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The community here at 355nation.net urges you to please use caution and seek professional assistance when performing modifications to your vehicle. Before attempting any modification it is advised that you refer to your Colorado or Canyon service manual or contact a certified mechanic as not all GMT355 trucks are the same. The staff and the associated members are in no way responsible for any damages, injuries or other harm inflicted to your vehicle or yourself which may result in attempting these modifications. The posts and content presented on this site reflect in no way the views of 355nation.net or it’s ownership.




A 355nation How To presented by
Canyon08Z71

Project Name
Exhaust Manifold Catalytic Converter Deletion

Project Description
This write up will walk you through removing a major restriction on the I4 and I5 exhaust system. This modification has been dyno proven to unleash 10+ horsepower on the I5 motor. Depending on your proficiency and working conditions, prepare to invest 2-6 hours on this project. There are two gaskets on the part of the exhaust system you will be dealing with and both are reusable. To be safe, I would recommend ordering at least the donut gasket before the project but that is up to you. I had both gaskets on hand but did not need them; they were less than $20 for both at GM.

Skill Level
Moderate

Project Vehicle
Make: GMC
Model: Canyon
Year: 2004
Engine: 3.5L
Power windows: Yes
Sun Roof: No

Tools Needed

Mask, protective glasses, and gloves (esp. during catalyst material removal)
3/8” Drive Ratchet
3/8" Drive Shallow and Deep Socket Sets
3/8” Drive 2” Extension
3/8" Drive Breaker bar
½” Drive Ratchet (used for collector nuts)
½” Drive Shallow and Deep Socket Sets
½” Drive Universal Joint Extension
½” Drive 8" extension
½” Drive 4" extension
½” Drive Breaker bar
8mm Socket
10mm Socket (deep and shallow)
13mm Socket (deep and shallow)
15mm Socket (deep and shallow)
Standard and Metric Wrench Sets
Wire wheel or Dremel with sanding disc
7/8” open end wrench or O2 socket
2’ Pry bar
2’ Flat head screwdriver
Air compressor w/ high-pressure output line/fitting attachment
Anti-seeze


Project Time
2-6 hours

Project Cost
Effort, Possbily Small Materials/Tools Cost


Dyno graphs courtesy of JLAUDIO29.


Before:


After:



Exhaust Manifold Catalytic Converter Deletion




Manifold Removal





1. Disconnect MAF and O2 sensors. (Unclip harness)
2. Remove intake and intake heat shield and all associated equipment. (8mm,10mm,13mm deep/shallow for K&N FIPK)
3. Remove transmission dipstick. Unlock and pull the dipstick 3 inches out of the tube before removing from transmission. (1- 10mm nut)
4. Remove manifold heat shield. (4 – 10mm nuts)



5. Loosen and detach manifold collector, remove donut gasket. (3 -15mm nuts)






a. A universal joint socket will be required to get at the top nut on the collector due to the front drive shaft interference. Other option is to remove the front drive shaft.






6. Loosen and remove manifold bolts. (9 - 13mm bolts)
a. Secure manifold with your hand during removal of last bolt to prevent it from falling.
7. Remove manifold from truck.








8. Remove O2 sensor from manifold. (7/8” open end wrench or O2 sensor socket)
9. Place manifold in a vice or work on a bench to remove the catalyst material inside the manifold. (2’ pry bar and 2’ flathead screwdriver will be the tools of choice)
a. Use the 2’ pry bar to get into the manifold through the collector to bust up the catalyst material. Use the 2’ flat head screwdriver to bust up the top end of the catalyst material.




10. Once complete with the catalyst material removal. Use compressed air to clean out the manifold of debris left behind.
11. Take a wire wheel to the manifold (where it mates to the cylinder head) and clean it up so the gasket can seat properly.
12. Use compressed air to clean up and debris left behind inside the manifold.
13. Install O2 sensor back onto manifold.

OPTIONAL:

When the manifold is off the truck you may decide to port the exhaust manifold.

A trip to the machine shop or some time with a dremel will work to widen the exhaust ports on the manifold.

See pic:
Picture Courtesy of Supermodulation




Manifold Installation


1. Set manifold down into place by the cylinder head.
2. Roll under truck and install donut gasket in the collector and assemble collector and start all three nuts onto the studs. (apply some anti seeze to threads)
3. Move back up to the engine bay and get manifold gasket in hand. Align the manifold and gasket to the head.
4. Install all manifold bolts by hand and hand tighten them all in. (GM recommends a drop of thread locker on the threads. I did NOT use it. I lightly coated them with anti seeze)
a. Verify gasket seated properly and ensure no bolts are going in cross-threaded.
5. Tighten all manifold bolts, using the GM recommended torque patter and torque specs. (See picture for sequence. 15 lb ft GM spec)

I5:


6. Roll back under the truck and tighten manifold collector nuts. (37 lb ft GM spec).
7. Install manifold heat shield (snake O2 sensor wiring back through hole in shield) and tighten down. (18 lb in GM spec) Reconnect O2 sensor wiring harness.
8. Install transmission dipstick tube grommet (if it came out with dipstick) into the transmission. (see pic) Release and pull the transmission dipstick back 3 inches and install the transmission dipstick back into the transmission. (this will require you place it in the general area from up top and finish the rest underneath) Tighten the transmission dipstick bracket down. (111 lb in GM spec)



9. Install air intake and associated components. Reconnect MAF sensor wiring harness.



---------- 2007 3.7L added coverage --------------


Primary Cat Deletion (Gutting) - '07 3.7L Motor
Courtesy SteveOZ71 (2007 3.7L Test)
Tools required: Same as 3.5L

O2 Sensor Non Fouler:
http://www.355nation.net/forum/how-...talytic-converter-deletion-10.html#post604260

There really is no need to go into any great detail here, as the installation is pretty much identical to the 3.5L, with only a few differences...so, no point in doing a totally separate "how to" for the 3.7's. I'll just try to mention the points that are necessary.

The most important is the secondary O2 sensor issue. On the '07's, this sensor monitors the primary cat function/efficiency, so you'll have to either relocate it downstream of the secondary cat, so it will have a "job" to do and thus, not throw a cat inefficiency code. You could also disable it with HP Tuners, if you have access to it. Now, if you go the PcmForLess route, just make sure to let them know you're defeating the cat, and they'll make sure it's disabled as part of your tune so you won't get any codes. I had an O2 bung welded in after the secondary cat when I replaced the muffler so that I could relocate the sensor and then use the original bung for my LM1 wideband. For now, though, we just disabled the O2 sensor with HPT.

The 3.7L's have a couple more emissions components than the pre-07 trucks to meet the '07 standards. The air injection system (A.I.R.), to the best of my limited knowledge, pumps air from the airbox through a carbon(?) canister and then injects it into the exhaust manifold via a solenoid valve, which lowers NOX, hydrocarbons, etc. Someone else here may have a better explanation of this system. This valve will need to be removed after removing the dipstick tube, as the tube bracket mounts to the left-hand stud for the valve. Pre-'07 models simply have a blank-off plate where this valve mounts. The only other thing is that the cat is a bit larger than the 3.5's, according to XHR Customs, and after seeing comparison pics-- it definately is. So it may take a bit longer removing the material on the '07+ trucks. I'm really not sure if GM up'd the cat sizing for the 2.9L, or not....haven't seen a comparo. For me, removing the catylist material was the most time-consuming part of the project.

I recommend wearing a mask as long as you can stand it. It was in the upper 90's when me and my brother did mine, so the mask was used on and off. I also recommend long sleeves, mainly due to the fiberglass you'll be dealing with. I tried not to have any direct contact with the material, but still managed to break out with a nasty rash on my left arm after gutting the cat.

The results: On a scale of 1 - 10.....I have to give it a 10, simply because it's a free mod (other than a little time and sweat) and the performance increase is definitely there, as shown with JLAUDIO29 's dyno sheets. I ran the truck hard for a few runs directly after this job and then loaded the new tune, as I wanted to see the difference of just this mod w/o adding any tuning. The truck ran noticeably harder throughout the entire rpm range. After we loaded the tune, I took her for a few more hard runs. I am very pleased with the result.

The only real downside I can think of is the weird hollow, tinny sound emitted at idle and light throttle. I immediately thought I had an exhaust leak upon firing it up the first time. Everyone seems to have this sound, described one way or another. I can't say if it's more pronounced with the 3.7's or not, since the cat is larger, and I haven't heard anyone else's truck with the 3.5 for comparison. Other than that, I'd say it simply gives the overall exhaust note a slightly deeper tone, inside and outside the cab.
 

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Discussion Starter #2 (Edited)
Emissions testing results:

I monitored O2 sensor 2 (post second catalytic converter) activity and mV levels.
Pre and post levels are steady at 750 mV, no change under load.
OBD II readiness tests for catalytic performance passed.

Good to go! :)

Exhaust sound:

The sound has greatly improved. It sounds a lot meaner on idle and especially on WOT. The decibel level does increase substantially. My worries of a "tinny" sounding exhaust are null. You can now hear the new "header" work under load but it does not sound "tinny", it sounds like a conventional header would sound under those conditions.

Performance:

This mod is noticeable but nothing that will change your life or make you think you drive an LS2.. Part throttle is improved in the under 2k range. The biggest improvements come in mid range and WOT gains.

Engine Bay heat reduction:

To be determined with further testing but I can imagine under hood temps will be reduced after losing the catalyst material that was a culprit of generating additional heat to the exhaust system.
 

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Discussion Starter #6
The JTR header is slightly different as the tubes are much longer and collect much further from the head. The power gains will be different than the stock "shorty" header. They will be more on the high end of the RPM spectrum. The JTR does remove the catalytic converter the same as this mod.

Is it worth the price tag? I am not sure. JTR proved 10 hp gain on the dyno (they did not have a tuner available when they developed it) but I believe a little more would be available post tuning. The JTR does look much better than the stock manifold if show is a factor when popping the hood.

The pro to this mod is that the exhaust retains its stock appearance which is key for states OTHER than Florida that have emissions testing visual inspections. Like my state.

The JTR header will not fit any 355 with 4x4.. So the JTR was not an option in my case. Cost is also a factor.. This mod costs nothing but your time.. JTR header cost over $400 brand new....

It all depends what you want to get out of it and how much money you are playing with....
 

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Discussion Starter #8 (Edited)
Factor in the statements below.. Looks like some fab work is required to fit the JTR also.

https://shell7.tdl.com/~jags/Pages/Parts_COL-headers.html

Installation requires modifying the heater hoses to provide clearance between the header and the heater hoses.

On trucks with automatic transmissions, the dipstick tube will need to be bent slightly, and the bracket that attaches the top of the dipstick tube to the cylinder head will have to be bent.

The exhaust system requires cutting and welding which can normally be done by most muffler shops.
 

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Discussion Starter #11 (Edited)
Great write up! :cool:

I'd do this, but am afraid of not being able to pass our very strict emissions testing. Only sure way to go about it is by buying a second header and mod that....
Thank you.

Hold off on the second manifold purchase.... Save the cash. I did purchase a second manifold because no one has posted up solid results in regards to emissions testing yet and I wanted the ability to swap back if this mod failed my inspection.

Let me do the dirty work for you. I will monitor all the sensors and ODBII readiness tests that the inspection process checks.
Allow me to put some more miles on my truck with the cat delete done and get you long term OBDII readiness test feedback. As of right now, I could pass inspection in my state and we follow California emissions standards.. Visual inspection will pass, there is no way you can tell it has been messed with by looking at it from the engine bay or from under the truck...
 

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I'm gonna try this mod like the day before our next Florida meet, should Zach be there. The reason I say that is I would like Zach to monitor mine and do any tuning necessary as was written above.

Whaddya say Zach?

NICE writeup man, props.
 

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Very nice writeup, I felt gains in the same area's as you. Do you have the part # for the donut gasket? I split mine, and haven't gone down and bought another one yet. A part # will make it easier to deal with the parts department.
 

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I'm gonna try this mod like the day before our next Florida meet, should Zach be there. The reason I say that is I would like Zach to monitor mine and do any tuning necessary as was written above.

Whaddya say Zach?

NICE writeup man, props.
I'll come help ya tear crap apart :)

The monitering i'll help to but odds are you won't see any issues. But like I said if you wanna go first i'll come up sometime and we'll tear both apart and gut em in a night or two (depending on if beers join the operation) :)
 

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Discussion Starter #16
Very nice writeup, I felt gains in the same area's as you. Do you have the part # for the donut gasket? I split mine, and haven't gone down and bought another one yet. A part # will make it easier to deal with the parts department.
It should only be like $6 or so.. Part #15105884 (have them double check that)
 

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So... I don't have a swivel socket. This I can use a 1/2" impact swivel and socket to get in there? I also only have 1/2" ratchets right now. Think it will be a problem?
 

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Discussion Starter #19 (Edited)
So... I don't have a swivel socket. This I can use a 1/2" impact swivel and socket to get in there? I also only have 1/2" ratchets right now. Think it will be a problem?
You need the univeral socket extension ONLY for the top nut on the collector. It is impossible with the body, drive shaft, and other stuff there to get on it straight. You will need an extension 6-8" also for the collector...

Just roll under and try your tools on the top nut on the collector first.. If you can get that, the "toughest" part to reach and worry is over and you can proceed with the project.

I used 3/8" drive rachet + 2" extension for the manifold bolts. 3/8" drive for the various nuts/bolts in the engine bay but yeah you could use 1/2" drive. BE VERY CAREFUL with the head bolts, 15 ft lbs of torque is not that much. :355:
 

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I'll come help ya tear crap apart :)

The monitering i'll help to but odds are you won't see any issues. But like I said if you wanna go first i'll come up sometime and we'll tear both apart and gut em in a night or two (depending on if beers join the operation) :)
That'll work. Maybe the day before the meet or something, or earlier that day. PM me what you think will work. Beer will be involved.
 
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