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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Big Bore Comet
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<blockquote data-quote="greg brillus" data-source="post: 176382" data-attributes="member: 597"><p>First thing to remember is that the hardest engine to gain more power from is a single, as it has to cop all the mechanical loses within that one piston.......Like any engine the Vincent can be tweeked for more power.......but there are many "Brick walls"........The head design is very shallow in height.......this means the port shape has too sharp an angle change for good gas flow.......The available room within the rocker/valve area is quite restrictive.......Moving the stem collar further up the stem and modifying the lower guide area will give more travel.......The timing chest is very restrictive.......again the available height from the cam spindle centers to the roof of the chest is limiting.......All these issues can be overcome to some extent, but it is a lot of work and takes much experimental work. The cylinder head is the biggest hurdle, as making up a new head is very expensive.......The Horners have done this very well........made the head taller thus allowing better port shapes.......and stood the valves up, thus allowing for a more shallow combustion chamber......this means a lower piston crown, less reciprocating weight, and so on.......The valve train......you can run a cam with a base circle of about 900 and a lift of about 400.......with ratio in the followers and on the rockers you can get good valve lift figures.......(we have 600 at the valves).......But all these parts need to be strong enough to cope.......the lubrication is not ideal.......in an engine with much more power it is going to over heat.........bigger head/barrel fins are needed ( a big downfall on the French engines)......The Horner engine suffer this big time.......overheating.......Running on petrol type fuels you really need an oil jet under the piston crown, not just to cool the piston but lube the small end which seems to be a common issue on these engines as well......One big issue I learnt recently was that running a big bore short stroke engine will result in major overheating issues........apparently, when using a short stroke whereby the piston does not actually come up and out from the crankcase mouth by much, the piston does not cool at all and the engine cooks itself.......It is a common problem on the 95 bore Manx's so I've been told.......This may explain why Peter Molloy here in Australia changed the bore/stroke combo on one of these French engines some years ago, extending the stroke from 75 mm to 86 mm.......But I'm thinking it was to give the engine more torque.......for track racing this drive out of the corners is really what you need........Madness really but all good fun.</p></blockquote><p></p>
[QUOTE="greg brillus, post: 176382, member: 597"] First thing to remember is that the hardest engine to gain more power from is a single, as it has to cop all the mechanical loses within that one piston.......Like any engine the Vincent can be tweeked for more power.......but there are many "Brick walls"........The head design is very shallow in height.......this means the port shape has too sharp an angle change for good gas flow.......The available room within the rocker/valve area is quite restrictive.......Moving the stem collar further up the stem and modifying the lower guide area will give more travel.......The timing chest is very restrictive.......again the available height from the cam spindle centers to the roof of the chest is limiting.......All these issues can be overcome to some extent, but it is a lot of work and takes much experimental work. The cylinder head is the biggest hurdle, as making up a new head is very expensive.......The Horners have done this very well........made the head taller thus allowing better port shapes.......and stood the valves up, thus allowing for a more shallow combustion chamber......this means a lower piston crown, less reciprocating weight, and so on.......The valve train......you can run a cam with a base circle of about 900 and a lift of about 400.......with ratio in the followers and on the rockers you can get good valve lift figures.......(we have 600 at the valves).......But all these parts need to be strong enough to cope.......the lubrication is not ideal.......in an engine with much more power it is going to over heat.........bigger head/barrel fins are needed ( a big downfall on the French engines)......The Horner engine suffer this big time.......overheating.......Running on petrol type fuels you really need an oil jet under the piston crown, not just to cool the piston but lube the small end which seems to be a common issue on these engines as well......One big issue I learnt recently was that running a big bore short stroke engine will result in major overheating issues........apparently, when using a short stroke whereby the piston does not actually come up and out from the crankcase mouth by much, the piston does not cool at all and the engine cooks itself.......It is a common problem on the 95 bore Manx's so I've been told.......This may explain why Peter Molloy here in Australia changed the bore/stroke combo on one of these French engines some years ago, extending the stroke from 75 mm to 86 mm.......But I'm thinking it was to give the engine more torque.......for track racing this drive out of the corners is really what you need........Madness really but all good fun. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Big Bore Comet
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