Well just in case anybody is interested , a small and short bit on cams and rockers as I made the image for someone at
turbominis.co.uk I might just as well use it here as well as I have never seen one like this in any engine book.
In a mini the cam is located halfway down in the block and is linked to the valves by push rods and rockers.
The rocker/follower allows for a multiplication of the motion as dictated by the cam.
As the absolute point where the lift starts is hard to measure (Initial Valve Opening is basically the point where lift is no longer zero) hence cam figures are given at a certain lift (mostly 1mm or 0.050 inch, but BMW also use 3mm lift).
As the rocker can only multiply the motion given by the cam this ”absolute” cam duration will not change one iota ( 1.5 times zero = zero). However as you measure a bit further up the lift curve motion is non zero and will thus be affected by the multiplication factor.
Above I sketched an exaggerated rocker effect. The thing to note is that the whole lift curve changes (including maximum valve lift, acceleration, jerk, ramps, etc. etc.), area under the curve is increased and the lift at a certain checking height other than zero is changed a bit.
For a good site with readable info on cams try this one : http://www.tildentechnologies.com
Furthermore it changes the lift on overlap.
As with a single cam you can not just dial out a bit of overlap without grinding a new cam this might be good or bad depending on what the particular combo wants.
The example that Vizard mentions about small bore engine and larger rockers concerns a Piper 270 cam (pretty big duration for a 998) that did not fare well with 1.5 rockers as the lift on overlap was to large as the LCA was too tight .
The whole object of the IC engine is to cram as much mass in the right mix into the cylinder as possible and then set fire to it. It you juggle all the variables right you can make use of wave tuning, inertia (although that only works really well at bike /F1 rpm’s) in combination with the proper valve events in relation to the piston speed etc etc to get more mass in the cylinder that it’s displacement would normally allow at ambient pressure .
Making sense of the wavy bits
If you look at the pressure traces (in this case simulated but you could buy sensors to put in your engine to really see what goes on)
The picture below is archetypical good wave behaviour of a well optimised touring car engine at 8000 rpm as described in G.P Blair’s book.
The things to look for are the valve events (IVO-IVC/EVO-EVC)
The overlap is the bit between IVO and EVC . Note the pressure trace of the cylinder and the exhaust and the intake.
Exhaust pressure is < pCyl ( i.e. it sucks) all the way from IVO to EVC
Intake pressure is > pCyl at IVO all the way until EVC. Note the big amplitude sine wave and the big peak just before IVC
So the exhaust is sucking ,while the intake simultaneously pushed air in ( quite hard .. look at the scale its about 0.5 bar, the intake in pushing 0.2 bar = 0.7 pressure differential which is a surprisingly high 10.15 psi)
After EVC the exhaust is not coupled to the system any more ( until the next cycle) and the intake ramming needs to finish the job of getting mass in the cylinder (marked by the horizontal red arrow) till the Inlet Valve Closes.
Low speed behaviour.
Below is a simulation output of a BMW 16v head mated to a 1360cc a series bottom end with tuned exhaust and intake tracts and a maximum of 115 Kw/154 bhp at 8250 rpm. The traces this time are at 1250 rpm so it is a fair bit out of it’s tuning range to say the least.
At low speed the intake ramming action is pretty much non existent (only a vague sinus like wave) and the exhaust suction pulse is likewise a lot smaller and because it is tuned to work at a totally different rpm it might be pushing back hard instead of sucking out towards the exhaust. If you have low overlap and/or low lift on overlap there is not much of a window for the gas to go where it should not, but at a price for efficiency at high rpm.
I personally think that it does not have to be all that bad all the time and that with a short cam with low overlap and not enough lift, the 1.5 rockers could work fine.
If it makes sense, money wise is another matter ( the cheaper rockers I have seen by Mini Sport tend to develop a large amount of play on the shaft after a disconcertingly short time, you get what you pay for..) and you might be better off sticking in a different cam for half the amount if you can do it yourself. However you need to pull the engine to change the cam so it does depend ( doesn’t it always..)