40 special dual exhaust

[RiKi5156B]

Has anyone done a dual exhaust and how were the pipes run? Getting ready to do mine with a 41 super dual carb and exhaust engine.

[Matt Harwood]

It has surely been done many times and you have to run the pipes very close together along the path of the original exhaust. Weaving it through the frame and around the pedals is a BIG challenge. You'll need a talented exhaust fabricator to make it look tidy.

 

I might advise simply going with a larger single exhaust system. That little engine and its associated breathing restrictions will not benefit from a dual exhaust system in any tangible way other than sound, and even then a straight-8 sounds kind of odd with dual exhaust. Going from the factory 2 inch pipes to 2.5 inch pipes will result in a significant improvement in flow without the added labor of true dual exhaust. I created a set of tubular headers for my '41 Limited and had a custom stainless exhaust system made that was larger than stock. Even with the headers and the dual carbs operating synchronously, I'm not seeing huge improvements in power, if any. The design of the head, intake, cam, and stock manifolds are too restrictive to benefit from anything on the exhaust side. Plus it's only 248 cubic inches, so there's not as much air flowing through it as on a larger engine. And remember that removing too much back pressure will result in a loss of low-end torque, something that engine can hardly afford.

 

 

If you haven't started, it's worth considering just doing a slightly larger single exhaust with a big muffler to improve flow. That will get you 90-95% of the performance of true duals without the expense and complexity.

[Skidplate]

I too just did my dual carb exhaust setup. Can’t remember the exact numbers but I calculated the area of the two stock exhaust diameters and found that a single (just a little larger diameter) exhaust was roughly the same combined area. So I just ordered a Y adaptor to fit my pipes diameters (I think 2.25’s in and 2.5 out) had my exhaust guy bend two shorties coming out of the manifolds (about three feet long ) at the proper angle about 45 deg from the ground. We fit the Y where it looked the best just below the manifold that would allow for the horizontal bend just past that. I picked out the muffler I wanted to use and he made the rest of the exhaust to the back bumper.

 

the hardest part is finding someone who will still bend custom exhaust.

[Mark Shaw]

Something I learned in the pump and valve business:  Doubling pipe diameter increases flow capacity four times.  

There are multiple internet sites with helpful "Do & Don'ts" with formulas for calculating exhaust systems.  

I suggest you follow Matt's recommendation.

Edited June 25, 2020 by Mark Shaw (see edit history)