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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
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