IMO the stock valve sizes are more than adequate for 90% of the bikes that are used on the street and operated below the red line. I find that engines that are run in the higher rpm range on a regular basis can benefit from a larger valve. A question to ask yourself would be how often if at all do you reach the rev. limit? Those that get to the red line occasionally, and those that never go there will not benefit from larger valves when operating in the lower rpms (IMO) and may see a loss of torque over the stock size valve (exhaust). Truth be told, most riders want more power everywhere, not just at the top, hence the popularity of the bolt-in cam build.
We run stock valve size in our CNC head. This is a d port exhaust head that will flow 150cfm at 10 inches. We could get more but, why??? They are a great head for the avg build they will work with a wide range of cubic inch set ups.
Well since I posted that we had been doing more testing and here is the latest size valve we use in our STAGE II CNC head for the 98 kits
Ferrea 1.90 int and a Manley 1.615 cut down to a 1.575 ( gives 80% margin) this current set up offer more to the end user in terms of cam selection and over all versatility. The head will make same numbers with smaller cams and yet offer more to the user that choose to run a larger type cam.
We donot want to produce a head that is a "one size fits all" as what you get there is something that is marginal overall.
Been saying it for a long time.......bigger is not always better. And valves are a prime example. Velocity is the key for good power, especially low end, and smaller valves that maintain good velocity and high intake pressure will have you running better in the sweet spot for most street bikes. Some of the best porters use the stock or 1.590 exhaust on even some larger builds. My own 107 uses a 1.590 exhaust and 1.94 intake. You guys want to use the 1.630 or 1.675 should get used to keeping the R's up to feel the juice. And forget quick throttle response and good low speed power.
We did find that the stock size ex worked very well with a cut down Manely valve it gets better. So through testing we have a great head that works very well for the avg build and will not break the bank in the process.