Hot XR750 cam comparison to Norton cams

Here’s the hot Harley XR750 race cam compared to some of the best Norton race cams. The flanks are about as steep as the JS3/D+ style. The ramps are the longest I’ve seen for a bike. Very long and easy. This is why its nicknamed “the lazy cam” (also called the ML or Mert Lawwill cam). XR 750s can revv to 9000 and this is the only cam they have that won’t bounce the valves or break the valves and valve train. I’ve been in touch with the designer of this cam and he told me that the long ramps make it possible to adjust the torque/RPM range for various tracks simply by adjusting the valve lash (which are adjustable over a wide lash range). The cam seems to have more point in the nose than most cams (could be more rounded) but there is no mistaking how much power the XR750 can put out and studying the profiles shows that other cams may have more or less duration & lift than necessary when pursuing XR 750 performance levels. The long duration at low lift is reminiscent of the high revving RX cam – lots of top end without worry about valve float. The early lift gets the fuel charge moving sooner and closes it off later for improved cylinder filling. The JS3/D+ cam is the closest but has less total lift and less duration at low lift. For example – the XR 750 has approx 20 deg more duration at .040” lift (where Britbike cams are measured). That’s a lot more duration at low lift. Duration at .040” checking clearance is a whopping 314 degrees. The duration of the XR750 cam is not so radical at mid lift around .150 to .300”. Getting things right at low lifts can be more important than higher maximum lift. The 480 Sifton dragster cam has so much duration throughout its lift that the lobe centers have to be very wide to avoid valve clash which may not be ideal – the 480 flanks are steeper and more stressfull as well. Note that the XR750 profile in the image below has been adjusted to accommodate the Norton rocker arm ratio.

Don''t beat about the bush - how do these cam profiles go in a Commando ?!

" the long ramps make it possible to adjust the torque/RPM range for various tracks simply by adjusting the valve lash) "
thats interesting. I gusess them XR750 boys have been at this for such a long time they certainly know how to come up with the goods.
Keep at it Jim.

I could modify the Harley XR 750 cam a bit to round out the nose (peak lift is not so important). It won't be necessary to have such extremely long ramps in order to adjust the valve lash for more or less top end/low end. But I like long gentle ramps to prevent valve bounce. It would have the smoothest closing ramps of any Norton cam. This cam would be somewhere between the Sifton 480 and the D+. With the Narley porting it could offer performance close to the XR750. New components like these could take Nortons to new HP & RPM levels and with todays higher quality parts everything should stay together.

The new profile below is under development. Approx 295 duration at .040" checking clearance is show. More duration is an option.