Fabricating an Exhaust for a 1985 Pontiac Fiero 3800
The available space for the exhaust on the Fiero is quite limited, most engine swaps end up with some form of modification to the trunk to make some room. On my car, I wanted to do something that I hadn’t seen on any other Fiero, a single exhaust outlet in the middle of the rear bumper, like on the Porsche Boxster.
I would not recommend fabricating exhaust manifolds/headers as it is quite involved to do it properly (e.g. each header pipe is supposed to be exactly the same length), requires special tools/methods to make and they are usually relatively easy to obtain. This article assumes that you already have manifolds (or headers) with the connecting (J) pipes if applicable, and you’ll also need the first few inches of the downpipe, or a flange that mates with the manifold for the start your exhaust.
Some exhausts have a donut type seal sandwiched between the flanges of the manifold to downpipe joint that facilitates a bit of movement to align the exhaust; if your application has a solid joint like mine, you can pick up a piece of flex tube to use instead. I got a small piece of flex tube from the local wrecker’s yard along with the rear section of a Ford F-150 exhaust where it goes over the rear axle because it provided a good length of 2″ pipe with several bends – non of which turned out to be tight enough.
The cost of a good pipe bender and limited potential for repeated use, ruled out purchasing one for me. At the time I didn’t consider renting a pipe bender because years ago a good friend of mine showed me a way to make corners out of straight pipe using the tools I already had. This method was to cut several V shape slots in the pipe that don’t go all the way through and then bend the tube so that the V slots close up before welding up the joins.
Don’t forget to incorporate a couple of hangers/mounts to support the exhaust, rather than loading up the weight of the exhaust on the manifold. If necessary, the mounts can also help with fine tuning the alignment.
Here are some general notes to consider during the design phase of your exhaust,
- keep exhaust gas flowing quickly and smoothly with minimum restriction/disruption
- review local emissions and noise laws – will you need a catalytic converter for example?
- as with everything, try to minimize weight
- remember that the exhaust is likely to get hot – consider the impact of heat on the surrounding areas
A Finished Exhaust
I bought my tailpipes used, I got two of them and was told that they were from an F-150. They came with a muffler as well but it was large and heavy and was a 1 in, 2 out design so I couldn’t use it. The muffler I used was found in a wrecker’s yard and was slightly smaller diameter than the 2″ pipe I was using, that’s why there is a step down in the exhaust just before the muffler. It’s not ideal but for a given pressure drop, the smaller pipe will be a little more restrictive while increasing the velocity of the flow and thus reducing dwell time. To be honest, at this point I was a bit fed up of working on the exhaust so I decided to just run with it and change it out later if it was no good. The fabrication process of the exhaust involved a great deal of welding, so much that I went through a whole grinding wheel cleaning all the welds up!
A common and easy way to modify the trunk is to simply cut the bottom half off at the ledge about half way down and then simply sit a sheet of steel on the ledge and weld it in place. Obviously the carpet will also need cutting to fit back in the smaller trunk. We have also seen applications where holes are cut into the wall of the trunk and the exhaust has been run through it so that when you pop the trunk, you can see the exhaust right there.
On my application, since I wanted a single exhaust exiting in the middle of the rear bumper, I made a channel in the bottom of the trunk for the exhaust to pass through with plenty of ground clearance. There is a bit of clearance on either side of the trunk, behind the rear wheels; with a dual exhaust exiting on each side, it might be possible to run the exhausts without modifying the trunk at all. The picture below shows the channel in the underside of the trunk.
The trunk sheet metal is fairly thin and likely to have a bit of corrosion, take the time to do a good job of preparing the area to minimize blowing through when welding. I was able to make most of the channel using the plate that was already in the trunk by making some strategic cuts and then bending the plate into position. Once the channel was made, I sealed the underside with some rubberized paint; there was a concern that the heat from the exhaust might melt it, but it has been fine so far and I’ve driven on some hot summer days. I was expecting a good deal of aggravation in getting the tip lined up, but I took my time, used lots of tack welds and offered it up numerous times to check alignment. Overall, it went surprisingly well and I really like how the end result looks on the car.