by Dan Cameron
Originally printed 9/87
For the performance buff that is always looking for that extra HP don’t overlook the exhaust restrictions.
With the small block Ford, this is the major bottleneck. It does not do any good to try to put more in than you can get out. We have previously discussed the desire to have smooth bends and a non-flattened (where it passes through the frame) exhaust system.
In addition, two other areas should be addressed. These are the mismatch of the header primary ports and the interface of the exhaust pipe tubing and the header collector cone.
Let’s discuss these one at a time. Initially, I disregarded the mismatching of the exhaust ports (heads) with the headers because dimensionally the header ports were larger. What I overlooked was the severe mismatch condition that occurred randomly, sometimes side-to-side and other times top-to-bottom. This is much more predominant when using the larger 351 W heads as in my case.
To remedy the situation, using a header gasket as a template, I determined the extent of the errors on a port-to-port basis. This was used as a guideline for metal removal. Using a 1/4″ drill (a little low in RPM but okay, a die grinder preferred) and a carbide rotary file I blended the header primary flange to the primary tube. Carbide rotary files can be purchased through most industrial tool supply houses.
In exercising the procedure above, take extreme caution and don’t get carried away. There is not much material there to begin with. If you take the grinding too far and the metal becomes too thin or a pinhole develops, take it to a welding shop to fill the outboard side of the flange at the union of the tubing.
All in all, this job was pretty easy.
As for the area where the exhaust pipe meets the header cone down, I was in for a real surprise. With the header removed, I noticed my good old muffler shop stuffed the tubing into the cone down so far, at an angle to boot, it acted as a baffle to restrict the exhaust flow.
As easy as the previous job was to accomplish, this one was a real turkey. The only solution was to lie for hours on the backside grinding away with a 1/4-inch drill and a coarse grained, 1 1/2″ aluminum oxide grinding wheel. The collector cone was blended to the exhaust tube at the point of the weld. Six grinding stones later, I had a smooth transition from the header to the exhaust pipe.