Does exhaust increase MPG?

kjmurphy · **Posted:** Tue Feb 07, 2006 8:58 pm

I was thinkin' about getting exhaust for my kj and i was wondering if it increases MPG. I definitely want it for the sound and performance but an increase in MPG would be great. Does more flow mean more MPG. I also heard adding exhaust gets rid of back pressure. Just wanted to weigh the goods vs. the bads before i would get it.

thanks for any info

kj in a tight spot · **Posted:** Tue Feb 07, 2006 11:00 pm

I got a Flowmaster 40 series and it did increase the MPG a little but not much. It is a good investment and is totally worth the money for the performance and sound! Get a Flowmaster.

nickmv · **Posted:** Tue Feb 07, 2006 11:50 pm

I don't have any personal experience in this field, but I've read several threads where people who got the Magnaflow Full Exhaust system experienced 1-3MPG increase, most of them being around 2-3.

NU ARTY BOY · **Posted:** Wed Feb 08, 2006 2:26 am

Any time you allow an engine to breath better you will increase HP, torque, and fuel economy. Flowmaster makes a KJ cat-back kit as well as Borla. Both are high quality stuff (I have a Borla system on my 86 Chevy pickup and I did notice HP and mpg increases). The question is comes down to whether its worth it $$$ wise. Is a $800-1,000 kit going to net that much in fuel savings? Only if you plan on driving your vehicle into the ground over 20 years.

MikeD0106 · **Posted:** Wed Feb 08, 2006 2:37 am

I'm planning to purchase the Borla dual exhaust setup. I like having the symmetrical look as well as the HP/MPG benefit. I'm just one of those people that the mod has to look good on my KJ as well as serve a purpose. If it does both then 9 times out of 10 it's worth it in the long run. IMO

dano104 · **Posted:** Sat Feb 11, 2006 6:08 pm

Lets not forget that some backpressure is actually a good thing, albeit even necessary. If you reduce the backpressure too much you will actually lose HP/MPG/Torque, and all the other good things.

ManicMechanicJoe · **Posted:** Sat Feb 11, 2006 11:31 pm

if you happen to own a turbo you don't need any back pressure....

dano104 · **Posted:** Sun Feb 12, 2006 12:51 am

Yeah you got me on that one

USAFCOP · **Posted:** Sun Feb 12, 2006 1:02 am

Too get the best results from an exhaust system, you need to make your intake better. Thast is the idea behind the cold air induction and other air intake systems. For us we have the drop in K&N or the cone type intakes, and then you have various throttle body spacers to help add turbulence to the air going into the injector cavity to mix with the fuel.

I installed the Flowmaster 40 and the K&N drop in filter and I had increased power at the wheels and an increase in MPG of 1-2 MPG on the highway, the city MPG had practically no change.

Guest · **Posted:** Sun Feb 12, 2006 6:15 am

dano104 wrote:

Lets not forget that some backpressure is actually a good thing, albeit even necessary. If you reduce the backpressure too much you will actually lose HP/MPG/Torque, and all the other good things.

backpressure is bad, end of story.

where that myth came from is going to too big of an exhaust will decrease your engine's efficiency, which is 100% true, but it's because of the velocity of the exhaust gases, NOT backpressure.

when the pipe gets too big, gases start to kind of swirl around and fight each other. that's what decreases efficiency, not lack of backpressure

ManicMechanicJoe · **Posted:** Sun Feb 12, 2006 8:20 am

my instructor (25 year veteran ASE certified mechanic) at night school said the same thing. He said the optimum setup would be something in the neighborhood of 2-3 inches for most V6s with no back pressure. Too much back pressure will kill an engine.

A cross over pipe (x or h style) is supposed to help gases leave the exhaust faster, thus improving efficiency. This would require a true dual exhaust from the manifolds (or hopefully soon, headers) to the tail pipe.

Mandrel bends (as opposed to crush bends) help the exhaust flow smoothly allowing them to leave the engine faster and reducing back pressure. I believe most OEM exhausts (KJ included) use crush bends.

A slightly larger muffler will help but again it cannot be too big. Most aftermarket mufflers are designed for a specific style of vehicle. The Flowmaster 40 is designed for V6 engines whereas the Flowmaster 50 (used in Rusty's Exhaust kit) is designed for SUVs and full size trucks.

Then we get into which material is best to make the exhaust out of. Aluminized steel is cheap but will rust. Pure aluminum would shed heat but it is a little soft and may damage easily. Stainless steel is tough and will last forever (just about). Titanium is light and strong. It doesn't rust and it will shed heat...

Building a perfect exhaust requires lots of R&D time on a dyno. It requires time and experience and is not something that should be messed with. Either buy an aftermarket kit or just replace the muffler and cat for best results.

Jeepjeepster · **Posted:** Sun Feb 12, 2006 9:43 am

The kj's exhaust IS Mandrel bent. You do not need to get a cat back, waste of your money. A muffler is all you need, unless youre doing duals.

Does exhaust increase MPG?

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