Should I do a Cat delete on '75 Duster

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John Mountain

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I've got a 33,*** mile '75 Duster with a 225 and 3 on the tree. Engine runs great but is gutless due to attempts to meet the upcoming emissions laws. 1975 and older cars are 100% emissions exempt here in California, and my car is the first year to have a catalytic converter. It also happens to suffer a 50hp drop from previous years of the same exact motor.

Could I do a Cat delete and get some of this lost power back? I don't want to invest too much in engine mods, since I eventually want to do a Gen III Hemi swap, but that may be a while. If I'm missing out on a 33% power gain I sure as hell want to get that power back.
 
I've got a 33,*** mile '75 Duster with a 225 and 3 on the tree. Engine runs great but is gutless due to attempts to meet the upcoming emissions laws. 1975 and older cars are 100% emissions exempt here in California, and my car is the first year to have a catalytic converter. It also happens to suffer a 50hp drop from previous years of the same exact motor.

Could I do a Cat delete and get some of this lost power back? I don't want to invest too much in engine mods, since I eventually want to do a Gen III Hemi swap, but that may be a while. If I'm missing out on a 33% power gain I sure as hell want to get that power back.
Just drive it. Sounds like inflated numbers to me. If nothing in the catylitic system is blocked ,I call BS. Not much gain to be had man. Its a slanty and now you cant pass emmisions.
 
My 75 had a catalytic converter that I removed when I had headers and dual exhaust installed in 81 or 82. I also swapped out the fuel filler tube so that I could run regular leaded gasoline(it was cheaper). But I don’t know if you’ll get much of an increase in horsepower as a result. I think a new cam would likely give more of a boost than pulling the converter. That being said I have zero experience with slant six engines, so hopefully someone that knows them will speak up.
 
Removing the catalytic converter and doing nothing else won’t add much, if any, horsepower.

Now, if you undo the other smog year changes- timing, compression, carb, etc AND remove the catalytic converter, you’d get some performance back. Nowhere near 50 hp though.

If you’re doing an engine swap, just drive it as is until then unless the catalytic converter is plugged up.
 
The large HP change occurred on all engines was due to how the HP was reported.
HP was reported as gross HP, basically, all the HP the engine made. That changed to reporting HP as net HP, the HP available after vehicle accessories were driven.
 
I pulled the CAT off my 76 Aspen with a slant 6 in it.Just like your Duster it was a gutless wonder .I put a carter 1bbl from a 65 Valiant on it and threw that smoged out Holly in the trash.It woke that car right up and it's now actually almost fun to drive it.Before getting on the interstate in traffic was downright dangerous and now when I hit the gas she actually gets up and goes.
 
The early cats were restrictive. In my view the big change in that aspect came with the Ford "5.0"
Now we can readily buy low restrictive cats, regular and '3-way', etc.
That said, for a 225 turning 2500 rpm, cruising, not much restriction.
IMO the biggest killer of torque and throttle response in a '75 (vs a '67) is the timing and timing delays.
Do some reading up on OSAC, and look at bypassing that. Delays when coasting a braking are no big deal, but not having the right timing for power as soon as the throttle is opened is a huge deal.
Chrysler acknowledged there might be complaints about drivability when system was introduced in 1973.
see here
Master Technician Service Conference - Chrysler's Training for Mechanics
 
If you say you are emmissions exempt, get rid of that catalytic converter.
Burn fuel and enjoy it.
 
The BIG performance drop from emissions came from the sa..........sloooooowwwwwwww timing curve in the distributors and late LATE (sometimes AFTER top dead center) timing, and lllllll.........lean !!!! carburetors

I doubt you would gain much removing it, and be DARN careful to check first what smog you have to pass down there before you remove anything
 
hey John, welcome aboard

one thing to keep in mind is that you are talking about a 45 year old car with a grocery getter engine

that engine was designed to run for ever, and the fact that you now have an engine that is almost 3 times your age proves that it does exactly what it was designed to do

however, you can not compare the slant 6 to a modern import 6 banger
in a lot of the cars you see today, specially the imports, the 6 is the performance engine

here, not so

even a great running slant 6 will not be a powerhouse
it will be fun, sure.
it will be adequate, of course

but it will not be a racecar


how, having said all that, following what 72blunblu suggested is a good start
and so is regular maintenance and tune ups, so do those first

oh, and post pictures
 
I agree with 72blunblu. There is a good chance your Cat is plugging up. Given you're exempt, axe it.
BUT, Chrysler Corp was notorious for lazy timing and very lean A/F ratios. (Lean Burn)
As others stated, get some timing in it AND jet it up. It will be overly lean with the Cat removed.
The HP rating reduction was mainly due to compression being lowered to be friendly with unleaded gas.
The gross/SAE hp ratings took place in 71 and 72
 
remove cat and replace the 1 7/8 pinched head pipe with a 2.25 super six size. While your at it, recurve the distributor..reset base timing to 70 specs, shave .090 off the head...wait, your getting a hemi? Chop the cat off and get an old carb and remove the timing delay thingy. When I was growing up in CA, the Smog cut off was 1966 so I got a 65. C.A.R.B. (smog nazis) in CA are assholes. Try and get a pre 83 car to pass smog nowadays with all the smog crap vacuum pots leaking and anti backfire valves not working anymore and are no longer available. You can plead your case to a referee (parts no longer available) but those guys are on the spot for passing cars too. One gets past them and they lose their status. My Mazda would only test in second gear and I had to go to a referee for that.."RPM to high at 30 mph" was the stupid machines excuse to fail, and they (test only station) could not test in second gear per their rules. "ever hear of gearing?" That is how STUPID CA smog laws are. OK, back to your Duster......
 
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Replace it with a quality 400 to 200 cell high flow cat. No need to destroy the earth "because you don't need to run one". I run dual high flow cats on my 76 F-150 with built 460 and I pass. Now I have to up the Idle to 1000 to pass but other wise the cat's are not holding it back. I run dual 300 cell cats on my GTS mostly because I can't stand the smell of raw exhaust and I'm not going to gas chamber my family on car rides. That odor get in your cloths and skin and isn't good for you or your loved ones.
 
It also happens to suffer a 50hp drop from previous years of the same exact motor.

No, there was a 5- to 10-horse drop in published rating for '75 (105 in '74…100 for the Federal '75, 95 for the California '75). If you're basing on the pre-'72 "145 hp" rating, be aware that number was artificially inflated due to the gross HP rating system in use before '72; see here.

Could I do a Cat delete

Not legally, but yes, you could.

and get some of this lost power back?

You won't gain anywhere near so much as you're hoping, unless your cat is clogged—which it very well might be; non-feedback fuel systems and catalytic converters were never a happy marriage.
 
The original poster is in California and his car is old enough now that it has aged out of the states smog testing requirements. At this point I believe he can legally perform any engine mods he wants. That’s one of the reasons I held on to my Duster for all these years. I was looking forward to the day it would age out. As fate would have it shortly after it did I moved out of state and brought the car with me to PA.
 
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Axe it. Unless your in California or some other communist state with draconian emissions laws.

Typical. Didn't even bother to read the details of the post and you don't know anything about California smog laws. The OP's car is exempt because of its age. His car never has to be smogged, or even inspected, ever again. As long as the title remains clear there's no reason anyone with any official capacity has to look at that car in person ever again in California.
 
Typical. Didn't even bother to read the details of the post and you don't know anything about California smog laws. The OP's car is exempt because of its age. His car never has to be smogged, or even inspected, ever again. As long as the title remains clear there's no reason anyone with any official capacity has to look at that car in person ever again in California.

That's why I bought it! It'll make engine swaps and the likes way easier, and getting smogged is such a hassle here
 
…unless/until the rules change, which has been known to happen.

Well let’s see, ‘75 and earlier cars were exempt when I got my driver’s license in 1995, so it hasn’t changed AT ALL in well over 25 years. So I won’t hold my breath?

And the last time a bill regarding smog exemption came up here a couple years back it was to RAISE the exemptions to 1981. It didn’t pass, but barely. So, yeah, not worried about it.

Anyone else that doesn’t have a clue because they don’t live in or daily drive a pre-smog classic in California want to comment?
 
…unless/until the rules change, which has been known to happen.
In spite of California’s recent moves to the left the car culture that the state is well known for, especially in Souther California, continues to have a fair amount of influence in certain policy decisions and the aging out of smog testing requirements for cars beyond a certain age(1975 and older at this point) is one of the concessions they won. But truthfully, even in California with its unsalted winter roads the percentage of cars on the road from 75 and earlier is pretty small. I don’t think John has anything to worry about going forward.
 
In spite of California’s recent moves to the left the car culture that the state is well known for, especially in Souther California, continues to have a fair amount of influence in certain policy decisions and the aging out of smog testing requirements for cars beyond a certain age(1975 and older at this point) is one of the concessions they won. But truthfully, even in California with its unsalted winter roads the percentage of cars on the road from 75 and earlier is pretty small. I don’t think John has anything to worry about going forward.

Exactly. These cars make up less than 1% of the cars on the road here (and everywhere else for that matter). There are very few people that use them as daily drivers like I do, and fewer and fewer every year. When I started driving my '72 Challenger daily about 12 years ago there were several other Mopars I would see frequently on the road, even during the winter months. If they weren't daily's they were close. Nowadays pretty much the only time I see other Mopars is during the summer, and typically only on the weekend when the local shows are going. I don't think I've seen another one during "commute time" for a couple years now. I'm not saying there aren't any at all, but probably not many.

SEMA and other organizations have a large lobby, and do a great job of making sure little of the new legislation applies to classics and hobby cars. With the tiny percent of the total they make up its not a difficult argument to make, and the trouble and backlash involved with including them isn't worth the tiny result. As I said, the most recent push has actually been to raise the number of exempted vehicles, there are a lot of the "truck guys" that would like to see up to 1981 included, as many of the truck bodystyles from the early to mid seventies carried through until then.
 
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