Re: V6 intake

[ethan schartman <eschartman@domain.elided>]

> Subject: re: V6 intake
> 
> On Tue, 12 Nov 2002, Joe Elliott wrote:
> > This overlooks the fact that the narrowest opening
> in the intake
> > system is inside the airflow meter--considerably
> smaller than the AFM
> 
> Actually, the smallest area in the intake system is
> usually valve area. I don't remember the valve sizes

> of the V6 off the top of my head, nor have a
measured 
> port area, but in the majority of engines I have
> measured, or seen measurements for, these usually
> restrict flow more than throttle area or the area of

> the any part of the intake system upstream
> of the throttle. 

i take it you mean total valve area (e.g. #of valves
times individual valve area)?


> Increasing the area of any of these parts will have
> no effect unless it's really the smallest part, and
> the throttle body and AFM are rarely the smallest 
> parts.

consider the intake system in three parts: air intake
to plenum, plenum, and valves. the plenum is an air
reservoir. the valves draw air out of the plenum at a
rate that is proportional to valve area and duty cycle
(related to cam lift, and rpm).  if the inlet's
maximum flow rate is less than the max flow rate to
the valves then engine power will plateau at that flow
rate. presumably a manufacturer will design the intake
system to have a max flow rate that is at least as
large as the max flow rate demanded by the valves. but
if you increase cam lift over stock (or max rpm), you
will increase the max flow rate - potentialy beyond
the intake capacity. all without altering valve size. 
incidentally, here's a non intuitive situation:
say you have a valve at the end of a long pipe and the
valve's max flow rate is sufficiently high that you
can get a pressure drop from one end of the pipe to
another (e.g. the valve opening is comparable to pipe
diameter). the opening and closing of the valve will
create compressional pressure waves in the pipe (i.e.
sound waves). at a fixed opening-closing frequency the
location of the pressure maximum may not be at the
valve opening - creating flow restriction across the
valve (because it is proportional to pressure drop
from one side to the other)! the flow rate can then be
tuned (or detuned) by adjusting the length of the
pipe. this is, incidentally, related to how musical
instruments such as flutes work.

this is also why it is possible to decrease engine
performance by porting a head! 

> 
> One should remember that inlet restrictors are a
> common form of power limiting used in some classes
of
> racing.  The restrictor used by the DTM is a mere 
> 28mm (1.1"), and these normally aspirated V8s make 
> around 450bhp.

that's a restrictor plate, though. a 1.1" hole in a
plate is less restrictive than a length of 1.1"
diameter tubing. 

IIRC,in an F1 engine, the plenum is at atmospheric
pressure, and flow rate to the valves is controlled by
varying the length of the runners from the plenum to
the valve.
 
-ethan schartman
'87 milano gold
princeton, NJ
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