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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Valve Timing
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<blockquote data-quote="davidd" data-source="post: 127269" data-attributes="member: 1177"><p>For some reason, Irving never tested the cams with megaphones Thus, the statement for a 6000 rpm limit is limited to straight pipes. The Mk2 will deliver power up to 8000 with a megaphone, but as far as I know, this should only be done on a single. The single crank can do 7000 RPM readily, but the wider twin crank should be limited to much lower RPM for all the reasons Glen cites. New flywheels made of better material will allow you to do high RPM on the twins.</p><p></p><p>Coburn Benson set the redline at 8000 for his Grey Flash with a Mk2 and a 10" Velo megaphone with a single taper. The chronometric was 300 rpm slow at that speed due to lag. He changed to an ATC tach, which was quicker, to eliminate his riders shifting at 8300. However, Coburn was racing the Flash in modern road races at the time and competing against factory machines even in the amateur classes. The races were often an hour long. Not really vintage racing.</p><p></p><p>The other limitation is piston speed. At 7000, on my racer, it is 4133 ft/min, at 8000 it is 4724 ft/min, which is fast enough to cause problems. Coburn did have some problems with pistons breaking and a few valve heads coming off.</p><p></p><p>David</p></blockquote><p></p>
[QUOTE="davidd, post: 127269, member: 1177"] For some reason, Irving never tested the cams with megaphones Thus, the statement for a 6000 rpm limit is limited to straight pipes. The Mk2 will deliver power up to 8000 with a megaphone, but as far as I know, this should only be done on a single. The single crank can do 7000 RPM readily, but the wider twin crank should be limited to much lower RPM for all the reasons Glen cites. New flywheels made of better material will allow you to do high RPM on the twins. Coburn Benson set the redline at 8000 for his Grey Flash with a Mk2 and a 10" Velo megaphone with a single taper. The chronometric was 300 rpm slow at that speed due to lag. He changed to an ATC tach, which was quicker, to eliminate his riders shifting at 8300. However, Coburn was racing the Flash in modern road races at the time and competing against factory machines even in the amateur classes. The races were often an hour long. Not really vintage racing. The other limitation is piston speed. At 7000, on my racer, it is 4133 ft/min, at 8000 it is 4724 ft/min, which is fast enough to cause problems. Coburn did have some problems with pistons breaking and a few valve heads coming off. David [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Valve Timing
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