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One of my best mates wrote this for a BMW site, I thought it was general enough to apply to our cars and thought it could spark some good discussion on the topic. It's geared toward naturally aspirated engines. I'm sure my mate would love to hear more opinions on the subject, so feel free to offer yours and let him know whatcha think!
It's a given that engine tuning has a lot to do with manipulating the
stream of intake charge. But to understand this in the right light I think
it's important to see the intake charge simply as a column of air. This
column of air has weight, and when you add velocity you have inertia.
Using this inertia to your advantage is the goal.
Cam timing, duration, lift, compression ratio, port dimensions, and RPM
are all used to manipulate inertia associated with that column of air.
Keep in mind it can't start and stop instantaneously, it sort of acts
elastically. We have to make compromises and design the intake system to
focus all of these factors to create the best cylinder filling within a
certain RPM range. Things like VANOS and variable valve lift go a long way
to broaden this range but there is always a compromise to some extent.
When everything is perfect, flow into the cylinder stops moving just as
the intake valve shuts having filled the cylinder as much as possible
before the flow reverses. But when it does close it would be nice if that
column of air, still in the intake track, actually maintained some of it's
velocity and 'packed up' behind the intake valve, ready for the next time
it opens.
When this process is optimized, air pressure behind the valve peaks just
as the valve begins to open. When it does open, you get this surge of air
into the cylinder. This surge of pressure on a naturally aspirated engine
is known as inertia supercharging and it is very important. When you think
about how velocity affects weight, creating inertia, you can begin to
understand how RPM is involved with coming 'on cam'.
You'll hear people talk about the importance of port velocity and it's
effect on inertia supercharging. Without velocity, you don't have inertia
and you loose that surge of air. Super oversized ports will not have the
port velocity necessary to make good use of inertia supercharging. This is
why some may see a loss of torque at low/mid RPM's after a big valve/port
modification. If they don't have the cylinder filling requirements its
just not going build the velocity needed to be effective.
So port dimensions affect port velocities, which in turn affects cylinder
filling but is controlled by valve timing and duration. RPM does its part
on port velocity and dictates when those cam specs are optimum.
Compression is the driver and what creates the 'pull'. At some point
everything will be maximized and for a brief period, the engine will be,
as they say, "on the cam".
It's a given that engine tuning has a lot to do with manipulating the
stream of intake charge. But to understand this in the right light I think
it's important to see the intake charge simply as a column of air. This
column of air has weight, and when you add velocity you have inertia.
Using this inertia to your advantage is the goal.
Cam timing, duration, lift, compression ratio, port dimensions, and RPM
are all used to manipulate inertia associated with that column of air.
Keep in mind it can't start and stop instantaneously, it sort of acts
elastically. We have to make compromises and design the intake system to
focus all of these factors to create the best cylinder filling within a
certain RPM range. Things like VANOS and variable valve lift go a long way
to broaden this range but there is always a compromise to some extent.
When everything is perfect, flow into the cylinder stops moving just as
the intake valve shuts having filled the cylinder as much as possible
before the flow reverses. But when it does close it would be nice if that
column of air, still in the intake track, actually maintained some of it's
velocity and 'packed up' behind the intake valve, ready for the next time
it opens.
When this process is optimized, air pressure behind the valve peaks just
as the valve begins to open. When it does open, you get this surge of air
into the cylinder. This surge of pressure on a naturally aspirated engine
is known as inertia supercharging and it is very important. When you think
about how velocity affects weight, creating inertia, you can begin to
understand how RPM is involved with coming 'on cam'.
You'll hear people talk about the importance of port velocity and it's
effect on inertia supercharging. Without velocity, you don't have inertia
and you loose that surge of air. Super oversized ports will not have the
port velocity necessary to make good use of inertia supercharging. This is
why some may see a loss of torque at low/mid RPM's after a big valve/port
modification. If they don't have the cylinder filling requirements its
just not going build the velocity needed to be effective.
So port dimensions affect port velocities, which in turn affects cylinder
filling but is controlled by valve timing and duration. RPM does its part
on port velocity and dictates when those cam specs are optimum.
Compression is the driver and what creates the 'pull'. At some point
everything will be maximized and for a brief period, the engine will be,
as they say, "on the cam".
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