Jamie7598

I recently purchased and installed an APN header due to the fact that my stock manifold was cracked in the typical spot. However it's been about 2 months since install and I have yet to see a difference in the gas milage that the crack was causing me to lose nor any preformance differences. I have a 96 xj. Any ideas/suggestions? Thanks in advance!

mschi772

An APN header alone won't have an impact on performance big enough to perceive. Hell, a full high-end exhaust system is only good for less than 10 hp usually--most people aren't going to feel that difference. If anyone promised you big gains, they were fooling you and/or themselves.

A cracked header often but not always harms gas mileage. Furthermore, it takes awhile for the computer to adapt to the new conditions (about 50 full warm-up cycles).

cruiser54

The factory exhaust manifolds were practically a header in the first place.

Why were you thinking the crack was affecting your mileage?

Jamie7598

I noticed the exhaust leak and a drastic decrease in gas milage at around the same time, from around 12 to 13 mph down to a little less than 8.

cruiser54

So, you're saying the exhaust leak wasn't the cause of the MPG decrease then? Makes sense to me. I've never understood how folks could think that exhaust leaking out of a manifold under pressure could affect an oxygen sensor reading downstream anyway.

mschi772

What happens to some is that fresh air gets sucked into the exhaust (sometimes the crack is positioned just right that it will be by a low pressure portion of the exhaust flow and the flowing exhaust basically scavenges fresh air from the outside world) which then gives the O2 sensor false info which leads to the computer enriching the mixture because it "thinks" you were running too lean. Doesn't always happen.

cruiser54

I can't see it ever happening.

What am I missing?

I've driven my Renix Jeeps with busted out donuts, cracked manifold, no exhaust pipe and never seen the O2 sensor do anything but it's normal stuff while monitoring it on a DRB tester.....

mr white

? It happened on mine.

Turbo X_J

Your crack must have been improperly positioned, in relation to the outside world.
It's when the exhaust gas is a false positive pressure, when it happened to me my engine ran backasswards.

cruiser54

Now we're getting somewhere.....LOL.

thatXJguy

Yeah you're not alone. I replaced my exhaust manifold, except I used a factory replacement from APN.

No real differences. Just less of an exhaust smell and less noise because of the absence of a crack. I will say though, going to a catback exhaust from Banks gave me an extra 20 miles to a tank.

XJUSMC

Ok so without messing around. When I replaced my header I went with an ebay special, while I was there I replaced my injectors with 784s, 62 MM TB, cowl intake and I installed a high flow Spin cat with Magnaflow from performance curve all within a week. Once I can install my longarms I plan on having the jeep dynoed at a local rice shop. Ill get back with numbers.

I have a 5.5 SA IRO left (Serious Long Arms are ordered just waiting for them to get here) with 265 65R 16.

I easily get between 17-21 mpg on the highway. I barley ever go over 65mph and off of red lights I hardly go over 2500 RPM.

If you want to see or feel gains it takes more than just a header.

cruiser54

What are you using the barley for? Isn't that for soup?

Turbo X_J

Barley is for Ricers? lol.

Longarms probably ain't gonna do much...

XJUSMC

Longarms probably ain't gonna do much...[/QUOTE]

What? plus 10 at least..... I knew someone would say that, I just don't like testing an unfinished product. I want to do the run the way I'll be driving it. I know its all mental but I'm paying so I get to decide.