I just installed a true dual exhaust system on my '76 L82.  This is a performance exhaust using magnaflow mufflers.  I purchased the kit from Corvette Central and it replaces everything from the manifold back (no cat anymore).  Amazingly it went on with no major problems and fit very well through the exhaust pipe holes in the central support.  The mufflers are marketed as "hidden" in design and actually are less exposed from the rear view then the OEM style.  The sound is good.  It is a thoaty but quite subdued unitl you push it and then they do wake up some.  I would say overall they would be great for those of us that want a little more umph in the sound without being loud inside the car with the windows up.

Aren't the outlet lines from the smog pump connected to the exhaust manifolds? 6 pipes go to 6 fittings in the manifolds. That system, AIR, should be pretty much still intact and operational even with the new dual exhaust. mkapp78792008-10-01 10:25:29

You are correct.  There are several lines (some the size of vacuum lines) that are still attached to various places.  I'd have to go back and look at each to see what's what.  But, there is approximately a 3/4" to 1" ID metal tube that ran down to and connected to the exhaust tube ahead of the cat and up close to the engine.  This is now just an open line not connected to anything. I assumed it is for exhaust gases coming from the smog pump.

I can't picture the open line you're talking about. I got my 78SA up on the lift right now so I'll go take a look at it.

There is a hose that comes off the "diverter valve" on the A.I.R. pump and has a check valve in the line.  From the check valve a tube goes down to the passenger side exhaust pipe just below the exhaust manifold.  The tube connects to the exhaust pipe before the pipe makes its turn toward the rear of the car.  I beginning to believe this set up is unique to the 75 and 76 year cars.

[QUOTE=mcgee0311]  I beginning to believe this set up is unique to the 75 and 76 year cars.[/QUOTE]

You can remove the smog pump and any tubing related and cap off the vacumm line, by removing the pump it will leave a cleaner apperance. 

The interesting thing in researching the pump is that it injects fresh air into both exhaust manifolds through small metal lines.  There is a line for each side and each main line splits into four lines that connect into each port on the exhaust manifold for that side.  If I take off the pump, I wouldn't have a clue what to do at the exhaust manifold to cap off the air injection holes on the manifold.  '75 and '76 being the first years for this smog stuff are kinda wierd.

All the AIR system did was pump fresh air from the air cleaner into the exhaust to provide oxygen to finish burning the stuff left in the exhaust.  It doesn't affect the tune to remove everything and just plug the various holes.  Since so many people took them off and threw them away years ago, you probably want to hold onto everything you take off, or sell it to some lucky restorer.

Larry

When I removed the cat from my '81 I only added a test pipe where the cat was located. The next step was to disconnect the belt and then the removal of the smog pump because it is nothing more than an eyesore and hides the nice valve-covers. Part of the removal of the pump was also the elimination of copper plumbing that led from the exhaust manifolds.  When I disconnected the pipes from the exhaust manifolds, I found some nice brass caps that screwed right on and sealed the manifolds.

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One more thing... here's the description of the Air Management System from the '81 shop manual (page 6E1-5)...

The Belt driven air pump, AIR, is controlled by signals from the ECM and vacuum.During cold operation, air flows to the exhaust ports to help control HC and CO and help warm the engine and catalytic converter more quickly. During deceleration and wide open throttle oprtation, air is directed to the air cleaner to avoid excessive noise. Excessive air pump pressure is also directed to the air cleaner. After the engine is warmed up, air is directed to the lower bed of the catalytic converter to provide an oxidizing atmospherre for effective catalyst operation.

The air management system has an electric air management valve or valves that divert air to the sir cleaner or switches air between the catalytic converter and the exhaust ports.

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Does your 4 tubes that go into the manifolds, still thread into the manifold, like a brake line and fitting. If so remove them and go to your local hardware store and buy some tappered brass plugs. If the lines cvan't be removed you can use a rubber exhaust 3/4" heat hose and put a brass or copper plug in the end of the hose with a clamp. The hose I am talking about is the one that was used to joint the exhaust tube diverter valve to the air pump hose.  You can also use the hose that is used to repair air tubes to the cat.

Thanks for all the advice.  I will take a look at the car and the AIM to see exactly what set up I have.  I figure that one of the options you all have presented will work for me.

I finally got into the smog pump this past weekend.  I got it removed cleanly and only had to plug one vacuum line after all was removed.  The exhaust tube that runs from the pump to the driver's side exhaust pipe is held in place with one of the exhaust manifold bolts.  You don't have to remove the bolt entirely (which I did) because the mount is sloted.  Simply loosen the bolt and the exhaust tube and backflow check valve will slide right off.  There are three bolts that hold the pump.  One is on the bracket that allows tension to be put on the belt; comes off easily.  The second one is the longest of the three and will not slide all the way out without taking the pulley off the pump.  The final bolt holds the tension bracket in place and is easily removed.   Now I have true duals, no cat, and no smog pump.  It sure feels like the engine has more pep; maybe it's just me.  But, sure like the sound.

The motor will have more pep without the drag of the pump and better exhaust flow.