hi fellow third geners.
as we can all see, the cam questions are plenty and abundant, many of us want to change our crappy stock cams for something more meaningful and so, we come here to ask our brothers.
now, there must be a mathematical solution to our problems, there must be some kind of formula, that we should be able to use to get our answers, for example (my case)
Q: what kind of duration and lift (specs in general) do i require to get 180 psi (or 190) of cranking compression on a 350 engine with 9.5 compression???
data: std. bore 350 (1986), dished pistons, 58cc. combustion chambers (heads 416 ported & 3 angled, stock 1.84 / 1.50 valves)
AND, what type of gas to use and timing?
there must be an answer somewhere, a formula, i wish we coukld ask "Mister Data" (star treck), but he's way out in deep space.
so if any human equivalent is here, please teach us, there will be hundreds of people here (like myself) that will be very, very, very grateful :hail: :hail: :hail:
Fernando.

 

you can alway ask me, i'll give you an answer, accuracy of the answer may be a problem, but if you want an answer i'll be glad to help. actually engine combos are about like fingerprints, everyones differant. i'm sure any of the better engine building softwear would be able to answer your questions, the trick is getting the softwear or finding someone to run the numbers for you. looking at other peoples results, or failures, would be a good bench mark to follow. any of the major cam grinders should be able to help you out.

 

dear ede...you're the man.
because, i've been around many cam entries here and some other places, i have learned quite a lot about them, but i'm still ignorant in the matter.
i've learned for example that a short duration gives more mass, therefore more cranking compression, so viceversa for the longer duration, but still nobody talks about numbers, 99% of what we can see is...feels awesome, made a big improvement, or it was crap.
but still, no numbers.
please dear ede, tell me the formula, the info is at my previous posting, or, maybe you can suggest a good compromise between good torque and top end HP? with say...112 degrees separation for a not so bad idle?, my actual stock cam for example gives me 20 inches of vacuum, but is far from a performer, i could live with for example 15 inches?
your advice is greatly apreciated, dear friend
Fernando.

 

I hope Ede will have it in his heart to forgive me if I misspeak,but I think what he is trying to tell you is that there are more variables involved in selecting a proper cam profile than you have listed. Also, cranking compression pressure may not be the best metric for judging the cam (and engine) performance.

One thing to remember is that cranking compression pressure (an indication of static compression ratio) can be fairly high, while dynamic compression pressure in that same engine at any kind of appreciable RPM can be relatively low, with a corresponding low peak HP output.

Trapping the air in the cylinder at low (cranking) RPM can be done best with a short duration camshaft with little or no valve event overlap. Almost no valve advance would make this phenomenon even more pronounced, creating even higher cranking pressure numbers. That will tend to create the highest cranking compression numbers, and would be great for a low RPM air compressor. However, that isn't necessarily what is needed to make power at higher engine RPMs. It would be great for a high torque, low HP truck engine that never exceeds 2,500 RPM, but would likely fall flat on its face above that RPM.

To understand the phenomena affecting the hypothetical "calculation", you should realize that the engine will produce power in direct proportion to the air that flows through it. While a quick analysis may indicate that the higher cranking compression pressure would be a good way to determine that air flow, the deeper analysis reveals that the air is a non-static fluid that doesn't act as a hydraulic function as the cranking compression would indicate. The air entering the engine has inertia and momentum, and since it can expand and compress (unlike a hydraulic state), there is some lapse between the opening of the intake valve and the beginning of movement of the column of air in the intake runner. Similarly, as the piston reaches bottom dead center (the point at which a purely hydraulic system would cease intake flow) the inertia of that column if air continues to move air into the cylinder, compressing the charge. Because of those two actions, it is desirable to open the intake valve slightly before the piston reaches TDC, so that the full vacuum created by the downward stroke of the piston can be used to move air toward the intake valve and into the cylinder. It is also desirable to hold the intake valve open for some time after the piston reaches BDC and starts moving upward, to allow the momentum of the air to compress more charge into the cylinder before the valve closes.

The timing of these valve events must consider the mass and inertia of the column of air in the intake runner, and the amount of time (in seconds) available to move that air from a relatively static state to a full-flow state. As you probably can project, the best adjustment of these valve timing events are therefore effective only in a relatively narrow band of engine RPMs, since it is the engine RPM that determines the amount of time between cycles.

The timing of the intake valve opening can also have another effect on cylinder charging when using a tuned length intake runner. As the piston approaches TDC on its exhaust stroke, there is still exhaust pressure in teh cylinder and chamber that is being pumped out by the piston. The pressure in the cylinder is higher than the intake, but the mass of exhaust gasses is moving toward and past the exhaust valve. Because of this, there is a pressure front (shock wave) of energy that travels backward past the intake valve when it is first opened, while the momentum of the mass of the exhaust gas continues toward the exhaust valve. The pressure wave that travels back through the intake runner will dissipate when it reaches an opening, such as the plenum area below a carburetor or the plenum of a TPI system. As the pressure wave dissipates, it leaves a slight vacuum in its wake, which turns to a milder pressure wave traveling in the forward direction, toward the intake valve. If the valve opening event can be timed to coincide with the point at which the piston starts its downward travel and the cylinder begins to charge, this pressure wave will help force more air into the cylinder faster. Again, the variables involved are time and distance, so the length of the intake runner and engine RPM must be considered. An intake runner at a given length will produce a better charging effect at a given RPM range. Typically, the longer the runner (more distance to be traveled) the lower the engine RPM will be at best charging effect, since the lower RPM allows more time between cycles for that pressure wave to travel up and back the intake runner. Conversely, the shorter runners are better suited to charging at higher RPMs.

Another consideration is the size of the intake runners. A larger runner will contain a larger mass of air, and a smaller runner will contain a smaller mass of air. Air in the larger runner will move more slowly at a given flow rate (CFM) than air in a smaller runner. Because of this, the air in a smaller runner will have more velocity and momentum, and will tend to charge the cylinder more effectively . However, air in the smaller runner will also take more time to overcome its inertia and start moving, so the maximum charging effect will be realized at lower engine RPMs, when there is more time to charge the cylinder. Making the runner too small can also starve the cylinder simply due to lower volume of air available in the runner. Conversely, air in the larger runner will move into the cylinder more readily, but the velocity and momentum of this column of air will be lower. The lower velocity will be increased at higher engine RPMs, at which point it will become beneficial as a charging effect.

If you study the exhaust system, you can discover a similar, but somewhat backward, set of events that occur, all of which will affect the best time to open the valve and produce the best cylinder scavenging. Emptying the cylinder is just as important, since it allows more space for a fresh cylinder charge, and therefore, more air flow. As in the intake, pressure fronts are also present, and if timed correctly, can actually lower the cylinder pressure and help draw more intake air into the cylinder.

As you have already noticed, there are several variables that affect the selection of valve events. As well as the timing, the rate at which the valves open and close can be factors, as well as the overall net lift. An intake valve that opens somewhat slowly but closes more quickly can also promote cylinder charging at some RPMs. An exhaust valve that opens a little later but more rapidly, then closes at a slower rate, can create a better scavenging effect and improve overall air flow at some RPMs. An engine so equipped and tuned, may actually show a comparatively weak cranking compression pressure, but when the engine is spinning at 5,800 RPM, the actual cylinder pressure is much higher due to dynamic forces and charging than it would be with a cam that produced a higher cylinder pressure number at 200 cranking RPM.

I suppose that someone has all these variables entered into a small program that will project the best valve timing for a given engine combination, but I've not yet seen such a program. I'm guessing that the major cam developers like Comp, Crane, Crower, etcetera have loaded and attempted to compute such data in an effort to provide guidelines for the best lobe profiles and valve timings for a given engine. I'll also guess that they've proven those projections wrong in actual dynamometer testing nearly as often as they have proven them correct. What is important is to realize the tendencies that each design feature of a particular engine will exhibit, and select the cam profile, valve timing, and supporting systems accordingly - then hope for the best once you hit the track or treadmill.

I apologize for the lengthy dissertation, but I hope you have a somewhat better idea of what is involved, and why it is so difficult to just quote numbers as an answer when all of the variables aren't known.