[alfa] Re: Porting in general...

[alfa@xxxxxxxxx]

Ah, Ron, to disagree, part of the essense of the Digest.

To consider porting without thinking of the cams is a big mistake.  If you port
the intake to give you great momentum dynamics at, say, 7000 rpm, and you put
cams in the engine that have short duration and low overlap, the car will not
perform well at all.  

The point of the cam timing is to take advantage of the momentum 1) while the
valve is still open, and to a lesser extent 2) when there is overlap.  If you
port the head for high momentum, you will need cams that close rather late, so
that the pressure in the intake is higher than the combustion chamber even
while the piston travels back up.  Obviously, if I had these same cams at half
the peak rpm (and half the air flow, and 1/4 of the momentum), the momentum
will be much less, and one would start to push out the charge back into the
intake.

But lets say I port the head to flow freely, to minimize the restrictions, and I
use shorter duration cams, I could say that even with the lower momentum, I can
put the same charge in at the high rpm assuming I also properly match the lift.
 One is about using the momentum to put the charge in, the other is about
minimizing the restriction.  Can one do both at the same time?  Sure- but I
would not want to drive it on a car with limited gear ratios...

The nice thing about my short duration cams is that at the lower rpms, where one
can not use the momentum as well, I still have a high charge.  Which is really
increasing the area under the power curve.

It is a SYSTEM, one can not cam without thinking of the ports, and one can not
port without thinking of the cams.  You can not optimize one wihtout the
other.

Also, I don't see how I contradicted myself when I mentioned that all CFD models
have problems modeling exhuast revision.  I've never known port flow to be
steady, ever- there is nothing steady about it.  Go from a somewhat higher than
atmosphere P, open a valve that has gas that is normally higher still, air goes
backwards, then it goes the other way, valve opens more, air flows more, valve
starts to close, velocity around the valve seat goes up, eventually hitting
sonic flow as the valve closes enough (assuming the flow is still there)... I
could go on, but my point was that there are no CFD packages there that
properly can model all the aspects of an individual port, yet.  It's worse with
any type of pressure charging.  Been there, done that, etc.  Tried to make a
turbo F- be faster and cleaner at the same time....

Again, Ron, unless you've actually done the research, one can not blanket say
that Jim K's book won't work.  Considering who it is, I'm fairly sure he has
done the work, not to the degree we would here at my employer, though...

Eric Storhok
Ann Arbor, MI
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