The Parade was a real "Blowout"!

A few weeks ago, my wife and I drove 'Nell' about 100 miles into the local mountains for a romantic weekend getaway.  The weather was spectacular, and the tbird was running great.  It was a much needed restful, relaxing outing.  On the way home, I noticed that the exhaust note was a bit louder than usual, but it really wasn't annoying so I thought nothing of it.

This week the exhaust noise has gotten progressively worse.  It's not an exhaust leak, more like a loud drone at certain engine speeds.  Yesterday I climbed under the car and checked for sound-shorts in the exhaust system... spots where the exhaust pipe or muffler may be touching something it shouldn't.  I ended up adjusting the right tail pipe, but the drone persisted!

Today my son and I drove Nell in a local parade, with my classic tbird club.

The exhaust noise was pretty bad!  Everyone in the club noticed it!  Something is definitely wrong with one or both mufflers... it's like the baffles have blown out, and are rattling around in the bottom of the can.  Not very attractive at all!  Glasspacks would sound better than this!

This is a disappointing development, especially since the mufflers were just installed in January!  Well, it's back to Muffler Guy next week to get this fixed.   I have to wonder what could possibly have caused the mufflers to fail so quickly?  I suppose that they could just have been faulty, but I need to be open to the possibility that it might be something else.

I have been running a Mallory racing distributor in the car for the last two years.  I had one dramatic failure with it, but that ended up being a very simple fix.  The thing about a racing distributor is that it has no vacuum advance... since it is designed to work with the throttle wide open, it doesn't need a vacuum advance.

For those who may be interested, here is a short tutorial on ignition systems:  Gasoline/Alcohol/Nitro internal combustion engines need a spark to ignite the fuel/air mixture in the cylinder just prior to the start of the power stroke.  Exactly how far in advance of the power stroke the spark should occur depends on two basic factors: 1) how fast the engine is turning, and 2) how dense the fuel/air mixture is in the cylinder.  The first factor is dictated by the fact that at its best, a cylinder full of fuel and air still takes a fixed amount of time to burn.  The engine is most efficient, both in power production and fuel economy, if the mixture has finished burning before the piston is a quarter of its way through the power stroke.  In order for this to happen, the spark has to happen sooner when the engine is turning faster.  In most 30's - 80's cars, this is determined by the "mechanical advance" mechanism of the distributor, a set of little weights and springs that automatically advances the spark timing based on engine RPM.  This is the only kind of advance mechanism my Mallory distributor has.

The second factor, fuel/air density, depends on several things that are set when the engine is built, such as compression ratio, intake & exhaust configuration, camshaft, carburetion, etc... but in practice, the only real factor determining fuel/air density in a running engine is throttle position.  More throttle means more air and more fuel, period.  More fuel and air means a faster burning mixture, and conversely less fuel and air means a slower burning mixture... when the throttle is open only a little, like when cruising down the freeway, the mixture burns a lot slower!  Why does this matter?  Well, when I am cruising down the freeway in Nell, much of the fuel/air mixture hasn't finished burning in the cylinder when the exhaust valve opens, so it flows out into the exhaust system and finishes burning there.  This not only wastes gasoline, it heats up the exhaust... a lot.  Racers don't really care, because they don't "cruise" very often.  The throttle is usually all the way open, or all the way closed.

I care because I suspect that this extra exhaust heat is damaging my mufflers... I'm on my second set in two years.  I would also like to start getting reasonable gas milage...  Nell has been averaging around 10 MPG, and I think she is capable of more like 16-18!  Most street cars from the 30's-80's accommodate this fuel/air density factor by using manifold vacuum to provide an additional way to advance the spark.  When the the throttle closes, manifold vacuum goes up (manifold pressure drops), and the distributor advances the spark to accommodate the slower burning mixture.

Back when I first had problems with the Mallory, I purchased a tbird compatible distributor that has both mechanical and vacuum advance.  I have put off getting it ready to install because the Mallory was working again.  With this exhaust problem, however, I think it's time to make the swap!  Some of the preparation of this new distributor has been easy, such as installing a new drive gear and an electronic ignition (points replacement) unit.  The hardest thing has been "re-curving"  the distributor, or changing the rate and limits to the mechanical advance mechanism, to accommodate the modifications I have made to the engine, such as the cam and high compression heads.  Based on discussions in this thread and others like it, I have settled in on 14° of initial advance and 26° of additional mechanical advance, which should all happen before 3000 RPM.  This is precisely how my Mallory has been set up, so I have confidence that these numbers are right for my engine.  However, the "stock" Ford/Mercury distributor I am using has more advance than this (about 40° on an initial advance of 6°), so I had to modify the slots in the mechanical advance mechanism to limit the advance.  Not limiting the advance risks severe engine damage at high RPM.  I used regular plumbing solder to do make the slots smaller:

The trick to soldering on steel is to get it really clean (I used starter fluid), lots of flux paste, and just the minimum amount of heat.  I got good blobs of solder and good flow on the steel.  I only needed to make the slots shorter by about 1/16".

I filed off the tops and bottoms of the of the blobs before attempting to size the slots. 

Measure twice, file just a little, measure twice more....

Continue until the slots are the right size... 0.450"

This is the completed mechanical advance mechanism.  There is another spring on the other side of the shaft... bending the tabs connected to the springs will change the advance rate.

This is the original points and condenser setup.  I will install an electronic substitute, but will keep the parts in the car as a backup.

The electronic ignition installed.  It's really just a points replacement, not a high-energy ignition system, but I did get a model with variable dwell.  This will give a better spark at high RPM.

With the rotor installed.  The vacuum advance unit is on the left.

The completed distributor.  Oh, and I had some fun with the buffing wheel.  Hey, if it's made of aluminum, I know how to make it shiny!  Note the tach drive socket below the vacuum advance unit... all early thirds have mechanical tachometers, driven off the distributor.

I'm looking forward to installing the new distributor in the next few days.  I will need to dial in the ignition timing, and maybe fiddle with the mechanical advance springs a little.  I'd like to get it running well before I take Nell back to the Muffler Guy.