some of this i knew, some I did not. I enjoyed reading and thought I would share

http://www.jdmcars.com/tech/lift_duration.pdf

One would think that the ideal camshaft would have 180 degrees of duration
and zero degrees of overlap. That is, the intake would open precisely at
TDC, close at TDC, and cycle through the compression and power strokes
until you were at BDC again, where the exhaust valve would open for another
180 degrees until you reached TDC at the beginning of the intake stroke.
Some have even commented that the ideal valvetrain would open the intake
valves instantaneously at TDC, leave them open for 180 degrees, and then
slam them closed instantaneously at BDC (likewise for the exhaust valves).
This would work fine on an engine up to about 2000 RPM, after which power
would be seriously compromised.
The main problem is that air has momentum. Because of this, the intake
valve must open before TDC in order to make sure that the valve is open far
enough to allow the incoming air/fuel charge in with the least amount of
restriction once the piston begins to move downward. Likewise, exhausted
gases that leave the combustion chamber create a vacuum behind them that is
used to assist the flow of the intake charge into the combustion chamber,
hence the need for a certain amount of overlap (ie: when both the intake
and exhaust valves are open between the end of the exhaust stroke and the
beginning of the intake stroke). This technique is called "scavenging" and
is present it properly designed exhaust manifolds as well (which time each
exhaust pulse so that each one helps draw the next ordered pulse out of the
engine). The opposite is also true... long overlap allows the intake
charge to help push the exhaust charge out of the cylinder if it is moving
fast enough.
Finally, there is funny physics going on between the crankshaft's circular
motion and the piston's linear up & down motion. Around TDC and BDC, the
crankshaft can rotate almost 20 degrees in either direction while moving
the piston downward very, very little. The piston does not move at a
fixed, linear velocity inside the cylinder. Rather, it follows the path of
a sin wave (eg: accelerating from TDC to a point exactly halfway down,
where it decelerates to a stop at BDC).
It all has to do with a factor called "volumetric efficiency", or "VE". VE
is a measurement of how efficient an engine is at drawing in air into the
cylinders. A cylinder has a fixed volume while the piston is at BDC, and
an engine with a VE of 100% should be able to cram in exactly the same
volume of air as the calculated volume of the cylinder. Because of
restrictions in the intake, however (primarily the venturis on carburetored
engines), most stock engines have a maximum VE of around 80%, while most
race engines hover around 95-100%. With the proper cam and/or ram airinduction, it is possible to achieve a VE greater than 100%. Forced
induction engines (ie: those that use turbos or superchargers) develop VE
ratings in excess of 100% the moment they develop boost, with each 14.7 PSI
equaling an additional 100%. Simply put, the higher the VE, the more air
you can cram into the cylinders, and the more power the engine can make.
A camshaft with a large duration (300+) can allow a normally aspirated
engine to get very close to a VE of 100% at high RPMs because the exhausted
gases leaving the cylinder help to draw in a larger intake charge. By
assisting the incoming air to enter the cylinder you cram more air into the
engine, and therefore increase the engine's VE. Port velocities are
critical, and velocities increase as engine RPMs increase. A long duration
camshaft usually has a power band way up the RPM range (6,000+), and a race
motor that sees frequent sustained high RPMs can really make a lot of power
with that high VE.
There is a tradeoff, however. A camshaft with a long duration doesn't run
well at all at idle or low RPMs. At low RPMs the port velocities aren't
nearly as high, and scavenging simply doesn't take place. In fact, since
there is a slight vacuum in the intake manifold and a slight pressure in
the exhaust manifold at all times, opening the intake valve too soon
creates a path of lesser resistance for the exhaust gases. Instead of
going out the open exhaust valve, the burned air/fuel charge tries to enter
the intake manifold instead (commonly called "reversion" because the air in
the manifold near the valve actually reverses direction). What results is
normally called "polluting of the fresh intake charge". Burnt exhaust
gases mix with the incoming unburned air/fuel charge and dilute it,
effectively reducing the amount of fuel and free oxygen entering the
cylinder (which means a lowered VE). Any drop in VE means an equal drop in
power. With long duration cams running at idle, the result is the typical
rough idle that you hear on many hopped up V8s. Some of them won't even
idle at all without raising the idle RPMs above what is considered normal
(in order to introduce more air/fuel, which increases port velocities,
which raises VE).
Intake manifold design (dual plane, single plane, high rise, velocity
stacks, etc.) can all contribute to engine tuning as much as header design
does. The choice of camshaft defines both. An engine's displacement and
camshaft define the style of intake and exhaust manifold used on an engine.
Get everything right and you get free power. Get one or two things wrong
and you lose power.
Remember way back when I mentioned the theory of instantaneous valve
openings and closings? Many people thought that this was an ideal (albeit
highly impractical) method of allowing the maximum amount of air in and
out. After all, how could a design that allowed the valves to slowly open
and close provide any real flow during those periods of low initial lift?
Actually, it has been determined that the slow openings and closings are
*necessary* for proper engine operation. When the piston begins its
downward travel from TDC during the intake stroke, the first few degrees of
crankshaft rotation create hardly any downward movement in the piston.Opening the valve instantaneously at this point would cause port velocities
to plummet, causing turbulence in the intake system and cause fuel to drop
out of suspension.
Now getting back to your question:
A low duration camshaft works best at lower RPMs. Even though the stock
cam has a duration of about 250 degrees, a mild aftermarket cam like the
Engle 100 (duration 270 degrees) will still move the point of maximum
torque/horsepower further up the RPM range. A cam such as the Engle 100
will idle smoothly and possess good port velocities at low-to-mid RPMs,
increasing the engine's VE within that range (a good guess-timate would be a
VE of about 85%). Eventually the low overlap of the 100 will hurt the
engine at high RPMs because the low overlap does not allow much scavenging
to take place. The valves are also open for less time over all, which
limits the engine's maximum VE (unless you resort to forced air induction
using ducts, turbos, or superchargers).
A super-high duration camshaft like the "320" you mentioned (the closest
Engle equivalent being an FK-87) is pretty much an all-out race cam. It
works best at very high RPMs where the scavenging effect could conceivably
push the VE up beyond 100%. 100% is greater than the 85% of the Engle 100,
and potential maximum torque and horsepower are much greater with the FK-87
as a result. The down side is that the FK-87 would idle like a poorly
tuned Harley (due to reversion) and have very poor off idle performance
(due to low port velocities at low-to-mid RPMs).
Lift is another matter entirely (are you getting tired of reading yet?).
Increasing lift has exactly the same effect as increasing the cam's
duration, except that you are not actually altering the opening and closing
points of the cam lobes. Increasing duration allows more air in & out, and
so does increasing your maximum lift (the former increases the size of the
valve opening while the latter simply increases the amount of time that
opening is open). A lot more goes into determining a cam's maximum lift
(or rate of valve opening) than does its lobe timing, though. Maximum
lift's main enemies are mechanical in nature, and have to do with a) coil
bind and B) valve-to-piston interference.
Coil bind is what you get when you attempt to open a valve so far that the
spring that normally holds the valve closed can't compress any further (the
coils end up coming in contact with each other until it is nothing else
than a solid column of spring steel). Valve-to-piston interference is a
no-brainer, since a valve that opens too far stands a good chance of coming
in contact with the top of the piston at TDC (the common solution being to
machine valve pockets in the piston dome to provide clearance).
That's the easy part. A cam's maximum lift doesn't just determine how far
the valves lift off their seats, however. It also determines how *fast*
the valve moves off the seat to the point of maximum lift and back down
onto the seat again (commonly referred to as "ramp speed"). A high lift,
high duration cam lobe is gentler on the valve train than a high lift, lowduration cam because the ramp speed isn't as quick. High ramp speeds
coupled with weak valve springs can result in a) "ski jumping" just after
maximum lift is achieved and B) valve float.
"Ski jumping" occurs when the lifter is accelerated off the tip of the lobe
so fast that the lifter actually leaves the surface of the lobe and becomes
"airborne". This will affect valve timing-- specifically altering the
moment that the valve closes. It also means that the valve will come down
on the valve seat harder than normal. "Ski jumping" often occurs at high
RPMs without sufficient valve spring tension and when matching high ratio
rockers on camshafts designed only for use with the stock 1.1:1 ratio.
Valve float happens when the valve isn't placed back on its seat gently
enough, causing it to bounce once or twice before the valve spring tension
holds it down firmly. This is "A Very Bad Thing"™, as it will pound out
seats, break the heads off of valves, and be generally rough on the rest of
the valve train components. Maximum lift isn't the sole contributor to ramp
speeds, however. The overall silhouette of the lobe is the key.
The camshaft is the heart of an engine, and it influences all the other
factors of your engine design. When building a motor, cams are almost
always chosen first (sometimes second when you are dealing with an engine
who's displacement is easily changed-- like the Type 1 ACVW).
PS: Not all identical model camshafts should be considered equal. It used
to be a common practice to "regrind" camshafts. The existing lobes were
shrunk in size to compensate for any areas on the original lobe which
experienced extensive wear (IOW, modified the original shape of the lobe).
Cam lobes contain an area called the "base circle" which is the part of the
lobe that the lifter rests against when the valve is fully closed (think of
it as the coast side of the lobe). The top of the lobe is what provides
the valve train with its maximum lift. Maximum lift is essentially the
difference between this point and the lobe's base circle. If you think
about it, it is easy to see that you can reshape the existing lobe and
create a cam with the same characteristics as it had before, by simply
"shrinking" the lobe. As long as the difference between the high point and
the base circle is the same as before, total maximum lift has not changed.
And as long as the rise and fall ramps of the lobe start in the same degree
positions as before, duration hasn't changed either. However, the lifters
will sit further out of their bores and you'll have to turn in your valve
adjusting screws another turn or so to make up the difference. Likewise,
it is conceivable that a camshaft manufacturer created a cam with lobes
larger than what your valve train can handle. Stories regarding lifters
that bottom out in their bores prior to the cam achieving maximum lift are
nothing new. My point? Don't assume that your new camshaft will simply
drop in. Measure everything at least three times and if you are ever in
doubt, ask someone else.

WALL OF TEXT!

I stopped reading at "One would think that the ideal camshaft would have 180 degrees of duration and zero degrees of overlap. That is, the intake would open precisely at TDC, close at TDC..."

So the ideal cam opens the valve, sucks in fuel and air, pushes it all back out again and closes the valve? I assume that they mean BDC for close? But when faced with a wall of text and an opener that causes my brain to fail - I decided it was time to stop reading and start being sarcastic.

A simple answer is that you need a period of time with all valves closed after the intake stroke sucks the air/fuel mixture into the cylinder, for the piston to push up & compress the air/fuel charge in the cylinder.


A full 180 degree cycle for each valve would leave zero time for compression.

One full piston cycle is 720 degrees of crankshaft rotation.
Plenty of time for compression.

Well I for one enjoyed reading that, thanks for posting it! I guess I'm a little more tolerant of "walls of text"

I knew some of it already, but it went a bit further and explained some more. I think there may be a typo there where it says:

"Increasing lift has exactly the same effect as increasing the cam's
duration, except that you are not actually altering the opening and closing
points of the cam lobes. Increasing duration allows more air in & out, and
so does increasing your maximum lift (the former increases the size of the
valve opening while the latter simply increases the amount of time that
opening is open)."

I think it meant to say that the former (referring to duration) increases the amount of time the valve is open and the latter (referring to maximum lift) increases the size of the valve opening.

One would think that the ideal camshaft would have 180 degrees of duration
and zero degrees of overlap. That is, the intake would open precisely at
TDC, close at TDC, and cycle through the compression and power strokes
until you were at BDC again, where the exhaust valve would open for another
180 degrees until you reached TDC at the beginning of the intake stroke.

Some have even commented that the ideal valvetrain would open the intake
valves instantaneously at TDC, leave them open for 180 degrees, and then
slam them closed instantaneously at BDC (likewise for the exhaust valves).
This would work fine on an engine up to about 2000 RPM, after which power
would be seriously compromised.

The main problem is that air has momentum. Because of this, the intake
valve must open before TDC in order to make sure that the valve is open far
enough to allow the incoming air/fuel charge in with the least amount of
restriction once the piston begins to move downward.

Likewise, exhausted gases that leave the combustion chamber create a vacuum behind them that is used to assist the flow of the intake charge into the combustion chamber, hence the need for a certain amount of overlap (ie: when both the intake
and exhaust valves are open between the end of the exhaust stroke and the
beginning of the intake stroke).

This technique is called "scavenging" and is present it properly designed exhaust manifolds as well (which time each exhaust pulse so that each one helps draw the next ordered pulse out of the engine).

The opposite is also true... long overlap allows the intake charge to help push the exhaust charge out of the cylinder if it is moving fast enough.

Finally, there is funny physics going on between the crankshaft's circular
motion and the piston's linear up & down motion. Around TDC and BDC, the
crankshaft can rotate almost 20 degrees in either direction while moving
the piston downward very, very little. The piston does not move at a
fixed, linear velocity inside the cylinder. Rather, it follows the path of
a sin wave (eg: accelerating from TDC to a point exactly halfway down,
where it decelerates to a stop at BDC).

It all has to do with a factor called "volumetric efficiency", or "VE".
VE is a measurement of how efficient an engine is at drawing in air into the
cylinders. A cylinder has a fixed volume while the piston is at BDC, and
an engine with a VE of 100% should be able to cram in exactly the same
volume of air as the calculated volume of the cylinder.

Because of restrictions in the intake, however (primarily the venturis on carburetored
engines), most stock engines have a maximum VE of around 80%, while most
race engines hover around 95-100%. With the proper cam and/or ram airinduction, it is possible to achieve a VE greater than 100%.

Forced induction engines (ie: those that use turbos or superchargers) develop VE
ratings in excess of 100% the moment they develop boost, with each 14.7 PSI
equaling an additional 100%. Simply put, the higher the VE, the more air
you can cram into the cylinders, and the more power the engine can make.

A camshaft with a large duration (300+) can allow a normally aspirated
engine to get very close to a VE of 100% at high RPMs because the exhausted
gases leaving the cylinder help to draw in a larger intake charge.

By assisting the incoming air to enter the cylinder you cram more air into the
engine, and therefore increase the engine's VE.

Port velocities are critical, and velocities increase as engine RPMs increase. A long duration camshaft usually has a power band way up the RPM range (6,000+), and a race motor that sees frequent sustained high RPMs can really make a lot of power
with that high VE.

There is a tradeoff, however. A camshaft with a long duration doesn't run
well at all at idle or low RPMs. At low RPMs the port velocities aren't
nearly as high, and scavenging simply doesn't take place. In fact, since
there is a slight vacuum in the intake manifold and a slight pressure in
the exhaust manifold at all times, opening the intake valve too soon
creates a path of lesser resistance for the exhaust gases.

Instead of going out the open exhaust valve, the burned air/fuel charge tries to enter
the intake manifold instead (commonly called "reversion" because the air in
the manifold near the valve actually reverses direction). What results is
normally called "polluting of the fresh intake charge".

Burnt exhaust gases mix with the incoming unburned air/fuel charge and dilute it,
effectively reducing the amount of fuel and free oxygen entering the
cylinder (which means a lowered VE). Any drop in VE means an equal drop in
power.

With long duration cams running at idle, the result is the typical
rough idle that you hear on many hopped up V8s. Some of them won't even
idle at all without raising the idle RPMs above what is considered normal
(in order to introduce more air/fuel, which increases port velocities,
which raises VE).

Intake manifold design (dual plane, single plane, high rise, velocity
stacks, etc.) can all contribute to engine tuning as much as header design
does.

The choice of camshaft defines both. An engine's displacement and
camshaft define the style of intake and exhaust manifold used on an engine.

Get everything right and you get free power. Get one or two things wrong
and you lose power.

Remember way back when I mentioned the theory of instantaneous valve
openings and closings? Many people thought that this was an ideal (albeit
highly impractical) method of allowing the maximum amount of air in and
out. After all, how could a design that allowed the valves to slowly open
and close provide any real flow during those periods of low initial lift?

Actually, it has been determined that the slow openings and closings are
*necessary* for proper engine operation. When the piston begins its
downward travel from TDC during the intake stroke, the first few degrees of
crankshaft rotation create hardly any downward movement in the piston.

Opening the valve instantaneously at this point would cause port velocities
to plummet, causing turbulence in the intake system and cause fuel to drop
out of suspension.

Now getting back to your question:
A low duration camshaft works best at lower RPMs. Even though the stock
cam has a duration of about 250 degrees, a mild aftermarket cam like the
Engle 100 (duration 270 degrees) will still move the point of maximum
torque/horsepower further up the RPM range.

A cam such as the Engle 100 will idle smoothly and possess good port velocities at low-to-mid RPMs, increasing the engine's VE within that range (a good guess-timate would be a VE of about 85%).

Eventually the low overlap of the 100 will hurt the engine at high RPMs because the low overlap does not allow much scavenging to take place. The valves are also open for less time over all, which limits the engine's maximum VE (unless you resort to forced air induction using ducts, turbos, or superchargers).

A super-high duration camshaft like the "320" you mentioned (the closest
Engle equivalent being an FK-87) is pretty much an all-out race cam. It
works best at very high RPMs where the scavenging effect could conceivably
push the VE up beyond 100%.

100% is greater than the 85% of the Engle 100, and potential maximum torque and horsepower are much greater with the FK-87 as a result. The down side is that the FK-87 would idle like a poorly tuned Harley (due to reversion) and have very poor off idle performance (due to low port velocities at low-to-mid RPMs).

Lift is another matter entirely (are you getting tired of reading yet?).
Increasing lift has exactly the same effect as increasing the cam's
duration, except that you are not actually altering the opening and closing
points of the cam lobes.

Increasing duration allows more air in & out, and so does increasing your maximum lift (the former increases the size of the valve opening while the latter simply increases the amount of time that opening is open). A lot more goes into determining a cam's maximum lift (or rate of valve opening) than does its lobe timing, though. Maximum lift's main enemies are mechanical in nature, and have to do with a) coil bind and B) valve-to-piston interference.

Coil bind is what you get when you attempt to open a valve so far that the
spring that normally holds the valve closed can't compress any further (the
coils end up coming in contact with each other until it is nothing else
than a solid column of spring steel). Valve-to-piston interference is a
no-brainer, since a valve that opens too far stands a good chance of coming
in contact with the top of the piston at TDC (the common solution being to
machine valve pockets in the piston dome to provide clearance).
That's the easy part. A cam's maximum lift doesn't just determine how far
the valves lift off their seats, however. It also determines how *fast*
the valve moves off the seat to the point of maximum lift and back down
onto the seat again (commonly referred to as "ramp speed"). A high lift,
high duration cam lobe is gentler on the valve train than a high lift, lowduration cam because the ramp speed isn't as quick. High ramp speeds
coupled with weak valve springs can result in a) "ski jumping" just after
maximum lift is achieved and B) valve float.
"Ski jumping" occurs when the lifter is accelerated off the tip of the lobe
so fast that the lifter actually leaves the surface of the lobe and becomes
"airborne". This will affect valve timing-- specifically altering the
moment that the valve closes. It also means that the valve will come down
on the valve seat harder than normal. "Ski jumping" often occurs at high
RPMs without sufficient valve spring tension and when matching high ratio
rockers on camshafts designed only for use with the stock 1.1:1 ratio.
Valve float happens when the valve isn't placed back on its seat gently
enough, causing it to bounce once or twice before the valve spring tension
holds it down firmly. This is "A Very Bad Thing"™, as it will pound out
seats, break the heads off of valves, and be generally rough on the rest of
the valve train components. Maximum lift isn't the sole contributor to ramp
speeds, however. The overall silhouette of the lobe is the key.
The camshaft is the heart of an engine, and it influences all the other
factors of your engine design. When building a motor, cams are almost
always chosen first (sometimes second when you are dealing with an engine
who's displacement is easily changed-- like the Type 1 ACVW).
PS: Not all identical model camshafts should be considered equal. It used
to be a common practice to "regrind" camshafts. The existing lobes were
shrunk in size to compensate for any areas on the original lobe which
experienced extensive wear (IOW, modified the original shape of the lobe).
Cam lobes contain an area called the "base circle" which is the part of the
lobe that the lifter rests against when the valve is fully closed (think of
it as the coast side of the lobe). The top of the lobe is what provides
the valve train with its maximum lift. Maximum lift is essentially the
difference between this point and the lobe's base circle. If you think
about it, it is easy to see that you can reshape the existing lobe and
create a cam with the same characteristics as it had before, by simply
"shrinking" the lobe. As long as the difference between the high point and
the base circle is the same as before, total maximum lift has not changed.
And as long as the rise and fall ramps of the lobe start in the same degree
positions as before, duration hasn't changed either. However, the lifters
will sit further out of their bores and you'll have to turn in your valve
adjusting screws another turn or so to make up the difference. Likewise,
it is conceivable that a camshaft manufacturer created a cam with lobes
larger than what your valve train can handle. Stories regarding lifters
that bottom out in their bores prior to the cam achieving maximum lift are
nothing new. My point? Don't assume that your new camshaft will simply
drop in. Measure everything at least three times and if you are ever in
doubt, ask someone else.
[/quote]

damn, got tired of editing....