On Sun, 18 Jan 1998 18:53:39 -0500 Trevor Boicey <tboicey@brit.ca>
writes:
>Rick Morrison wrote:
>> The exhaust and intake are so tied. Simply put, what goes in,
>must
>> come out.
>
> That's just not true. What goes in comes out, along with a whole
>lot more. Gasoline goes in as a vapour and comes out as various
>gases which are much more voluminous.
Please explain how a whole lot more goes out than comes in. Maybe I
missed something, but I was under the impression that matter can neither
be created nor destroyed - only changed.
> That is the central point.
>
> As well, the exhaust and intake systems are isolated by the
>valves. There is no meaningful amount of time when both
>valves are open and the intake and exhaust are part of the
>same air flow system.
No meaningful amout of time? the camshaft on my Midget has an overlap
period of 83 degrees. That comes to 12% of the entire 720 degree cycle.
But even that is immaterial. Be cause we are not talking about an airflow
system, in the sense that the flow is from intake directly to exhaust.
We are talking about an air pump. Sometimes called an engine.
>> If you increase the intake volume (increase volumetric
>effeciencey), and
>> nothing to the exhaust
>
> This statement is in itself illogical. This statement within itself
>assumes that the intake and exhaust ARE balanced, which is an
>illegal assumption when you are trying to prove or disprove it.
> (ie: You can't prove A=B by saying "well, I know that A+1 = B+1
>so therefore A=B!" because you are using the proof within itself)
If it is illogical to make my statement, the logic in your refutation, is
at best strang.
because the next thing you say is:
> Obviously, improvements to both are a best. As well, an unbalanced
>amount of improvement in one area without the other will not
>pay off.
If as you say, improvements in one have no need of improvements in the
other, there are a lot of engineers out there who have made a lot of
money doing things that were absolutely unneccessary.
> However, it is simply not true that the intake and exhaust have
>to be "balanced" in any scientific way. They don't have the
>same volume of flow, not the same rate of flow, and not the
>same gas temperature either
No one ever said that the intake and exhaust should be "balanced" in the
sense that they have the same flow characteristics. In fact a general
rule of thumb in valve/port sizing is to have the exhaust flow about 75%
of the intake valve/port flow. The reason is that due to the heat and
pressure behind the exiting gases, there is more energy (about 25% of the
energy an engine produces goes out the tail pipe by the way) and thus
more velocity.
Assume a cylinder has a maximum volume of say 500cc,and further that the
volumetric effeciency of the engine is 80% (a good VE for LBC engines),
then on the intake cycle the cylinder will ingest compress and fire 400
cc of fuel/air mix. On the exhaust cycle, that 400cc of burnt gases will
have to be removed by the exhaust system.
Now, we increase the VE of the cylinder from 80-90% by improvements to
the intake. The charge is now 450cc . But we've done nothing to the
exhaust which is only capable of handling 400cc in the alloted time
period. The result is 50cc of unburnt gases remaining in the cylinder,
heating all the components. The next intake cycle can only ingest 400 cc
of fresh mix. Right back where we started. All that work for nothing.
There are several very good books on the theory and practice of
intake/exhaust matching, sizing and improvement. One of the best is by
David Vizard. Most of what he covers in his book on heads, he also covers
in the A series book.
Try it, you'll like it.
Rick Morrison
72 MGBGT
74 Midget
>Trevor Boicey
>Ottawa, Canada
>tboicey@brit.ca
>http://www.brit.ca/~tboicey/
>
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