See inserted text within old text.
Quoting "John G. Napoli" <john@xxxxxxxxx>:
> > John N. had a comment about the need to enrich the carburetor if you
> replace
> > the exhaust with a less restrictive system. I am not sure I agree
> with
> that.
>
> I don't want to start an argument, but this does need to be addressed.
John, this is a civilized discussion. I do not modify or hotrod cars for a
living, so my point of view is not based on experience. I see this from the
perspective of engine theory and design, which I know well. Those not
interested in engine theory need not read any further!
> If
> you improve the exhaust enough to make a difference, you must
> compensate
> with richer mixtures, whether carb or FI. Yes, most engines are
> calibrated
> on the rich side as a safety measure at WOT. With 'better' exhaust you
> begin to use up this margin of safety. If you use this up and then
> continue
> to run at heavy loads and WOT you risk burning a hole in a piston.
>
OK, we are discussing here resistance in engine knock or detonation. Yes, at
WOT you need a mixture richer than stoichiometric to avoid/postpone knock. I
can't see how a less restrictive exhaust will create conditions more favorable
to knock though, other than the slightly higher volumetric efficiency. In
fact, the lower residual fraction due to reduced back pressure may in fact
reduce the tendency to knock (residual fraction is the percentage of trapped
gasses in the cylinder from the previous cycle. The higher this percentage,
the larger the concentration of "radicals" and the higher the tendency to
knock). At any rate, after such large modification, one would tend to readjust
the spark timing anyway. The way this is done is advancing the timing till
audible knock at WOT, and then retard a bit. If this is done, I see no danger
of damaging the engine through knock and no reason to enrich the mixture
further. In fact, you may be pleasently surprised that your engine will
tolerate more spark advance than the factory specs. On the black LeBaron I
have 13 degrees even though the factory specs call for 8 (of course, the
biggest part of this is probably due to the cam "upgrade"). No audible knock.
> Carbs need fatter jets. FI engines need different mappings. Carbs
> can't
> fatten their jets on their own, so I am puzzled by your statement that
> only
> FI systems need recalibrating.
OK, sorry, I did not state my position properly. Injection systems run on
closed loop at WOT, and usually they use maps based on rpm and manifold
pressure, and the output of the map is an injector pulse width, which in turn
regulates the fuel flow and mixture strength for each engine condition. Now,
say you modify this engine (say larger cam, better flowing heads, or to some
lesser extend some exhaust work, or whatever). In such a system, the
electronics would not know that for a given rpm and MAP (14.7 psi or zero
vacuum in the case of WOT) the amount of air flow is now higher. So, an
increase in the stock pulse width would be necessary in this case, just to
maintain the mixture strength. If you don't do that, you may end up lean (air
flow increases but injector pulse width is at stock values), and then you are
in trouble (and in these "high tech" cars, you may not be able to retard the
timing by just turning the distributor! If the car has a knock sensor, it will
retard for you, but it will retard a BUNCH!). A carburetor on the other hand,
is a device that simply delivers fuel to the air flow proportional to the
amount of air passing through it. So, if you do any mods to increase the
engine air flow through volumetric efficiency improvement, the carb will
automatically adjust/increase the fuel flow to maintain the mixture strength,
which is already on the rich side by design. In other words, the carb does not
know that the increased air flow is due to the better volumetric efficiency or
is it due to higher operating rpm. It will simply damp more gas in the air
stream! Of course, carburetors have their limitations. You may end up
exeeding the carburetor cfm, in chich case it will restrict a bit the engine at
higher rpm, but nothing dangerous here. Most carbs are sized to be a bit
larger than needed, so if you do small mods, the carb is rarely an issue.
> you add headers and so on, you will as you improve things. This
> information
> can be verified from any of the header manufacturers or speed merchants
> or
> engine designers.
Now, as I said, I have no racing experience, so I may be missing something.
Usually, when the theory fails to explain something is either something wrong
with the theory (I doubt it in this case) or the theory does not consider all
the facts. May be the "speed merchants" suggest a richer mixture as a
precaution. The engine power is a strong function of equivalence ratio when
you are below 1.2 and it peaks right around 1.2 (Equivalence ratio is Air Fuel
Ratio/Stoichiometric Air Fuel Ratio, 1.0 is stoich, less than 1 is lean, higher
than one is rich). However, when you exceed 1.2 or so (ie you are more than
20% rich), the power drops but only slightly (unless you are say, 60% rich!).
So, the "speed merchants" know that its better to be a bit too rich rather than
a bit too lean (in addition to reducing detonation chances). The main reason
by the way why power peaks on the rich side is due to the charge cooling effect
of the evaporating gasoline. The more gasoline you damp, the more the charge
cooling (unless of course, you damp so much in that you choke the engine, and
then you are over-rich). Another thing I have noticed is that many "speed
merchants" in addition to them not being as well informed as they think they
are, they often have as their primary objective to sell their merchandise
rather than helping you really improve your car! Also, their engineering
background is usually ... weak. But there are esceptions.
>
> Happy New Year, All!!!
Agreed! D^2
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