 The last item in my pursuit
for getting extra power out of my aging 4.0 is a Borla Header. If
you have spent any time around the automotive accessory industry
you know Borla well. They make probably one of the broadest lines
of high end stainless steel street legal headers. If you have been
around Cherokees for very long you also know that the exhaust
manifold is the most apt part of the 4.0 to break down with age.
Mine had cracked and made this change not only a good performance
move, but a good repair move. Borla makes a very good product. I have yet to hear of anyone who has had a bad experience with a Borla header. The quality and durability of stainless steel is well known. As an exhaust component material it negates the deterioration of rust and holds up to heat well. In general the Borla header is far superior to the stock manifold. The most unfortunate tidbit about the replacement of the exhaust header is that everything has to come off of that side of the engine. At first glance this thought alone may stop many of you or lead you to the expense of having it done. But before you get too discouraged let me assure you that this is an installation that most of the mechanically inclined can do. My biggest fear was getting the original, never before removed, nuts and bolts off of the existing manifold. I have "been there, done that" many times before and can tell you that some of the hardest bolts I have ever removed were heat welded exhaust bolts. To my great surprise this was not the case with the 4.0 exhaust manifold. All bolts came off easily. I did soak them liberally with WD-40 the night before and that might have helped. Basically, you just need to mark all hoses and wires and put nuts and bolts in separate marked containers so that you will know where you removed them from . The two toughest items to get off will be the power steering pump and the catalytic converter exhaust connector.  The key to getting the power steering pump off is the center bracket bolt off. This bolt is in the front hidden behind the pump itself. You have to take the slider bolts off and move the pump out of the way to get to the center bolt. This center bolt connects the power steering pump bracket directly to the intake. I have to admit that I never got the exhaust connector that mounts to the frame at the catalytic converter off. I cut the pipe. I got 3 of the 4 bolts off but the last one was rust and heat welded on and had to be cut off. After I installed everything I ran it open header to the exhaust shop and had them do this and weld the header to the supplied pipe connector. I could not get the two to line up enough to negate a slight exhaust leak. Welding it was the easiest thing to do. I can tell you that the 4.0 does not run well with an open header. Unlike most V8's that can benefit from a cut out for short open header runs, the 4.0 evidently depends on some back pressure. I also imagine that the placement of the O2 sensor has something to do with this. ANYHOW, it just doesn't run well at all without the exhaust system connected. While I had everything torn down I took the intake in and had it chemically dipped to remove all of the old carbon build up. It came out looking new as you can see from the pictures. This is also a good time to get your injectors cleaned if they have not been for some time. Look for any worn or broken hoses or connector clips and get replacements before you start putting things back together.  You can see from the picture the overlap of the exhaust and intake manifolds. Since they employee the same bolts replacement can be a bit of a bear on the bottom. Actually, the only one that posed a real problem was on the number four pipe (this is the rear of the two center pipes). The pipe obscures a direct attack on the bolt from the bottom and the intake from the top. I had to get it started first then install the intake. To tighten it I used a long extension on my ratchet with a crow's foot on the end. If you don't know what a crows foot is just ask your local tool store, they can point one out. A swivel attachment wont work and the crow's foot is the only thing I know of that will. After starting all of the nuts and bolts its important to torque every one in order. Check a manual for the tightening order. This will keep the manifolds from torque warping, which could cause a leak. Past this its just reconnecting everything back up. Once complete and with the exhaust system welded my low end torque has improved. You will notice better pickup and stronger pull. I think on a street setup this would be more noticeable. With the lockers in the front and rear I testing off the line acceleration is a challenge. I have to do more of a rolling start acceleration test. There is only one negative to note, and I am not sure yet whether this is a fuel deliver problem that I may have created. Since installing the header I have noticed at highway speeds in overdrive, when having to push it to get around someone there is a dead spot in acceleration. It seems that dropping into 3rd for this works best and the motor pulls well. If I do not drop down and let the throttle kick in the passing gear I don't get as much response from the engine. I have not had time since doing the install to track this problem down. Again, it may be a fuel delivery problem as I am now noting a secondary sound from the fuel rail. HEY, if I don't create at least one problem while making an upgrade or fixing another problem, I normally did something wrong. Overall I am very happy with the Borla header. Power has improved and the exhaust note is actually, at least to me, smoother and quieter. This may be because of the noise that the original cracked manifold was making. The Borla header teamed with my high flow Cat and Flowmaster muffler has a nice growl. I think that this is a great replacement for aging exhaust manifolds or just for the power improvement. Install, while a bit daunting, is not that difficult and can be performed by most of you. 4.0 Power Series Conclusion Well after going through the hassles of putting this together am I happy with the outcome. Yes, although I am still working on trying to fine tune and dyno the setup, I am. I can tell you that during preliminary dyno test we were able to get almost 200 hp at 4500 rpm. This was measured at the back tire and can be compared to the stock rating of 170 hp at the flywheel. That was prior to the Borla install and I would imagine that right now that my 4.0 with over 118,000 miles on it is putting out a solid 200+ hp at 4500 rpm. Not to shabby! I hope to get back on the dyno soon to verify this. That is after I work out the fuel rail problem. The only item that I have gotten mixed dyno results from was the MSD unit. Initial results were not consistent. After discussions with MSD I have switched to a Champion Platinum tipped plug, that is over gapped by .10, and hope to work this out as well. I will post these results when I get them. For more information contact; Borla Performance Industries 5901 Edison Drive Oxnard, CA 93033 805-986-8600 Dyno time courtesy of; All Chevy Engines 424B West Fork Drive Arlington, TX 76012 Order Line: 800-861-3960 Tech Line: 817-861-3969 All Chevy Engines builds from 285 hp street engines to 650 hp full race engines. Prices run from $1799 to $8999. Follow us on Twitter at www.twitter.com/OffRoadDotCom
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Posted Jul 15 2008 01:23PM
