I'll start by saying that each cam does its own function. The lnlet has to provide enough lift and duration to suit the rpm that we want the engine to produce its miximum power at. The exhaust has to be able to release the gas pressure that is still in the cylinder, it enough time, so that there is no pumping of exhaust gas as the piston rises on the exhaust stroke.
The two cams are seperate, they don't have any relationship together, they are two separate functions, and have to be seen that way. Now I know that this is not what you read from the "tuners", as Tom has said they are tied up in 'valve overlap'. This is a result of the two cam settings, and have little effect on the way the cams are set.
If we look at what Tom has quoted," that we can just vary the overlap to do the job. If we incress the overlap with the standard cams, we would advanced the inlet lobe center, and retard the exhaust, which would have the torque appear lower in the rev range, not higher.
This is going to go against 'what they say' but this is the case. The change from two valve to four valves, has changed the way we do and think, about it. The four valve engine has superior breathing at high rpms. To give an example, a two valve engine, of 550cc cylinder size, would use a lift of 10 to 12mm, and a duration of 360* to have the torque at 7500. The same cylinder with four valves uses 9mm lift and 260* to produce more torque at the same rpm.
Harvey.
