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Motorcycle Engines: Why is it so?

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by MV, Feb 16, 2009.

  1. Two things that I find weird about motorcycle engines:

    Why is there a carby for each cylinder? Surely one carby would be easier & cheaper? I suspect performance...?

    Why are the exhausts on the front & carbys on the rear? You could have some killer forced induction at speed by having the carbs on the front. I presume the exhaust would get very hot by not having the airflow over them? You could around that pretty easily though.

    Is there something I am missing?

    :?: Does any one make an engine with one big carby & scoop on the front & exhausts at the rear? :?:


    Any ideas?

  2. Performance, yeah - more accurate mixture control, less 'laggy' throttle response - the engines even have individual throttle bodies (ITBs), one for each cylinder! Usually gives a better throttle response.

    Also, a large intake manifold with a single throttle body may be too bulky to fit onto a bike nicely. I remember the manifolds we built for Formula SAE (a car racing format which uses motorcycle engines) - they'd be difficult to fit on a bike.

    Mostly packaging, I suppose - Having the airbox on top of the caburettors/throttle-bodies is an efficient way to package things together, especially for high-revving motorcycle engines - the intake runners need to be very very short on fast-revving engines.

    Mind you, some sportsbikes do a little bit of ram-induction by ducting the airbox's intake to a high-pressure part of the fairings, which (allegedly) raises the intake pressure a little at >200kph. The individual intakes then breathe from the pressurised airbox.
  3. The simple answer is "marketing".

    the more complicated answer is "Performance and timing overlap".

    On a 360 deg. twin you don't need two carbies as there is no timing overlap. There is some argument that shorter intakes mean less fuel drop out and thus better performance, but it's pretty week. You'd be lucky to dyno the difference. The truth is the old twins went this way because it was trendy.

    After that it gets more complicated. A 180 twin would carburate differently on each cylinder if it didn't have two carbies.

    A 4 cylinder would need a pretty big carby and it wouldn't be very good at wide range as a result. You could go twin carbies on a 4 cylinder, but the inlet manifold would need to be complicated on a account of the firing order. hence it's easier just to have 4 carbies. And keep marketing in mind.

    Interestingly the first belt drive 750ss' had a single carbie( might have been the Paso or the 900 ss)

    not with an air cooled engine you couldn't. Well not before heat coating you couldn't. Exhaust valves were a problem among other things

    these day you could do it with water cooled engines and heat coated exhausts and I too am a little suprised no-one has done it. it says something about the Japanese.

    people used to muck around with reversed British top ends, for early ram air reasons.
  4. I seem to remember something about the Chinese playing with the idea of a single cylinder laying flat so the carbs are effectively at the front of the bike, with the exhaust running underneath.
    Seemed to make a lot more sense than having a vertical single with the exhaust having to make a 180 degree turn.
  5. So multiple carbys = more complicated but necessary for most things.

    I assume the current way of doing things worked the way it was & no-one bothered messing with it too much. FLipping inlet & outlet around seems to make sense to me, although I'm sure you would run into all sorts of different problems to what we have now.

    Very cool info, any more details on the reversed British stuff? I see no reason why it wouldn't work so far!
  6. They stole the idea from Moto Guzzi and more lately Honda.
  7. Sounds good to me! Could get the centre of gravity right down with an engine like that.

    (Hang on a moment... Wasn't the Honda CT110 like that?)

  8. That still has the exhaust gas doing a full 180 degree - and then a 90 degree bend.

    The Guzzi looked like it was onto something though.
  9. Companies like Keihin get a range of carbies working really well and these sorts of things aren't infinitely scalable. Even if a company wanted to ignore the difficulties of tuning across different cylinders and run a big single carby, there is a whole new run of development difficulties. What would happen when someone went to race it? They'd rip it off and fit 2 or 4 or whatever smaller carbies which are a perfectly known quantity, if not perfect. What do people want in their road bike??? Usually the same thing as what would be in the factory race bike.

    But basically, it is the ideal solution. Otherwise you'd see performance fuel injected motors with one or two big ass injectors, one throttle body, and a manifold.
  10. True enough, but that's 'cause it's an ag-bike to be ridden over stuff which might tear the exhaust off of the bottom of the bike.

    I see no reason why a CTR110RR couldn't have the exhaust run under the bike the whole way. ;)

    But what I was thinking was - we have had bikes with horizontal layouts, etc, but the motorcycles used for serious racing have evolved in a very specific direction - I3/I4/V4/V2/V5 engines with the engine sitting upright, forward and low, the airbox above, the fueltank above that, and so on. There must surely be some advantage in packaging, chassis design, et al for them to use that layout. :-k

    Alternately it could just be a vestigial design decision that was made decades ago and has been carried on "because that's how it has always been done" or something that's been steered by the older rules of the sport constraining innovation, I'm not sure.
  11. The fun of side-valve engines:
    Both the carb AND exhaust mounted on the front
  12. The new husabergs have the cylinder laying forward 70 degrees.

    And either Kawasaki or Suzuki have got some drawings in the patent office with i4's with the cylinders laying forward much the same.
  13. Yeah, but reversed head Trumpys, whilst a piece of the proverbial to do, didn't actually work terribly well in the vast majority of cases, like most of the horrors committed in the early 70s in the name of chop building. Open bellmouths pointing forward will scupper any chance whatsoever of maintaining accurate carburetion over any kind of speed or load range. I think it was used successfully on the odd drag bike which only ever had to work flat out, but for road bikes it was a dead loss.

    Didn't yamaha produce a back to front TZR250 (almost certainly never officially sold here)?
  14. Yes/No. Carbies rely on vacuum to carburate. FI doesn't. So there is no advantage in running a big single FI from a carburation point of view. Crbies on the other hand perform better with a more consistent load.

    But your right, in that long and complicated inlets mean fuel drop out.

    Interestingly one of the more common British mods on the High performance models was to switch back to the single carbies of the more cooking models for better ridability and torque.
  15. I had always imagined that the exhaust exiting to the front was a function of packaging, to get the requisite length of pipe to optimise the exhaust flow.

    Of course, with modern HP engines running enormous RPM, the pipes can be shorter now, as with the underslung stubby pipes that are in fashion... (?)
  16. And of course the Ducati L-twins have their engine literally |_ shaped, with one cylinder straight up and the other lying (nearly) flat. :)

    That husaberg is some really impressive packaging. :eek:

    A place for everything, and everything in its (incredibly efficiently packed) place.
  17. I didn't consider the more consistent load. But then there's lots of things I didn't consider. :LOL:

    Interesting thread. One only has to look at bikes like the Britten to realise that the accepted standard isn't necessarily the best.
  18. looks a bit arse backwards. Or arse upside down. Because the engine is on top of the gearbox the wieght looks very high.

    Better to put the engine down low with the sprocket shaft and put the input shaft high IMO.
  19. This setup actually lowers the centre of gravity whilst increasing ground clearance.

    I know it doesnt look it, but that's what they say.