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Discussion Starter · #1 ·
Here's some hypothetical exhaust mod plans...

(Disclaimer: These hypothetical modifications are for hypothetical OFF ROAD USE ONLY. Federal Regulations prohibit actually disabling emmissions control equipent of street-driven vehicles. Cheers!)

I've edited the pic posted by AutisMDXTypeS to show my hypothetical modification plans. I'll hypothetically monitor mpg's between stages.

Stage 1: Hypothetically gut the 3rd cat. I think it's only there to get the "Ultra" part of ULEV. Without it, it hypothetically meets the regular emmissions requirement. It then oughtta flow a little better, hypothetically. MPG test hypothetically in progress.

Stage 2: Hypothetically remove the factory 3rd cat and feed a replacement splitting-muffler directly, Magnaflow 1in-2out (3"in-2.5"out). This will give better flow compared to the kludge of a splitter after the factory muffler. The splitting muffler's outs will transition to 2" flexible
stainless tubing and stepped down to the 1-5/8"OD pipes leading to the factory mufflers.

Stage 3: Replace rear mufflers with magnaflow 2" units. More flow, more noise, might be too loud for the wife? (I know my f350 with just those two mufflers is good-n-loud in WOT).

Stage 4: Install 3" magnaflow cat at location of factory 3rd cat, install two O2 sensor fittings after it and move the two after-manifold-cat sensors to that location. Then hypothetically gut the manifold-cats. Emmissions are hypothetically fine as long as there's an adequate cat in the 3rd location. A correctly-selected magnaflow cat would do the job.

Or, skip 3 and just hypothetically do 4, get a quieter ride and maximum power/efficiency for that quiet-tude.



Here's the original pic...

 

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Too many hypotheticals in this...
 

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My suggestion, just replace the 3rd cat with a straight piece of pipe. In case you dont like the sound, you can always have the shop reinstall the cat. Go from there. It will still pass emissions since it is not monitored.

Personally i wouldn't mess with the primary cats and their sensors.

Seriously, we know these wont be for off road use only. :29:
Nobody is here to judge or go after you.

Post clips of the sound if you make any changes.
 

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Discussion Starter · #5 ·
If I want the 3rd cat there, I'd use a 3" magnaflow because the factory cat housing has an output hole that's actually narrower than the input. While I'm sure the Honda engineers knew what they were doing when they chose such a narrow size tubing between the J-pipe and the mufflers, I think their goal was different than mine... they designed for minimal noise and emissions (ie ULEV) while trying not to sacrifice much power (but there's some power choke as a result, I think), but my goal is to accept a little more noise and emissions (yet still meet the basic emissions requirement) and free up some of those horses.

Here's a nifty chart of exhaust tubing and power capabilities... The cat narrows down to 2-1/4" and the pipe between it and the first muffler is narrower. That piping is rated to handle about 150HP. The MDX is supposed to put out 265HP.

How To Calculate Muffler Size and Exhaust Pipe Diameter | Exhaust Videos
 

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I went with a 2.5 inch straight pipe. I like the sound of it lol. Thought about getting some better mufflers later down the road and upgrading the piping from those to the tips.
 

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Discussion Starter · #7 ·
I'm using 3" pipe just because the muffler I've got is 3"in/2x2.5"out. This is a Magnaflow copy from Speedmotoring on ebay, half the price of Magnaflow but just as good. Speedmotoring's 1in2out only comes in this size. I used a couple of their 2.5" mufflers on my truck and they are nice build and sound.
 

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Discussion Starter · #8 ·
Well, with Stage 1 done (gut the 3rd cat), I got no obvious benefit to mpg's. It looks like the greater constriction to exhaust flow is the pipe, 1-7/8" OD, between that cat and the mid muffler.
I just completed Stage 2 (replace cat, mid pipe and mid-muffler). It sounds much better. Before, I could start the motor, and if I didn't look at the tach, I might think it didn't start and I'd crank it again, grinding the starter. Now, it's got a nice mild rumble at idle, and overall is just barely louder than stock. I'm in the process of checking mpg impact.
 

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I would leave it at that. Once you take out the mufflers in the back, you will get a good amount of noise. Much more incremental than stock to mid-muffler/cat delete. Plus, there may be drone, as well.

If you want more noise, you can look into deleting the resonator that's part of your intake.
 

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Discussion Starter · #10 ·
MPG update: Stage 2 has netted ~2mpg improvement. My baseline is the family trip to the inlaws, about 240 miles round trip. First trip mpg net was ~20. This past Thanksgiving visit I got 22.
My next modification will be converting to true dual setup, retaining factory mufflers and primary cats. I'll hack up the J-pipe and run a pair of 2" pipes all the way back to the mufflers, with a crossover pipe just before the rear differential. I think this will get a good improvement in cruising torque (and hence mpg) due to the scavenging boost. I decided this because I noticed that the aftermarket performance J-pipes available have longer paths before merging to the collector and I figure fully dual pipes might be even better.
 

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Interesting work! Will you please post some pics or videos of your setup and new setup? Maybe it will be less confusing than reading all the specs for each section of the exhaust! Thanks!!! Good luck with your project!


Sent from my iPhone using AutoGuide.com Free App
 

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anyone else do this? I would like to hear a before and after sound difference of the "Stage 2". If I cut out the cat and mid muffler and put in a straight 2.5" pipe. I wonder the difference? I dont want anything excessively loud..
 

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Do not do it ! leave mdx exhaust system stock ! Trust me you will regret it when driving down the road hearing drone noise...Unless you a ghetto person and like those kind of noise !
 

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Discussion Starter · #17 ·
anyone else do this? I would like to hear a before and after sound difference of the "Stage 2". If I cut out the cat and mid muffler and put in a straight 2.5" pipe. I wonder the difference? I dont want anything excessively loud..
Before I stated my project here, I found a vid on youtube of the midmuffler replaced with a y-pipe. It's just a little louder than stock with a little more rumble. My setup (1in-2out muffler, instead of a y-pipe, replacing the stock midmuffler) is barely louder than stock. I get no droning at any rpm, the 1in-2out muffler serves as a resonator.

It's taking a while (due to family time/money considerations) to get to the next step in my project but I've got all the parts I need at this point. I'm not going to hack up the factory J-pipe (in case I want to revert). Instead I'll make flanges and weld up my own head pipes. I'll end up with a true dual (not X-pipe, just a crossover tube) setup, retaining the factory rear mufflers. I am using a couple glasspack mufflers in line for resonators. I hate drone. (Summit has Thrush stainless glasspacks on sale for $23. That's 75% off regular.) It'll sound barely louder than stock, and give me better mpg's on the highway.
 

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I have To redo My complete exhaust piping on My X...even though I am in Puerto Rico it Was imported from Canada...salts plus snow...rusty all the way...even the chassis has presented a few rust spots...so I'll be grabbing some ideas from here to so mine and keep ya'll posted!!


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