A few years back I had replaced the stock exhaust with DeLorean Performance Industries' SPEC I exhaust. It is a high-quality stainless steel free flowing performance exhaust. What this means is that the exhaust gases can exit the engine as quickly as possible. This reduces the back pressure that normally resists the piston's upward movement on the compression stroke, instead transferring that energy to the transmission.
In retrospect, I should have realized that "free flowing" also means "really loud". Some people like loud cars; I'm not one of them. I'd prefer a quiet hum, but an electric conversion isn't something I want to do just yet. I got an offer to quiet it down (mostly by stuffing it with stainless steel wool), but after waiting for over a year and a half for it to come back (no money changed hands, thankfully), I decided to just source my own.
I still had my DPI headers, which got around the hardest part of the installation: actually mating it to the engine. These have standard three-bolt flanges on them that are easy to source.
I wanted to keep it street legal, emissions-friendly and quiet. My first plan was to install catalytic converters on each side, with each going up to a resonator, through an "X" style cross pipe, then down the other side through a Borla Pro XS muffler to the tailpipe. I even blocked it out with PVC pipe, but once I actually go the parts I quickly found that there simply wasn't enough space to even just the two mufflers without the resonators.
I would up switching with a much simpler implementation that is closer to the original DeLorean and other aftermarket setups, such as the current DeLorean stainless exhaust and the DPI SPEC I exhaust. This included a Borla Pro XS muffler, two Walker catalytic converters, and two Car Chemistry silencer inserts.
The DPI headers were for 2.5" pipes, so I built my exhaust with 2.5" parts as well. I also went with stainless steel for everything that I didn't have to weld to.
The catalytic converters are Walker 15028 EPA certified units. They are 50 state certified, meaning that they support California emissions requirements, and therefore the requirements of all other states that use those specifications. I chose these partly for that reason, but also for their relatively small size. Smaller cats can be found, but they are not EPA certified like this, or are much more expensive racing cats. Having cats installed should also reduce the noise from the engine a bit, although not as much as the muffler will.
In the end, the limiting factor in fitting the exhaust system was the catalytic converters. These just barely fit, with the final installation having mere millimeters of space to work with. In retrospect, I likely could have saved a lot of trouble if I could have found some smaller 50 state certified cats, but my searches came up fruitless.
For the muffler, I got a dual in/dual out Borla Pro XS 400286. I chose the Borla because of some tests that showed it to be the quietest of the mufflers out of a range of aftermarket options. It is a free flow muffler, but pretty much all aftermarket mufflers are. The catch with this Borla is that it's designed to have the engine exhaust enter from the same side of the muffler. This makes sense in a conventional engine/exhaust layout where the engine is in the front and the exhaust is in the back, but the rear engine design of the DeLorean makes it difficult to use it this way.
To compensate for this, I did a little extra pipework so that when the exhaust is mounted horizontally behind the engine, the driver's side engine exhaust would enter the bottom of the muffler, exiting out the bottom of the other side and leaving through the tailpipe. The passenger side would enter through the top of the muffler, existing through the top of the other side before turning down to the tailpipe.
Another trick with this muffler is that it has is a cross-flow muffler, meaning the top and bottom paths are linked with a hole between. This reduces droning and quiets the exhaust a bit more. However, it was designed so that the gases for both paths are moving the same direction. With my mounting, the gasses are moving in opposite directions, which will lead to some extra turbulence between the two chambers. I'm not overly concerned by this, as majority of the gas flow will be straight, and it might even further reduce the engine noise.
To make this even quieter, I ordered a pair of three disc Car Chemistry silencer inserts. These introduce restriction into the exhaust, breaking up the exhaust flow. The claim is that they don't affect power, but honestly, my goal is to reduce the noise over everything else, and I'm not overly concerned about having a huge amount of power. My car is an automatic, so it can only get so much power anyway. I'd rather have a car I enjoy driving while being able to have a conversation at cruising speed. The three disc model is meant for street applications and further reduces the volume over the one and two disc models.
The CC inserts come with two caps, one with a hole and one without. The idea is that you can tack weld one or both onto each end to further reduce the noise (I did both). On top of that, you can pack the space between the discs with stainless steel wool to even further dampen the noise, which I did as well.
Reviews of these suggest that these can reduce noise levels by anywhere from 5 dB to 20 dB, although I don't really expect to see the high end of that. Every 10 dB reduction halves the amount of sound you hear. All told, I'm hoping for a total noise level of 70 dB at 50 feet, which is about the same as a diesel truck. Ideally, I'd be at 60 dB or lower, but I'm not sure I'll be able to do that. The Borla mufflers are reportedly about 80 dB at idle without cats on a Mustang, so hopefully I can get somewhere near that.
The inserts are painted black, but the paint burns off during normal use, and is just to protect them during storage. The paint also made the inserts just a little too wide to fit in the pipe, but a quick hit with the bench grinder solved that problem (sand paper would have worked just as well, I imagine). The insert instructions specifically mention that the inserts might have to be ground down to fit.
The inserts can either be welded in place or riveted. I went with rivets, buying an inexpensive blind rivet gun from Amazon and some stainless steel closed-end dome rivets from McMaster-Carr. Be sure to get closed rivets, as open rivets have a hole in the center that will leak exhaust gasses and create more noise..
I bought 2.5" stainless steel exhaust pipes from the cats to the muffler, and from the muffler to the tailpipes. Most of these were 90 degree mandrel bends with a 2.5" bend radius. The tight bend radius wound up being critical for the 90 degree bend from the headers to the cats, and another 90 degrees from the cats to the muffler. A 4" radius increased length just enough that there wasn't enough room to reach the muffler. The mandrel bends are just a higher-quality bend that doesn't compress the pipe. In addition to the 90 degree pipes, I had one 45 degree bend from the driver's side of the muffler to the tailpipe.
For mating the cats to the engine, I used 90 degree aluminized steel pipes with 2.5" bend radii. Here I used steel only because I needed to weld on a three-bolt flange to attach it to the headers, and I couldn't find any pre-installed. It seems that it's tricky to weld stainless, as the backside tends to oxidize unless you take special precautions.
Rather conveniently, the standard seems to be for the pipes have a 2.5" outer dimeter (OD), while the cats and mufflers have a 2.5" inner diameter (ID). This means that you can easily slide the pipes into the other parts.
Welding vs. Clamping
Ideally, I'd have welded the pipes to the the cats and mufflers, and bent the pipes to ideal angles. However, the equipment to bend pipes is very expensive, and other techniques (such as heating and bending pipes) requires a lot of experience to pull off properly. A muffler shop could have done it, but I wanted to see what I could do on my own. And as mentioned above, welding is stainless steel tricky enough that I didn't want to risk it.
So I went with pre-bent pipes and clamps, specifically:
- Lap clamps, to secure the 2.5" OD pipes to the 2.5" ID muffler joints.
- Butt clamps, to join two 2.5" OD pipes together, mostly for the driver's side tail pipe.
- 1" band clamps for tight spaces, notably for the very short space from the 2.5" OD pipes to the 2.5" ID catalytic converter joints.
The clamps are all stainless steel.
I needed two hangers to hold the muffler up. I bought two universal hangers from JEGS that clamp over the pipe. Again, welding would have been ideal, but there's the stainless steel issue again. I also bought two silicone hanger inserts from JEGS for a few dollars each.
The idea is that the the hangers' rods would run through the silicone inserts mounted on brackets attached to the engine block. I was able to reuse the passenger side exhaust bracket from the DPI exhaust, after fabricating a mount for the silicone insert. I could have bought one from DeLorean, but they were $30 each, and that seemed a little steep when I was already fabricating a bunch of other things anyway.
For the driver's side, I first tried to modify the DPI bracket, but the hanger stuck out just a bit too far for it to work. I wound up ordering some 1/8" stainless steel stock from McMaster-Carr and made my own, using the original as a template. I cut it down to size with an angle grinder, and drilled the mounting holes with progressively larger drill bits until I had the hanger I needed.
The purpose of the silicone inserts (or the rubber ones, if you use those from the DeLorean) is to vibration isolate the engine and the exhaust. This further reduces muffler noise and keeps vibrations from the exhaust from resonating into the cabin.
For the stainless parts, I left them as-is. For the steel pipes and flanges, I spray painted them with high-temperature black paint for corrosion resistance.
Along with the usual set of wrenches, I needed a few new tools:
- A chop saw, to cut the pipes down to size. It's not a good idea to put a chop blade in a wood saw, as they aren't designed to handle the metal shavings. I bought an inexpensive saw for about $100 at Harbor Freight.
- A pipe expander. This is a small tool that can be used to work around problems where the slip-fit pipes won't fit in the ends of the muffler or cats. You will not be able to use an inexpensive pipe expander to expand one pipe so it can fit in another pipe; those expanders are far more expensive. The one I got was about $30 on Amazon, and was extremely useful for situations where I couldn't QUITE get the pipe to fit in the muffler or cat. To expand the ends of the cats, I needed to cut the bolt at the end of the expander to avoid damaging the honeycomb of the cat.
- Bench grinder. I already had one, and it's great for getting rid of burs after cutting pipes.
The first thing I did was make sure there was enough room for the cat and its piping, specifically with regards to the space between the cats and the rear fascia. For the muffler end, I used a 90 degree 2.5" bend radius stainless steel pipe for one end of the cat, cutting it as short as possible with a 1" band clamp, as there wasn't enough space to fit the longer lap clamps. A similar pipe was used on the other end, but steel, as I needed to weld a flange onto it for the headers. I started by only cutting the cat ends of the pipe, leaving the other ends long until I could get the fit figured out.
Cutting the Pipes
The chop saw tended to leave two two burs on the cut, one on the inside of the top part of the pipe where the saw pushed down on it, and another coming off the bottom part of the pipe. The bottom one was easy to grind off with the bench grinder. The top one was trickier, since it was inside the pipe. At first I used a drill with a wire brush to bend the burs back out, then grinding them off with the bench grinder again. Later I switched to a Dremel with a grinding bit, which let me remove the bur in one go.
I wound up having to cut down the ends of the cats pipes to further reduce how much space was needed for installation. After cutting, the ends had deformed slightly, which made it difficult to fit the pipe into it. This is where the pipe extender came in handy -- it easily returned the end to round, and let the exhaust pipe slip in without any problems.
Fitting and Mounting the Muffler
To figure out the lengths of the other ends of the pipes, I first needed to mount the muffler to the engine. On the passenger side I was able to reuse the passenger side muffler bracket from the DPI exhaust with a custom mount to hold the silicone insert. I had to fabricate a new fabricated a new driver's side bracket with an piece of stainless steel sheet stock, angle grinder, and a power drill. Once installed, I had the final mounting position of the muffler.
A side effect of using these 90 degree bends is that the cats run under the frame and the fiberglass body. If I could have found some pipes with 125 degree and 45 degree bends around a 2.5" radius, I might have been able to avoid the body and saved another fractions of an inch of space, but I wasn't able to find anything appropriate. Getting pipes bent for this purpose was another option, and is what the aftermarket exhausts like DPI's SPEC-I do. Because I didn't have that luxury, I had to worry about clearance issues, which resulted in the muffler sitting a couple of inches lower than would have been ideal.
Next I mounted the muffler to the engine. I clamped the hangers to the muffler, and attached my fabricated brackets to the engine. The rods from the muffler slipped through the silicone inserts on the brackets and held the muffler in place. I used extra 3/8" stainless nuts and washers to keep everything in place.
This revealed that that my mounting brackets weren't perfectly designed, as the driver's side of the muffler sat a bit lower than the passenger side. I was able to work around this a little through the placement of my hanger hanger mounts, clamping one to the top pipe of the muffler facing down while the other clamped to the bottom side facing up. This still resulted in a couple of degrees of tilt, but it was the best I could do without engineering a new plate, and I was willing to live with a slightly uneven exhaust. I can always address it in the future if it's really bugging me.
Flanges and Headers
With the muffler positioned, I could figure out the lengths of the cat pipes. Since I was worried I might screw up my welds on the flanges, I did that first. Only after that was good did I cut the pipes from the cats to the muffler.
I cut the driver's side steel cat pipe as short as I could, then slid the flange onto it and test fit it in the car. The goal here was to figure out the angle that the flange needed to be welded to the pipe. I marked it with a sharpie, then tack welded it and did another test fit before doing a final welding on both sides of the flange. I used a few coats of high-temperature black spray paint to protect the steel pipe from corrosion.
This led to the next problem: the total angle between from the header to the muffler though the cat isn't exactly 180 degrees, or my pipes weren't 180 degrees. As I tightened the bolts on the header, the pipe in the muffler would slide out until it finally popped free of the muffler itself.
At first I tried to heat and bend one of the stainless steel pipes. Even with oxyacetylene I couldn't get enough of the pipe hot enough to actually bend it, and I imagine I would have just made a mess of things anyway.
In the end, I "bent" the steel pipe by cutting a wedge-shaped piece out of it, then welding it back together. Ideally, I'd have made two cuts, removing half the angle from each end of the cut. This is important because cutting a tube at an creates an elliptical cross section, so a straight cut and an angled cut would be difficult to weld. I got close, and didn't leave too much of a gap to fill when welding.
After welding the pipe back together, the final installation went smoothly and everything fit as expected. I needed to do the same for the passenger side, but I planned ahead this time -- I angled the flange itself on the end of the pipe and welded it directly. It was a bit tricky to force the flange on at an angle, but it was just enough to fit.
The tail pipe for the passenger side was easier than the driver's side, since it just needed to make a 90 degree turn out the back. The metal plate under the muffler ends just before the tailpipes, so positioning was easy too. The driver's side one was a little trickier, and it took me a while to hit on using a 90 degree bend with a 45 degree bend. This was much simpler than my original plan, which was to create a kind of "U" that wrapped around the cat pipe. This avoided all that, used far fewer parts, and let the muffler sit lower than wrapping around the cat pipe would have allowed.
After buying the tips, I noticed that the DPI exhaust and the stock exhaust simply used cut the exhaust pipes at a slant and rounded them over, rather than using any tips at all. That would have saved me $50, but I don't have a good way to round the ends of the pipes, so it wouldn't have looked quite as clean.
My original plan was to use a metal heat shield, possibly the one from my old DPI exhaust, but I quickly discovered that I had no place to really mount it. Instead, I bout some Heat Shield Products Exhaust Armor. Three metal ties secure it to the muffler and reflect heat back towards it and away from the engine. The amount of material in the kit was perfect for covering the side of the muffler towards the engine, and avoided the mounting problems of a simple metal plate, although at an increased cost.
I wrapped the heat shield around the top of the muffler and the side facing the engine, on the logic that this would help keep it from slipping down over time. I didn't have there special tool that is normally used for metal ties, but Heat Shield Products has a video showing how to bend them around tight enough that they won't come loose. I almost forgot to fold the metal part of the shield over at the edges, as per the instructions, to keep dirt from getting into the softer part of the shield that is up against the muffler. Installation was quick, and much simpler than fabricating a metal shield and figuring out mounting, and probably about the same price once you figuring in the extra metal and screws needed to mount a sheet metal shield.
I'm pretty happy with this design, although it's certainly not perfect. The final parts list is as follows:
- One (1) Borla Pro XS dual in/dual out muffler, part 400286. Also known as the Turbo XL; they're the same thing just with different branding for consumers vs. shops.
- Two (2) Walker 50 state catalytic converters, part 15028
- Two (2) Walker exhaust flanges, part 31900
- Two (2) 2.5" OD 90 degree 2.5" bend radius aluminized steel exhaust pipes
- Four (4) 2.5" OD 90 degree 2.5" bend radius stainless steel exhaust pipes
- One (1) 2.5" OD 45 degree 5" bend exhaust pipe
- Four (4) 4" long 2.5" stainless steel lap clamps
- Four (4) 1" long 1.5" stainless steel exhaust clamps
- One (1) 4" wide 2.5" stainless steel butt clamp
- Two (2) Dynomax stainless steel exhaust tips
- Two (2) Car Chemistry three-disc silencer inserts, with both sets of caps welded on
- One (1) box of stainless steel wool
- Two (2) Pypes 2.5" C-clamp style stainless steel exhaust hangers, 7" long, part HVH11S
- Two (2) Summit Racing silicone exhaust hanger grommets, part SUM-610320
- One (1) Heat Shield Products Muffler Armor kit, part 177101
- Stainless steel sheet to build the driver's side exhaust bracket
- Aluminum sheet to build the adaptor from the DPI exhaust bracket to the Summit Racing grommet
- DPI Headers and exhaust bracket, from my old exhaust
I also bought a chop saw, a hand rivet gun, and used my angle grinder, bench grinder, Dremel, drill, oxyacetylene torch, and other assorted tools to make this work. Since I knew I'd only use these tools a few times, I bought cheap ones from Harbor Freight in most cases.
I haven't turned the engine over yet, so I don't know what the sound is like. Hopefully it's not too loud. The total cost was around $800 or so, probably a bit more, especially if you include all the extra and wrong parts I bought and couldn't return.
Still there are some things that I'm not happy with:
- The muffler sits lower than I'd like, and doesn't quite sit level due to how I cut the brackets.
- The muffler sits too close to the rear heat shield, making it difficult (or impossible) to install the three central screws that hold the fascia in place. A shorter catalytic converter, or using bent pipes or a combination fo 135 degree and 45 degree bends, would have allowed the muffler to sit slightly closer to the engine.
- If I'd cut another inch off of the driver's side steel pipe, the bottom edge of the fascia near the tail pipe wouldn't touch the pipe near the driver's side cat. This problem with "fix" itself after a few runs (by melting that corner of the fascia, although I'll probably cut it off before then), and won't result in any visible cosmetic issue, since it's well under the car.
None of these are deal breakers, and are more a side effect of my inexperience, making due with the parts that I could easily research on my own, and my desire to have a primarily stainless steel exhaust while doing as little welding as possible. If I had someone bend pipes for me, or had a muffler shop weld the stainless, or I'd learned to weld stainless myself, I likely could have both dropped the price and made for a cleaner installation. I can go back later and refine this system if need be, but I'm expect that it will be fine for my purposes.