Voxwagen
Exhaust
Exhaust
Volkswagen did many things to limit the performance of the Fox. The car was marketed as their entry level / economy car. A car that would likely be your second car. They made sure that it played second fiddle to their other cars, making it easy to up sell to the Golf and Jetta. One area of performance impedance was the exhaust system.
The stock exhaust manifold is a 4 into 1 design. It is similar in execution to the manifold used on later Rabbits. That manifold is often described as the most restrictive exhaust piece VW ever built.
The folks that use those terms to describe the Rabbit manifold must have been unaware what simply has to be the most restrictive exhaust part VW ever built. Often called the "donut", it is a gasket used on the Fox between the downpipe and the catalytic converter. This gasket effectively reduces the diameter of the exhaust to about one inch.
After the donut, the exhaust system meanders towards the rear of the car taking two 90 degree bends, and in places it is squished to ensure clearance.
Fortunately, one company still supports the small group of people interested in improving the performance of a Fox. Techtonics Tuning. They make an exhaust system which is a direct replacement of the factory exhaust yet produces more horsepower.
This was to be the first performance modification I had performed on the car and the fact that the exhaust was a direct replacement gave me peace of mind that it would fit. Fit well and not require any half assed fixes.
The first phase was the replacement of the Cat back part of the system. I also had the Cat replaced at the same time. At some point the Cat had been tack welded to the center pipe and it had to go with old exhaust.
When I fired the car up with these new parts for the first time, the new exhaust note was very pleasing to the ear. Unfortunately, pressing the accelerator did not provide the same pleasure enhancement. Upon reflection, it seems that the greatest restriction in the system is in the manifold and downpipe. Replacing the Cat back with a larger pipe cannot make up for the loss on the manifold.
Later, when the manifold and downpipe had also been replaced, the car was generating grins whenever the gas pedal was stomped. The ass dyno was satisfied for now. And the free flowing exhaust lays the base for maximizing the improvement of future mods.
Here is a picture comparing the stock Fox manifold (the Rusty one) to the Dasher Manifold (the Shiny one).
The dual outlet manifold has an area of about 9.6 square cm per outlet. Or about 19.2 square cm total. About 20% greater area.
Though the separate outlets are smaller on the dual exhaust, each one is dealing with only half the amount of exhaust.
The exhaust pulses are evenly divided between the two outlets. The gases from the engines 1-3-4-2 firing order are split in such away that they alternate between the two ports. 1 and 4 go into one port. 3 and 2 go into the other. This "spreading out" of the exhaust pulses makes them less likely to interfere with each other. The design somewhat resembles that of a true 4-2-1 exhaust header.
This was the main performance modification on the car when it was dyno tested for the first time. A stock Fox is rumoured to produce about 69 HP at the wheels. The car was measured at 82 HP in second gear. 78 HP in third. The Techtonics exhaust is good for 9-13 HP on a stock motor. At the low end that would be a 13% increase, 18% at the high end. A significant and welcome increase.
It has me thinking of getting a manifold extrude honed and port matched. I would like to find out how much more power could be extracted form the exhaust.