Maryland Cherokee Club
04-27-2012, 08:30 AM
☠ CF Sheriff ☠
you guys need to remember the primary rule of engines - what goes in must come out
so if you add a bigger TB, you also need a better flowing exhaust to see a benefit. And to use an even bigger (larger that 62mm) you really need a stoker engine, headers, and free exhaust
so if you add a bigger TB, you also need a better flowing exhaust to see a benefit. And to use an even bigger (larger that 62mm) you really need a stoker engine, headers, and free exhaust
04-27-2012, 08:34 AM
☠ CF Sheriff ☠
04-27-2012, 08:34 AM
☠ CF Sheriff ☠
04-27-2012, 08:34 AM
CF Veteran
Join Date: Feb 2012
Location: Forest Hill, Maryland
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Year: 1998
Model: Cherokee
Engine: 4.0L
well not for me = D I'm looking forward to the new car
you mean a cutout?
i only get 13, and thats on a good day
I will talk to you about regearing later
dont worry I got him a 24 pack to give to him today = D
me? or another one?
me? or another one?
04-27-2012, 08:37 AM
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Join Date: Feb 2012
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Year: 1998
Model: Cherokee
Engine: 4.0L
you guys need to remember the primary rule of engines - what goes in must come out
so if you add a bigger TB, you also need a better flowing exhaust to see a benefit. And to use an even bigger (larger that 62mm) you really need a stoker engine, headers, and free exhaust
so if you add a bigger TB, you also need a better flowing exhaust to see a benefit. And to use an even bigger (larger that 62mm) you really need a stoker engine, headers, and free exhaust
04-27-2012, 09:02 AM
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Join Date: Mar 2009
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Year: 1998
Model: Cherokee
Engine: 4.0
I'm going to try to cover this without getting too technical.
The stock exhaust is a compromise between efficiency, power and quietness and does have a considerable amount back pressure. In this case, the back pressure actually aids in quieting the engine, but decreasing the torque.
Back pressure is not an engines friend as it causes hot exhaust to remain in the engines exhaust port for a longer period of time. Granted, it just for a fraction of a second, but that's all it needs.
A well balanced exhaust system will have a 'tuned' exhaust manifold, exhaust pipes of sufficient diameter to maintain exhaust velocity and cat and muffler with minimum restriction.
A 'cat back' system generally refers to the components after the catalytic converter. Reducing the restrictions of the muffler and tail pipe will improve engine torque. But, the cat still remains as a restrictive device and can be solved by replacing the stock converter with a high flow version.
The stock cat has sufficient restriction to maintain the burn in cleaning up the exhaust. This is probably more true of the '97 and newer XJs with the dual O2 sensors. However, a high flow cat is designed so that the exhaust has less restriction while doing a proper burn.
A tuned exhaust manifold is designed so that as the cylinder releases the exhaust gas it aids in pulling in the next air/fuel component, called 'scavenging'. It does this by having timed equal lengh exhaust tubes.
To clarify this, as one cylinder pushes out the exhaust, the next cylinders exhaust won't collide with the previous going out the exhaust pipe. For illustration, look at the following manifold. You'll note that cylinders 3 & 4 are closer to the exhaust pipe port than 1 and 6. In a factory manifold, the exhaust is just dumped into a common pipe and the exhaust exists the best it can. With the referenced manifold, each pipe is designed in length and diameter to reduce exhaust collision and exit freely.
That's all I'm going to get into here. If you want to learn more here are two excellent articles (and not too complicated) that explain in detail the exhaust flow.
http://www.veryuseful.com/mustang/te...Scavenging.pdf
http://www.bigcitythunder.com/pages/...ng_exhaust.pdf
A restrictive (too small) pipe, will cause too much back pressure and impede free movement of the gasses. The pulses begin to overlap, and you lose HP.
You want to minimize backpressure, but keep just enough so as not to have free flowing gases that remain in the exhaust system and not remaining in a line of pulses that aid in expulsion. For this reason, too large of an exhaust pipe is not desirable.
I highly recommend for a GOOD well tuned XJ exhaust system you keep the 2.25" piping, but get mandrel bent downpipe and tailpipe as well as a free flow cat/muffler to increase and smooth out flow. Going to 2.5" piping CAN actually be counter productive.
Hope this helps SOMEONE since I have been seeing a lot of talk on the subject lately.
Also, unless you get a header with a 2.5" exit at the collector, you won't achieve a true 2.5" exhaust.
STOCK tailpipes are only 2" as an FYI...it narrows after the muffler....
The stock exhaust is a compromise between efficiency, power and quietness and does have a considerable amount back pressure. In this case, the back pressure actually aids in quieting the engine, but decreasing the torque.
Back pressure is not an engines friend as it causes hot exhaust to remain in the engines exhaust port for a longer period of time. Granted, it just for a fraction of a second, but that's all it needs.
A well balanced exhaust system will have a 'tuned' exhaust manifold, exhaust pipes of sufficient diameter to maintain exhaust velocity and cat and muffler with minimum restriction.
A 'cat back' system generally refers to the components after the catalytic converter. Reducing the restrictions of the muffler and tail pipe will improve engine torque. But, the cat still remains as a restrictive device and can be solved by replacing the stock converter with a high flow version.
The stock cat has sufficient restriction to maintain the burn in cleaning up the exhaust. This is probably more true of the '97 and newer XJs with the dual O2 sensors. However, a high flow cat is designed so that the exhaust has less restriction while doing a proper burn.
A tuned exhaust manifold is designed so that as the cylinder releases the exhaust gas it aids in pulling in the next air/fuel component, called 'scavenging'. It does this by having timed equal lengh exhaust tubes.
To clarify this, as one cylinder pushes out the exhaust, the next cylinders exhaust won't collide with the previous going out the exhaust pipe. For illustration, look at the following manifold. You'll note that cylinders 3 & 4 are closer to the exhaust pipe port than 1 and 6. In a factory manifold, the exhaust is just dumped into a common pipe and the exhaust exists the best it can. With the referenced manifold, each pipe is designed in length and diameter to reduce exhaust collision and exit freely.
That's all I'm going to get into here. If you want to learn more here are two excellent articles (and not too complicated) that explain in detail the exhaust flow.
http://www.veryuseful.com/mustang/te...Scavenging.pdf
http://www.bigcitythunder.com/pages/...ng_exhaust.pdf
A restrictive (too small) pipe, will cause too much back pressure and impede free movement of the gasses. The pulses begin to overlap, and you lose HP.
You want to minimize backpressure, but keep just enough so as not to have free flowing gases that remain in the exhaust system and not remaining in a line of pulses that aid in expulsion. For this reason, too large of an exhaust pipe is not desirable.
I highly recommend for a GOOD well tuned XJ exhaust system you keep the 2.25" piping, but get mandrel bent downpipe and tailpipe as well as a free flow cat/muffler to increase and smooth out flow. Going to 2.5" piping CAN actually be counter productive.
Hope this helps SOMEONE since I have been seeing a lot of talk on the subject lately.
Also, unless you get a header with a 2.5" exit at the collector, you won't achieve a true 2.5" exhaust.
STOCK tailpipes are only 2" as an FYI...it narrows after the muffler....
04-27-2012, 09:05 AM
CF Veteran
Join Date: Feb 2012
Location: Forest Hill, Maryland
Posts: 6,468
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Received 18 Likes
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14 Posts
Year: 1998
Model: Cherokee
Engine: 4.0L
I could use a header like that one = D. I'd like to get a hi-flow cat, and then from the muffler back do 2.5" tailpipe
