<VV> Lifter Question (Fascinating stuff)

[BobHelt@aol.com]

In a message dated 8/30/04 9:42:54 AM US Mountain Standard Time, 
qcc65@charter.net writes:

> Is there an optimum degrees BTDC to open and optimum degrees ABDC to close 
> a valve or is this the difference between low end torque and high RPM power 
> and can it be explained so we all understand it. What is different about 
> intake and exhaust and their requirements. What are the effects of higher lift, 
> pro and con, and quick closing versus slower closing of the valves. All things 
> I have wondered about. Again, fascinating stuff.
> Ron Guy
> 

Hi Ron,
Well you're getting into a book on valve timing and flows. Of course there is 
an optimum because the end goal is to fill the cylinder as completely as 
possible with gas/air, and expell the exhaust gasses as completely as possible. 
The problem is that this optimum valve timing changes with rpm for a given 
engine. So that what is optimum at 2000 rpm is far less than optimum at say, 5000 
rpm, and so on. That is why different cams are required for different 
applications. That is why a torque curve is curved somewhat like an unside down U. In 
addition different cams and timings are required whenever changes are made to 
the engine (porting, desired rpm range, CR, carburetion, exhaust system, etc. 
etc.)

Cylinder filling on the intakes is dependent on the pressure differential 
between atmospheric and the cylinder (and of course the time avail to do so). 
Exhaust removal is dependent on the pressure differential within the cylinder and 
atmospheric. But when the exhaust valve is opened, the cyl pressure is still 
very high, say several times atmospheric. So the exhaust is "blown" out. And 
to complicate matters both intake and exhaust systems have pressure waves that 
vary with engine speed. These pressure waves can help, or hinder, the flows at 
certain rpms depending on the design of the systems.

Higher valve lifts allow for more flow (up to a point), gaining better 
efficiency. The downsides are high valve train inertia and wear (including guide 
wear). Faster valve action also increases the flow (think of it as the 
combination of valve-open time and the amount of lift). But greater accelerations cause 
increased wear, and valve float and bounce (both of which are bad). That is 
why OHCs are better than pushrod operated valves. They eliminate a lot of valve 
train inertia.
Regards,
Bob Helt