John is correct. Making the exhaust less restrictive will have no benefit in
low speed acceleration, even WOT. The restriction wil start creating back
pressure only when the flowrate through the system becomes very large. The air
flow rate through the engine depends upon rpm and throttle position. It is
possible however that the load on the engine will be reduced with the less
restrictive exhaust at non-WOT in high speed cruising (at say 3000 rpm and more
which would be 85-90 and above on an Imperial). Unlike restrictions in the
intake system (that tend to make the engine work "less" hard), restrictions in
the exhaust system increase what its called "pumping" work. In this case, this
is the energy it takes to move the piston up during the exhaust stroke. A
slightly more restrictive exhaust will have some small effect in volumetric
efficiency, but the main reason of the performance gain is the reduction in
pumping work.
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 can see that at low rpm and heavy throttle, the more restrictive exhaust
would tend to slightly increase the flow instability in the intake due to the
valve overlap, which would tend to enrichen the mixture. Sure, with the less
restrictive exhaust you loose that, but at the mid range (close to the peak
torque) this effect is very small or non-existent since the valve overlap at
these speeds does not create back flow to the intake system. And of course,
close to peak torque is where the most dangerous detonation occurs. And at low
speeds, most carburetors are richer than what they need to be anyway. So, I
can't see why you would have to mess with the carburetor, or any parts of the
intake system. Remember, this is not an injection system. The carburetor will
automatically feed you more gas as a small improvement in volumetric efficiency
will take place.
When I first got my 68 Sedan, it had a single exhaust. This engine was at
least partly rebuilt before I got it, and it appears that a fairly large cam
was installed. When I added a second 2.25" pipe, there was a huge improvement
at high speed, but I could feel no difference below 2500-3000 rpm, and the
improvement was larger the higher the rpm. I did a very cheap installation
which included some "Turbo" mufflers. The result is a cool but rather loud
sound at low rpm (city driving) but the car gets quite as soon as you hit
highway speeds. Even at WOT and over 2500-3000 rpm, there is hardly any
exhaust sound, most of the sound is from the intake. Above 4000 rpm, you can
barely hear a slight rumble from the exhaust. My 68 LeBaron has a dual exhaust
system that the owner told me was 2", but looks more like 2.25 to me. It has
two very large resonators and no mufflers. This car has a truly unique sound.
Unlike the �Turbo� mufflers, at idle or light load, it is quieter, and as the
load and rpm increases, it gets progressively louder. At WOT and about 3500-
4000 rpm and up has a race car sound to it with a LOT of rumble. Now, for the
Imperial purist this is very "unimperial". But I really like it. I have heard
very few cars that sound as "mean" as the black LeBaron. And given how sound
proofed the body is, it is reasonably quiet inside the car unless you cruise at
over 85 or 90.
D^2
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