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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Setting valve timing
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<blockquote data-quote="timetraveller" data-source="post: 18703" data-attributes="member: 456"><p>HHHMMMMMM!!!!! I am very suspicious about this. At the end of the day what counts is what happens on a brake, a rolling road or at a sprint meeting where the same rider and conditions prevail. All else is either subjective or due to the rider of the day at the racetrack and the competition etc. Some facts and figures might help. I have looked back through some of my graphs for Mk II and Mk III cams and what I consider to be the relevant figure in this context is the lift in the middle of the overlap, typically at about 4 to 6 degree BTDC. For the Mk IIIs the figure is 0.050" and for the Mk IIs the figure is 0.180". This means that if one were to advance the cam to the max allowed, less 50 thou, then one would have very different timings for the two cams. The situation is a little bit complicated because Cyril seems to put the engine at exactly TDC. Now with the Mk III cam the valve is lifting at about one thou per degree of engine rotation at this stage while with the Mk II it is lifting at slightly more than double that figure. So by doing the adjustment at TDC different cams are going to have yet a further variation in their timing. Therefore John is quite correct to say that this method of timing the cams is going to be variable. The rate of lift, the timing of the cam and the depth of the piston cut out are just the first three things that I can think of which will cause some variation. It might be fair to say that what Cyril is doing is to advance the whole of the cam timing, inlet and exhaust, as much as possible consistent with the inlet valve not hitting the piston. How does this compare with what is regarded as the optimum for valve timing in modern books on the subject? If one looks at the book by John Robinson, (Motorcycle Tuning, Four Strokes, ISBN 0-7506-1805-1) then on page 55 you will see a series of graphs do do with the duration of the inlet valve opening and the engine timing of the inlet valve closing versus engine revs. The latter graph shows power increasing with revs as the closing of the inlet valve becomes later. That is the higher the revs the later the valve should close. If I have understood this correctly, what Cyril is doing is advancing the cam to the maximum amount and thus making the inlet closing point earlier. For a 500cc pot the graphs suggest that a 30º retardation in the time of inlet valve closure will result in maximum power dropping from about 6,000 rpm to about 5,000 rpm. If that is where the maximum power is required then fine. However, looking at one of the other graphs on the same page shows that if one wants a 500cc pot to produce maximum power at 5,000 rpm then the inlet valve opening duration should be 240º. The figure for Vincent cams is more like 280º for Mk IIIs and 303º for Mk IIs. None of this looks like a good way to optimise ones valve timing but if Cyril can actually demonstrate from either rolling road or sprint times that his system produces better results then I could be persuaded that there is much still to be learnt.</p></blockquote><p></p>
[QUOTE="timetraveller, post: 18703, member: 456"] HHHMMMMMM!!!!! I am very suspicious about this. At the end of the day what counts is what happens on a brake, a rolling road or at a sprint meeting where the same rider and conditions prevail. All else is either subjective or due to the rider of the day at the racetrack and the competition etc. Some facts and figures might help. I have looked back through some of my graphs for Mk II and Mk III cams and what I consider to be the relevant figure in this context is the lift in the middle of the overlap, typically at about 4 to 6 degree BTDC. For the Mk IIIs the figure is 0.050" and for the Mk IIs the figure is 0.180". This means that if one were to advance the cam to the max allowed, less 50 thou, then one would have very different timings for the two cams. The situation is a little bit complicated because Cyril seems to put the engine at exactly TDC. Now with the Mk III cam the valve is lifting at about one thou per degree of engine rotation at this stage while with the Mk II it is lifting at slightly more than double that figure. So by doing the adjustment at TDC different cams are going to have yet a further variation in their timing. Therefore John is quite correct to say that this method of timing the cams is going to be variable. The rate of lift, the timing of the cam and the depth of the piston cut out are just the first three things that I can think of which will cause some variation. It might be fair to say that what Cyril is doing is to advance the whole of the cam timing, inlet and exhaust, as much as possible consistent with the inlet valve not hitting the piston. How does this compare with what is regarded as the optimum for valve timing in modern books on the subject? If one looks at the book by John Robinson, (Motorcycle Tuning, Four Strokes, ISBN 0-7506-1805-1) then on page 55 you will see a series of graphs do do with the duration of the inlet valve opening and the engine timing of the inlet valve closing versus engine revs. The latter graph shows power increasing with revs as the closing of the inlet valve becomes later. That is the higher the revs the later the valve should close. If I have understood this correctly, what Cyril is doing is advancing the cam to the maximum amount and thus making the inlet closing point earlier. For a 500cc pot the graphs suggest that a 30º retardation in the time of inlet valve closure will result in maximum power dropping from about 6,000 rpm to about 5,000 rpm. If that is where the maximum power is required then fine. However, looking at one of the other graphs on the same page shows that if one wants a 500cc pot to produce maximum power at 5,000 rpm then the inlet valve opening duration should be 240º. The figure for Vincent cams is more like 280º for Mk IIIs and 303º for Mk IIs. None of this looks like a good way to optimise ones valve timing but if Cyril can actually demonstrate from either rolling road or sprint times that his system produces better results then I could be persuaded that there is much still to be learnt. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Setting valve timing
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