Archive for March 7th, 2014

Designing & Fabricating Exhaust Baffles for Hot Rod External Side Pipes

Friday, March 7th, 2014

Designing & Fabricating Exhaust Baffles for Hot Rod External Side Pipes
I thought it might be interesting to explain a bit about the exhaust system I decided on for the 1929 Ford roadster hot rod. The idea of having an external exhaust system on the hot rod appealed to me. What came to mind was to use roadster headers with external side pipes. With a bit of searching and price comparison, I decided on a set of Patriot roadster headers. Ordering a set of pre-made headers didn’t make sense as the shipping cost would be out of this world due to the size of the package. I opted for the weld up kit and ordered it for a fair price at a local hot rod shop. When I received the box, it was an amazing jigsaw puzzle. Figuring out where each pipe went took a bit of time. In the end, the headers looked great on the 29 Roadster, but I had one tiny problem. With the C4 Corvette suspension from the 1986 Corvette, stock Model A Ford wheel base, and the huge inward wheel offset, the inside of the tires where just touching the front exhaust tubes. One option to solve this problem would be to use one inch wheel spacers. Using wheel spacers would effect of dynamics and performance of the front suspension. This is something I didn’t want and coming this far with the build why not do it right? I started pricing Lakester headers and thought these would look good on my 29 Ford. Not wanting to inhale exhaust fumes, the idea of side pipes continued. This time it was a bit more of a challenge to achieve. With a lot of thought and making small models using plastic pipe I came up with a solution to incorporate the side pipes on the roadster. This will all be explained in another post. For now, lets start with something simple, designing and fabricating the exhaust baffles to fit into the 3.5” side pipes I am using on the hot rod.
Over the years I have designed and built many exhaust systems which included mufflers. Some were successful with reducing the exhaust noise level and some were not and needed a bit of massaging. This was all part of the learning experience. Having an electronics background, the theory of waveform addition and subtraction can be applied to an automotive exhaust system. Using the idea that the amplitude of a forward waveform can be reduced by an out of phase reverse waveform, I came up with an idea to apply to my exhaust system. For example if a forward waveform had a frequency of 500 hertz with an amplitude of 1 volt peak to peak and the reverse waveform had exactly the same frequency and an amplitude of 0.75 volts peak to peak but was 180 degrees out of phase, the resulting waveform would be only 0.25 volts peak to peak. Using this theory to the exhaust system, the exhaust coming from the engine would be the same as the forward waveform. If I can create a reverse path in the exhaust pipe, I will achieve a reduction in exhaust noise. There is software available to design exhaust system that I do not have. So I will attempt the trial and error route and a bit of common sense.

exhaust gas path & flow pattern
CAD drawing of an exhaust baffle

Below is a video illustrating the fabrication process I undertook for this project. In the end I did achieve my goal of a quiet exhaust maybe a bit too quiet while testing this in the shop at idle and medium rpms. I will ultimately know for sure, once the car is on the road and I can test this out at higher rpm levels and at various engine torque conditions.

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