???Freeflow Muffler = Worse MPG ???
08-23-2008, 07:10 AM
#13
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Hes getting a lot worse than 12 mpg if he burned a whole tank in 150 miles.
08-23-2008, 07:19 AM
#15
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I didn't see what size tires your running anywhere.. What do you run?? There must be some difference in the muffler system to give you 17+ and me a sucky 12 mpg..!!
08-23-2008, 07:49 AM
#16
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100% agree. There is something else in play here. Contrary to an increasingly popular belief, modern 4-stroke engines never really "like" to have back pressure. That is, none will really operate more efficiently with impending pressure in the exhaust system.
The OEM exhaust system in the JK really isn't all that bad, frankly. The JK has mandrel bent 2.5" stainless tubing that is hard to improve upon. (Larger tubing isn't gonna help on a NA 3.8L motor and unless you want to pop for titanium, stainless is a pretty good next-best solution.
The main disadvantages of the JK's OEM exhaust system is the size, location, weight, and lastly, flow (in that order) of the OEM muffler. Replacing the muffler itself with a quality 2.5" "in" and 2.5" "out" muffler will yield benefits, albeit small.
The Cool Tech JK exhaust kit, for example, targets to address all of the OEM deficiencies, namely;
Owners choose one exhaust mod over another for various reasons. Primary factors include OEM appearance, sound, "bling", improved clearance, performance, etc. Properly installed, the performance differences alone between the aftermarket alternatives is very small and should be *weighted* accordingly in the decision process.
The OEM exhaust system in the JK really isn't all that bad, frankly. The JK has mandrel bent 2.5" stainless tubing that is hard to improve upon. (Larger tubing isn't gonna help on a NA 3.8L motor and unless you want to pop for titanium, stainless is a pretty good next-best solution.
The main disadvantages of the JK's OEM exhaust system is the size, location, weight, and lastly, flow (in that order) of the OEM muffler. Replacing the muffler itself with a quality 2.5" "in" and 2.5" "out" muffler will yield benefits, albeit small.
The Cool Tech JK exhaust kit, for example, targets to address all of the OEM deficiencies, namely;
- The muffler is replaced with a double-wall, all-stainless Magnaflow which is both significanlty lighter and better flowing than the OEM muffler.
- The effective length of the exhaust system is shortened by 3' or more. (The exhaust "exit" no longer traverses from the driver's side over to the passenger side via the muffler.)
- The muffler is relocated to underneath the Jeep and above the frame rails so the Jeep has MUCH better off-road clearances, aka departure angle.
- The result is about 3-4 HP and ~5 lb-ft TQ improvement over stock. Additional mods such as CAI, may improve power benefits but likely very modestly.
Owners choose one exhaust mod over another for various reasons. Primary factors include OEM appearance, sound, "bling", improved clearance, performance, etc. Properly installed, the performance differences alone between the aftermarket alternatives is very small and should be *weighted* accordingly in the decision process.
Wow, that is a nice write up on exhausts, I may copy and keep it
08-23-2008, 10:51 AM
#18
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I can understand if he dorve like Miss Daisy, then like Tony Stewart after the change, but not to the degree of difference he is seeing. It just seems like there should be a more technical answer to what happened.
He wants badly to replace it back to stock but like I mentioned earlier - he had it done by a muffler shop and they surely tossed it already.
08-23-2008, 11:43 AM
#19
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There's a lot to be said for exhaust scavenging, that is each exhaust pulse "pulling" the one behind it down the pipe. I've heard of guys getting worse mileage/performance after adding a big ole honkin pipe/pipes and cherry bomb muffler at the lower RPM's where Jeeps generally operate but I can't see it being a huge drop.
A few years ago I had a CJ-5 with a 304, headers, and side pipes with cherry bombs. Loud as hell and gutless at low RPM's. I added turbo mufflers and that seemed to calm my headache somewhat but it was still gutless. Then I read about proper exhaust design in a 4 wheeling mag and decided to return it to stock. Someone traded me stock manifolds for the nasty headers and I had a single pipe installed with a generic muffler. Wow what a difference. I could actually carry on a conversation and suddenly I had low end torque. I could burp the tires on pavement just by tapping the gas a little and my mileage went up some too. Suddenly there were a thousand other noises that needed to be fixed as well, now that I could hear them.
A few years ago I had a CJ-5 with a 304, headers, and side pipes with cherry bombs. Loud as hell and gutless at low RPM's. I added turbo mufflers and that seemed to calm my headache somewhat but it was still gutless. Then I read about proper exhaust design in a 4 wheeling mag and decided to return it to stock. Someone traded me stock manifolds for the nasty headers and I had a single pipe installed with a generic muffler. Wow what a difference. I could actually carry on a conversation and suddenly I had low end torque. I could burp the tires on pavement just by tapping the gas a little and my mileage went up some too. Suddenly there were a thousand other noises that needed to be fixed as well, now that I could hear them.
08-23-2008, 12:42 PM
#20
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It would be interesting to document the difference between the Magnaflow and stock setup.. If I knew that i could go back to getting 17 mpg I would go back to stock..!!!