I can’t help but butt in on this talk about valve sizes and volumetric
efficiency. Pardon me a moment while I mount my soapbox.
Volumetric Efficiency -- (VE) the definition is simple enough. If a 1.8
liter motor expels 1.8 liters of exhaust on every cycle, it would have
100% volumetric efficiency. Racing engines are lucky to hit 90% VE at
their peak power range. Many things contribute to high VE and one of
those things is the size ratios between the intake and the exhaust.
Consider:
What goes in is not what comes out. A relatively cold and dense fuel /
air mixture enters the engine. Occasionally your Lucas dizzy ignites
this mixture at the proper time. This produces heat. Heat makes pressure
that will try to make gases bigger. Thus a higher volume of gas is
leaving the engine than what entered. I suggest a rather simple
experiment for those wishing to dispute this: Expose only the wick of a
firecracker in your closed fist. Light the firecracker. Notice after the
firecracker explodes that there is no longer a complete and compact
firecracker in your hand. You may remember feeling some pressure inside
your fist as the firecracker went off.
When the exhaust valve opens, a couple of things are going to move the
spent mixture into the exhaust manifold: There will be residual pressure
of about 60 to 70 pounds or more from the combustion process. These
gases are in a hurry to leave and their pushing and shoving past the
exhaust port is what gives the internal combustion engine it’s
characteristic sound. Also, there is the very positive influence of the
upwardly moving piston that is rather efficient at forcing the remaining
gases out of the cylinder.
The intake of gases is much more lethargic. When the intake valve opens,
the air/fuel mixture may have just been standing around doing nothing
and not feeling any great urgency to move. There is only 14.7 psi
(atmospheric pressure) over a vacuum to influence this movement of the
incoming charge. The opening of the intake valve and the downward
movement of the piston are the only things drawing in the new charge.
Furthermore, the piston is at a dead stop at both the top and bottom of
its travel. It is at its greatest velocity half way through the stroke.
Thus it can do the most work only around the time of its greatest
velocity. On the exhaust stroke, the hot and compressed gases are
motivated to leave the moment the exhaust valve opens. Once the piston
approaches half of its upward movement, it can merely reinforce what the
gases have a mind to do anyway. On the intake down stroke, there is no
such predilection of the incoming air/fuel charge. The piston sucking
the mixture into the cylinder must do virtually all of the cylinder
filling of a naturally aspirated motor.
For the above reasons, the intake valve should always be larger than the
exhaust valve. How much larger depends upon the head design. Overhead
cam designs can have the exhaust valve about 90 percent of the size of
the intake valve for peak VE. There is more leeway in port design
because there isn’t any area being consumed in the head by the pushrod
towers. If you have to leave room for the pushrods, then the exhaust
valve is squeezed down to about 85 percent of the size of the intake
valve for maximum VE.
However, on our beloved LBCs that have a siamesed intake port, the
intake process is further hampered and now the exhaust valve should only
be about 80 percent of the intake valve size for maximum VE. Why?
Because any benefits of intake port ram-air tuning are practically nil
with the siamesed head.
Ram air intake tuning -- When the intake valve is open the downwardly
moving piston is sucking the intake charge out of the intake manifold
and into the cylinder. The incoming charge just gets a good head of
speed when the intake valve slams shut. This causes a bounce of the air
inside the intake manifold. You can use this bounce when tuning the
length of the intake runners to improve torque at some desired RPM
range. That is, you can get a slightly denser intake charge on the next
cycle if the pressure wave of the bounce does not make it to the other
side of the throttle butterfly before the intake valve opens again on
the next cycle. However, our LBC motors have siamesed intake ports. That
is, a single port feeds two intake valves. The potential of implementing
ram air induction is just about nil. So much for the myth of "properly
designed LBC motors."
That is my contribution to valve sizes and ratios. The cam plays a major
part in the process as it controls when the valves open and close.
Obstructions in the head and the radius turns of the plumbing play a big
part also. Finally, the mechanical condition plays a major part -- the
more efficient the sealing qualities of the rings and valves, the better
then engine will act like an air pump.
</Soapbox>
Back to the great beer debate. . .
--
Bob Allen, Kansas City, '69CGT, '75TR6, '61Elva(?)
"You should expect to get out of something only what you see pictured on
the box."
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