"gutting" catalytic converters: OK w/ fuel-inj. cars??

[Maynard Krebs]

I recall hearing or reading advice NOT to 'gut' catalytic converters on cars with carburetors (that don't have to pass emissions testing)... but what about Cads with fuel-injection instead of a carburetor?

Specifically, I'm thinking that a fuel-injected engine is not nearly so dependent on "back pressure" in the exhaust, and/or intake manifold vacuum.... which are all related, performance-wise.

I remember advice against reducing exhaust back pressure below what an engine is designed to have... even though all engines must "breathe".. and properly.

Maybe members here can advise?

[Dave Shepherd]

From a emissions and legality standpoint, in Nys, this would fail inspection if the car was under 25 years old.

[Scot Minesinger]

I think this question applies to Cadillacs older than 25 years, and would like to know myself.

I have acquired an intake manifold from a 75 Cadillac with fuel injection that could be bolted onto my 1970 Cadillac and make a 472 fuel injected if I wanted to (with some work of course, yes (fuel pump in tank and etc.)).  Since the car already has good power unlikely to ever do it, but nice to know it is possible.  However, all years in 1970's and newer where Cadillac offered EFI also a cat convt. was also factory installed.
 

[Dan LeBlanc]

This question has come up before. On any car originally equipped with a catalytic converter from factory,  it is a federal offense to remove or disable it.

[TJ Hopland]

That is an interesting question, scientifically speaking of course since none of us would consider altering the emissions equipment on our cars.

One question I have to ask is why are you considering gutting one?   Performance?  Sound?  Its damaged and just don't want to spend the money on a replacement?

Over the last couple years I have been chasing a problem on a 93 so its got a MAF (mass airflow) but no rear O2's and at one point had the theory that the cat was plugged so I removed it.   All that seemed to happen is the thing got loud and a sort of drone to it on the highway.   Tuned out the cat was fine and the issue was somewhere else which eventually got fixed.   The noise finally got to me so I got a replacement cat.   Nothing seemed to change except the sound.   Probably put 3-4k miles on it without and maybe 1500 so far with it back again.  Performance is unchanged.  MPG unchanged.   Sounds stock again.

[Scot Minesinger]

Of course no one would alter the emission system on our cars.  The question is hypothetical.  If the exhaust back pressure is reduced in a carbureted car by removing the cat conv. (hypothetically) the performance improves, is this also true for a fuel injected Cadillac?  The question would apply to 1975 thru 1979 Cadillacs.  Does anyone know the answer to the hypothetical question?  Presume the answer is that performance would improve, but back pressure may be essential to operation - just do not know for sure.

[TJ Hopland]

So why would you need or want back pressure?   And how does that tie into performance?   Is it because it takes more energy from the crank to push the pistons up on the exhaust stroke?     Does it effect the manifold pressure (vacuum)?     

[Eric DeVirgilis CLC# 8621]

I doubt the absence of the catalytic converter would pose any issues on cars equipped with FI (or FI added) of this era. 

I seem to recall the concern about converter removal centered around burned exhaust valves at one time but I'm sure other aspects of operation would be affected with modern cars.

Frankly Scot, I'd just keep the carburetor on.  The increase is HP is minimal for the relatively small for the amount of complexity.   

[Scot Minesinger]

Eric,

Yes not a big believer in modification to increase power unless it retains stock look and is easy.  Reason is these 4,500 lb non-aerodynamic cars still are not rocket ships even after a significant power increase while retaining all the comforts such as ac and all.  Would I enjoy my car more if it was increased from 375hp to 475, if that is even possible easily, probably not.

I have a nice modern daily driver Dodge Charger with Hemi as a daily driver that is my rocket ship if ever so inclined.

The 1970 Cadillacs are enjoyed driving frequently on local errands, car shows and the like with plenty of thumbs up along the way.  Beating other cars at the stop light is not the staple of classic car enjoyment.  It is nice that these 46 year old cars still keep up with modern traffic without much trouble.

[fishnjim]

You didn't hear this from me (legally speaking).
But if the catalyst hasn't been changed/replaced after that long, that thing is not much of a convertor.   The early ones on high sulfur fuels made lots of nasty tail pipe eating acids too which are verbotten today as well.   
So what good is it now but you can get penalized for removing it?   Makes Govt sense but not reality.   The laws aren't designed for common sense situations.   EPA just wants these old "clunkers" to go away so they don't have to deal with the issues...   

[TJ Hopland]

Burnt valves is something I remember hearing too.  Same thing on some engines from removing smog pumps.   Never heard any reason why it was just one of those things you were supposed to believe. 

Hopefully someone here knows some of the science and can splane it to us.

[Eric DeVirgilis CLC# 8621]

The explanation I got was that with the converter removed, hot exhaust moves more freely out of the engine past the valves resulting in higher temperature than that for which the valves had been designed causing heat damage to the valves.

I seem to recall this mainly affecting smaller 4-cyl engines and maybe some 6-cyls, not so much big beefy American V-8s.

A few years ago I had a really mint '80 CdV on which I am 99% certain had it guts taken out by the original owner as well as having the fuel filler neck punched out in order to accept leaded gasoline. Car ran perfect (or better) with 22,000 miles although I have no way of knowing when the owner had made the "improvements".

[TJ Hopland]

So why would more flow equal higher temps at the valves?  Seems like more flow would tend to give them a better chance to cool?   If these were two strokes I could see lots of stuff going on a 4 stroke it would seem to have minimal effect on the whole process.   Look what happens when you get a really restricted exhaust like from a plugged cat,  very smooth hot running engine that just does not make power.   

[Eric DeVirgilis CLC# 8621]

I can only relate that is the explanation that had been given to me once upon a time.

For all I know it may be complete hogwash.

[Chas]

TJ and Eric........I'm gonna reach back to my old racin' days and maybe answer your questions. First thought is that you would want zero or negative back pressure to evacuate the combustion chamber quickly after the spark explosion. In real life, not true however. Remember your 4 cycle is intake, compression, power, exhaust. Without getting REALLY involved, if you reduce back pressure too much, you're sucking out the incoming air/fuel charge before it gets detonated by the spark plug. Also has a lot to do with cam profile/valve timing. Getting back to the question of the "hot" valves, remember that the intake charge of air/fuel is a good deal cooler then exhaust gases, thus, you want that charge to hang around awhile for its cooling properties. When I say " hang around"' I'm talking milliseconds. Based on cam profile, compression, Rpm run.......we would experiment with header collector length to get that optimum amount of back pressure..........enough to evacuate the chamber quickly, but not too much to quickly suck out the new air/fuel charge.

As to the original  question about hollowing out a cat.......I have never ever ever ever broken ANY laws as they pertain to Federal emission equipment, or even the speed limit for that matter. I even deny that old accusation about me one time removing a that tag from a mattress. I'm the last guy you should ask about breaking the law!

[TJ Hopland]

Is that the mattress tag police I see headed down Chas's block?   

I think I can follow that explanation.  The 'sucking' happens just from the hot air expansion if there wasn't anything to slow it down?  But with an exhaust system you also get some sucking action from the flow from that and other cylinders? 

Sounds pretty complex and likely took a lot of smart people a lot of time to get right when your car left the factory.   Also sounds like everything else, if you change 1 thing (like the gas you put in) all bets are off as far as it working like it did at the factory. 

It kinda seems like with the modest power and efficiency we are talking about in most cases here that we really are not going to change much either way.   A clogged cat is going to hurt but if you have a properly flowing one its not really hurting?   Modern cats seem to flow just fine and are not especially expensive so why not just replace it if yours is plugged or rusted out?   

In the 70's and early 80's era stuff I think the cats are the lest of the problems.   You look at the cam profiles and can see that they were trying all sorts of odd stuff which plays into the stuff Chas was saying.   There was one year, maybe 73 there is something like a 180* overlap.  Its very slight lift but its there.  That must be part of that in and out flow thing and maybe related to the egr which started that year too. 

I really don't see any difference between a carb and typical pre 96 EFI system.   They both work and respond mostly off of manifold pressure and flow.   If that changes on a carb that changes how much fuel is pulled in.  On an EFI the computer changes the fuel but the result is the same.   Post 96 they added MAF sensors and post cat O2 sensors so there is more monitoring available and more chance to anger the computer.       

[bill06447]

This discussion shouldn't even be taking place. Moderators?

[55 cadi]

Just another side of this.

I know people would gut the cat convert because of money.

They have i believe titanium and/or other previous metals in it that is worth some money.

Think this is more about performance or lack there of, but yes it is against the law to dismantle them.

I believe taking the insides out would also make the valves work harder, the back pressure with the insides intact slows the valves because of the pressure,  if there is no pressure they run faster making it harder on the engine.

And for blow outs/backs

I could be wrong

[The Tassie Devil(le)]

If Cat Converters were for increasing power, then every race car would have them.

Personally, I don't have them on my cars, because of age, and whenever I want to make more power, I install a good intake system, either aftermarket, good compression and valve timing, and a tuned exhaust system to allow the gas to escape.

Bruce.

PS.  It is nice to live in a Country where there are no testing done for emissions, no matter how new or old the car is.   Actually, no Vehicle Inspections, unless the registration is allowed to lapse for longer than three months, or the vehicle is needing re-registration when coming in from another State or Country.

[dochawk]

This discussion shouldn't even be taking place. Moderators?

By this point, discussion is about how and why combustion works, and why it's important not to interfere with that.

Not an issue that should alarm moderators . . .

hawk