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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
600cc Comet Carbie?
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<blockquote data-quote="Monkeypants" data-source="post: 116695" data-attributes="member: 2708"><p>Just read an old MPH interview of PCV on the big port heads, which PCV said they first made in 1950.</p><p>Same as in nineteen sixties and seventies hotrodding , both ports and valves were enlarged for power.</p><p>We now know that making the exhaust port too big kills power. Making the inlet valve and port can definitely add power, as long as the shape is right.</p><p>Finding that shape is the tricky part.</p><p></p><p>Terry's current heads use standard sized Vincent exhaust valves and a smaller than stock exhaust port with the floor filled in to create the sideways D.</p><p>Inlet ports are greatly enlarged (38 mm vs 28)</p><p>Inlet valves are stock Vincent size but get lifted pretty high by some of his cams. Stock Vincent valves are big valves, I guess intended to work well with low lift MK1 cams.</p><p></p><p>Add to this the TP squish band mentioned by TT and combustion chamber shape differences. Looking at all of that, while both types are big port, there are more differences than similarities.</p><p>Did the factory use two plug ignition on their big port heads?</p><p>I couldn't find reference to this in the PCV interview.</p><p>He did mention that the oversized exhaust valves caused problems for them by breaking.</p><p></p><p>Glen</p></blockquote><p></p>
[QUOTE="Monkeypants, post: 116695, member: 2708"] Just read an old MPH interview of PCV on the big port heads, which PCV said they first made in 1950. Same as in nineteen sixties and seventies hotrodding , both ports and valves were enlarged for power. We now know that making the exhaust port too big kills power. Making the inlet valve and port can definitely add power, as long as the shape is right. Finding that shape is the tricky part. Terry's current heads use standard sized Vincent exhaust valves and a smaller than stock exhaust port with the floor filled in to create the sideways D. Inlet ports are greatly enlarged (38 mm vs 28) Inlet valves are stock Vincent size but get lifted pretty high by some of his cams. Stock Vincent valves are big valves, I guess intended to work well with low lift MK1 cams. Add to this the TP squish band mentioned by TT and combustion chamber shape differences. Looking at all of that, while both types are big port, there are more differences than similarities. Did the factory use two plug ignition on their big port heads? I couldn't find reference to this in the PCV interview. He did mention that the oversized exhaust valves caused problems for them by breaking. Glen [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
600cc Comet Carbie?
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