Muffler Question
July-18th-2003, 04:06 PM
#16
Guest
Posts: n/a
Originally posted by Protegé Menacé
Bruce, I guess what i meant is some cars have an O2 before and after the main cat, ours have them before, and backpressure isnt needed.
.
Bruce, I guess what i meant is some cars have an O2 before and after the main cat, ours have them before, and backpressure isnt needed.
.
crawl under the front and tell me if you have 2 O2 sensors or 1 ..
if there are 2 .. one will be in the down pipe and the other in the upper manifold ....
my 95 1.5L has 2 ...
my 96 1.8L has 1
July-18th-2003, 08:59 PM
#17
Registered User
Join Date: May 2002
Location: Mt. Pearl NF
Posts: 167
A protege that is OBDII has 2 and OBDI has 1 . OBDII uses one in frt of the cat for lean - rich monitor and one behind the cat for cat effiecency monitor. On the Z5 one in manifold then cat and one in frt pipe a BP would have one in manifold and other under car with the plug behind the pass frt seat as the cat is downstream further.
August-6th-2003, 06:13 PM
#19
Registered User
Join Date: Aug 2003
Posts: 21
Q: Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.
The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.
Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.
Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.
This is a good read for you guys.
Edit: *More information on the subject*
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Last edited by FlySwat; August-6th-2003 at 06:20 PM.
August-7th-2003, 02:01 PM
#20
Protege Enthusiast
Join Date: Aug 2003
Posts: 68
i got 3a racing cat - back exhaust
Whats up guys,
i got a 96 protege
bubba, i got a 3a racing cat-back exhaust...... sounds ******* awsome......def look into 3a racing
it costed me 175 but thats because my neighbor owns an exhaust company
ducom
i got a 96 protege
bubba, i got a 3a racing cat-back exhaust...... sounds ******* awsome......def look into 3a racing
it costed me 175 but thats because my neighbor owns an exhaust company
ducom
August-19th-2003, 10:53 PM
#21
Protege Newbie
Join Date: Aug 2003
Location: Orlando
Posts: 7
backpressure
An engine does not need any backpressure. What is required for optimum exhaust velocity is what's known as scavenging. Exhaust comes out in pulses, and scavenging is when the frequency of exhaust pulses causes a vacuum (or low pressure area) just outside of the exhaust port, so that when the valve opens, the exhaust gas is sucked out. If you have a 1.5l, I wouldn't go any bigger than 2"o.d. tubing. Unless you've figured out a way to spin your engine at 15,000 rpm, you won't be moving enough air to need any more than that. 2.25" is the accepted max for a naturally aspirated four banger, but that's a much higher flow rate than we need. The 3rd gen guys or even a hot rod 1.8, maybe, but not a 92hp 1.5l. Also, Unless you like having black soot all over your bumper, I wouldn't remove the cat. The stock design may be restrictive (I honestly don't know, I haven't had my wife's off yet), but catco makes a straight through cat that flows very well and still runs clean (relatively). So, with what I know, my plans are a brospeed header pipe, catco cat, 2" cat back (mandrel bent), resonator (basically a glass pack), and a flowmaster or borla muffler. That should do it.
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