High Flow Cat Install question 4.0 SOHC

[JOEZ33]

My 98 SOHC 4.0 has stock exhaust for now, but I'm about ready to order all the parts to upgrade my exhaust system including the catalytic converters. My thoughts are High Flow Cats and a 70 series Flowmaster. I'm not aiming for power, and want to keep it quiet on the interior. I don't care if it's dual outlet either.

My main question is- What is the stock diameter of the catalytic converter inlet and outlets?

A little bit about my reasons for doing this. A while ago while troubleshooting a suspected "Plugged" catalytic converter, I emptied one of them out (because it was 400 degrees hotter than the other). Although in the end the cat was NOT my cause of the problem, I didn't replace it for over a year after emptying it. During the last year I did some minor aerodynamic and computer upgrades to help the mileage, and got it up to a consistent 29 MPG if I stayed 55 miles per hour or less. I finally replaced the catalytic converter with one on eBay, brand new made by magnaflow. Immediately I had lost a ton of power and my mileage is now limited to 23 MPG at 55 MPH. It's been like this for the last 6 months now- I want my mileage back. I blame it on this catalytic converter, which happens to be much smaller overall size than the stock one.

Thanks guys for your info.

 

[CDW6212R]

Interesting, the OEM piping should be 2.25", so select that inlet and outlet size. What size was the converter that you used?

 

[JOEZ33]

Not sure what size the converter was, but it was a direct bolt in replacement including the flange for the manifold, and the other pipe for the drivers side as one big assembly. It has the same exact pipe size, but the outside diameter of the converter is much smaller. I know this was the only change I made when my mileage took a huge dive. I've given it plenty of time to relearn the computer, and have done quite a few little tune ups etc. to get teh mileage back up.... I'm lucky to even get 23 MPG @50-55 mph now, it's horrible. I've tried retuning the computer with the sct, not much I can do except WOT mixture and timing advance. I'm thinking about putting the stock program back in to see what happens to the mileage.

 

[CDW6212R]

FYI, the pre-2002 PCM's do not have learning capability like you read about. It is interesting that you would lose mileage from a new converter. I wonder if the brick design was more restrictive than OEM.

 

[JOEZ33]

I had gotten used to how the truck drove and how it pulled out into traffic, merged onto the interstate, and how much fuel it consistently burned. Then one weekend I finally had the time to install this thing, unwillingly, and have seriously regretted it. That day after completing the install, I took it onto the interstate to burn off that new exhaust pipe smell. I was so pissed when I tried to merge into traffic and almost couldn't merge without slowing everyone else down- I have to practically floor it now, before I only lightly stepped on it. It must definitely be more restrictive, by a large margin. It's not just in my head, the gas mileage proves it also.

 

[JOEZ33]

Ok, guess I'll go with the metal substrate 2 1/4 inch high flow cats, and a 70 series big block muffler. The muffler has dual 2 1/4 inch inlets and a single 3 inch outlet. That should get things flowing pretty good. My only other hope is that its still just as quiet on the interior.

 

[Cobraguy]

You know, Magnaflow makes some pretty darn good stuff. Have you contacted them about this? I'm sure they would be very interested in hearing your story. You have to have a defective unit...or a unit designed for a different application.

 

[CDW6212R]

They do make a huge number of cats, various sizes etc. I'd like to know what would cause that much of a difference with a converter that should be correct.

 

[aldive]

When removing/replacing the cat, was there any chance you damaged the O2 sensor(s) wiring?

 

[JOEZ33]

well... it's been so long that I didnt think it was worth contacting magnaflow. The O2 wires are fine, and no CEL either. There was a problem with the O2 mount though- they put it on top of the pipe so that the O2 sensor was hitting the floorboard. I was forced to not use the exhaust support right at the aft of the transmission just so that the exhaust system would stay down far enough to avoid hitting the floorboard. It wasn't built very well.

 

[Afboy143]

I personally dont run cats, but when I did I used these

http://www.performance-curve.com/index.asp?PageAction=VIEWPROD&ProdID=1058&HS=1

I never had a problem with them- Always worked without a flaw. Get one and have a shop weld it up. Are you getting a CEL light with the new cat installed?

 

[JOEZ33]

that converter is what I'm planning on using. Does it need a heat shield?

 

[CDW6212R]

The O2 sensors would be more likely to make such a difference, but they should throw a CEL light if they weren't working properly. I have two of those linked above, in 2.5" size, to go in my V8 truck. They look well made and the brick isn't very long. At 4" in diameter they shouldn't need a heat shield. If you do that try to get the cat out from under the trans pan, to keep the heat away.

 

[JOEZ33]

Yeah, I plan on keeping the left one further away from the tranny, possibly a heat shield under the transmission anyway.

 

[CDW6212R]

I would love to have bought an aftermarket trans pan to cool the fluid better. The cat is way too close, and underneath it.

 

[Afboy143]

no heat sheild needed. The cat I linked above is very high flow and pretty small so itll fit pretty much anywhere a straight piece is. Good luck!

 

[JOEZ33]

Thanks, I'll post some before and after pictures of the setup, and keep a more accurate record of mileage and driving conditions before and after. Maybe I'll get my 29 mpg back (at 55 mph)...

 

[Ray Lobato]

Just a quick note here.
The stock OEM Cats are high flow.
Unless you are going to race your truck, you really don't need to be replaced them, as it won't make much difference, if any for daily driving.
Spend you money on something else.

 

[aldive]

Just a quick note here.
The stock OEM Cats are high flow.
Unless you are going to race your truck, you really don't need to be replaced them, as it won't make much difference, if any for daily driving.
Spend you money on something else.

Amen.

 

[foxy]

The only thing I didn't like was the placement of the oe cats.Out here in OZ we have only 2 o2 sensors and they are just before the cats .I up graded my system to 2 1/2 " system and put a single 3"high flo cat after the exhaust box . Yes it does sound throaty but it is one of the better mods I've done Cheers Barry