pokeytemplar said:
Isn't more information needed? 300 at what lift? What about flow at .200? I do know that average flow means more than peak flow. I don't know how you would spec a cam around those numbers though. The cam is the area of greatest importance and the area I know the least about.
There are so many variable that effect it. Weight, gearing, heads, R/S ratio, intended use, automatic or manual tranny, etc. How do each of these affect the cam choice?
If the heads hit peak flow, say 300 c.f.m., @ .585" lift, you could choose a cam and rocker ratio that provides a total lift of .600" at the valve. Much higher than that punishes the valve train for no real good reason, because the port can't move anymore air/fuel with a higher valve lift anyway.
It is only at maximum lift for an EXTREMELY short time.
Flow gains in well designed heads (like the NASCAR guys use) concentrate flow improvements in the mid-lift range which will provide usable power over a wider r.p.m. range. I'll put in a plug for Strikers here because they have pretty significant gains in flow in the mid-lift area. So you are right pokeyT, while peak flow at MAX lift is a guide, it doesn't really tell the whole story.
Also, you could compare the Intake Port with the Exhaust ports (a ratio) and have the cam ground to match that. So say the exhaust port flowed 25% less than the intake, you could add more duration to the exhaust side of the cam to help compensate. That is the second part of matching cam & heads.
"We" won't get much of a power improvement by ONLY changing the cam when using the o.e.m. Viper heads because the cam & heads are a pretty good match already.
Increasing overlap (a tighter lobe separation angle) will create a peakier power band that tends to kill torque (cylinder pressure) down low and then creates more power (cylinder pressure) once the r.p.m. is high enough to move the column of air THROUGH the engine, in the normal direction.
When an engine is idling and the piston is still rising for the very last part of the Exhaust stroke,
during overlap (just when the intake also starts to open) the column of air can be pushed back up into the intake manifold. Then it goes back into the port again towards the cylinder, once the piston descends enough to encourage flow. The boogety-boogety idle is because the airflow from the intake into the cylinder isn't yet stable enough to flow in one direction. So it bounces back and forth until once r.p.m. climbs somewhere above idle, inertia helps carry it one way (
into the port and cylinder), the chaos diminishes and the engine smooths out. As r.p.m. rises, during w.o.t., engine power (torque) typically climbs until max r.p.m. is reached and somewhere before that point, maximum cylinder pressure, or Peak TQ. is achieved.
Increased Overlap effects vacuum readings as well (they are typically lower) because, when the I & E valves are open at the same time, the engine sees atmospheric pressure. Anyway, overall, airflow through an engine is quite "elastic".
The reason for overlap is as follows: Somewhere (based on cam design)
before the piston reaches the bottom of it's POWER stroke, the exhaust valve opens. As the piston starts to rise again, somewhere before it reaches the top, (again based on cam design), the intake valve opens. This is when overlap is happening. The more overlap the longer the intake and exhaust valves are open at the same time. Anyway, the reason for this is because the exhaust gasses are leaving the cylinder and that column of gasses helps start the column of air coming INTO the engine through the open intake valve. This is all happening
before the actual intake stroke, when the piston descends and the exhaust valve finally closes. The overlap trade-off is a lumpy idle (but I like it anyway!)
Well designed headers help prevent any excess scavenging (or when the Air & Fuel that is supposed to stay INSIDE the cylinder to produce power, leaves with the exhaust). That is where header pipe diameter that is too big (even a bit too big) can really screw things up.
If you look at a cam card- Add the number when the Intake Opens to the number when the Exhaust Closes, that is overlap. If you get a weird negative degree number it is because it is being measured from a certain lift (say .050").
Torque is just cylinder pressure. And horsepower is just torque measured over time.
This is really simplified but may provide some more info to
Stinker's School of Camshafts...
Long winded explanation. Someone else please take a turn now!!
Rotten
