'79 Coupe DeVille exhaust/tuning question

[0maha]

The original exhaust on my '79 is pretty much past it, so time for a new one.

I intend to just take it to my exhaust guy and have him fabricate new stuff to more or less replicate the factory setup. This isn't a show car or a hot rod, so neither absolute originality or high performance are factors.

I assume the stock cat unit on there was highly restrictive from the factory, and has only gotten worse over time. I'm inclined to have him just delete it. Or maybe have him put a modern cat on there, which I assume will also be much less restrictive than what is on there now.

My question is whether or not that will influence AFR enough to require tuning/re-jetting in the carb.

Is that something I should plan on?

Thanks!

[TJ Hopland]

I have had cars where as part of troubleshooting the cat was removed.  In many cases I ended up replacing them because of the sound.   Some systems it makes a really big difference especially if your preferences lean more to a stock Cadillac sound.   I have not done it on a car from the era you are talking about so I don't know what the sound difference would be.

On the cars from that era I have always heard how terrible the cats were and how they are always plugged up.   Usually dealing with it is on my list when I first get them but so far there has never been any reason to suspect a problem so I have just worked the rest of the list and enjoyed the cars. 

I don't know what others have experienced but I have never actually felt any sort of performance gain when they were removed unless they were actually plugged.   If they are plugged there was a reason so you also have to try and figure out why and if its still an issue and correct that problem.

If you are having an exhaust shop do the rest of the system I would consider just having them put a new one in.   They are not very expensive in the context of a new custom made system and its just safer from a legal standpoint for everyone involved now and in the future.  What if you move to a state with inspections or want to sell the car to someone that has to have an inspection?  Would have cost you $100 now but in the future could cost a lot more.   You can also ask the shop how much credit they will give you for the old one.  I believe those old pellet ones are among the most valuable recycling wise so the scrap value could be in the range of the replacement cost especially since the labor cost won't really change since they will be doing the rest of the system.

My experience has pretty much been the same with all the 'emissions crap'.   When I have bought cars where that stuff has been disconnected and messed with they usually have not run very well or sometimes at all and that is where I have had plugged cats and other issues.  If things can be reconnected and parts are available putting most or all back has got me back to an enjoyable car.   

My opinion is you need to treat the cars as a full system.  You either need to more or less keep it stock and keep everything working or you need to start basically from scratch and replace or at least re calibrate everything.   

["Cadillac Kid" Greg Surfas 15364]

Jeff,
I would go someplace that has an AFR (possibly a hot rod chassis dyno shop) and check your system out after you complete the exhaust system.  I believe it is still illegal (Federal) to remove a Catalytic converter from a vehicle so equipped, but that may be wrong.  My '79 came to me sans cat and that is the way it is now.
As far as sound you might have the muffler shop mock up the assembly with no resonator and see how that sound.  It will be a bit throaty, so if it is too loud the resonator can go in.
Greg Surfas

[0maha]

Appreciate the thoughts, gentlemen.

We don't have emissions testing here in Nebraska, so I hadn't considered that. Might be best to have a new CC installed just to keep the car sellable for other states.

I'm still hoping someone will chime in on the tuning question. I'm pretty sure I could find a shop with a dyno to do an AFR test, but I'm not as optimistic about finding someone who has experience tuning these old Rochester carbs. And the car runs really well now, so I'm hesitant to get in there and start screwing it up.

["Cadillac Kid" Greg Surfas 15364]

Lacking any nearby carburetor shops I might suggest contacting Jet Performance   
https://jetchip.com/product-category/performance-carburetors/rochester-quadrajet/big-block-quadrajet/

Let them know what you have, what you want and they can build the QJ that will perform as desired "out of the box".  I know they have done a few for me and I have been extremely happy.  One of the features of their carbs is an external  adjustable part throttle (APT) that lets you tune the carb without taking it apart.
Greg Surfas

[TJ Hopland]

I don't know if every state's titles have them but the few I have seen have a check box that says something to the effect of I certify that all the emissions and safety equipment are in place.   I'm not a lawyer but once there is evidence that someone lied on a legal document it opens up a whole legal can of worms.   $1000 beater car is one thing but you start talking $10k things could get ugly.

If was me I don't think I would be too worried about the tune if you are just going to a new cat and not too different than stock exhaust.   I don't think Cadillac had as delicate of a tune to meet emissions as some of the other manufacturers did at the time so we have a little more of a window to play in than some others do. 

Now if you were going to do 3" dual exhaust with no cats and removing the egr you would want to look into tuning.   EGR is a big one.  My limited testing showed disabling the egr system makes things go real lean.