#3 Cylinder Head
The FS (and FP engine) utilizes a dual overhead cam 16 valve cylinder head...which is pretty common among modern engines, although unlike most current engine lines...this Mazda engine family never used any form of variable valve timing control...The profile of the cam lobes are all you get, and there is nothing controlling them other than a belt connected to the crankshaft...Entirely 'physical' design like this doesn't limit your control as much as you might think, but it will always involve some compromise on how you want the engine to perform...Systems like V-TEC are NOT why a Honda K20 can spin to 9000rpm...its simply why a K20 can spin that high, yet still have a completely quiet and smooth idle...
in our case...we don't have that option...if you want revs...you have to sacrifice some smoothness and torque availability down low...
the way a head works is straight forward...there are 4 valves per cylinder...2 intake, 2 exhaust...2 cam lobes are positioned for each cylinder, per cam...so the intake cam has 8 lobes total, 2 for each cylinder...same for the exhaust...and the lobes are ground as pairs, so that each intake or exhaust valve per cylinder is moved at the same time, duration, and length...basically, it all works in pairs...
This is about as simple as a DOHC head could be...As far as the computer is involved, the only thing it is watching is the position of the exhaust cam (a sensor that reads magnets on the cam shaft's sprocket)...and the position of the crank...by comparing that data as the crankshaft turns, it knows which cylinder is at what position, and therefor when to inject fuel, and when to ignite the mixture...this, commonly, is refered to an engine management system's 'triggering'...
The cams themselves control the valves...and because of that, the specifications of the cam lobes are directly related with how the engine breathes...They are the gate keepers for letting air in, and letting exhaust gas out...and are easily the most complex parts of the entire engine, design wise...small changes to certain specs of a cam can make some really good...or really bad...changes to its performance...The lobes interact with 'tappets' that are mounted onto the valve's stem...the lobe pushed the valves into the chambers. Under the tappets, a retainer and stem lock is in place, that houses valve springs...the springs push the valve back to closed as the cam lobe leaves the tappet surface...head design such as this will have every valve closed when the cam is either not in place, or the lobes are not pushing on the tappets...
For the most part...there are 3 different specs to worry about with a particular pair of cams (there are more, but I'll have a specific cam article soon)...
duration: the amount of time, in rotational degrees, that a cam 'lifts' the valves (usually measured at .050" of lift for comparison to other cams)
lift: The amount of measured distance the valve is physically 'pushed' into the chamber
overlap: The amount of time, in rotational degrees, that the exhaust valve remains open while the intake valve begins to open
As far as modifications to the head itself, it gets pretty limited for the DIY type of guy...Nearly everything, other than assembly or removal of the parts...needs to be done by a proper machine shop...The head has a cast-in series of ports for the intake charge and the exhaust pulses...These ports, in certain cases, can be machined to larger diameters, have angles in them changed or removed, and the surfaces polished...again though, work such as this is pretty much impossible to do accurately yourself...and its very expensive and time consuming to have done correctly...Same with the valves...you can increase the size of the valve seats, and use larger diameter valves...but at a significant cost...for those reasons, this will be one of the very last modifications i look in to...
Modifactions to these areas are actually very complex when it comes to tuning. A streetable engine has to be able to operate effectively in a lot of different speed ranges...It needs to have the flow speed to get a nice gulp of air right off idle to get the car rolling, and has to be unrestrictive enough to breath effectively that high revs i'll be running...Getting all these variables right...isn't easy...bigger cross-sectional area of the ports slows intake charge speed down, which dramatically decreases low end performance...you can even get it so wrong that you can create intake 'reversion', which is where the air will simply stop moving in the correct direction...fouling up the maf readings...and making the car literally inoperable...
With that said, its not something to take lightly...and the LAST thing to do is do these mods before i have the parts in place in other areas to take advantage of it...as in...i'd end up with all of the disadvantages of a free flowing head at idle and low rpm...and couldn't yet take any advantage of the high rpm benefits...
but since the cams are all directly controlling those valves...you can use different cams to change all kinds of things related to the opening and closing of them...
So to quickly go over that (i'll get more specific later on)...you can run cams with more duration to keep the valves opened longer, you can run (very slightly in our case) higher lift on the lobes to open the valves a little bit farther...and you can incorporate some more overlap to have the exhaust cam open just a little longer when the intake valves begin to open...this overlap number will correlate with something know as scavenging, which is where exhaust pulse vacuum in the exhaust system will begin 'pulling' elastic air volume into the chambers...even BEFORE the intake stroke utilizes its own vacuum to move air in...
there will be A LOT of info coming on overlap soon...Now that we have all the physical parts of the engine wrapped up, i can go into further detail about how all this stuff works together...things like dynamic compression, pressure bleed, scavenging by using proper exhaust diameters, intake systems, etc...as well as simple adjustment to cam timing by using adjustable cam gears...