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November 28, 2015

Exhaust System

One of the last things left to do before reuniting body and frame is to install the exhaust system.  Though this could be done later, it seems that it would be much easier now.

I bought a stainless steel system that is a copy of the original factory system.  It consists of a dual downpipe weldment, a pair of intermediate pipes, a pair of rear pipes, and  the dual input dual output muffler.

As with most things I've bought for this project, there were a few things to clean up.  One item was sloppy dressing of the joint between the down pipes and the flange.  Another was some damage to one of the muffler stubs, probably in shipping.  That short piece of pipe made a nice mandrel to reshape the collapsed tubing.

The system I ordered was supposed to be for a '74, but what I got was for an earlier car.  Luckily, the only difference is that the earlier intermediate pipes had hanger tabs welded on, while the '74 system used a separate hanger for the intermediate pipes, so I just cut the superfluous tabs off.  The hanger kit I bought had the correct intermediate hanger.

The next job was to add bungs for oxygen sensors.  The TR6 engine can for some purposes be thought of as a pair of three cylinder engines, each with its own carburettor and exhaust.  There are balance passages between the two halves of the engine in both the intake and exhaust manifold, but in a well tuned engine, there shouldn't be too much intermixing of air/fuel mixture or exhaust gasses, especially at midrange and higher RPMs.  This dual system just cries out for two oxygen sensors so that air/fuel ratios can be determined for individual carbs.  I had originally considered installing the bungs in the exhaust manifold, but putting in two of them was going to be tricky.  I decided it would be easier and just as effective to put the sensors high up on the downpipe.  A mockup showed me that there was a little more room to put the sensors on the front side ofthe pipes rather than on the rear, where the sensors would make removing the starter a pain.  Even in the front, there was a potential issue with fouling the motor mount.

I first bought a pretty common Bosch sensor, but found that it was a little too long.  I ended up with a slightly smaller universal replacement sensor that would just fit.

For the bungs, I got a short piece of 1-inch stainless steel and fashioned a pair of saddle-shaped fittings that matched the diameter of the down tubes.

Then cut a pair of matching holes in the tubes, and made a little slug to align the holes in the bungs with those in the tube.

I'm a mediocre MIG weldor at best, and I quickly found that welding stainless (with stainless wire) has its own challenges.  While I don't fret the strength of the welds, I decided to dress them to avoid publicly embarassing myself.  Then threaded the bungs to the standard (for oxygen sensors) M18-1.5.

I mounted the downpipes on the manifold and installed the sensors with liberal anti-sieze.  I decided to drill a couple of holes on the internal web of the motor mount for the wires.  I'll install grommets, but I didn't have them on hand.

After that, it was just laying pipe all the way to the back of the car.  The intermediate pipes were next.  The stainless band clamps provide a good seal without distorting the pipes the way that more traditional clamps can.  I found that one of the clamps, though the correct size (1.75 inch/44.5 mm), wouldn't clamp down far enough to eliminate motion in the joint.  I had to put a layer of thin shim stock under the clamp.

This is the intermediate hanger.  It attaches to one of the rear transmission mount bolts.  Some builders find this hanger troublesome and just leave it out.  I'll just watch it and if it becomes a problem, I'll modify it somehow, but I want to keep its function.

The rear tubes take an upward jog and disappear into the tunnel between the cruciate plates.  I'm still considering whether to install some kind of heat shield in the tunnel to keep some of the exhaust heat away from the fuel and brake lines.

The next hanger is where the rear pipes connect to the front of the muffler.  This hangar incorporates the clamps, so I couldn't use the band clamps here.  I did modify the clamps slightly,though, so they could be side by side without interfering with each other.  This hanger mounts to the bridge that supports the rear of the differential.  Since I had boxed in that bridge, I lost access to the inside where the bolt head had to go.  I cut a one inch access hole so I could get a wrench in there.

The last hanger holds up the tailpipe.

The tailpipe seems very close to the frame tail--about 1/4" clearance.  I can probably get some more space, but this is clearly where it wants to be.  I might even fashion a rubber pad with a hanger here so it can't thump against the frame.

Exhaust pretty much all done, except for possible tweaking mentioned above.  I think I'll wait until the body is on for that.

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