Ancient History

[Arnaldo Echevarria]

>On every engine - a small portion of exhaust gas is pulsed into the 
>intake manifold the amount depending on cam timing, load, and engine
>RPM.  This is then sucked back into the cylinder diluting the charge. 
>For you old timers, this is why you must richen a carb when putting 
>headers on an engine - because the reversion pulse is damped - leaving
>less flow thru the carb (the first pulse backwards - followed by the normal
>forward pulse.)  Untimed fuel systems have a fuel air mixture that is 
>pushed backwards up the manifold (reverted) before being sucked back
>in.  As this is very hot gas with no oxygen present (e.g. no flame!!) and 
>lots of CO2, and this hot gas is quenched by contact with relatively cold 
>(far below ignition temperature) matter in the intake tract - cold mixture 
>cold intake valve, cold head, all that happens thru almost all ranges of
>performance is charge dilution and therefore less than theoretical power
>is developed.  Aside - a very rich fuel mixture such as present when 
>fuel is "puddled" on the valve is not combustible. A lean - i.e. 
>minimally vaporized mixture such as from a mistuned carb or timed
>injector is explosive!!
>
>What timed fuel injection in conjunction with proper cam timing does is
>allow this reversion pulse to go up and come back out the exhaust port
>before adding fuel to the incoming air.  Properly done, no air pump is 
>needed for the cat converter as the reverted air pulse shoves oxygen 
>out the exhaust.  The chamber gets a full, dense properly mixed charge
>and makes better power, fuel efficiency and lower emissions.  
>
Could someone elaborate more on this?  Excuse my ignorance, but I'm trying to
learn more about combustion theory.  I've been hot rodding cars for years 
now and have played with different carb / efi setups, but I'm trying to learn
a little more on what "really" goes on (as in good techniques vs bad techniques
of getting fuel to burn using injectors or throttle bodies etc).  What I'm going
to build (no laughs, please) is a fuel efficient pontiac 455 (If that ever
existed) using some sort of fuel injection. Right now the car runs low 13's
in the quarter
and gets 12mpg (city) using a Quadrajet 750cfm; I want to improve that to
15-17mpg.
I think the cheapest way to go is to use a tri-power manifold and hook up three
2barrel tbi's. I'm pretty good with building microcontrollers so now I'm just
trying to decide a baseline by which the fuel injectors will fire (before
tdc? during?
after? during exhaust cycle too? in between? how long?) Of course I cannot get
specific numbers, each engine is different, but I'm just trying to get an
idea on
how to approach the problem.  The system will be a closed loop using o2
sensors and
a map sensor (small cam, good vacuum).

I'm interested to learn more about when should I inject fuel for best power
and when 
should I do so for best efficiency.  It seems that a Hillborn system just dumps
fuel and fuel guaranteeing full combustion but at the expense of efficiency.
I have so many questions but no words to put them together; I guess I'm just
looking for a little "knowledge".

>In all endeavors we tend to forget what has worked and is working in our 
>efforts to achieve the new.  We get enamored with new technology and 
>scorn the old.  We should instead - understand the baseline from where
>we want to go and use that to our advantage.  Use only from the new what
>is needed to solve the unsolvable with the old - progress - not perfection.
>
I totally agree.

Arnaldo E.
aec at ao.net