If your memory is short like ours, know that in 2012 we showed how Marshall Woolery at Thun Field Rod & Custom in Tacoma, Washington, transformed the suspension and chassis of a 1939 Chevrolet coupe ("The Stovebolt Saga", May-Aug. '12 issues). Math, more than memory, shows that we haven't visited the subject in nearly a year.
The Stovebolt Saga is back for a proper exhaust system. Any ol' exhaust shop could've bent up a workable system in an afternoon but we think this application warrants something more.
It's largely because of the engine. It's a healthy example of a pre-'64 six-cylinder Chevrolet, aka a Stovebolt. These engines really aren't different from any other but their relatively small size underscores the importance of preserving as much power as possible. In exhaust terms that means building a system with pipes just big enough to handle the engine's volume at fast speeds yet small enough to generate the torque-building velocity at slow speeds. It's a balancing act that's challenged builders for time immemorial.
It's also something functionally impossible to achieve with tubing bent by conventional means. Press benders at exhaust shops and even the fancy tube benders at chassis shops basically mash the tube into an oval through the bend. Circumference being equal, an oval has less surface area than a circle. That restricts flow with predictable consequences. Increasing tube size to compensate for the bends would kill the gas velocity through the straight sections, which would let the torque dribble lazily out the tailpipe.
The solution is a mandrel bend. A mandrel bender resembles the tube benders in chassis shops but it has a die that supports the inside of the tube so it stays round.
Unfortunately, true mandrel benders are heavy-industry tools exclusively. Luckily companies like Patriot Exhaust Products sell mandrel-bent tubing segments. Since the tubes remain round at all points they can be cut and assembled to straights or other bends in near countless combinations. Read the "How to Cut Mandrel Bends" entry in this article to learn one way how to do it.
Piecing together a system from mandrel bends costs more time and money but the advantages are significant. Anyone with a saw, a grinder, and a welder can do it. And a system built of mandrel bend segments can potentially fit a lot better than a press-bent system.
Why Not a Y?
This engine's header inspired us to think creatively. It divides six pipes into two collectors, which technically isn't as efficient as routing all of the tubes into one. In really rudimentary terms a collector gives each cylinder the benefit of pumping its charge into a low-pressure, high-velocity stream generated by the exhaust event that preceded it.
Merging the pipes from two collectors into one larger pipe (a Y-pipe) achieves much of the benefit of running all of the header pipes into a single collector. In fact, that's what most vehicle manufacturers did for years to build street-friendly torque with V-8 engines.
Some really elemental calculations indicated that two primary pipes with a 1 7/8-inch ID feeding a single pipe with a 2 1/4-inch ID would flow enough yet still build fair velocity. Assuming a 16-gauge wall thickness, that's two 2-inch primary pipes and one 2 3/8-inch main pipe.
But there's a flaw with that secondary-pipe diameter. A 2 3/8-inch-diameter pipe is quite hard to find, regardless of wall thickness. Larger pipes like that can also make a Stovebolt drone like a boat or tractor. They're also more likely to induce interior resonance. At the very least large pipes rob Stovebolts of their characteristic exhaust rap, which, for some, is reason enough to endure such an antiquated engine.
1. The headers in this application dump straight down but because the pipes need to go straight back Marshall Woolery cut a 2-inch-diameter U-bend to make two 90-degree elbows. Refer to the sidebar on page 30 to learn how to cut tubes properly.
2. The elbows slip into flanges that bolt to the header. They appear to sit at different heights but they're even. They do, however, point in slightly different directions for a reason.
3. Here's why the pipes point two ways: the collectors line up in the direction of the pipes. So Woolery let the rear one (left) point straight back while the forward one points a few degrees to the passenger side.
4. Because the rear pipe can go straight back at the engine's mounting angle, Woolery simply jacked up a straight pipe to it and marked the fit points. Technically he could've tacked the pipe in place but he left it loose for the time being.
5. The other pipe requires a little finesse. It needs a slight bend to straighten it. Woolery measured the offset and cut a piece slightly longer than necessary. Again, refer to the sidebar to learn how to cut bends properly.
6. Woolery cut the piece long so he could trim it until the second pipe landed about 3/4-inch away from the first. The gaps at the inside radius indicate that this pipe needs to be trimmed a bit more to maintain the 3/4-inch gap between the pipes.
7. Because the downpipes turn at 90-degree angles the first part of the exhaust follows the engine-mounting angle. To level the pipes Woolery cut another bend. The fixture in the sidebar shows how to mark a tube cut at a pretty precise angle.
8. Here's where the two tubes came together at the weld.
9. Cut and rotate a bend 180 degrees to create straight offsets of various amounts. You can do it by distance. First determine the required offset and half that figure. Mark each tube where its centerline rises or falls by that amount. Then draw a line through that mark along the radius centerline.
10. You can also calculate offset by angle if you know the bend radius. A 180 cut into two 90s will create an offset equal to the bend's radius, in this case 3 inches. The pipe needed to rise 2 1/2 inches or 83 percent of 3 inches. Well 83 percent of 90 degrees works out to 75 degrees. So Woolery cut both pipes to 75 degrees to create his 2 1/2-inch offset. Note that this shot shows the X attached.