Actually the dip and curve was changed from a differnt cam grind.I modified the stock cam and used a diffenent cam timing for the opening and closing of the valve.Have dynoed other ones with the duals and the dip was still there and fell off at the same rpm,but gave about hp from 8000 to red line and the same power 8000rpm and less.The next mod that I'm trying is the same opening and closing timing as this but a more agressive lobe.
(For the benefit of readers as Timbo no doubt knows already)...... Dips and peaks are caused by pressure waves travelling up and down the intake and exhaust tracts and the way they interact with the cylinder on the overlap phase (helping or hindering the cylinder scavenge). Change the resonant lengths or the cam timing and the dips and peaks are likely to move.
The main 'culprit' is the very strong pressure wave that sets off down the exhaust system as the exhaust valves open when the piston is part-way down the power stroke. There is still a lot of cylinder pressure at this point and the 'bang' going down the pipe is reflected by the silencer (usually) as a returning positive wave (the pipe-end reflects it as a negative).
At certain rpms this positive wave could arrive back at the exhaust valves when they are just at the end of their duration, when the inlets are also open (the 'overlap' period). This hinders cylinder scavenging considerably, keeping spent gasses trapped in the cylinder, and often making a strong 'barking' sound in the intake (also causes lots of 'stand-off').
If you make the pipe longer it takes longer for the wave to return and, by that time, the valves may have closed, so you would not get the dip (at least not at the same rpm).
Likewise, if you make it shorter, the wave may come back before the overlap and be 'pushed away' by the gas exiting the cylinder being pushed by the piston. Just 'behind' the strong positive wave there is a strong negative, and this can be used to help scavenge the cylinder, and this helps to make more power.
When the positive wave reflects off the open end of the exhaust it returns as a negative and, by 'tuning' the exhaust system in the correct way you can optimise the power delivery of the engine. This goes for the design of the silencer as well as the pipe length.
Clearly, with such a close interaction between the pressure waves in the pipe and the valve's opening times, changing EITHER the cam timing OR the exhaust/intake lengths will change the position of the dips and peaks through the rpm band.
Often engines are designed so that when the exhaust is working to help the breathing at a particular rpm, the inlet is such a length that it is hindering performance at the same speed and vice-versa. This makes for a 'smoother' power curve as one 'peak' counters the 'trough' of the other tract.
By making a change of either length in this case 'peak' power can often be improved considerably but, as the 'trough' before and after the peak is usually deeper, the 'area under the curve' is no better.
Area under the curve through the rpm band that gearbox forces the engine to use is the real measure of performance, not just the 'peak' power......