RTorrez1

OK, This may be a stupid question. I have a 98 XJ sport, 4X4, 4.0, auto trans with a 3" lift. I have been thinking about installing a header to replace the stock exhaust manifold. I have never installed headers before. My question is would I have to change out anything else to install them. Such as exhaust piping, cat. or the muffler?
Thanks in advance.

Merry Christmas to everyone.

wmmccall

The downpipe is a press bend, looks like it was squeezed. If you can get someone to make a downpipe for you that would be awesome for a little more flow.

There is a company that makes a new downpipe without the press bend in it
Walker Exhaust 55277 Front Pipe.

Mods to compliment the header (Of course); Open air intake (K&N, AEM, etc.), high-flow exhuast, a new CAT is good too...but anything would be better than running old stock with a new header.

RTorrez1

Thanks for the info. I'll check into it.

Firestorm500

The stock exhaust manifold is already essentially a header.

The money and time spent replacing it would not gain you enough power, if any, to warrant the changeover.

wiggles

I would just stick with the stock manifold but replace the downpipe

1991Jeep_Man

Hey guys??? APN Header? CHEAP?!?!

Idk what your alls experiences were, but after installing my APN header to replace the cracked stock manifold, I could tell a slightly difference. This was before I did all my exhaust work. Engine sound was a tad throatier, performance felt a tad bit better. But the REAL seat of the pants kicker came with the rest of the exhaust work. 2.5" exhaust all the way back (minus the downpipe.... getting that as soon as funds permit), including Flowmaster Super 44 Muffler and High Flow Cat. The difference is def there, and I am 100% sure the header compliments the rest of the work.

Just my $0.02

1991Jeep_Man

And as far as the stock unit being a header already, I must disagree. Look at the designs of the various header manufacturers for the 4.0 vs the stock manifold replacement.

The OEM replacement (with the addition of flex joints) has the typical runner style of any other OEM manifold, going down into a tight squeeze collector from two joining pipes.

Now, look at the header replacements. Hell, for the sake of cheapness, look at the inexpensive APN. The runners are slightly beefier, and have a more verticle and equal length collecting down into 2 main collectors, then collecting down into one big collector near the flange. Aerodynamics would favor this setup over the stock anyday.

Once again, just my own $0.02.

Firestorm500

Anybody got any certified flow numbers of a stock manifold versus aftermarket?

Just looking at something tells you nothing. Let's see some dyno results.

1991Jeep_Man

Too late for most for dyno results. Unless someone wants to buy an OEM replacement and swap it on, test, swap the header on, test again.

And visuals can offer some hint of how an object will perform. If you look at a 1/2" pipe vs a 3/4" pipe for a drain, which do you think will flow better?

Firestorm500

It's not just a question of flow rates. You also have to deal with sonic waves and a certain amount of backpressure for the most efficient scavenging of the cylinders.

If open flow was the only consideration, then race engines would not have any exhaust manifold(s) at all.

ol'DixieJeep

stock ones arent equal length... equal length=better flow since it uses a vaccume effect to draw out exhaust gasses. also use 2.5 in pipe. i use the stock dual cat downpipe to straight 2.5 straight pipe and thats it. sounds awesome. mines a stock manifold at the minute since i put all my money into forged internals for the stroker.

Firestorm500

All equal length=better?

Not necessarily. Consideration also has to be given to the firing order.

1991Jeep_Man

x2 on this one. If this was the case, many Chevy V8 shorty headers would not exist.

ol'DixieJeep

for I6 equal length is better... we dont have smallblocks in our jeeps do we?

firing order can be taken into consideration when deciding which two primaries to put together and go 6-3-1 to the downpipe to increase the scavaging