| To go fast and win races, you need horsepower! To make horsepower, you you need a high flowing,
high velocity, efficient induction system. Minimizing port volume, while keeping enough cross sectional area to provide enough
cfm to produce maximum horsepower and high rpm torque, along with great throttle response, is what makes an efficient induction
system and a strong running engine. When looking at a flow sheet from a ported cylinder head, you see that numbers increase as
valve lift increases. This is because the valve becomes less and less of a restriction as it leaves the seat. But don't be fooled from the high number at peak lift. Lets say it's 310cfm @.700" lift, with a 220cc intake port.
Well unless your doing some serious racing, your not running a valve train that's going to produce that kind of lift.
If your building a street engine or a mild race engine, you will probably be limited to say .550" to .600" lift, in which case
you're probably moving around 275-285cfm with the same port. These numbers are still respectable; however, the valve is only at peak
lift for a short amount of time. With that being said, the area of the lift curve in which you want to see improvement is mid lift
numbers from .250" to .450" lift. The valve will spend most of it's time at these points, so improvements here is where more
horsepower will be made. High flow numbers are important, but even more important is port swirl, port velocity, and of course
a good port volume to airflow ratio. Any large port will move a lot of air, but the smaller port moving just as much air will
create a flatter torque curve and be more effective with high velocity and great throttle response (important in bracket racing).
Now, those of you racing a big block sportsman drag race engine, you may be running valve lifts near .800"to .850"
lift. In this type of application anything under .400" lift is about useless! On the intake stroke, from the time the valve comes of
the seat, to .400" lift the piston is either coming up the cylinder (to end the exhaust stroke) going the wrong way, or hovering near
TDC. So it's not until the piston starts its way down the cylinder that it's going to produce enough of a pressure differential to
move some air through the intake port. In this case the higher valve lift airflow figures are very important.
One more thing we would like to mention is that airflow numbers are not the most important variable when choosing or comparing one
cylinder head to the next. More importantly is port volume, port crossection and shape, air speed, and the relationship between the
size of the throat and the valve seat. Head A may flow 20cfm more than head B, but B being more efficient will most likely produce
more horsepower. Bigger is not always better. When in doubt, go smaller.
For a given CID, a long stroke engine does not produce more torque than a short stroke engine. It may produce the
torque more "smoothly" at lower rpms, but thats it! The argument that a longer stroke has greater leverage is offset by the fact
that there is less push, because the piston area is smaller. If anything, the short stroke engine is better at producing torque
because there are less "frictional losses" do to piston circumference than due to stroke length.
Torque per cubic-inch is directly related to volumetric efficiency (VE) and compression. It has not increased nearly as much as
horsepower per cubic Inch in the last 20 years. This is due to today's heads being able to breathe to a far greater RPM.
This means that (though the torque has only gone up a little) the rpm at which the torque is produced has gone up a lot.
In many forms of racing today the engines never come down as low as the rpm at which peak torque is produced. With that in mind,
we need to concentrate on the form of the power curve past peak power. This is a very important area. An engine that can hang on
longer past peak power can also hang on a lower gear longer. This equals greater acceleration, which wins races! |
