DIY Borla CAT back + aftermarket Davidco CAT

[brokencase]

Background:

When I purchased my Scorpio back in late 2015 I had discovered that the previous owner had replaced the exhaust system from the CAT back by going to some muffler shop and had something custom fabricated.

It was ordinary steel with those cheap mandrel bends that are very restrictive (as you will see in the photos that follow).

The muffler was rusted out on the bottom seam, so in early 2016 I replaced that with a Maganflow 9"x14". This was in 409 stainless steel.

After coming back from Carlisle this year I noticed that the exhaust sound was louder than normal. Sure enough the pipe had rotted out just before entry into the muffler.

Dang...and PA inspection due in June. Gotta do something.

So, I really missed the Borla CAT back setup I had in my previous Scorpio. As it turns out my brother had purchased one of these TIG welders:https://www.ebay.com/itm/273154563738

I figured I would bite the bullet and try my hand at TIG welding stainless exhaust pipe. I figured it would be a nice skill to master.

I ordered the following items:

https://www.amazon.com/gp/product/B07BF4K5VK
https://www.amazon.com/gp/product/B001KZYT4C
https://www.amazon.com/gp/product/B0006HMBYU

I also decided to install a new aftermarket CAT made by DAvidco and sold through Catalyticconverter.net
$124 plus free shipping. Not too bad.

I then had to buy 1/16 308L stainless filler wire ($35), 1/16 tungsten electrodes ($8) , and some alternate ceramic cup nozzles ($6) from local welding suppliers in my area.

The nozzle that comes with the welder is too small for exhaust pipe work.

I have a MIG welder and I used it's argon tank for this work.

[brokencase]

First let me talk about the Davidco CAT. I recall someone else on this forum installing this unit and claiming that they believed it flowed better than the stock CAT.

This may or may not be the case. But I am a little disappointed with the fitment. The holes on the forward flange were of the wrong spacing (too narrow). This necessitated that I cut off the edges off the flange, and use large washers to get it all to work. I also cut a 1/2" long section of the stainless pipe that fit on the inside of the joining pipes to act as a locating ring to keep the flanges aligned.

The Davidco gaskets provided were worthless so I reused the existing flange gasket with liberal exhaust putty.

Other than that the cat fit reasonably well. Can't really complaint because the $124 price was pretty good.

There was probably nothing wrong with my original CAT other than being 35 years old. The Davidco CAT is probably the last aftermarket CAT ever to be available and supplies are drying up. Rockauto used to have them and now they don't. There was a Davidco up on ebay going for $195.

davidco2.jpg (111.38 KiB) Viewed 4090 times

davidco1.jpg (87.9 KiB) Viewed 4090 times

The old Scorpio CAT was worth $20 from my local metal recycler. I think they ripped me off. I think it was worth more than that...maybe $45 at least!

[PeterP11c]

Impressive list. Over time, I replaced 3 sets of mufflers. Had them done at the shop and so far so good. It helps that we have dry, warm weather so it reduces the rusting process quite a bit.

[brokencase]

Here is the old system removed. As you can see it was very restrictive with the lousy mandrel bends reveled in photos.

If you do the research the 2" pipe diameter is a little undersized for the power level of the Scorpio. It really should have at least 2 1/4" pipe.

But since the CAT is 2" you don't really have the option.

In hindsight I think a better approach would have been to do the "full monty". That is to install Ranger headers into a 2 into1 universal CAT that exits to a 2-1/4" pipe. Maybe some other day!

oldexhaust2.jpg (164.78 KiB) Viewed 4088 times

oldexhaust1.jpg (164.14 KiB) Viewed 4088 times

[brokencase]

The Borla Muffler is 304 stainless and a "straight through" glass pack design. It is "center inlet/offset outlet" to match the original Scorpio muffler.

The Bozo muffler shop only had offset/offset when they made their system. This makes inlet routing more difficult. For this reason I threw away my Magnaflow replacement. In addition, the Magnaflow is 409 stainless...which is junk IMHO. I'll never buy 409 stainless anything again.

I had to weld on a 1/4" 304 stainless mounting hook. I happen to pick up a bunch of stainless rod they were throwing away at work.

borlamuf1.jpg (140.07 KiB) Viewed 4088 times

[PeterP11c]

Nice work. So, recapping you suggest 2 1/4" diameter for the entire exhaust piping?

[brokencase]

I only used about half of the pipe kit. I am left over with a long straight and two U- pipes and one 90 degree bend.
I actually cut a few inch sections from one of the U-bends to practice TIG welding on.

I used one 90 degree bend and the two 45 degree bends along with the straight.

But when you are jumping into this you don't know what you are going to need to get the job done. I'm not sure you will save anything by buying only the pieces you need individually.

The process was to fit a a 90 degree to the front of the muffler and work my way forward with the two 45 degree bends. I had to cut the 45 bends by a few inches each to get a straight shot forward to the CAT exit. Everything was fitted under the car and then the pipes were tack welded into place. One set the pipes were removed and complete welding was done someplace more accommodating.

The TIG welding went very easy. I just followed the advice of some YouTube videos. Set the welder to around 38 amps. Used a wider nozzle.
I never blew through the pipe. My welds were not the cleanest, but they improved with each iteration. TIG welding stainless is much like brazing or soldering. I am glad I used TIG instead of attempting MIG.

For ease of disassembly I did not weld the slip joint between the long straight and the 45 bends. This would get fastened with a 2" clamp.

The original Borla Scorpio CAT back system would shift over to the passenger side where the resonator was in order to utilize the factory exhaust hanger on that side.

I opted to go straight back from the CAT under the diff, thus eliminating extra bends. However this required modifying an off the shelf exhaust hanger from Advanced Auto such that it would bolt to an existing hole in the differential mount. This hanger was then "unbent" and a hole was drilled that allowed it to bolt to a 2" pipe clamp.

Although the underside picture is deceiving, it hangs no lower than the CAT and is no lower than the original system. Ground clearance is fine.

The prior muffler shop had welded to the body a hanger just in front of the muffler. I cut this off when I removed the original exhaust. However I left enough of it to allow me to bolt another new hanger to it.

$49 might seem like a lot of money for a Borla muffler tip. But after all this effort I wanted that "Borla Look". However I was astounded to hear the muffling effect that Borla tip provides. It really takes the raspy edge off the sound of the exhaust.

One other item. I had no flare for the ball exit of the CAT, so I cut the steel one off the original system. Cleaned it up and welded it onto the straight stainless pipe. I should have ordered a stainless flare, but I was out of time. Need to get the car inspected. I can always change this later.

Wish me luck.

pipeswelded.jpg (266.4 KiB) Viewed 4086 times

muffshot1.jpg (116.35 KiB) Viewed 4086 times

undercar1.jpg (118.6 KiB) Viewed 4086 times

[brokencase]

Nice work. So, recapping you suggest 2 1/4" diameter for the entire exhaust piping?

So Peter..if you look here.
https://www.exhaustvideos.com/faq/how-t ... -diameter/

You will see that max hp for 2" pipe is 144. That is what a Scorpio makes stock.
It would be nice to get a little more flow...Right?

But the Scorpio CAT is 2" pipe, so it you won't see real any improvement by fitting 2.25" pipe after the CAT.

So for a Scorpio exhaust system to really provide some horse power gains you have to go "full monty".

That is to install/modify Ranger headers and run them down to a 2 into 1 universal CAT that then exits into 2.25"
Then do everything I did but with 2.25" pipe and muffler.

The other thing you mentioned about climate is meaningless. Exhaust pipe rots from the inside out.

[brokencase]

I guess I should add my post install impressions...

Yes I definitely feel a performance improvement, especially over 2000 rpm. But I suspect this is only because my previous setup was so lousy and restrictive.

It sounds just like the original Borla CAT back that was offered back in the 90's.

With this experience now under my belt, I will never buy an aftermarket exhaust system ever again. 304 stainless is the only way to go.

[brokencase]

Some may be wondering what the original Borla CAT back system offered back in the 90's looked like.
Here is a picture comparing the original Scorpio system with the Borla.

exhaustcomp.jpg (39.66 KiB) Viewed 4078 times

As you can infer, my setup is a straighter and shorter design.

[PeterP11c]

I am impressed. The rear muffler looks cool and yes Scorpio has 144hp. The original exhaust system carried 2 mufflers and one catalytic converter. Did you keep the same configuration? From the picture it looks like you cut down one. What about the biennial smog test? Can you pass one with this system? Good luck. Keep posted how it works after completion. Curious to find out if 2 1/4" gets you more power.

[brokencase]

The original resonator is just to make things quieter. It does not impact emissions.

I don't plan on going to 2.25" any time soon.

[PeterP11c]

Just to be the devil's advocate. Is it possible to install dual exhaust on Scorpio? I know present configuration is what we are used to, but using 2" pipes and dual muffler will raise the limit to 185hp as per the chart you provided.

[brokencase]

You could but then you would have to run two catalytic converters. But you only have one O2 sensor.
What are you going to do? Connect it to only one side of the dual system?
You will also have the much larger difficulty of routing the two pipes to the back of the car and fitting two separate mufflers.
I don't think it will get you very much for the amount of effort and cost involved. The system would also add appreciable weight.

The solution is to use a universal CAT like this...since it has the single O2 sensor fitting on the dual inlet side.

The real horsepower benefit would come about by having headers going into this dual inlet CAT. The Scorpio stock cast iron manifolds are really lousy.

If my Y-pipe ever rots away I will be forced into this approach.

dualinletcat.jpg (16.25 KiB) Viewed 4066 times

[PeterP11c]

I let my imagination run wild for a moment and forgot we are dealing with old car design and old engines. Original cast iron manifold look old too and gave me trouble with gasket wearing out prematurely. So, I'd rather keep it nice and easy and not modify the car too much.