Four Stroke Engine Valve Timing Calculations

One of the more challenging areas to understand concerning the spark-ignition internal combustion engine is that of valve timing. If you've been into muscle cars very long, I bet you have considered "hopping up" your engine, and with it, you have considered a cam change... Then you start looking through your Mopar Performance Handbook, your Crane Cam book, etc. looking at the cam possibilities, and start reading things like advertised duration, duration at 0.050" lift, overlap, intake centreline, etc. You know generally what a "bumpstick" does and maybe even how to index one into an engine, but all the terminology has you "blown away"... Then to make matters worse, once you get a "feel" for what a particular cam can do in an engine, from "word of mouth", recommendations, or experience, someone asks you about another cam and gives you specs like EVO, IVO, etc. What in the hell do those terms mean? Or some cam software you are trying to use to "see" what you can expect from your selected cam asks for the EVO, IVO, etc. specs, and all you know are the advertised numbers... How do you convert advertised numbers over to the numbers the program wants?

 Actually, why would the program want those numbers anyway? Well, never fear, this page is here to try to "set you straight" on some basic cam/valve timing terminology and to give you the ability to do some basic calculations... So, let's go ahead and begin...

The first thing to do is understand how our 4-STROKE engine works. So thinking about the physical nature of a basic muscle car V-8, with overhead valves and a single cam, we know the spark plugs fire a combustible mixture of about 14.7 parts air to one part gas and that those "explosions" (or better-termed, expansions of gases) move pistons, through which rods move a crankshaft to power the car... Air and exhaust are let in and out of the engine with valves controlled by the movement of a camshaft, translated to the valves through pushrods and rocker arms, and that a timing chain ties the camshaft to the crankshaft so timing between the movement of the pistons and the valves are coordinated...

Great, but what are the FOUR STROKES? Well, they are summarized in the table below, in relation to both crankshaft and camshaft positions for any given piston in that awesome muscle car V-8 of ours...

So for every revolution of the camshaft the crankshaft turns twice? Yep, that makes sense because the timing gear attached to the camshaft is twice the diameter of the gear on the crankshaft... How about everything else in the table?
Ok, let's be more specific:

At top dead centre (TDC), the POWER stroke occurs and the spark plug fires, expanding the combustible mixture in this piston's chamber, moving the piston down to bottom dead centre (BDC).

Then, at BDC, the EXHAUST stroke occurs as another piston fires, where the exhaust valve opens for this piston's chamber and the piston moves up to TDC forcing out the exhaust gases.

Then, at TDC, the INTAKE stroke occurs as another piston fires, where the intake valve opens and a combustible mixture is sucked into this piston's chamber as the piston moves to BDC.

Finally, at BDC, as another piston fires, with both the intake and exhaust valves closed for this piston's chamber, the COMPRESSION stroke occurs where the piston moves up to TDC and the combustible mixture is compressed...

So now we see that in a 4-stroke engine it takes one camshaft revolution (360 degrees camshaft) and two crankshaft revolutions (720 degrees crankshaft) for a piston to fire and be ready to fire again. OK, now we understand the process and the timing...

But wait... The above representation isn't exactly right, is it? We know that the valves cannot instantly open when they should be open, nor close when they should be closed... They have some inherent opening and closing time... For example, to efficiently push out exhaust, we need to open the exhaust valve some time before the power stroke is complete and the piston reaches bottom dead centre (BDC) so it is open long enough to allow the spent gases be pushed out during the entire exhaust stroke... Likewise, we probably need to have the intake valve open sometime during the exhaust stroke to ensure efficient cylinder filling during the intake stroke... That means the exhaust valve and the intake valve will be open at the same time, with some OVERLAP... And that also means, if we want to maximize our flow efficiencies that the exhaust valve will still be open some time during the intake stroke, and the intake valve has yet to close when the compression stroke has begun...

These are the events that generate the manifold signals that the Carburetor must read and deliver the correct amount of fuel to maintain a smooth crisp idle

1) Auto Math Handbook, John Lawlor, HP Books, 1992.

2) The Step-by-Step Guide to: Engine Blueprinting, Rick Voegelin, S-A Design

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