Custom Cat-back Exhaust

Intro: Ahhh custom... I do so love making things myself. Some people call their cars custom just because they put different parts on it but a truly custom car has hand fabricated parts on it that no other car has. I suppose my cat-back is almost a Frankenstein mishmash of parts that might not look pretty but it is functional and is better than most exhausts that can be purchased. If you are interested in how and why I did it then read on.

In the Past: Naturally every car comes with a stock exhaust and in the case of the Rx-7 it is a 2.5" mild steel unit. I am convinced that car manufactures have a secret deal with muffler shops so that they won't spend a few extra dollars to make exhausts out of stainless. Anyway when I bought my car I was given the old stock exhaust, which was in fine working order. However, installed on my car was an old mild steel 3" system produced by GReddy. I actually really liked this exhaust. I guess small changes over time aren't as noticeable as catastrophic failures so as the top of my muffler rusted away I didn't notice. However one day after a race event I noticed erratic changes in tone to my car's sound. I also thought my car seemed to be getting louder over the years because I remembered a time when the sound didn't annoy me so much. I thought maybe I was becoming a sissy. Either that or I had a problem with my car so I attributed it to a blown exhaust gasket. I took my car to KD Rotary to have it checked out. Initially they agreed it was an exhaust gasket but later found that only the downpipe gasket was blown. They also informed me that my muffler had a hole in it big enough to fit your hand in to. I inspect my car regularly but missed this hole because it was on the top of the muffler instead of the bottom where I would expect to find more rust. Now that I think back I was finding random chunks of fiberglass in odd places back there that were being blown out of the ever enlarging hole. I was planning on replacing the GReddy with a custom unit in a year or two anyway so this just accelerated the project and pushed back my lingering vented hood project.

Materials: The design for a cat-back exhaust is rather simple. 3" pipe is the only acceptable choice for the 3rd gen Rx-7 except under extreme circumstances. Naturally it should be all stainless with a muffler that can withstand the heat from a rotary. I found that Magnaflow had a good reputation for this and their website's info on how they are constructed convinced me it was the right choice. They also have a wide array of sizes and a lifetime warranty. I decided on a twin tip system because of appearance and that a dual tip system can be quieter. It is more restrictive though. I went with 2 1/4" inch tips, whose combined area is greater than a single 3" tip. It took me a lot of time to pick a set of tips that I liked. I eventually decided on a double walled angle cut polished tip. Honestly I ended up with pretty much the most expensive tips out there that I came across. They are "hand crafted" polished stainless tips from Thermal R&D. Most stainless systems come with a mild steel exhaust flange for some reason but I found a stainless one from Racing Beat. For simplicity I reused the hangers from my old GReddy. Here is a list of my parts and the vendors I found with the best prices. (Prices include shipping)

The total cost of parts was 268.13, which is about half the cost of any of the reputable cat-back system available on the market. I also had to refill a tank of gas for welding and buy another spool of stainless wire. These things ran me about $30.

Construction: Just as with anything custom, construction is difficult and time consuming. I started out with two examples of exhaust systems that fit, so I used the curves as a template. Every detail was measured and considered because I couldn't fabricate the system one curve at a time with the muffler attached to my car while on a lift. Wouldn't that be nice to just put the parts on, consider an angle and then start welding? Oh well, if I was that rich I'd probably be spending my time making custom cars, not custom exhausts. So anyway I cut the pipe and laid it out as I thought would be best. I then used masking tape to hold it together for a test fit under my car. Once satisfied I took the parts to my friend's shop who owns a welding machine. A few minutes later and I had some parts together. I wasn't comfortable doing it all at once so I made several trips after additional test fittings. The angle of the flange was particularly difficult and took some modifications. The hangers also needed modified. Only the tips were easy and were angled slightly to the left to lineup with the car better.

Conclusion: I don't have any dyno charts for this cat-back but I have no reason to believe I introduced any restrictions in flow, unless that massive hole was producing horsepower. The appearance is great and is such an improvement over the boring single walled tip on my old GReddy. The sound is stellar. It is so quiet compared to a broken muffler that I can actually hear myself think. Now that is not all together a good thing considering the things I think about. Anyway it is a deep throaty sound that almost hides the menacing power that would turn any normal car's driveshaft in to a Twizzler.

Exhausted? Then go home