So my question is this. hopefully simple and easy to answer i'm hoping!

A while back (early October) I got a small stainless steel high flow cat. Much tinier than stock...never mind way smaller than stock.

After i got it put in i noticed that my durango doesn't down shift as quick. if ever at all.

I could be driving at 50 mph and my rpm is around 2000 instead being around 1450 or so.

its really killing my gas miles. I'm getting maybe 10 mpg now.

I'm certain that when i put my high flow cat in it did something to the airflow or back pressure that's making my durango stay in gear.

Has anyone ever seen this? i mean the stock cat had to be 18-20 inches long and have a round 8-9 inch round body vs my high flow which is only 11 inches long and has a 4 inch round body.

I don't wanna spend 60 bucks to have a shop put my stock cat back on and not solve the problem.

A buddy suggested getting my exhaust tuned but i don't know if that's gonna help anything?

DESPERATE HELP GUYS! IN A WEEK HERE I'LL BE DRIVING AROUND 25-30 MILES A DAY AND WHEN YOU WORK PART TIME AT MIN WAGE 10 MPG WILL KILL YOUR WALLET!!!

 

I would imagine that way smaller cat is giving your engine more back pressure. The OEM cat convertor is a high flow cat anyways and I'm not sure why you went so small.

 

yea idk why you changed the cat unless it clogged up, OEM on these trucks is high flow, no need to change it, sounds like you got a highflow cat, only its highflow in name only, not in function

 

i know shrp i know! it was that "young, loud is better" phase i was going through! i think its gone now. guy at work thought i had dual straights though when he heard the durango go buy this morning!

Well I had heard about it and when i looked it up i only found a few people who used it. BOTH DURANGO'S!!!! one is actually a 2002 4.7l. I thought it would be close to stock size. but it wasn't. and it was kinda cheap only 67 dollars shipped. it did what i wanted at the time...hope 45Autopete isn't around...it made my exhaust louder!

Which is what i wanted. but not at the cost of 2 mpg. which adds up quickly!!

But does this sound right? would a much smaller cat change the shifting like this? I'm assuming it has to do with the back pressure. its so much smaller that the air is being forced through much harder which is causing some sort of an issue?

 

I can't seem to think of a reason why a backpressure change would effect the shifting. The excess RPM's sounds to me like either OD is not working or the lockup convertor is not locking.

 

The exhaust giving back pressure to the engine can often times throw off the cam position sensor, thus reducing gas economy and power output. The power output that your cam sensor is sending can throw off the PCM and it cause weird problems, I've seen it before.

Typically is the cam sensor was acting up though it would send a code (which its not I'm guessing) so that leads me to believe that it might possibly be your muffler.

 

my muffler was a super 44, and is now a super 10. i don't think a muffler would cause any problems? i mean stock dodge durango mufflers flow good anyways right? I think its the cat. i know a guy who can weld and i talked to him tonight he says he's gonna look at my exhaust setup. if its not to bad he says he'll weld in my stock cat converter for free so i might be in luck? hopefully...we'll see.

 

It's a PCM thing.

Yes, back pressure could cause the inputs to not be in line when it's time to shift. Basically, the truck is thinking it's getting you foot stomped in it and wont shift.


Never change to a smaller CAT, always stay at least the same size and flow.

 

Yeah, See what hydra's saying is what I'm thinking. Alright well I'll hopefully get this taken care of. i was looking at my cat and realized my muffler is covered in rust after two months...the funny thing is its supposed to be 409 s stainless steel...

 

Oh and whitewidow when i ordered the cat i didn't realize it would be so small compared to stock. not until i actually got it in hand did i notice. I probably shouldn't have got it put on but i did. at least it didn't give me a check engine light. thankfully.