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Message: [QUOTE="Rich Rohrich, post: 144265, member: 16241"] Up to a point that's probably true. Problems start to occur when the rpm, port, valve, and bore sizes combine to generate velocities where sonic boundaries are crossed. It's relatively easy to reach a point where the flow stops completely due to shock waves in the port or valve throat. So it's a system that can be manipulated but too much of a good thing is still too much :) I haven't spent any real time with the XR400s but the bigger XRs have good overall flow but they seems to lack good low speed combustion. So the increase in port velocity makes them more lean tolerant on the jetting and less octane sensitive at low speeds and high loads. Low speed high load is a common cause of knock in these engines when they get hot. With the stock cam Honda shuts the intake fairly early so there is little to be gained from the ram effects inherent with the increased displacement, but the HRC grind cam closes the intake much later and is an excellent match to the bigger bore. IMHO the HRC cam is a poor choice for most people running a stock piston, but it's the best choice for those running the additional compression and a bigger bore. The XR4 and XR6 series seem to have the same basic design philosiphy applied to them so I believe this transfers fairly well between the two. [/QUOTE]