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post #1 of (permalink) Old 04-03-2008, 12:15 PM Thread Starter
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Question Exhaust/air flow questions

will try to keep this short as possible....

Jeep: '92 YJ, 4.0, I6, 3" BDS suspension lift, all else stock

Problem: Cracked manifold and big dent in front pipe where it bends off the manifold to parallel the ground (which may be restricting flow).

Questions:
1. Difference between manifold and header? (from what i've read, headers provide more air flow and can be tuned, but don't offer any benifit to low end torque and benifits are only in the upper rpm ranges anyway, is that all?)
2. Sounds like if I replace my manifold with an OEM part, then it will just crack again. Gale Banks and Borla have lifetime warranties on their manifolds/headers, but are these headers or manifolds, cause they all look the same? It seems these terms are sometimes interchangable.
3. Since I have to replace manifold and front pipe, I might as well upgrade the whole system right? But it sounds like if I put on headers, there are no benifits without intake upgrades as well. Is it the same with the high end manifolds?
4. Suggestions to intake upgrades? (I've read that K&N filters aren't what they're cracked up to be)

I could care less how it sounds, looks, or how fast I can go on the highway, but I wouldn't mind a little more power for the trails.

sorry for the long post.........
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post #2 of (permalink) Old 04-03-2008, 02:13 PM
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My opinions:

Header is a fancy name for a fancy manifold. There's no sharp, definitive line between the two. Most stock manifolds are made of cast iron. Most headers are made of welded steel tube. Aftermarket steel tube manifolds have a reputation for cracking and, if they're allowed to get old enough, rusting through because the material is thinner. Cast iron manifolds seldom rust out because of their thickness, but they will also crack, especially if doused with water while very hot.

Exhaust manifolds can be tuned to operate best at a particular engine speed, as can intake manifolds. If both are tuned to the same speed and matched with a camsghaft, a significant power increase is possible. Serious aftermarket designers typically work for the highest maximum power, which will occur in the upper RPM range, but it's possible to design manifolds for maximum power down low, although I don't recall ever seeing any touted as being tuned for low-speed. And the fact is that at low speed the potential gains are pretty small anyway.

Normally I recommend stock iron manifolds, but if you can't protect the pipe some way and can find an aftermarket part that will cure the problem, go for it. Just be prepared to lose some low-speed grunt unless you can find a manifold specifically designed for that.

The 4-liter is a terrific off-road engine with a lot of torque, a flat torque curve and good throttle response. It's doubtful that you can do anything to improve that significantly. With a complete re-do of the intake and exhaust you can probably get significant maximum power gains at the expense of off-road tractability.

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post #3 of (permalink) Old 04-04-2008, 04:26 PM Thread Starter
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What about JBA? Anyone had any experience with JBA Exhaust systems? I just got off the phone with them and they've got lifetime warranty headers that claim some low end torque gains as well as gains in the upper rpm range. There was a nice write up about them in the Sep 07 issue of Off-Road Adventures. And they're almost $100 cheaper than the Banks manifolds, and more than $100 less than Borla.
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post #4 of (permalink) Old 04-05-2008, 05:05 AM
 
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I havent heard of JBA however i bet they use the same idea as the torque step headers that headman makes for small block chevys. if your not into spending alot of money there are ways that you can weld the 4.0 manifold so it wont crack as easily. i would recomend an aftermarket header and full exaust but it will take a chunk out of your wallet. as for the intake, within the past 6 months JP magizine did a very informative intake shootout.

remember, loud pipes save lives.
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post #5 of (permalink) Old 04-05-2008, 12:18 PM
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"""""I could care less how it sounds, looks, or how fast I can go on the highway, but I wouldn't mind a little more power for the trails."""""

Power on the trails - most of the time you are at "grunt" speed, not at high R's, not even low mid range. You are close to idle, sometimes even at sub-idle.

Headers work by using the exhaust pulses to pull or "scavenge" exhaust gasses from the cylinders.
At low speeds those "pulses" are so far apart that to make a header work the collector would have to be behind the rear bumper!

The best exhaust for low R's is the log type stock manifold. Best to fix or replace what you have.
Make sure to torque the bolts down slowly and evenly with a torque wrench! Do not overtighten!


Intake manifolds are virtually the same idea - they work on pulses - and they should be matched with the exhaust header to make a significant increase. They work in unison along with the cam and valve train.

And then, once they are matched together, you have something that has more power - at that one speed - but other speeds may even have a decrease in power!

Besides, those mods are expensive.



Not sure if you have FI on it now - that would make a difference you can really feel.

If you have a carb, going to a better one than stock is probably the best thing you can do - use the MC2100 - do a search here - lots of info.

And - get the distributor curve correct for power, not for emissions like you have. Find and use direct PORTED SPARK vacuum, bypassing the relays.

And - use a better ignition system - use an HEI conversion or hybrid HEI/TFI.

And - disconnect and plug the vacuum line to the EGR valve.

Those will give a much bigger grin on your face than doing expensive things.

Info on all those easy mods are available using the search here.

Caution - they may not be smog legal in your area - so make it easy to revert back to stock when it needs to be tested.
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