I feel this warrants its own thread. Perhaps this can become a sticky or Tech Info article in the future. I do not believe we have readily-available information on properly choosing a catalytic converter for our cars, and it would benefit myself and others to have a healthy conversation where we can discuss what we know about converter size in relation to engine displacement/output.


I just got off the phone with a Magnaflow tech adviser. I asked him which cat converter would be appropriate for a '97 Bonneville application, and he recommended a 94000 series that is rated for up to a 378 CID motor. I thought this was a bit unusual, because a 3800 only displaces 231 cubic inches, and there are smaller Magnaflows that are rated for less displacement.

I asked, "Wouldn't a larger body restrict my exhaust more than necessary? I'm only running a 231 CID engine." His reply was that there is a negligible difference in flow between larger and smaller models, and that a smaller one "would get too hot" on my car. His opinion was that the Bonneville needs a 94000 series.

Example: Here is the information on the Magnaflow 94000 series converters




...and here are the stats for the 91000 series converters



As you can plainly see, the 91000 series should be able to handle our engine displacement. However, the weight rating could become an issue. It would be quite easy to exceed 4500 lbs. with a full load. Perhaps this is why the larger 94000 series was recommended?

The only troubling factor is that CarSound'* website (when the server is actually up :?) recommends a 91000 series cat for a '97 Bonneville, which is contrary to the Magnaflow tech'* advice. I'm not sure what to believe, whether a larger cat actually flows the same or not. My fear is that a large converter would be overkill, and would keep things from flowing efficiently. I would like to understand more about catalytic converter selection.

Another topic that should be addressed is O2 sensor placement. Magnaflow does not recommend a converter with an O2 sensor bung for our application, because the stock O2 installation is in the exhaust pipe. Is this valid advice, or is it easier to order a converter with a bung?

Let'* talk about this kind of information and decide what it means for us in choosing a high flow catalytic converter. I want my exhaust to flow optimally, to maintain legal emissions standards while allowing the most power in a given setup. I want to spend my money on the right product the first time around, so that I'm not forced to buy another unit when I discover that I purchased the wrong item initially.

I'm eager to see what we can find out about this topic!

I am running the Magnaflow P/N 94106 on my car.

I'm also running a 94106 and haven't seen any loss in low-end torque.

Seems that is a 'shorter version of the 94000 series' as listed by Magnaflow, and has specifications identical to the 94000 series shown above. I assume those do not have an O2 bung?

Very cool. Keep it coming, guys!

EDIT: Perhaps we can also elaborate on the difference between a 2-way and 3-way converter, and the reasons for choosing one over the other.

I google.
"A Two-Way converter, used on American cars between 1975 - 1980, oxidizes unburned harmful hydrocarbons and carbon monoxide into water and carbon dioxide. The first vehicles with catalytic converters had Two-Way reduction only capabilities.

A Three-Way converter is a triple purpose converter. It reduces nitrous oxides into nitrogen and oxygen. And, like the two-way converter, it oxidizes unburned harmful hydrocarbons and carbon monoxide into water and carbon dioxide.

A Three-Way + Air converter performs the same functions as the Three-Way converter. It oxidizes and reduces. The difference is the addition of secondary air between the two internal catalyst substrates that improves the oxidation capabilities of the converter. The secondary air is pumped into the middle of the converter between two separate catalyst coated ceramic substrates. The front ceramic performs the reduction and the back ceramic performs the oxidisation. Its like having two converters in one."
http://www.bba-reman.com/cats.htm

I'm using a round 53000 Series 3" in/out cat right now.
Specs:
-------------EPA -----------------------------CARB------------
256 CID / 4.2L / 4250 Lbs | 244 CID / 4.0L / 4250 Lbs

Ok..... I deserved that one

Based on the info from www.justmagnaflow.com, the 94106 that ssei1995 and 96SSEi_tampa use is only a 2-way converter. So, aside from simple definitions, why would it be better to choose a 2-way, a 3-way, or a 3-way + air? Are there flow advantages to any one of them? Will you pass emissions more readily with one kind over another? How does price factor in?

Mike, although Google can answer most questions, I think it would benefit members to have the information here in this thread. That way there is a one-stop information source for catalytic converter advice, and people can make their decisions based on Bonneville-specific info. So thanks for pasting the information into your post.

Keep posting guys! We need opinions on the best converter models and whether 2-, 3-, or 3-way + air is the best choice. Let'* turn this into something worthy to be placed in Tech Info.

I'm guessing just by looking at the information there:

3-way + air > 3-way > 2-way when it comes to emmisions and the environment, but material has to be added to the converter to accomplish reduction in a 3-way so they probably flow a little less than a 2-way and cost more. With modern technology I don't think a 3-way would impede exhaust flow that much more than a 2-way though, so it would all come down to cost, how strict emmissions in your state are, and your personal feelings about emmissions.

I wasn't really poking fun ben, I'm mainly laughing at myself for how much I use it.

Well I figured I should have used Google to find out myself. It'* all good. Sorry for the misunderstanding! I'm glad you posted the info.

Can we determine any scientific advantages of one type of converter over another? They are a few dollars more expensive to get a 3-way, but I'm not sure how flow is actually affected.

I'm all for helping out the environment, but frankly, I want the minimum restriction to still be legal here. If the 2 way passes NY State Emissions and is less restrictive, thats the one I'm putting on my car.