Hi Uncle Indy452,

Your 1991 Volvo 240. like all 240s, save for most turbo 240s, from the factory came with two "mufflers" after the catalytic converter (two words I can't ever spell correctly).

The front muffler just down stream of the catalytic converter is an exhaust resonator (another word for muffler). Inside the resonator are chambered baffles to cancel or reduce portions of the exhaust sound spectrum. The rear muffler does the same thing; a chamber to baffler the exhaust pulses. So, when the exhaust escapes the tailpipe, it is a quiet sound.

In the above board after market, places like iPD sell a 'cat back' exhaust using some custom piping of a larger diameter connecting the catalytic converter out pipe to a heavier duty muffler of a design similar to the turbo 240 rear and (usually only) muffler.

Is your 84 240 a normally aspirated or turbo 240?

What you may consider a reduced flow or pressure or exhaust gas volume and velocity at the tailpipe on your fabulous 1991 240, in spite of a seemingly hollow catalytic converter, may be the ceramic substrate formally in your catalytic converter has collected inside the front muffler causing blockage and obstructing escaping exhaust gas. You may need to remove the exhaust piping rearward and see if sooty chunky crap comes out the front muffler input pipe as well as the rear muffler.

You can sometimes give the piping a good shake or hard tap with a fist to hear the crap rattling around inside the front and rear muffler. The baffle design inside the front and rear 240, 740, and 940 mufflers are terrific at keeping sound and crunched and powered up catalytic converter guttiwuts.

The catalytic converter ceramic substrate is formed in a honeycomb-like matrix. Usually a rich running condition soft and hard, almost gooey when hot, combustible hydrocarbon-rich crap. That stuff usually collects when the owner allows the engine to run in poor tune. When you motor, then, on the highway at sustained speeds, your catalytic converter heats up to dangerous temps, causing a failure of the ceramic substrate.

You could also have a replacement catalytic converter; some sort of cheap after market. These can fail for any reason.

Your 1991 240 has what some call the metal spin gasket where it meets the header pipe output. The factory hardware used to secure the header pipe output to the catalytic converter input was woefully undersized for the task and the wrong alloy. It gets soft over time with repeated heating and cooling. That exhaust union on all 240/740/940 is a feeble point on the entire over head cam Volvo four cylinder exhaust system using the spin gasket.

The prior method connecting the header pipe output to the catalytic converter input found on some early 1990 240s (dunno the cut over for 700s) is somewhat stronger as it uses larger diameter securing hardware.

On the triangular interface with round metal 'doughnut' between the header pipe output and the catalytic converter, you'll note the hardware studs are anchored to the catalytic converter side. You say you had to cut one of the studs off. You may want to remove the other two studs so you have three holes and use the largest and longest sized stainless (I forget the proper stainless type - something .300 or .400 series) or heat tolerant high carbon hardware to re-secure the header pipe output to the catalytic converter input. The two pieces of triangular metal, one on the header pipe out put and the other on the catalytic converter input, should be in a beefy condition, if coated in rust, yet hard steel underneath. If either triangular metal piece is of a crumbly nature, you'll have to have some welding done to replace the triangular metal pieces. Either a machinist or a trusted exhaust shop; and I don't trust any exhaust shop.

Near the end of the header pipe is a threaded anchor of some sort that connects the header pipe end to a bracket array that secures to the two lower left North American passenger side (when viewing the car from the rear -> forward) transmission bell housing bolts (compounds nouns galore; best written in a Germanic language, what?). This connection secures the rather heavy header pipe as well as bracing the somewhat long exhaust run to the first set of rubber hangers. Also, while most let this critical support, the connection serves to better ground the exhaust piping up to that point and that's useful for 02 sensor operation.

Whether or not you have a functioning catalytic converter, your engine economy and performance depend, in part, on a well functioning 02 sensor.

For a longer lasting exhaust, use some braided stainless or tinned copper braided conductor and electrically connect the exhaust to the chassis. The grounding connections seriously slow the corrosion inherit to all RWD Volvo exhaust on models with catalytic converter systems.

Check your county and state emissions requirements. Depending on the emissions limits and whether you must have your 1991 240 emissions inspected, a well-tuned 1991 240 may pass inspection with a hollow catalytic converter.

Questions and comments.

Hope that help.

cheers,

MacDuff
--
- Currently back in Slumkane.
- My 1991 Volvo 240 (M47 II, moon roof, EGR) gets now 26-28 MPG thanks to a new 02 sensor and intake vacuum leak banishment. I also got an 016 MAF (or AMM) from a 1995 940 (generation 3 or 4 design) and that helps.
- Portland has deteriorated in the last 25 years.
- Vancouver, WA remains alright.
- The stupid, rickety I-5 lift bridge connecting Vancouver and Portland is a liability.
- As is the Cascadia Subduction Zone.
-The PacNW RULES over all!