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Why such a power difference?

Discussion in 'Naked' started by rcheli32, Oct 19, 2013.

  1. I know this is a dumb question, yet I'm going to ask it anyway...

    Usually there is a trade off for everything, but I can't see how the gsx600 has more power and plenty of torque, compared to the Kawasaki er6 which is a 650. Is it inherent with having more and smaller cylinders? Is it engine design, chambers, compression, timing.... what? Will the lower power er6 out last the gsx (I doubt it)....?

  2. The torque developed by an engine is dependant on a lot of things. Port design, compression, fuel control, intake and exhaust design, ignition timing, combustion chamber design ... everything is a trade-off and it's not a simple thing to pin down.
    Power is a function of the torque produced at a given engine speed, they're tightly related.
    As for why a specific 600 has more punch than a specific 650, engine displacement is only one factor. Fours can usually make more power than a twin of similar capacity and tuning, ie both max power, but that's just a rule of thumb, not a hard law of physics. A more softly tuned engine will also, as a rule of thumb, last longer than a highly strung rev beast, all things being equal. That doesn't mean YOUR peaky sports machine won't outlast a specific twin, it's just a rough generalisation. Variations in the quality of design and materials make a huge difference, plus there are other factors that are harder to define. Many people have heard the term "Wednesday bike" or "Monday bike", or something similar, to refer to two bikes that SHOULD be identical but one just feels like it was put together with care and passion (the Wednesday bike) while the other feels like it was built with a bad hangover and a burning sensation when the assembly line worker had to pee (Monday bike).
  3. Or a Friday bike: chucked together as quickly as possible so the workers could piss off and get onto the piss...

    I had a company car once that had a few troubles, and someone jokingly said it must've been a Friday car. I work for the company that built it, so I looked up its build info, and it rolled off the production line at about 4:00pm on Friday 13 April 2007... I loved the car, but it had a sh1t-load of problems!
    • Funny Funny x 2
  4. and they were tired and fed up with management's BS at the end of the week, hated the foreman, fought with the missus after she blew all the pay on Thursday at the pokies etc etc. Never want a Friday bike. In Italy they supposedly had wine served in the factory canteen, so not only all of that but after lunch Friday they were pissed off AND drunk!
    • Like Like x 1
  5. Have you ever wondered where the Italian bikes got their poor reputation for reliability? It's becaise everyday, they have a 3 hour lunch, wine (Or should that be Whine?) included.
    • Disagree Disagree x 1
  6. First off, unless you test both bikes side by side on the same dyno under identical conditions, you can't be certain of the exact magnitude of any difference in the figures. The one set of figures you certainly can't believe is the set provided by the manufacturer. It's why, when I was regularly buying modern bike mags, I liked Performance Bikes as all their test bikes were independently dyno'd by one of the top operators in the UK and independently tested for top speed and standing quarter. All figures were invariably less than those quoted in the sales blurb.

    Having got that out of the way, there are many differences in engine design which contribute to the gap you mention, as noted by posters above. It's also a good rule of thumb that, for a given state of tune, more cylinders will produce more power. There are various reasons for this. Bigger cylinders require heavier pistons which, in turn need heavier con-rods and bigger bearings so the reciprocating mass goes up, placing limits on the rev ceiling. Power is a function of engine sped so you take a hit right there. Big cylinders also have big combustion chambers which are harder to fill efficiently than small ones and also can have issues because a combustion flame takes so long to travel across the whole thing.

    It;s a huge subject and, although Sir Harry Ricardo laid the foundations when he literally wrote the book on how the petrol engine works c1920, development continues and will do so for the forseeable future.
  7. Phil Irving's "Tuning For Speed" is an excellent text on this sort of thing.
    It's old but a lot of is still relevant.
    And, I just found out you can get it as a free e-book.
    I read and re-read that book many times as a teenager.
    • Like Like x 1
  8. Since no-one has put it simply.

    The 600 is an inline 4 which revs a lot higher and makes more power than a twin like the 650.
    • Informative Informative x 1
  9. The power is measured with your typical rider on it. As the gixxer riders thongs and singlet are much more aerodynamic the bike had more power.
    • Like Like x 2
  10. I thought smooth leather would be more aerodynamic compare to exposed body hair?
  11. Can someone enlighten me as to why Ducatis and KTM twins put out as much power as litre bikes?

    Also why did v-twin superbikes come and go in the 90s
  12. Gixxer riders also wax.
    • Funny Funny x 1
  13. They have bigger capacity engines, 1198, 1199, 1190 etc
  14. I suspect that the Ducs and KTMs also won't have the same engine longevity as a (cared for) litre Jap four. Some of the more barking motards push out huge power from singles too, but need a top end rebuild every fortnight if you actually use it.
  15. Partly compression and ignition timing, but mostly valve timing.

    Small cylinders are advantageous because they have both less inertia and less friction, so they can rev higher without special treatment. To go higher requires low-friction wall coatings.
    • Informative Informative x 1
  16. Another old book that explains a lot about engine design and power production is "The design and tuning of competition engines" by Phillip H Smith. Another one I read several times as a teenager.
  17. Simple maths.

    Torque is proportional to capacity.

    Power = torque x rotational speed.

    An i4 can rev higher than a twin, for a given capacity.

    Therefor an i4 can make more power than a twin, all else being equal.

    Lots of other things influence it, but that's the crux of it.
  18. Superbikes World Championship regulations at the time favored v twins (999cc twins were allowed vs 750cc fours)
    • Informative Informative x 1
  19. The sort where the maintenance schedule is in hours, not kilometres?

    I'm not sure most people could even survive those things :p.
  20. Engines are designed for different tasks. A 4 cylinder will have a longer power curve that develops less torque at low RPMs and much more horsepower at high revs along with more torque. A twin cylinder will have more torque down low, at the same speed, to a certain point. Twin cylinder motors are lighter and can achieve better fuel mileage, cost less to manufacture, and fill a need for that sort of use. It isn't uncommon for a large V-twin, like the Harley Davidson, to achieve 55 miles to a gallon at cruise speeds. Very few, if any, four cylinder engines can reach that kind of efficiency, pulling that much weight. In short, tractor vs. sports car. The new Indian Thunder Stroke, makes 125 Ft lbs. torque at 3000 Rpms, with only 71 HP max. The Kawasaki ZX-14 only makes 27 Ft lbs. at 3000 Rpms and is barely off idle whereas the Indian is near red line. Hope this helps, cheers.