This will help you understand the basics of Cams.  If you want the optimum performance from your cam purchase then ask a cam engineer.  Do not consult someone in a internet chat board or some kid reading off a computer screen at a mail order house... Talk To A Real Cam Engineer!!


Jim Dowell the owner of Racer Brown Cam Technologies is probably the best in the business, you find a cam catalog in our office anywhere we we need a cam we want it right the 1st time we pick up the phone and call Jim.  I recommend you do the same: 410-866-7660 after 2pm EST

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 50" lift, overlap, intake centerline, 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, expansion of gases) move the 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...

Stroke          PistonDirection    IntakePort     ExhaustPort    CrankshaftDegrees    CamshaftDegrees    
Power                   Down              TDC toBDC        Closed                   Closed                    0 to 180    0 to 90    
Exhaust                   Up                 BDC toTDC        Closed                     Open                180 to 360    90 to 180    
Intake                    Down              TDC toBDC          Open                     Closed              360 to 540    180 to 270    
Compression         Up                BDC to TDC         Closed                  Closed              540 to 720    270 to 36    

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 center (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 center (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 center (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

3) How to Build Mopar Big Blocks ia another good reference book SA171 by Andy Finkbeimer.

4) The FBO Tuning Guide is another must read