Adding a Cat

So my GMW has a dual exhaust setup that goes to the OEM rear muffler. It's straight piped, and welded solid on the passenger side of the car, no flex joints, the OEM muffler has broken its mounting from the muffler body, and it leaks, badly. 

I'd like to fix it so it's not leaking and hopefully not smell as bad and maybe change the tone of the exhaust a bit.

With previous searching I found a link to this site:

How To Calculate Muffler Size and Exhaust Pipe Diameter

And it looks like I can run dual 2 1/4 piping on both sides (rating of 371hp, beyond the power goal, but ballpark of what the engine is at presently)

So the plan is flex joints off the headers, then I'd like to Y into a single cat.

So if I run dual 2 1/4 piping, what size pipe do I need for the catalytic? The same chart lists 3" for comparable power levels, but even a hi-flo cat is more restrictive than straight piping. And after the cat, should I Y it back to dual or stick with a single pipe?

Engine is a crate 350 rated between 333hp-360hp since I purchased the car with it already installed and the previous owner told me one number from when he purchased it vs checking the same part number on GM Performance. (swap was also done about 6-7 years ago so GM may have made changes/adjustments).

It is running MSD Atomic EFI with a Edelbrock intake manifold and headers. 

Any advice/insight would be greatly appreciated. I won't be tackling the installation myself, but the local muffler shop is okay with guys bringing in specific parts for a job (mufflers, cats)

Get a high flow cat like a Dynomax

Wait, wait, wait.

Adding a cat is not just adding a cat.  If the engine management is not spot on, your catalyst will become a useless platinum cork in 1000 miles.  OEMs have years of experience and each little change they make to a tune requires months of ASTM testing to make sure it will work.  Cats require very specific temperatures and fuel mixtures to work at all, let alone not turn into a solid carbon plug.  Even with millions of dollars of testing and tuning, there have been hundreds of catalyst and engine management recalls, service bulletins, and known problems because once the cars hit the real world, the catalysts fail because of a tuning issue.

I very strongly applaud your wishes to help my lungs, but it's not just an "add catalyst, save the earth" proposition.  It's like tossing a random carburetor on something.  It might run OK out of the box if you know what to buy, but will still need tuning, but adding any old catalyst to it might be like bolting a chainsaw carb onto a big block.

Absolute best case scenario, adding a catalyst at home might do a tiny bit of good, but it likely won't help at all and you run the risk of wasting your money on an expensive part that ends up becoming packed full of carbon in a matter of months.

OEMs go to extreme lengths to tune the engine management to delicately balance NOx, CO, HC, and other factors.  Aftermarket EFI kits for old-school V8s focus on running well with zero R&D on emissions.  Aftermarket EFI is an electronic carburetor replacement and lacks the complexity needed for emissions control.

Go for it if you want (and thank you for even considering it) but my guess is no more than 1500 miles before you're taking it off because your engine starts sputtering and giving you trouble.

Can you better describe what your powertrain is?  It's too hard to tune it to be really effective and run safe.  BTDT.  

Year, what kind system is running it, sensors, etc.  

If I were to set it up, I'd run a catalyst on each bank.  Easy to set up, less restriction, etc.  

In reply to Curtis73 :

FWIW I ran 4000 miles with a cat on a self-tuned turbo Volvo, and when I took apart the car to v8 swap it, the cat looks perfect.  It's not THAT hard to do an acceptable job at tuning.

Well, there isn't exactly an OEM recommended cat for a crate small block chevy in an E36 coupe, so I'm kind of left to attempt at figuring something out.

The MSD does have at least one oxygen sensor as it is now, what information it shows me I haven't monitored too closely as readings from it are skewed due to it being installed into the pipe that has various leaks, and I have focused on the general running condition of the engine overall.

I have to get the exhaust repaired before taking the car in for proper tuning, and it needs proper tuning due to various strange running issues it currently has and the previous owner told me it was 'never optimized or tuned beyond getting it to run' and the builder telling me 'it was tuned to the best it could be tuned' while also admitting that things like the fuel pump probably should have been replaced and the random stalling when decelerating had been happening since day one so he figured it was just a fault of the MSD.

So I want to get it properly tuned. With some kind proper exhaust installed so that it makes enjoyable noises and doesn't get me excluded from the possibility of using it on track IF there is a cat required.

I will be researching this extensively before I can a) afford to get the exhaust pieces b) afford the proper tuning. I'd ideally like there to be maybe a weeks time between exhaust install and getting it properly tuned. But I will definitely see what the present o2 sensor IS monitoring and see if any universal high flow/High effiency cats have listings for optimal temperature range, be it magnaflow, dynamax, borla etc.

Cool.  

Before you put the catalyst on it, get all of the demand table to run 14.7 for driving conditions up to about 80-90 part pedal operation.  Just doing that will make it run reasonably well- we are not talking SULEV30, but it will work.

At WOT, as long as your are putting them back far enough, running at peak power 12.5:1, you will be good to go.  

Once you get it running well have have a nice sealed exhaust, then put the catalyst on.

As far as I can tell, it's THIS engine, and I would believe THIS MSD variation, I do know it has further MSD ignition components as well. This is all packaged into an E36 coupe 

You can make it work reasonably well.  Better than the same set ups 30 years ago, for sure, with the WB sensor in the exhaust.

Just did a quick look online for what's available- given the prices of them, they don't have much on them, but they have EO's to suggest they have something....

And I'm seeing options that are reasonable enough that you can get one for each bank- so how big of a pipe do you have from your headers?   And then do you have 1-2" larger somewhere down stream of the header for space?

My Rabbit with CIS designed to go full rich at full throttle never killed its cat, so I would expect you to be good, but I would also vote for just putting in two.

Matthew

I came here expecting this:

Leaving disappointed.

Matthew Kennedy said:

In reply to Curtis73 :

FWIW I ran 4000 miles with a cat on a self-tuned turbo Volvo, and when I took apart the car to v8 swap it, the cat looks perfect.  It's not THAT hard to do an acceptable job at tuning.

We'll just agree to disagree I suppose.  It may have looked perfect, but chances are it was likely completely spent and no longer a catalyst.  Once that happens, the honeycomb goes from "looks good" to "solid chunk of carbon" pretty quickly.

I bought an S10 that had an LS1/T56 swap in it.  It had a Holley carb and an MSD LS spark box on it.  Granted, not the same as what we're discussing, but I'll expand on that in a minute.  The guy who built it put cats in it and he drove it twice before he sold it to pay for college loans.  Within 3 days and a few hundred miles of buying it, the catalyst was spent and completely coated with carbon.  It still looked perfect, but was completely ineffective.  If the catalyst doesn't reach its proper temps before too much HC passes over it, it's done.  Spent.  Kaput.

The difference we're discussing is huge.  The difference between an aftermarket EFI and OE EFI is massive.  Couple that with the fact that an aftermarket EFI doesn't have the 35 engineers, 8 months, and several million dollars of equipment to test different tunes, and it just has such a slim chance of working.

A carburetor is like pooping in the woods on a log and wiping with a pine cone.  OE EFI and emissions is like a marble bathroom with an android-operated bluetooth bidet.  People think that an aftermarket EFI is like that same marble bathroom, just maybe with cultured marble and without the bluetooth bidet.  It's not.  Its more like a nice outhouse with a corn cob.  Its much closer to the log and the pine cone than people think.

In reply to Curtis73 :

FWIW, for an OE calibration, there's maybe a half dozen engineers who work on it.  2 for dyno at the beginning, one dyno engineer as it goes on, and the 4 to 5 (depending on the timing) for the vehicle development.  And there's no "slim chance of working" about it- you have to make sure it is reliable and is going to pass at 150,000 miles.  It has to be spot on and really robust.  

The nice thing about Dj's likely goal is the easy part of development- fully warmed up emissions.  And he's not requiring ~100% efficiency.  It may not make any requirement that I've ever had to do, but it will work, and it will make the car a lot more pleasant to drive for long distances.

I don't disagree, I just think it's a real pipe dream (pun intended) that he'll get it to work at all.  I'm afraid he'll spend thousands of dollars and a year to get it to work poorly, or maybe not at all, and he'll have to remove it anyway.

Like I said, I applaud his motivation, just offering a counterpoint to prevent disappointment.

My 35 engineers number was an exaggeration.  We had about 32 at Ford and 30 at GM, but that was for the full line and multiple projects.  I don't think we had more than about 12 on any engine at one time.

In reply to Curtis73 :

I know how both aftermarket and OE EFI work.  I'm a developer on the rusEfi open source ECU project.

I'm not sure what sort of improvement you think there is to make once the car is running within +-0.2 AFR of target at all times, within about 10 seconds of startup.  Closed loop fuel the whole time, yes even at wide open.  The light-off of course isn't going to be as fast as an OE calibration, but it's going to work fine once things are warm.  Nobody here is trying to go for SULEV, we're just trying to make the car not completely terrible for the environment, just mostly terrible for the environment.

Thousands of people have used aftermarket tunes (and turbos, and...) with the stock catalytic converters in their cars without jamming them up.  The point is conceded that the catalyst may not be operating within optimal parameters, but killing the cat is far from a foregone conclusion. 

In reply to Curtis73 :

He's got the EFI system already- so there's $0.  The catalyst I'm seeing that have EO- which say they are tested to be effective - not great, but they at least will do something- are less than $100/side.  So it's not going to be thousands.  I fully know that the $100 aftermarket catalyst is nothing like what I put into production, and they will likely not get lit off all that well.

But relative to just running when he's driving around, they will work, and it's not all that expensive.

As for the time, again, given what that EFI can do, there's no need to spend a year calibrating it.  I know for me, if I have a good map of the engine already, getting it to make emissions does not take a year.  W/o the testing I can do and the extra tools that the EFI I work with brings, there's no need to spend the time.  It's all about getting the base map right, and then following that up to get the transient compensation spot on.  Once you do that, that's all that can be done.  If it runs around the target +- more than people think, it will work.  The less it hangs on one side or the other, the better.

Caitlyn Jenner has subscribed.  

Cats on the Molvo with Fitech EFI are gone. I put them on because I did not want the smell of unburnt fuel like I have with the MGB. EFI and Cats, how much more civilized could it be?

 Spent all last year chasing ghost with the Fitech TBI and yes, most of it was systems related. But  when you dont even know what you dont know, it is a bitch to sort things out; especially when the system is changing the whole time. 

In reply to alfadriver :

Presently, the pipes from the headers are a mess of various diameters, so the plan is to completely replace the entire system with new and uniform pipes. As for space, I should be able fit something closer to the headers for optimal heat to the cats. Ideally it would be 'header - v-band - flex joint - cat' then route the rest of the piping to the rear for a muffler.

NOHOME said:

Cats on the Molvo with Fitech EFI are gone. I put them on because I did not want the smell of unburnt fuel like I have with the MGB. EFI and Cats, how much more civilized could it be?

 Spent all last year chasing ghost with the Fitech TBI and yes, most of it was systems related. But  when you dont even know what you dont know, it is a bitch to sort things out; especially when the system is changing the whole time. 

Which is why I don't recommend those systems, Or TBI, TBI is only marginally better than a carb.

 I think it is a carrot on a stick to get guys to try EFI, but it falls short.

In reply to bentwrench :

TBI has the POTENTIAL to be better than a carb. If not installed properly by people who dont know what they dont know, it has the potential to be an expensive Mongolian Clusterberkeley. Or a great learning opportunity depending on how you see this stuff.

Pete

Consider contacting Solo Performance. They offer a line of high flow CATs that seem to last well and do the job (I have one on my Pontiac).  Their service is very high quality.

https://shop-solo-performance.com/en/search?controller=search&s=cat