#5.1 General Camshaft Info
This is going to take a few write-ups. The camshafts of a particular engine are BY FAR the most complex, least understood, and overall confusing critical engine part throughout most automotive communities. To attempt to explain them, i'll start by stating: I really wish modern engine's didn't even have them...
If 'our' generation of car enthusiasts has anything similar to a carburetor...as in something that needs to be scrapped entirely...and a better 'system' used in its place...the cams are next on the list...
In principle, camshafts do something relatively simple...they 100% control every single valve's opening and closing event. The part itself is simply a metal shaft with a series of journals that it rides on, with a distinctly noticeable series of lumps sticking out of it...the lumps lift the valves off their seats and physically push them into the combustion chambers; letting air/fuel in...and burned exhaust gas out...the valves are housed in a small cylinder cast into the head, and retainers are used to hold springs around the valve stems...so when the cam lobes are not pushing and 'lifting' the valves, the spring tension returns them to a closed position...
So in terms of how they actually work...thats it...simple, right?...Right off the bat that simplicity is where problems start...We've already lightly gone over all the problems with air properties, and also how the abilities of an engine to suck air in actually change with engine speed...so immediately you can see that something that can't physically change shape or dimensions isn't going to be that efficient at meeting the demands of the engine through ALL speed ranges...nor can it effectively deal with all the problems with air in the first place...and by the way, this 'simple' cam shaft set up is exactly what our engine's have.
A very common trend in modern engine design has been centered around taking these simple parts...and making them a lot more complex, yet still holding on to the original idea surrounding them. EVERY production continues to use some form of a cam shaft...they're still lumpy sticks that physically push valves open...But modern tooling equipment, modern computer controls, etc...have allowed most manufacturers to some how manipulate the cam shaft while the engine is running...these systems are all pretty different, but attempt to accomplish the same thing. While we've all heard of V-TEC, things like VarioCam Plus and Double Vanos may not ring a bell quite as much. To keep it short, these systems change the cam's physical position in the head, switch to a different set of cam lobes, and adjust the timing of the open/close events...while the engine is actually running...some systems do all of that, some only do part of that...but its always in an attempt to better match the cam's physical attributes at a specific engine speed...rather than the one size fits all issue we're faced with...
My argument since college has been...they need to STOP doing that...cams need to be gone. Technology is literally already here to allow complete valve control by a completely different system. Computer controlled pneumatic actuators that use pressurized air to open and close the valves was introduced to F1 back in the late 80's, but it was deemed unfair and back burnered to keep the field leveled...Systems like this would give 100% control of every aspect of the valves...the timing...the lift...the duration that they are opened, the speed with which they are opened...everything...and could be adjusted on the fly as the engine revs...but for now, its no where in sight...
Ok, so with that out of the way...forget that, as well as things like V-Tec...its not relevant to these cars...
When dealing with a typical 'fixed' DOHC engine...there are several geometrical figures related to the cam grind that is important...I always mix some of this up, so please correct me if something is backwards...Also, keep in mind that a 4 stroke engine creates one universal constant: A rotation of 2 crank degrees has to create only 1 degree of cam rotation. The cams have to spin at half the speed of the crank, or it would quickly be incorrectly timed. This relationship is kept in check by the tooth count on the timing belt, crank sprocket, and cam gears.
Base Circle: BC is a fairly simple measurement of the overall cam lobe. The lobes 'reach' out from the cam's center origin, and the opposite side of this reach remains a perfect circle. the BC is just the diameter of that circle, without any of the reach. Its harder to explain than it is to just look at a picture:
https://encrypted-tbn0.gstatic.com/...b05ndTVIAJ6Zs3PbQisTw0MUUOK08SxA-faABOunrpjU7
Duration: The time in crank degrees that a cam lobe physically opens the valves. The degree figure can be different depending on how its measured. Some manufacturer's start with duration at .003" of lift (nearly immeasurably pushed off the valve seat), some start with .006" (usually described as 'advertised duration')...while others, for sake of comparison, also list a duration number of degrees at .050" lift
Max Lift: The measurement of the farthest length that the valves are pushed open. Measurements for this can be made two different ways. the first would be max valve lift; how far the valve is pushed away from its seat in the cylinder head...and max lobe lift; the difference of a cam's base circle reduced from the lobe's 'nose to heel' (refer to the pic above) measurement. Both numbers are usually slightly different, so when comparing a cam to cam, you'd go with the max lobe lift...comparing head to head, you'd go with the max valve lift...if that makes sense...max valve lift is not just effected by the cam lobes, but also the 'lash' measurement...which is the physical space between the cam's base circle and the tappet it pushes on...with our solid lifter/tappet design, lash is adjusted by shims which are available in various thicknesses...So using a thicker shim will not only reduce valve/lobe lash from the base circle, it will also add more material for the cam lobe to push on... For example, running a smaller lash clearance...will result in HIGHER max valve lift, running 'looser' lash will create slightly less max valve lift...but the cam lift values remain constant.
Center Line: This is a very important, and often overlooked measurement of a cam shafts relationship with the pistons...the center line of an intake cam shaft is expressed differently than an exhaust cam shaft, but they are both a way of 'timing' either camshafts point of max lobe lift. Intake cam center line is measured in degrees AFTER Top Dead Center. Which is simply how many degrees of crank shaft rotation after TDC that the intake valves are opened all the way. Exhaust cam center line is a measured in degrees BEFORE TDC. That makes sense as the intake cam needs to be fully opened as the intake stroke (piston travelling down the cylinder) leaves top dead center...while the exhaust cam needs to open the exhaust valves as the piston is travelling up in the cylinder, pushing exhaust out...you'll see a center line figure in crank degrees with a before or after description, like mentioned...but sometimes see an CL figure in cam degrees...both mean the same thing, just make sure you're comparing apples to apples.
Ramp Angle: RA is a measurement of the lobe's face, which translates into how aggressively it opens and closes the valves. High RA go from seated to max lift very quickly, low RAs do the opposite. Most cylinder heads and valvetrains are designed around a particular RA, so its not something that is usually messed with in terms of aftermarket cams. For our purposes, a cam with nearly indentical RA should be used if possible. Adjustments to tappets and other parts may be needed if you stray too far from that.
That makes up the main 5 measurements of each cam, but just as importantly...there needs to be a few measurements that illustrate their relationship with each other...
Lobe Separation: (sometimes referred to as LSA) While this number is generally used for a single cam that uses both intake and exhaust lobes, the principle behind applies to dual cam engines...the only thing to remember is that this measurement is a sum of angles on both cams, as there is no way to simultaneously measure both. The LSA is the measurement in CAM degrees between the intake lobe's max lift, and exhaust lobe max lift. In our factory head with factory cam gears, this number is held constant (assuming the timing belt is installed properly and doesn't slip). Its also the basis for making adjustments to the timing by using adjustable cam gears, which i'll get into later...
Overlap: The holy grail of Natural Aspiration...overlap is a function of the LSA. To make it more useful this measurement is determined by knowing the LSA, and incorporating it into crank degrees. So overlap is expressed with a number X stated in crank degrees. Increasing duration and/or reducing LSA both INCREASE overlap. Decreasing duration and/or increasing LSA DECREASE overlap. Overlap, basically, is the number in crank degrees that the intake valves are opened while the exhaust valves are opened. When thinking of overlap, you start with the exhaust stroke from BDC. The piston moves up, the exhaust valves fully open, and begin to close as the piston approaches TDC...as this is happening, the intake cam begins to push open the intake valves...the number of degrees the crank rotates with both slightly opened is your overlap. Like duration; overlap can be expressed at a few different points of lift...it may be overlap at .003", .006', etc., but the quantifiable number is crank degrees.
So with the basics out of the way, i'll get into what all this means in the next one. At least now we have it down what the actual measurements are, and what they represent...
Most thorough cam explanations are related to popular engines for modifications...So if you dig deeper on this, you're quickly going to run in to very in depth descriptions of V8's using a single cam...everything above is mostly the same in principle, its just the way the measurements are made...as well as the way things are adjusted...that are different.
