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Exhaust Manifold: Stock vs Headers for MPG

Old 08-26-2015, 02:54 PM
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Default Exhaust Manifold: Stock vs Headers for MPG

A while back I swapped my stock exhaust manifold for an APN header, wrapped it with exhaust wrap as well. It definitely added some performance along with my 99+ intake manifold, but now its cracked, and I assume the exhaust wrap is partly to blame.

I'm also getting horrendous MPG lately, engine is giving oxygen sensor code, and a lean condition code.

So I have a new NTK O2 sensor in the mail, but I'm worried that my cracked exhaust manifold may also be to blame for the bad MPG.

Now, I kept my stock exhaust manifold because when I took it off I didn't see any cracks, although I didn't inspect it super thoroughly or anything. So I'm actually considering possibly reinstalling it, mostly cause I'm short on cash.

So, here's my questions:

1) What's a good way to check the stock manifold and ensure there are no cracks?
2) Would it be completely stupid to reinstall the stock manifold?
3) Would a stock manifold affect MPG compared to headers? I never tried to measure any difference when I installed the headers, so I have no idea. Would a stock manifold increase MPG? Would it reduce it?
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Old 08-26-2015, 04:07 PM
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Default Exhaust Manifold: Stock vs Headers for MPG

A crack can definitely reduce mpg. Air gets sucked in, the O2 sensor thinks you are too lean because of the extra fresh air, your engine runs richer than it should to compensate for a lean condition that doesn't really exist...you end-up wasting fuel and killing your MPG.

There is no trick to finding cracks in a header. Just look very carefully. They are USUALLY around the collector.

A well designed header should provide some power and mpg benefits compared to stock assuming both are in good condition. My Thorley header did and has been great so far.

I would not recommend wrapping your header. The primary cause of header cracks is a bad motor mount, but the wrap can exacerbate the situation for a header that isn't able to withstand all that heat coursing through it.

Last edited by mschi772; 08-26-2015 at 05:06 PM.
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Old 08-26-2015, 04:47 PM
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Default Exhaust Manifold: Stock vs Headers for MPG

My APN cracked this week. I had my ceramic coated before install , I'm getting a PO171 code now dumping fuel like crazy 15miles to 1/4 tank. I'm trying to find another brand that fits the 99. If not I'll just go back to a stock manifold or buy the intake off my neighbor who's parting a 98. and yes my engine mounts are good.
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Old 08-26-2015, 05:02 PM
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I don't think anything near the price of the APN fits with a 99+intake without any massaging or grinding other than Dorman/OEM. Plus, all the other options in that price range are either APN clones or unreliable at best like Pacesetter which is pricier and has a reputation for unreliable quality and rust. At higher prices, I think there are ones that fit, but those prices are MUCH higher. My favorites after obsessing over the topic for awhile became, for various reasons, Dorman, APN, Borla, Thorley, and AFE. I would have given Borla a go given my large budget for the project (because I love the design--longer primaries, included downpipe, 304 stainless), but Thorley's most recent design has such a spectacular reputation for durability and is offered by Kolak, so I couldn't say no to it. Borla's durability did sound about as crack-risky as any other option out there; even with a lifetime warranty, having to replace a cracked header sucks.

A section of flex pipe before or immediately after the cat can also help reduce stress on the header from the shaking of the exhaust system. It's a simple and relatively cheap way of further reducing the odds of header cracking.

Last edited by mschi772; 08-26-2015 at 05:04 PM.
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Old 08-26-2015, 05:48 PM
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Gibson. Been VERY happy so far. ZERO fitment issues. I have a 97, but it is good up to 99. I got the SS ceramic coated, and while costly, tons better than the Borla wrapped headers that came off (yes they were cracked, at the collectors) the stroker when I bought it. Avoid wrapping. That stuff sucks.
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Old 08-26-2015, 07:31 PM
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I've had good luck with mineral spirits for similar projects.
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Old 08-26-2015, 08:53 PM
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Default Exhaust Manifold: Stock vs Headers for MPG

I probably won't go apn again I'll buy something better but I want to know if it will fit without modification using the newer style intake.
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Old 08-27-2015, 07:22 AM
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Originally Posted by Tom95YJ View Post
I probably won't go apn again I'll buy something better but I want to know if it will fit without modification using the newer style intake.

For a stock 4.0 and even a lot of the mild strokers the design of the stock 99 header is a fairly good design. The cheap headers are just that and shows from what I have saw posted on several sites for the 4.0. Only the good headers like Banks and such coated by people like "Jet Hot" who only do that is the way to go.


Wrapping a header was done yrs ago on race cars to keep heat in the header I used it all the time, but it ruined the tubes in just a year or two either rusted or cracked. At that time we ran Hooker Super Comps cost $5xx when we started coating them another $500+ then they lasted 5-6 yrs. and worked so much better overall.
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Old 08-31-2015, 07:10 PM
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My stock manifold is ripped off and doesn't meet the down pipe on one side and I have absolutely nothing behind the cat, needless to say, she's loud as ****, but im averaging around 10-12 per gallon and im only on 33s got a quote for $1500 to replace and install everything but I barely have $15 let alone $1500, any recommendations?
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Old 09-01-2015, 02:33 AM
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my stock manifold was cracked. I don't buy the lean condition caused by a cracked manifold. how does air get sucked into a highly pressurized system through an opening thinner than a sheet of paper? I saw no different mpg after repairing the manifold. I can only assume a more efficient header will afford a mileage increase assuming you aren't frequently employing the newfound efficiency with the skinny pedal.
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Old 09-01-2015, 05:49 AM
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With the manifold out of the truck, you're basically relegated to visual inspections to see if it's cracked. As previously stated, the OEM manifold isn't that bad. If you know the APN to be cracked, swap them. What ever mileage you lose because of the restriction, it won't compare to the mileage you're losing from having a cracked manifold and constantly fouling O2 sensors.

As far as an APN replacement, save your pennies or check craigslist and other classifieds often. So far, I'm very pleased with my Gibson GP400S-C but I highly doubt I ever would have paid full retail on it.

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Old 09-01-2015, 12:45 PM
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Default Exhaust Manifold: Stock vs Headers for MPG

Originally Posted by s346k View Post
my stock manifold was cracked. I don't buy the lean condition caused by a cracked manifold. how does air get sucked into a highly pressurized system through an opening thinner than a sheet of paper? I saw no different mpg after repairing the manifold. I can only assume a more efficient header will afford a mileage increase assuming you aren't frequently employing the newfound efficiency with the skinny pedal.
The smaller the crack the more i t will suck air in. It's the venturi effect that is in place. Same effect that makes a carb suck fuel out of a jet. When a fluid moves over an opening at 90* to the opening it will create a vacuum in the opening.
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Old 09-01-2015, 01:46 PM
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Originally Posted by toasterknight View Post
The smaller the crack the more i t will suck air in. It's the venturi effect that is in place. Same effect that makes a carb suck fuel out of a jet. When a fluid moves over an opening at 90* to the opening it will create a vacuum in the opening.
i didn't know the exhaust manifold was full of fluid. if this lean condition theory was correct at some point of the 30,000 mile span of a cracked header my jeep would've thrown a lean code, followed by "dumping fuel" to compensate. none of that happened. not to mention once the code was met with fuel it would go away bc the mix would no longer be lean. then you'd have 0 codes and 6 mpg. as mentioned before, none of that happened. I welded the damaged portion of the exhaust and guess what, mileage was unaffected.

edit: it's nothing like the Venturi effect. that deals with fluid pressures altered by restriction.

Last edited by s346k; 09-01-2015 at 01:58 PM.
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Old 09-01-2015, 02:00 PM
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Default Exhaust Manifold: Stock vs Headers for MPG

Originally Posted by s346k View Post
i didn't know the exhaust manifold was full of fluid. if this lean condition theory was correct at some point of the 30,000 mile span of a cracked header my jeep would've thrown a lean code, followed by "dumping fuel" to compensate. none of that happened. not to mention once the code was met with fuel it would go away bc the mix would no longer be lean. then you'd have 0 codes and 6 mpg. as mentioned before, none of that happened. I welded the damaged portion of the exhaust and guess what, mileage was unaffected.
Air is considered a fluid in physics which is where the effect is describes. The ECU only throws a lean code when it cannot compensate. If it was only sucking enough air to modify the mixture by a relatively small percentage the ECU would just add more fuel to the issue thinking it was lean and making the mix rich. That is what mine is doing. The only way I could tell is I change the tailpipe recently and it was black inside withing 20 miles.
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Old 09-01-2015, 02:05 PM
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Default Exhaust Manifold: Stock vs Headers for MPG

Originally Posted by s346k View Post
edit: it's nothing like the Venturi effect. that deals with fluid pressures altered by restriction.
At the collector there is a restriction that can cause this effect.

edit:

Last edited by toasterknight; 09-01-2015 at 02:10 PM. Reason: adding information.
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