Is it a stock one?  Cause I have a stock one on it right now, no problems.

when i talked to Chad from Golen engines who built my stroker, he said on his 4.6L he expects about 15 - 20 HP increase AT LEAST and recommends the Banks or the Borla. I grabbed a Banks header for about 455 with a lifetime warranty. im not sure what it would do on a stock 4 litre though

Gar, the flowmaster was VERY cheap.  Like the cost of shipping.  I don't really care how it sounds, as long as it works.  Also have someone that can bend the pipe, weld, so on and so forth.  So not to worried about that either.  Thanks for the offer though.  Also would be a loud ride up to your place.  The stock headers are all the exhaust that is on it...

The factory manifold is basically a header... using firing orders and stuff, they have some that's engineered in a 6-3-2-1 or something so that each exhaust pulse doesn't collide with a corresponding exhaust pulse, rather it's tuned to that they 'line up' rather than 'crash' like in most 6-2-1 manifolds. I don't think on a stockish engine it'd be very noticeable though.

I run a flowmaster 40 series... I like the durability, idle and under load sound is nice, but no load and rev a little, and I don't like it. I've heard better and worse.

A lot of guys like 2.25 inch pipe for scavenging the exhaust, I'm running a 2.5" pipe but this was before I really looked into it. If you're concerned about mpg do a tune up, and if that's done alreday, cut up the stock airbox like I did (take dremel tool, extend hole around to drivers side of airbox).

I don't think I gained anything from the exhaust, but I'm running stock cat as well... then I cut the airbox and noticed almost 1mpg and more power from about 2500rpm and up, so perhaps the stock exhaust was enough to expel what little air the stock crappy intake can inhale. Also added a nice intake growl.

I'd like to have a flange ahead and behind my muffler, so I could swap it out for straight pipe (cat quiets it down some anyway), glasspack, etc... easily. I personally like classic chambered mufflers... I think it's http://www.classicchambered.com/ I only wish there was a way to remove the louvers which screw up airflow... then again, did you ever see a flowmaster display? lol... magnaflow seems to look the best outside of a perforated (non-louvered) glasspack.

ChevelleSSLS6 wrote:

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The factory manifold is basically a header...

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Not even close to being true...

There is a lot to be gained going to an aftermarket header.

1) Solves the prone factory "cracking" issue. I'd say a good portion (80% or more) of the XJ's on the road have a cracked factory manifold right now. The factory manifold can't handle the stress that the Jeeps have seen over time and just simply fail. An aftermarket header most of the time is "beefed up" to handle the stress.

2) There is something to be gained out of the better material used with an aftermarket header. Heat dissipation is different and most of the time going to a stainless header will help with underhood temps.

3) Size of the actual tubes. Most aftermarket headers go to a slightly larger tube size, thus helping with exhaust flow. Thus increasing HP and gas mileage.

4) Scavenging of the exhaust is often much better with an aftermarket header where the tubes are actually the correct length. This will help with exhaust flow, thus increasing mileage.

I think an aftermarket header is a great investment; even on a stock motor. I think the gains are worth it. If you want to talk bang per buck when it comes to the motor I think the header is on the top of the list.

 

Yeah, I didn't mean to sound like i was against a header over a manifold-I was just saying with 33's and stock gearing(assuming you have 3.55:1) its doubtful you would see 20mpg's. Like Lead said about cracking manifolds, I am sure that mine is cracked. I hear a exhaust leak from motor.