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quick and easy noob question

Discussion in 'Technical and Troubleshooting Torque' started by En, Oct 6, 2006.

  1. hey,
    what is bore x stroke. and compression ratio? i know everything else but finally remembered to get around to ask what those are.

  2. also just incase my brother doesn't know everything, whats a strearing dampener do.
  3. Bore x stroke are aspects of piston and cylinder interaction.
    I can't tell you how to tell what's good and not if someone gives you the figures, but bore will be the internal diameter of the cylinder/piston and the stroke is the max length the piston will travel into the cylinder.

    I imagine those figures would essentially provide pretty close to the figure of what the engine's displacement is.

    A compression test of your cylinders measures how much pressure is produced in a cylinder when the piston descends into it.
    The compression of each individual cylinder should be identical... or at least pretty close in older bikes :)
    A "loss of compression" or "low compression" indicates that a cylinder isn't completely sealing properly, which can mean valves need adjusting, reseating or replacing, or there may be scoring on the piston allowing pressure to leak back up past it. This means that when fuel explodes in the cylinder, instead of all the force hitting the piston and being transferred to your drive-train for moving power, some leaks past the cylinder - resulting in power loss for your motorcycle.

    A steering dampener/damper [I've heard both said] tightens up the steering on a bike the faster it goes.
    IE at low speed you want to be able to turn the handle-bars lots because that's how you control where you're going. At high speed, you want to lean to turn, and you definitely DON'T want the handle-bars to be able to move much - as turning the front wheel at high-speed will result in something expensive and painful.

    [and now I'll hit Submit and watch someone else's post appear with a couple of links in it and a 'use Google moran!' comment while I spent 5 minutes typing this --> edit: huh, nevermind then]
  4. If I remember rightly the stroke is how far the piston has to travel (i.e. how long the cylinder is). Bore is how wide the cylinder is. Feel free to correct me if I'm wrong.

    The compression ratio is the amount of space within the cylinder when the piston is at the bottom of it (e.g. the intake stroke of the piston) as opposed to how much space there is when the piston is at top dead centre (TDC). Example, if there is 100cc (cubic centimeters) when the piston is at the bottom and 10cc when it is at TDC then the compression ratio is 10:1.

    As for the steering dampener I don't know enough about them to explain them, the best I could do is hazard a guess.
  5. Damn your worthless hide Ktulu!
  6. Hey, I just talked about compression in general. Not the ratio.

    I used my psychic powers to know you were explaining that and didn't want to steal your thunder.

    BTW, those dreams you've been having. Yeah. Disgusting. You should be ashamed.

  7. Correct. Some engines have a bore that's equal to the stroke - others are defined as either oversquare (larger bore than stroke) or undersquare (stroke is greater than the bore). Typically oversquare engines rev higher, produce more power and wear less - but run hotter (and therefore can't have as high a compression ratio). Undersquare engines produce better low speed torque and can run a higher compression ratio - but are heavier and taller and so aren't used that much.
  8. But JD, my pistons are extreely small i know, but I would have assumed they travel furthar than the diameter of the piston, but mine isn't low speed torque (i dont think)....

    Have I been delusional all this time? :? (Rhetorical question, dont answer that)
  9. Once again correct me if I'm wrong but the engine configuration is also a large factor in how it performs. Inline-4s produce more high end power at the expense of low-end torque (if you want torque, get a twin :) ).
  10. yeah, for an engine of X capacity, the more sylinders it has the harder you can rev it, because each cylinder is smaller, and the pistons, rods, e.t.c have less weight.

    When it comes to revs, the reciprocating components suffer, the smaller the stroke, and the smaller the mass of the reciprocating components the harder it can rev. the more an engine revs the more power it makes, but the harder the components wear.

    But an engine can only ever really be tuned for a specific rev range (a specific airflow rate). The valve timing, fuel maps, intake and exhaust vales and ports and the combustion chamber itself can be changed to effect the engines efficiency at different flow rates(RPM).

    So, if you have an engine making peak power ar 16K, it's going to have huge valves, and a free flowing head, to let it breathe, but it will suffer at low RPM.

    If you have 'lazier' engine, with less cylinders, more stroke, smaller valves (to increase gas velocity) it's going to make great low RPM power.

    With engines everything ias a comprimise, as technology advances though it gets better, EFI was a huge step, and variable valve timing and (in the future) electronically controlled valves will be another huge step.

  11. Eloquently explained, jd!
  12. Yes but you'll find most inline-4 motorcycle engines are an oversquare design. Trying to run one piston with a large bore produces problems with cooling - four seperate pistons can be cooled more efficiently. So the adavantage of using more cylinders is that you can effectively run a larger bore (of course there's other factors involved too).