter than the engine stroke and vice versa
>>13881240
Manufacturers have been moving to undersquare designs for fuel economy reasons for the past 30 years, at least. The advantages of the smaller surface area for heat loss and the extra crank arm leverage make up for all the inefficiencies.
One advantage of long stroke not listed on your post, is that they have more turbulence and swirl, and keep the air/fuel charge better mixed than oversquare (particularly at low RPM). Complete combustion is always more efficient than incomplete.
Oversquare is all about high RPM and peak performance. Undersquare is about low rpm and efficiency.
>>13881240
What the fuck are you talking about ?
>>13881240
Op I don't know why everyone is pretending this is which craft
My take away on this topic? Different strokes for different folks
>>13881240
I like this thread. It seems to separate the intelligence of /o/.
square>oversquare>>>>>>>>>>>>>>>>>>>>>>>>>>>undersquare
>>13881240
The big one you didn't mention is piston speeds. An oversquare engine will have a lower RMS piston speed than an undersquare one of the same displacement, and hence lower peak acceleration.
Generally what limits piston life in our race engines is the peak piston speed, the max limit is about 25m/s. We run massively oversquare bores to keep the small end loads down
>>13881240
seeing as you copypastad the question i'll do the same with my answer :^)
the torque will be the same, maybe a bit more for oversqaure because of the advantages you mentioned.
the advantages for undersquare are mostly thermodynamic but don't apply in your example because of the caveats you gave.
the undersquare advantages you mentioned are mostly nonsense. was it a trick question?
>>13886653
This. For example, NASCAR engines (long stroke) and F1 engines (short stroke), have about the same piston speed: see Figure 2 here: http://www.epi-eng.com/piston_engine_technology/comparison_of_cup_to_f1.htm
Would oversquare have an advantage in how high it can rev since it has a shorter stroke? Or would the larger bore nullify that?
>>13887803
big bore helps with high rpm breathing due to room for larger valves
>>13881240
Larger bores have a tendency of detonation, depending on how big we are talking.
