Magnaflow 15620 3 Inch Full Conversion

With Bullet Muffler

This page will detail the conversion of the stock 2.25 exhaust system of my 2001 Firebird to a complete three-inch system from Y pipe to tail. Thanks to all the forum members who’s shared knowledge and information on this topic made this modification so simple and affordable.

TOOLS USED: Metric sockets and wrenches, SAE deep sockets and wrenches (9/16, ½, and 7/16) hack saw, sawzall, thread cutting oil, tin snips, file, two jack stands, wheel chocks, floor jack, normal drop light but with fluorescent bulb…not prone to shock damage…cool.

The key component of this mod is obviously the catback system itself. If you have read my Basic Exhaust Overview then you already know that not all catback systems are equal in terms of actual components supplied in the kit such as including S pipe or not including S pipe, etc. This is where something like the Magnaflow # 15620 1994-97 LT1 V-8 3” catback is hard to beat. Once you replace the stock 3x2.25 cat converter with a 3/3 cat the 15620 will mate right up. The tail pipe hangers are different but that is an easy fix and the horizontal S pipe to tranny mount hanger will have to be modified but those are both very simple ‘do it yourself’ mods. Oh, yea…you will have to do something about the lack of a rear O2 sensor boss in the new system but that has an easy solution as well. Here we go.

This is what the kit looks like when it is all out of the box. I got a free Magnaflow/Carsound CAT with my order so it is in the pic as well. The included install kit comes with stainless steel u bolts clamps for all joints. This is almost everything we need for the project.

Notice the hangers on the tail pipe and cross over section. Those are different than stock ’99-02 hangers and will have to be dealt with. Also notice that there is no O2 Boss in the S pipe but that can easily be remedied.

Yep, you guessed it. My solution to the lack of a supplied O2 sensor boss in the new kit was to order a CAT that came with a rear O2 Boss. (magnaflow/carsound universal 12” oval 3”x3” no air tube #94039…not for California) You could also order an O2 sensor boss online for about $10 and have it welded in the S pipe by a muffler shop or that same muffler shop most likely has plenty of them on hand ready to go.

My solution for the tail pipe hanger issue was to use a couple of universal tail pipe hangers on each side. I removed the chassis mounted hanger support and used those stock bolts and threaded chassis holes to mount the aftermarket hangers. You could also cut off the V-8 style hangers from the new tips and weld on your old ‘Pin/hook’ type hangers and use your stock chassis mounts. Either way it is nothing to worry about. I like the range of adjustment granted by using two aftermarket hangers on each side as well as it being stronger than just one. I hate exhaust rattles. Hate em!

Once the stock 2.25” S pipe is replaced with the new 3” S pipe you will notice that the car is lacking a horizontal S pipe hanger. Don’t worry. This is easy. A picture is worth a thousand words, right? Here you are.

Hey, what is this flange on the end of my new 3” S pipe? Just cut that flange off as close to the end as possible and the new 3” CAT will slip right over it ready to be welded or clamped.

Your stock Y pipe and CAT needs to be cut as close to the weld line as possible. My 2001 Y pipe and CAT were like one single piece…no collar on the cat…just a robotic weld line right up against the CAT. Make your cut right there. You want to leave as much meat on the Y pipe as possible. Feel the top of the Y pipe…just a couple of inches from the CAT…that dimple like depression will just be a mandatory weld and fill job if you do not leave enough room on the Y pipe for a solid joint between the new CAT collar and the end of the Y pipe. If this doesn’t make sense it will when you are physically inspecting the stock exhaust. Just make your cut as close to the CAT weld line as possible. I own a sawz-all and used a 6” metal cutting blade. Butter. I would have rented one if need be or used this as an excuse to buy one. Either way…PLEASE BE VERY CAREFUL! You could severely injure yourself or your car in the blink of an eye wielding something like a sawzall.

The stock S pipe is band clamped to the mid pipe. Once that was removed I made a single cut in the over axle portion of the stock exhaust pipe in order to remove the two halves from the car. I left the muffler and tail pipe hangers on before making the cut to help keep the stock exhaust pipe in place while cutting. Be careful. There are brake lines and stuff up in that area.

The stock bolts and fasteners are metric. The U-bolt clamps are SAE…9/16 I believe.

You only need to jack the car up on the passenger side…front and rear. Two appropriately placed jack stands, parking brake set, wheel chocks on both tires…front and rear of tires as well. I also have a habit of placing a car ramp under whatever side of vehicle I will be working under as well as jack stands.

The 3” stainless u-bolt clamps supplied by Magnaflow were somewhat ‘over cut’ on the clamp portion. Basically the holes in the clamp portion were too big for the 9/16 nuts. Can you say washer?

When it comes time to start hanging the new muffler and pipes it is a little awkward but you can do it yourself. Just use your knee to support the driver’s side and hang the passenger side first.

I used tin snips to remove some material from the chassis mounted sheet metal heat shield above the CAT for better fitment of the rear O2 sensor.

Just get all the clamps snug… not super tight at first. Get all the components installed and connected and then adjust system so that nothing is rubbing or touching the frame or body if possible. The new 3” over axle pipe is a snug fit at best but if the entire system is adjusted and the tail pipes are securely mounted then there should be no vibration rubbing from the over axle pipe. Once everything seems to be well aligned go back and start tightening down the clamps. Leave the muffler/tail pipe adjustments for last. Adjust the hangers and position the ‘depth’ of tail pipe collar over muffler as needed for proper tail pipe alignment. Now that everything is on and clamped down tight it might be time for a test run. If everything is good to go then you can stick with the clamps or you can easily and affordably have the entire system welded.

Ooohhh, another tip: Cut off the excess ‘u-bolt’ ends for better fitment. I got them all pretty tight and pointing down then cut off the ends. When and if you remove the clamps the nut will self thread where you made the cut. Once the ends of the u-bolt clamps are trimmed down loosen the clamps and turn the clamps up toward the underside of the chassis so as to give more clearance under the car.

I was unable to re install the S pipe cover plate. I still have the u-bolts clamps on and that takes some room and the new 3” pipes are just too big for the plate to fit. It looks easy enough to hammer out that plate a bit to re shape it. Perhaps some spacers on the mounting holes will help add some clearance as well. Until I get the system welded and can remove the u-bolt clamps that guard plate will have to stay on my workbench.

That was all the steps that require special planning or modification. Here is a rundown from start to finish.

With the exhaust system cold, set up jack stands on front and rear passenger side. Brake set, wheels chocked. Disconnect negative battery cable. Cut off stock CAT as close to weld line of Y pipe as possible. Remove S pipe plate guard. Remove rear O2 sensor. Remove bolts from the S pipe portion of the horizontal tranny mounted exhaust hanger. Cut the over axle pipe in half…you guessed it…somewhere over the axle. Remove S pipe to mid pipe band clamp. You can use some oil on the ends of the stock tail pipe hanger pins to help get them out of the rubber hangers. Remove stock chassis hanger mounts and save bolts for use with universal hangers.

Now that the stock exhaust system has been completely gutted start off buy installing the aftermarket 3x3 cat. Get it snug tight. Next is the S pipe. (You can save installing the O2 sensor until the end) Get the mid pipe on and then the over axle pipe. Put a hanger on the end of the over axle pipe and bolt it to the chassis. You can adjust it later. Put the drivers side tail pipe and eye it out for a temp fit. Get a hanger on that driver’s side tail pipe. Ok, ready…put up that muffler…into the over axle pipe and the hang the drivers side. You can tighten the connections later…just keep the muffler supported for now. Phew! Next is the passenger side tail pipe. Once you have all the joints snug and the different components in place it is time adjust the system for clearance a good ‘no rattle’ fit. It is also a good time to trim off the ends of the u-bolt clamps. For the final torqueing of the clamps start at the Y pipe to Cat connection and work your way backwards. Keep pushing and shoving the sections to keep everything aligned well and not rubbing the underside of car, etc. (A chunk of 2/4 is handy to keep the S pipe/mid pipe from turning and sagging before final torqueing of clamps.) Install the O2 sensor. When you get to the mid pipe to over axle pipe joint…examine the muffler mounting. I ended up going back to the mid pipe/over axle pipe joint to re adjust. So now it's time for the muffler and tail pipe adjustments. Push and pull, mount some extra hangers, visually inspect tail pipe positioning while standing behind car and repeat as necessary. If everything is golden then it is time to rig up a custom mount for the horizontal S pipe hanger. Don’t skip this step if you hate rattles.

Double check that everything is on and where it should be and is TIGHT. Test drive time. If the new system is all rattles and bumps…no big deal…just adjust that stuff again. If all seems good it is recommended to re torque all the clamps after fifty miles or so. (that’s if you are not getting things welded up any time soon.)

“But, dude! It still sounds all ricey and **** up top!”

To deepen the overall tone of the engine as well as reduce some of the high-end harmonics common to smaller engines such as L36 some people choose to add an additional muffler or resonator to the system. I detail bullet mufflers on the Basic Exhaust Overview page.

I chose to incorporate an 18” Dynomax bullet into the system. It is 3x3 and it pretty much takes the place of the mid pipe provided in the Magnaflow kit. I ordered mine from www.summitracing.com for about $35. Line up the bullet muffler next to the mid pipe and mark lines on the mid pipe for cutting guides. Remember to allow 2 to 3 inches for the collars on the muffler…the muffler ends will slide OVER the mid pipe.

That pretty much covers it.

If I ever make a sound bite it will be right……..

HOME BACK TO GUIDE MAIN BASIC EXHAUST OVERVIEW