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Discussion in 'Technical and Troubleshooting Torque' started by russ, Apr 10, 2006.

  1. My g/f just got her license & splashed out on an xt250 enduro. (seat height being a major factor)
    she got a great deal when comparing to the bikes in the category, but I was concerned about the 2 valves (as opposed to the 4 valve bikes)

    wouldn't mind a rundown of the goods/bads of 2valve systems and a list things to make sure I do (or not do) to keep the bike in good nick (that i wouldn't need to worry about if it were 4valve.

    I'm used to efi sportsbikes but will be switching into the enduro world (htough I'll be getting a BMW Dakar) sometime soon so any info would be aprreciated
  2. Well the good is that that don't use as much fuel...which is also the bad :LOL:

    You don't have to do any 'extra' maintenance with a 2 valve, in fact it should require less maintenance than a 4 valve because it won't be as highly strung.
  3. Wow, that's a bigger question then you realise.

    Firstly I'm not model specific knowledgable, so forgive any misstype related to that.

    Ok. The number of cams is not neccarily related to the number of valves. You can have single cam 4-valves or twin cam 2 valve motors. Add 3 and five in there too. Although I don't know of any 5 valve, single cam motors, though it's certainly possible.

    2 Valve motors generally produce less power. This is because they can't fill the cylinder as well. Some people will tell you that they produce more torque, but this is misinformation (here we go). For a given state of tune a 4-valve motor will generally produce about 20% more torque. Remember for a given state of tune.

    That noted, 2-valve motors are generally in a lower state of tune and as such MAY produce more torque.

    Complexity may vary, depending on the configuration. For example a 4-valve, single cam can be quite complex, due to rockers and cam followers needed.

    Whereas a 4-valve may be quite simple, if it uses bucket and shim valve adjustment.

    Having said that, most 2-valve, single cams will generally have screw adjust rockers, which is what you want if you are doing the maintenance yourself.

    You only want bucket and shim adjustment if you want a high reving engine and are less fussy about long term maintenance.

    Because there will be a lot of rubbing going on in the cam mechanism of this thing, you might need a slightly better oil, but other then that it should be easier to maintain then the current crop of high performance nose picker chariots.

    I think the XT250 has been around for ever, so it should be a good bike. Theres probably heeps of new and used parts around.
  4. hi guys thanks for the replies.

    I had heard whispers of the XT having oiling problems at high speeds but wasn't sure if that was related (not that the intention is to be trvelling at 100+ on this bike :)

    thanks for the info
  5. Hang on! You just said that they don't.

    I think that what is a more correct assumption to make is that 2 valve head engines produce their torque at a lower range than a similar capacity 4 valve head engine.

    Honda's VTEC system is where 2 valves operate down low and for high speed operation where the greater air/fuel flow is required, 4 valves come into play. A mate just bought a VFR800. I've yet to ride it but he says that it is quite grunty down low, more so in fact than the aging FZR1000 that it replaced.

    Look at the Ducati ST2 or ST3 with 2 and 3 valve heads. They say that they're more grunty down low compared to the ST4 or ST4S, which apparently are no longer made, if the Ducati website is anything to go by.

    And it has less mechanical inertia compared to an engine with rocker arms, etc. A shimmed engine can rev a lot higher without things becoming all pear shaped inside.

    I'd amend that to say that ALL SOHC 2 v/v head engines have rocker arms. I dunno how else you can operate the valves.

    So, for a greater spread of torque and power, which equals greater tractibility and probably a bit less maintenance, go for a 2 v/v head engine. For performance and where you don't mind the extra maintenance, then a 4 v/v head is the way to go.
  6. OK I'll reword it a little.

    IF, note IF, a 2-valve motor is in a lower state of tune then it MAY produce more torque. However if you had 2 motors equivalent in all respects, including state of tune, then the 4-valve motor will produce more torque.

    They key to my statement was "state of tune".

    They only produce more torque, due to the fact that they must be generally tuned lower, otherwise they become ineffecient.

    The Ducati 2-valve motor makes more bottom end torque, because they produce their peak power a few thousand rpm lower then the 4-valve motors. If they had their peak power in the same spot as the 4-valve they would be all but unridable.
    These days I can't imagine there are any, but the Sunbeam twin is one that comes to mind. The cortina 2L may have been, but don't quote me on that one. The Ford 427 FE OHC was also, but that was a pretty rare beastie.