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Other than the hangers, the spot where the aluminum exhaust meets the steel exhaust will be the highest stress point. One simple way to make the steel/aluminum transition is simply to use a stainless V-band assembly on one side, and an aluminum one on the other. I was paranoid, though, that the stress concentration on the weld between pipe and V-band would be a failure point, so I came up with a more convoluted solution.
The idea was to make a slip joint, where the aluminum slips over the last section of steel pipe and a T-bolt clamp holds them together. This spreads the interface out over a large surface and seems like the least likely to concentrate loads enough to cause a crack. The only problem is that you can't find aluminum tube with a 3″ ID. Steel mufflers are often made with one side swedged out to slip over a standard tube size, but Vibrant's aluminum mufflers are made with standard 3″ ends and have to be butt welded to the exhaust pipe. If I wanted a slip joint, I'd have to make my own.
Here's where I show what a genius I am and what an idiot I am at the same time. Pay attention: I started by digging around in the scrap bin at Road/Race Engineering, where I found a few inches of 4″ OD aluminum tube. I have no idea what monstrosity they had been fabricating with this, but it was perfect for my needs. Next, I grabbed two small lengths of 3″ stainless. The plan was to weld these stainless bits to the Vibrant V-band. One would weld to the end of the RX-8 silencer, and the other would have the aluminum tube slip fit over it.
Cutting the 4″ OD tube down to a 3″ ID tube was a trick. Just making a straight slit lengthwise down the tube was tricky. Mike Welch just stood the tube on end and shoved it through the band saw. Making a 6' long cut all at once like this took about 5 minutes.
To figure out how much to cut off the aluminum to get to a 3″ ID, I simply made a Sharpee mark on the 3″ stainless tube it had to wrap around, put the stainless tube inside the aluminum one with the Sharpee mark aligned with the slit in the aluminum tube, and started rolling. Roling the stainless tube inside the aluminum one without any slippage was a little tricky, but once I got it, this trick perfectly measured the target circumference around the inside of the aluminum. Once the Sharpee mark touched down on the aluminum again, I simply transferred that mark to the aluminum and I knew where to cut.
That was me being a genius. This is me being an idiot: Returning to MD Automotive, where I was building the exhaust, I tried to fit these little chunks of 3″ stainless into the Vibrant V-bands only to find out they weren't 3 OD at all. They were 3″ ID! These scraps were apparently cut off a muffler that had its ends swedged to slip fit over a 3″ tube. I had just fabricated a piece of aluminum to slip fit over a piece of stainless to slip fit over a 3″ tube. Oops.
And that's how i ended up with this. A short section of aluminized steel in the middle of my otherwise exotic stainless and aluminum exhaust. I was under the gun to finish this project and get it off MD Automotive's lift before I disappeared for a few weeks of business travel, so I had to dive into my personal exhaust fab scrap bin, which sadly had no 3″ stainless in it.
As strange as the marriage of materials is, this transition from aluminized steel to stainless to aluminized to aluminum is remarkably smooth and clean from the viewpoint of exhaust flow, if not metallurgy.
Once I re-measured and re-cut the aluminum sleeve, I slipped it over the aluminized steel stub and the first section of aluminum exhaust tubing and held it tight with two V-band clamps.
The section over the aluminum tube was welded shut (and simultaneously welded to the tube). Later, I welded the circumference of the slip fit to the aluminum tube too. The slit in the aluminum remains where the aluminum and steel overlap, which allows the remaining hose clamp to clamp it tight against the steel. Inside the sleeve, the inner steel and aluminium tubes butt up against each other, making a smooth transition.
The polished finish is how all Virbant aluminum tubing comes. If you actually want to maintain this nice finish you have to 1: not use it on an exhaust, and 2: be really careful during fabrication. By the time I was done, all the pretty polished bits were gouged and scratched, but they're sure to be dulled by exhaust heat before long anyway, so I didn't really care.
1 comment
Hey,
Do you have any updates on the reliability of this? Thanks!