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'Undersquare' vs. 'Oversquare' pistons? Need Help

  1. Aug 27, 2007 #1
    'Undersquare' vs. 'Oversquare' pistons?? Need Help!!!

    I'm a high school student doing a little research on the performance applications of oversquare and undersquare pistons in car engines...
    btw, oversquare = pistons with bore:stroke ratio > 1
    undersquare = pistons with bore:Stroke ratio < 1

    I've found that several sports models tend to have oversquare pistons, while the slower and rather heavy duty cars prefer undersquare pistons.
    Could someone explain the mechanics of this...?

    I've heard that, for every cubic centimeter or inch of displacement volume, oversquare engines can operate at much higher speeds than undersquare ones. Does this necessarily imply more power, if so how??

    I mean if we were to create a 400 cc cylinder (not engine), and decide whether to make it undersquared (eg. bore= 7.14 cm, stroke = 10 cm, volume = 400cc) or oversquared (eg. bore = 9.21 cm, stroke = 6 cm, volume = 400cc) and want maximum power out of it... would we prefer the oversquare design?
    Is this simply because it can 'rev to more RPM'?? Please note I'm only concerned about the performance aspect of this comparison.

    Could someone kindly explain this to me in detail?

  2. jcsd
  3. Aug 27, 2007 #2


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  4. Aug 27, 2007 #3


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    Welcome to PF, Crazyevox.
    There is no power advantage to either design. Where they differ is in how that power is produced. As a general rule, a 'stroker' (undersquare) makes it's maximum hp/torque at lower rpm's. An oversquare motor of equal displacement can make the same amount, but you might have to rev it three times faster to reach the power band.
  5. Aug 27, 2007 #4
    Thanks for your reply!
    I've googled this a bit and I find several sources claiming the same... but I havent found any source saying both types produce the same power if they have same displacement.

    If that was the case, why would say, sports car makers prefer oversquare designs... wouldn't they wish that power would come earlier in the rpm band? Wouldnt producing that large amount of power at high rpm damage the engine?
    A lot of the responsive engines such as porsches and several others seem to use that design.

    well kindly correct me if I'm wrong... but this was what I have heard..

    >>>>An oversquare engine produces 'more' power than an undersqaure one because it can 'rev higher'.

    Since the volume of fuel involved is the same for both engines (as they have similar displacement), we could say that..
    'Work' done by the fuel (gas) on the piston= p x V (p = gas pressure, V = gas volume)

    Power = Work/Time
    So if the gas can do more work per second, it obviously makes more power.
    By revving high, the gas would do more pushing and pulling of the piston in a second, hence more work, and more power.<<<<

    However, I'm afraid this could be incorrect because the 'work' at a given rpm would be different for undersquare and oversquare engines...
    because work = p x v
    the volume should obviously be same for both (because of equal displacement)

    But I find the pressure value difficult to understand :(
    Would the gas pressure be constant both engines too, because they have same displacement volume and presumably same temperatures (if temp and volume are constant, I think pressure should be too) ??

    So this is what I believe:::::
    If the pressure IS constant for both engines, so should the work done at a given RPM. So if RPM is increased, work done per second increases, hence more power! Since oversquare engines can rev very high... they should make higher max power. :)

    By the way..... it can be proved that oversquare engines are naturally 'capable' of running higher rpm than undersquare ones.
    Apparently oversquare pistons would be heavier than undersquare pistons of the same displacement due to their larger radius. This would create large stress loads on the engine. However, 'strokers' undergo a larger momentum change at BDC and TDC of the stroke. The overall effect is that increasing stroke length causes a greater increase in stress load on the engine than increasing the bore length. Hence, undersquare pistons cannot rev as high.

    Pleeeeeeaase correct me if I'm wrong!!

    Cheers again!!
    Last edited: Aug 27, 2007
  6. Aug 27, 2007 #5


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    Read that Wiki article again. Undersquare engines are the strokers. They have smaller-diameter pistons than short-stroke engines of the same displacement.
  7. Aug 27, 2007 #6
    oh cheers turbo!

    That was a careless mistake on my part. I've got it sorted out now.
    Hopefully its more clear.
  8. Aug 29, 2007 #7
    Does anyone believe this is incorrect? :(
  9. Aug 29, 2007 #8


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    Remember that a stroker takes longer to extract the energy from the expanding gas because there is less surface area to the pistons. Since the volume is the same as the shortie, the pistons travel farther to achieve it.
  10. Aug 29, 2007 #9
    Could you kindly elaborate a bit further?

    So does that imply that oversquare pistons are indeed capable of producing more max power than strokers of the same displacement volume?

    Basically, for the same disp volume, given a fixed number of revs, shouldnt both pistons produce the same power? They're doing the same amount of work (gas pressure x volume are same for both cases, given the compression ratios are equal too), in the same amount of time (because of the same revs).
    SO theoretically if we increase the revs, they should produce more power. But it can be proved that the shortie is capable of doing more revs than the stroker because of physical strains on the system.
    Hence the shortie (oversquare) should make more max power.
  11. Aug 29, 2007 #10


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    Horsepower is calculated from torque and time. A stroker has more torque because the crankshaft 'throw' gives a longer lever arm on the power stroke. An oversquare engine has to spin faster to get the same horsepower from less torque.
    That longer throw is also why a stroker will have a higher compression ratio if other design considerations are the same, and why it can't match the rpm's of a spinner.
    Last edited: Aug 29, 2007
  12. Aug 29, 2007 #11
    Well I kind of disagree with the fact that the stroker provides higher torque because of its leverage.

    I think theres something that compensates for that extra leverage effect...
    Oversquare pistons have a larger surface area affected by the gas pressure.

    Hence Force on piston = Pressure x Surface Area

    So strokers, having lesser surface area, will thus experience lesser force than a wide bore oversquare piston.

    Torque = (Force on piston) x throw length
    So between strokers and oversquare pistons, there is a trade off between the force and throw length. And this effectively makes the torque produced by both types the same, given that the gas does the same amount of work (pressure x volume) on both.

    Now on to the pressure issue,
    I really wanted to keep the same compression ratios for both types... although it could seem a bit unfair....
    if trading off between bore and stroke (while keeping displacement constant), makes the strokers have better compression ratios... I'd want to change the combustion chamber volume to ensure the ratio is the same for both. Hopefully that doesnt have any significant impact on the way power is produced.

    So equal compression ratios imply equal gas pressures right?
    Hence same work done for both pistons at given rpm. But since shorties can rev more,
    This should eventually mean that shorties are indeed capable of more max power.
  13. Sep 1, 2007 #12
    I have another issue I need help with...

    I've heard that piston speed is directly proportional to stroke length. Could someone prove this, possibly by derivation?
  14. Sep 11, 2007 #13
  15. Sep 11, 2007 #14


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    In one revolution, the piston moves a distance of twice the stroke. After N revolutions, it travels 2N(stroke). But N revolutions takes a time (N/rpm) in minutes. The average speed of the piston is therefore

    (distance)/(time) = 2N(stroke)/(N/rpm) = 2 (stroke)(rpm)

    Of course, this forces the time units to be in minutes. It's easy to fix that if you want, though.
  16. Sep 23, 2007 #15
    Something I didn't see anyone mention is that the larger bore offres better breathing not only by allowing lager valves for a given displacment but also through less valve shrouding. I ran a quick dyno one on a 302 chevy and the other on a chevy 305 both exteremly mild motors with the same parts the 302 which has a 4 in bore and 3in stroke made 3 horsepower more and lost one foot pound of torque then the 305 with it's 3.735 bore and 3.48 in stroke
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