On the question of porting, we generally refer to the size at the outside ofvthe head, ie, stock 1 -1/8"(28mm+-), or ported to 33, 34 or even 35 mm.
On reading Irving's writings on port sizes and shapes for various engine types/usages, it is clear that a lot of thoughful design went into the shape and size of the original Vincent ports, valve size and seat design. It was made the way it was in order to perform really well in most conditions on the road, but not for racing. In tuning for speed he describes how to carefully make changes to the port shape and size in order to get more ultimate horsepower, always at the cost of horsepower and tractibility lower in the rpm range. And that is only when porting is done correctly by someone with knowledge. If badly done, port velocity is lost, charge density decrease, and you have created a gutless wonder that has giant ports.
The old rule was 'If in doubt, port it out" Now it is more commonly understood that port shape , not just size, is the big factor.
What we have with a lot of these ported out bikes, like my ex sprinter, is something that was attacked by someone with a die grinder and no knowledge of flow. No wonder the results are so variable!
On reading Irving's writings on port sizes and shapes for various engine types/usages, it is clear that a lot of thoughful design went into the shape and size of the original Vincent ports, valve size and seat design. It was made the way it was in order to perform really well in most conditions on the road, but not for racing. In tuning for speed he describes how to carefully make changes to the port shape and size in order to get more ultimate horsepower, always at the cost of horsepower and tractibility lower in the rpm range. And that is only when porting is done correctly by someone with knowledge. If badly done, port velocity is lost, charge density decrease, and you have created a gutless wonder that has giant ports.
The old rule was 'If in doubt, port it out" Now it is more commonly understood that port shape , not just size, is the big factor.
What we have with a lot of these ported out bikes, like my ex sprinter, is something that was attacked by someone with a die grinder and no knowledge of flow. No wonder the results are so variable!
