I myself would go with a factory replacement. The motor doesn't make enough horsepower to require an actual header to let it breathe better. Besides, if you do a long tube header, or even a shorty, not only do you have to get a custom head pipe made, but it's way too close to the front driveshaft.
I have seen more than 1 get crunched by the driveshaft, or scraped off on a rock because it was down too low.
The horsepower gain to money ratio is so far off that it doesn't require a rocket science degree to figure out that its just a waste of money.
Then you have to get a header that matches the thickness of the flange on your stock manifold. Most header aren't that thick, and you have to build up the difference with washers because the bolt that holds it on also holds the intake in place and the thickness has to match or it will leak. I have installed a few sets on customers vehicles just cause they "had to have them", but I think they are just a PITA.
Just my 2 cents. Don't let my negativity discourage you from getting a header if that's what you really want.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...
I've been running Borla Headers for probably 10 years. They are not cheap but have a lifetime warranty. California smog rules are tough and when Cali required the O2 sensor to measure all six pipes Borla sent me a new header no charge. I did the install myself and everything bolted right up without a problem. According to JP magazine dyno test, headers was one of the better power increasing mods though I beleive the power is at the upper end not low end torgue. I like the looks and sound of my Borla and Flowmaster, not a V8 but much better than stock sound.
Went with the Dorman factory type exhaust manifold. It has the flex joints welded into the pipes to help prevent cracking in the future. I don't see that the 4.0 is going to get another $300 worth of performance with a header. I did buy Bosch 4 hole injectors and those seem to have made things smoother as well. Can't speaks for mileage as I have only put 50 miles on it. I did VHT the manifold before install.
A header is one of those things that guys just want to get. Ive gotten headers for my GTO, and my 72 CJ5 V8. Just because. I know that its not quite worth the time or expense on the straight six, so I've always just talked myself out of it.
That being said, the YJ I just got had a header already installed by the PO. Its a Doug Thorley unit. It has the super thick flange to match the intake thickness. Its been on there a while yet its not cracked or leaking, and my new OE style front pipe fit on perfectly. Its a quality piece. I cant comment on any performance benefits, other than maybe a point or two of cool factor. FWIW.
'91 YJ Sahara 4.0L/AX-15/NP231, D30/D35 @3.07
'12 KK Cherokee 3.7L/42RLE/MP1522, D30M/8.25 @3.73
"Non vi sed, Arte"
I had a banks stainless one on my last jeep. I loved it. No installation issues but I replaced the whole system. Off roaded a lot and never had any clearance issues. Will do it again if my manifold ever cracks.
Do not listen to the guys telling headers are pointless, Pacesetter makes a great set of headers, that work well together with w CAI, and actually re-route the exhaust around the front of the oil pan to clear the driveshaft...