How do aftermarket headers affect the low end?

I tried searching, found all kinds of threads on headers, and how to get more power. What I couldn't find is any discussion on what headers do to low end torque. My XJ spends a LOT of time in the rocks, and one of the big reasons I'm running a Jeep is the 4.0L engine. Like many others, I have the typical cracked factory exhaust header.

I'm considering replacing it with one of the aftermarket headers, but I absolutely do not want to lose the low end power (I really like being able to idle up most obstacles right now without having to run super deep gears ). Knowing that most headers typically sacrifice low end to make gains in the mid and high end, I'm curious if there are any for the 4.0L that at least don't lose the low end, or possibly even make gains on the low end?

with a crack in the header, your O2 sensor isnt reading all the exhaust...

I'd bet you'd make gains in the low end of torque just by getting your engine managment working better...

Replacing the manifold is something that I definately need to do as well - I can fit my small finger into the gap :anon:

Keep in mind that rated power is max power at that RPM. When you're idling over rocks you're no where near max power, so whether or not you loose a little max power is meaningless, you won't loose any power for what you're doing. Also, the 4.0L has so much low end that you wouldn't notice a slight loss in the situations you're talking about, or in any situation.

If anything, the header would move maximum torque up a few RPM, and the only time you'd notice (if it could be noticed at all by seat of the pants) would be under full or near full throttle acceleration. In that situation it could be better.

Basically, it's a non-issue, put on the header.

I put a pacesetter header on my 87 XJ. I did not notice a lack of low end. What I did notice was a smoothing-out of the powerband. Before header, there was a definite increase in acceleration at about 3k, after, it's just smooth power all the way up, no kick in the ass at a certain rpm. Performs the same installed on the 93 motor that's in my Jeep now, and that motor seemed the same way before. Get the header. Smooth is good, it also sounds pretty cool.

i was under the general impression that long tube headers were good for high end horsepower and short tube headers were for low end torque.

From what I learned in my Subaru Impreza tuning days, the less back pressure you have, the higher up the low end RPM range moves. To a certain point of course. Then, you just start losing torque. This would make sense for the statement above that a long tube header is good for high end and a short tube good for low end.

Heres the example to my reasoning:

On my Impreza (N/A, 2.5L boxer engine) I had a stock exhaust minus the Borla header. With the exhaust in place, I had a good low end pull. Now, every once in a while, I would remove the axle back portion of the exhaust...basically leaving a header, cat section, and a mid pipe that exited right before the rear wheels under the car...the muffler section and about 2-3 ft of exhaust pipe was removed. There was a noticable loss in low end, BUT (BUT) BUT!!!, the torque and power was still there...just higher up in the RPM range. This helped dramatically when I went to Deals Gap (Tail of the Dragon, Highway 129, awesome curvy famous road for all the retards) b/c I didnt EVER have to shift past 3rd gear...In conclusion...the more you open up the exhaust, the higher the power is going to move in the RPM range.

This is a very complicated subject if you really get into it b/c it doesnt have a lot to do with backpressure, which a lot of people talk about, but about scavenging of the exhaust system.

i have a 99+ intake, APN, hi flow cat, and dynomax muffler.

no loss in low end, put the motor screams at 4K+ RPMs. no difference when wheeling.. but when i put the 62mm TB on i had to learn to be easier on the gas pedal since throttle tip end was really sensitive, but manageable since i have an auto.

although, since i have managed to rip my exhaust of multiple times off road, i can say with confidence that without even just the muffler you lose a LOT of power. i lost everything up to the x member last time out and i had no power, even on the higher end.

89xj said:

i was under the general impression that long tube headers were good for high end horsepower and short tube headers were for low end torque.

Click to expand...

I always thought it was the opposite. As said before it has more to do with velocity and scavenging than backpressure. I always thought longer, skinnier primaries provide better low end than shorter fatter ones. But of course, exhaust is much more complicated than most believe.

I have a pretty decent grasp on exhaust theory, and I know that backpressure is bunk (it's all about velocity). But it's not like I can open up the Summit catalog and choose from a few dozen different headers. We've got what? half a dozen options? I haven't closely compared them all, but they all look pretty close and are of long tube design. On that note, it's been my experience that short tube headers tend to be more for clearance and sound then any real power gain any where in the band.

In my case, I'm really don't want to bump the power band up any more than I have to. I'm extremely happy with the performance of the engine stock, just hate replacing a cracked factory manifold with another one that's likely to do the same thing.