Header info, right or wrong

[honesT]

My 2cents from a newbie that only has research on his side :moon:

From what I have read the best set up is a header that flows from 3 to 2 to 1 like this one http://rustysoffroad.com/mm5/merchant.mvc?Screen=PROD&Store_Code=rustys&Product_Code=FD409&Category_Code=headers

As far as going to a larger than 2.5 diameter pipe it has been stated that all is good with longer pipes to improve exhuasting the spent combustion chamber gasses thus allowing more fresh air into the combustion chamber giving a better explosion, BUT the larger than 2.5 diameter pipes are said to effect the back pressure of the system ( I`m guessing it has something to do with the compression like a vaccum cleaner going to big will only cause a tornato affect instead of a straight flow ) and rob the engine of it`s good gass mileage due to the combustion chamber still being full of gasses from the last explosion...

This is just what I can gather from what I have read, Myself I have decided not to go with a header as my stock manifold is still in good condition and my 96 cherokee is only a stock engine and the only noticable gain I would get is a louder growl... I have up graded my exhuast system to a 2.5 high flow from the bottom of the stock manifold to the tail pipe and this setup will help my engine breath better and improve my gas mileage a little :driving:, My next upgrade is a cold air intake and larger throttle body to better improve the fresh air flow into the combustion chamber, of course after I has researched the crap out of everything I hate waisting time and money :nuke:

[4.3LXJ]

The stock exhaust manifold is somewhat like a header already, a "shorty". The tri wye design is a tried and proven method for low end and mid range torque, which fits the design of the engine. Not sure about the gas milage situation, but the small 2.25" pipe is very restrictive as evidenced by the level of noise you get from a straight pipe. But the whole engine is somewhat detuned anyway.

[honesT]

The stock exhaust manifold is somewhat like a header already, a "shorty". The tri wye design is a tried and proven method for low end and mid range torque, which fits the design of the engine. Not sure about the gas milage situation, but the small 2.25" pipe is very restrictive as evidenced by the level of noise you get from a straight pipe. But the whole engine is somewhat detuned anyway.

I`ve been doing alot of thinking on this and it seems that the engine is not as good on higher rpms because of all the left over gases still inside the combustion chamber due to the smaller diameter pipe, in turn getting worse gas mileage & more money for the top brass :boohoo: or am I just paraniod...

4.3LXJ thanks for pointing out that the stock manifold is like a small header I never really looked at it that way, I just thought it was crap :coffee:

[4.3LXJ]

I`ve been doing alot of thinking on this and it seems that the engine is not as good on higher rpms because of all the left over gases still inside the combustion chamber due to the smaller diameter pipe, in turn getting worse gas mileage & more money for the top brass :boohoo: or am I just paraniod...

4.3LXJ thanks for pointing out that the stock manifold is like a small header I never really looked at it that way, I just thought it was crap :coffee:

In a sense, they are. They are prone to cracking and considered unfixable when they do.

[bluedragon436]

Well I know that I am running a 2.5" cat back... and have had no loss of gas mileage... I figured it out on my first tank of gas after the exhaust install... and on that run I did see some gain in gas mileage... will see if it is roughly the same on the current tank just to make sure it wasn't just a fluke... can't wait to install my 99+ intake, Neon injectors, and 60mm TB.. should see some nice gains with that too...

[honesT]

I`m still learning so I could be a little off but it makes sense for now...
4.3 LXJ, I now understand what you stated when you said low end and mid range torque, as the exhaust air pressure changes through the RPMs the diameter and or length of the pipe makes the difference on the back pressure causing the spent combustion gases to ether be (fully or partly pushed out) / (fully or partly sucked back in). in turn affecting the strength of each explosion or combustion due to ether fresh or half fresh air mixed with the gas upon combustion...

More insite I found online: A shorter pipe improves torque after the peak (reduces it at lower RPM), preventing the
torque curve from falling off so quickly as speed increases. A longer pipe extends the torque curve
backwards to improve the engine's flexibility, at the expense of after-peak torque.

For anyone who wants more detailed information check out this site and scroll down to Exhuast manifold : http://www.scribd.com/doc/15418414/jeep-Engine-and-Cam-specs

[4.3LXJ]

Just to add to your confusion, a longer header pipe also increases rpms available. So while technically you might have a little less torque, you get it back through increased rpms. Another side note, less exhaust gasses increases combustion chamber temps and improves efficiency. With the right cam, a long tube will scavenge the chamber by pulling a partial vacuum on it due to the inertia of the gasses in the tube. The best headers ever made were all six feet long

[honesT]

Just to add to your confusion, a longer header pipe also increases rpms available. So while technically you might have a little less torque, you get it back through increased rpms. Another side note, less exhaust gasses increases combustion chamber temps and improves efficiency. With the right cam, a long tube will scavenge the chamber by pulling a partial vacuum on it due to the inertia of the gasses in the tube. The best headers ever made were all six feet long

So does this make any sense:
If you want to have ur rig full throttled all the time then you will need a different setup then someone who wants the best of both low & high rpm ranges... It`s due to how fast the spent gases are traveling wether the air flows straight, tornato or tumble due to the diameter, lenght and curves of the header pipe, in turn affecting how much fresh air is in the combustion chamber upon the next explosion, I`m guessing


So just to keep things clear :stars: The gains that are made from a header or exhuast system upgrade are all because they have a direct affect on how much spent combustion gases or dirty air is pushed or pulled out of the combustion chamber so the next explosion has the max amount of fresh air to mix with the gas :shocker:

And the exhaust from high RPMs have different push pull affect then low RPM exhaust...

I`m still learning so don`t hold me to it :thumbsup:

[4.3LXJ]

So just to keep things clear :stars: The gains that are made from a header or exhuast system upgrade are all because they have a direct affect on how much spent combustion gases or dirty air is pushed or pulled out of the combustion chamber so the next explosion has the max amount of fresh air to mix with the gas :shocker:

And the exhaust from high RPMs have different push pull affect then low RPM exhaust...

I`m still learning so don`t hold me to it :thumbsup:

Yes that is the way it works. I once knew a guy that raced on the same off road course but different class than me. He had a built 350 just like everyone else. But he had 6' headers with Spin Tech mufflers. He also had a cam shaft that was ground for the long headers. The intake and exhaust were open at the same time for a little while. The exhaust would actually pull a vacuum on the carburetor. He was kind of doggy at low rpms, but as soon as the exhaust went whoosh, he took off like a rocket and left all the rest of those Chevys behind in a very short distance. So long story short, there is quite a science to headers and exhaust.

[honesT]

reduction in backpressure is causing more air to be drawn into the combustion chamber than before.

Here :
http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

[honesT]

Sweet video of inside of combustion chamber on a real engine.
http://www.youtube.com/watch?v=sEf8v...eature=related

[honesT]

1st 30 seconds of video most informative :
http://www.youtube.com/watch?v=ICAWQPLNFAc

[honesT]

Another : http://www.youtube.com/watch?v=9A6S1NUjg_A