91Civic_Joe

Looking to upgrade your Cam(s)?
There is Three things to look at when buying a cam(s)
(Duration, lift, and overlap).
I will explain below what you will wanna take in mind when purchasing the perfect cam for your application.

Duration-
Is the number of degrees of the crank rotation that the valve is held
open by a cam (don’t forget that the cam spins at half the speed that
the crank spins at). A general rule of thumb is that the more duration
the cam has, the more top-end power it will create, and this does
cause you to lose low-end power. The more duration the cam has the
more the valve will stay open during the cranks rotation. The longer
the valve stays open the longer the time the cylinder can be filled.
This is important at high rpm, since there is less and less time for
the cylinder to fill.

Lift-
Is the height that the valve is lifted of the valve seat. Usually the
more lift (with in reason) as the higher the valve is lifted the more
flow that can go by. Look at it this way, if you have your window open
half way you let in less air then if the window is open ¾ of the way.
The draw back to having more lift is that the valve opening and closing
speeds become high increasing the chance of valve float. This is why
you should upgrade to stiffer valve springs if the cams lift is
moderately aggressive.

Overlap-
Is the point at which both intake valves and exhaust valves are open
at the same time (at the end of the exhaust stroke and the beginning
of the intake stroke). Overlap is important, because having both
intake and exhaust valves open at the same time creates better
scavenging of stale exhaust to occur. This is because the flow from
the head into the cylinder (from the intake valves) creates a good
push to get rid of stale exhaust out of the cylinder. Just like
anything though, too much of a good thing is bad. If you get to much
overlap the flow from the intake valves will push gas out of the
cylinder before it has been used, thus wasting fuel, which equates to
power. One more draw back of to much overlap is reversion into the
intake ports. Reversion is when fuel/air is pushed back into the
intake ports. This reversion causes intake charge dilution at low rpm
as the backflow in the intake port gets in the way of cylinder filling.
Two things will occur because of this, cylinder pressure becomes poor
at low speeds because of the incomplete filling and the fuel/air
intake charge becomes diluted because of air rushing back into the
intake ports causes low speed misfire. This is why aggressive cams
that have big overlapping, create a ruff loopy idle. This misfiring
causes the motor to skip a beat in rhythmic fashion at idle and low
rpm. Usually firing once every four revolutions when the cylinder gets
enough fuel to touch off. This occurrence is called 8-storking,
because of the misfiring and the car skipping a beat it fires every
other stroke of the 4-stroke cycle

rjngo

thanks

Nova_Dust

Great info.

zeeman

iTrader: (1)

its also very important to match the valve train you are using to the cams. Meaning, that if you use a cam with lets say 12.6 lift you need valve springs that can handle that much lift. Certain valve springs can only handle so much, if you exceed that limit you're playing with fire.
There are also other things to consider when picking cams. Where do you want the powerband? Do you want a broad powerband or for the cams to make power up to 9500rpms? Do you want stock-like idle? Whats the static compression ratio of the engine? Can your head flow enough?
You wouldn't want to use stage 3-4 cams on a stock engine or one with low compression b/c it would be overcamming the engine. Or if you're running like 13:1 CR you wouldn't want to use a cam with lower lift, less duration, and not alot of overlap or on an unported head really.