Bigger isn't always better. Despite what you may have heard having
a 3" cat back pipe on a 4 cylinder Honda isn't a good idea, unless it's
making over 300hp. If the pipe diameter is too large you then run the risk of
losing horsepower. No back pressure causes a loss of low end torque. Some back
pressure is required. Your goal is to increase the flow, without robbing the
engine of the power gain.
An exhaust that creates too much
back pressure is called a Restrictive Exhaust. Most OEM exhaust are built as a
restrictive type with cost and gas economy in mind. A restrictive exhaust
hampers performance in two ways. One is that the engine has to work harder to
push out the exhaust gases from the cylinders. Second is the engine has left
over gases in the camber which then get diluted with new incoming air and fuel
mixtures. This can help gas mileage, but performance suffers.
Yet, backpressure is needed to remove gases from the engine. Back
pressure creates something of a suction to help remove spent gases when it
isn't too restrictive. Too much backpressure and it can't all escape, too
little and the engine will pull it back in through the exhaust valves when
open. It's a balance you’re trying to achieve as you increase the power of the
engine.
It is usually associated with a popping or burble noise coming
from the exhaust, not a backfire. You should expect properly tuned engine to
burble or pop when the throttle is close from a high RPM. Think of a top fuel
dragster and how the engine at idle makes the popping noise. That engine is
creating a lot of backpressure. That is also the reason for the short open
headers and with the remaining small amount of back pressure the gases are
removed easily. If your car is making loud pops under hard
accleration/decleration then that is backfiring.
Backfiring is not a good thing. We've all heard of a car being
called a fart can. It's due to the unburnt fuel getting into the exhaust system
and then it is being ignited. Commonly it's a small air leak in the exhaust
system connections. It can also be due to a lean air fuel mixture or it could
even be related to the ignition system. The ignition sequence could be off, or
the plug wires worn, and the coils themselves could be bad.
If you plan to upgrade your
exhaust then you should look for a system that is .25 to .5 inches larger than
the existing stock exhaust piping. This will give you the best increase. Also
to consider is what RPM your car or truck will operate at, most of the time. An
exhaust pipe with a smaller diameter will produce more low to mid-range RPM
torque, where as a larger pipe diameter will increase mid to high range RPM
torque.
Below is a chart that will help
you also determine what diameter is needed depending on the horsepower your
engine creates.
Mufflers and Resonators:
A free flow muffler can help the
removal of exhaust gases, but sound increases. You can use a resonator or spiral
flow gas pack muffler to help reduce sound before the muffler. I've ran a 4 cylinder
car with just two gas pack mufflers, which helped a lot in performance, but
created little more noise than a normal burble.
A muffler can be straight through
or have an S-turn. Just make sure the pipe diameter of the in/out is upgraded
to size of the piping used. The gases are NOT trapped by an S-turn muffler.
Sound is trapped. Once you nail the throttle everyone will know, but before
that the normal sound of the car won't attract much of the wrong kind
attention, usually from law enforcement.
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