: Dual vs Single Exhaust on a 425

04-28-12, 12:13 AM
California says I can't do true duals, and they also say that even though my Deville is Federal emissions, as of 2009 if I get an aftermarket cat I still have to get the California approved one. Then again how would they know when I put it on? Magnaflow has a 2.5 in out California one for like $250 or so on summitracing, but the Federal ones are half that. I really want to do the exhaust that will be best for the car in the long run. Some people run dual 3", making me feel like a single 2.5" wouldn't be enough, but no one makes a 3" cat that's approved. I thought maybe I would try to do true duals with two Federal cats and just see if I could get away with it, and if not, have it set up so it's easy to take off one side and throw a Y pipe in for smog day. My main goals are to improve mpg as much as I can, and eventually get the hp/torque closer to 300/400 or more if it's not too hard. What would you guys suggest? If you guys think the single Magnaflow would be enough then maybe I'll just go that route in order to avoid the hassle in the future. I'm sure it will flow way better than stock, but will it be enough?

04-29-12, 01:37 AM
My own experience is, a single 2.5" isn't big enough for a 400 up engine, and a monolithic
cat is a big improvement over the original "bucket of gravel". Don't know how that fits
into the CA plan. Bruce Roe

04-29-12, 12:49 PM
I don't suppose I could take the cat to a welding shop, and have them cut the ends off where it tapers up to 3", and then have them weld on some 3" ends.

Or, what if I used a 2.5 into 3 Y pipe that tapered to 2.5 at the very end, and then immediately after the cat split into 2 2.5s. Then maybe the cat would act as a sort of long x pipe lol.

I think my best option is two federal cats with an easy to swap out setup.


After more research, I've managed to dig up some interesting bits. So the only converter's that magnaflow specifically lists as "proper" for a 1979 425 Cadillac is model 36006 and a "direct-fit" version which costs more, both of which are 2.5". They also make a 36009 which is a 3" converter, but it's only listed for late 80s f-bodies and caprice 9c1s with the 5.7. However, if you look at the executive orders on arb.ca.gov, they only have one for the entire Magnaflow 36000 series of converters, making me wonder if I couldn't get away with running the 36009. But then it says to contact the manufacturer for specific vehicle applications, so I probably couldn't.

The CARB EO number is D-193-86.

Here's some links if anyone is interested:


http://www.arb.ca.gov/msprog/aftermkt/devices/aftermktdevice.php?sortOrder=EO&DevSelect=75&submitDevice=Search+for+EOs (do a page search for "36000" or the EO number)



04-29-12, 10:40 PM
You could do true dual exhaust IF someone in the aftermarket had gone to trouble of certifying a system as CARB compliant. It has happened in other cases.


I doubt that anyone is going to check the model of the CAT to see if its the right one or call Magnaflow to check.
If you want to really be a member of the underground, fabricating a dual exhaust that looks factory, right down to the resonators, oddball curves and twists, quiet mufflers and outlets that look COMPLETELY factory, it can be done and has fooled inspectors in the recent past. I even saw this person stamp a GM logo on the pipes to make them truly appear legit.

04-29-12, 11:31 PM
I'd put on the 3" converter, run 3" most of the way back will help without being
obvious. That is for my 79 Eldo (403), FWD has no room for duals. Duals may
not help much with a single cat, attract more attention. If that doesn't work,
move to ILL. Bruce Roe

04-30-12, 02:46 AM
Yeah doing a factory looking dual system wouldn't be hard since there are already pre-made ones for the Impala SS crowd. They're 2.5", but for duals I think that would be fine. The question then would be if an H or X pipe would tip them off. Anyway, I guess I know what the answers are, and I just need to think about how many risks I'm willing to take.

05-03-12, 11:20 PM
I was reading some more and maybe a single 2.5" cat wouldn't be so bad. There was a test with some after market cat backs on a 5.4 F150. It had a 2.5" system stock, and some of the cat backs went to 3" after the stock cat/y-pipe which was 2.5" and they still saw some good gains. If I couple it with a nice mandrel bent Y from the manifolds, and then go to a 3" after the cat or y out again to dual 2.5" pipes I should be pretty well off right?

06-14-12, 04:01 AM
Okay, so if I get a dual hump crossmember, I noticed there's a brace that runs from the middle of the trans mount on a 45 degree angle towards the left front tire, but the Impala SS doesn't have that, so if I get a good enough crossmember I'm wondering if I need it. It's definitely in the way of running a pipe on that side.

06-17-12, 01:41 AM
So just another update in case anyone is interested. Got the 472/500 manifolds on finally, and I put on the head shield for the drivers side. I used the 500 style one. I had to take apart my steering in order to get it in. Now it's in, and I barely have 1/16" of clearance between the steering shaft, and the bulged out part of the heat shield. I guess I'll just hit that part real hard with a block of wood and a hammer to push it in a little more. On the other side, rather than cut my A-arm bolt or reverse it, I just got a shorter bolt. I've heard the A-arm itself can be an issue once the car is on the ground, so I'll see. It could be since it's lowered.