New exhaust, performance issues

[Johnnyb]

I went ahead and pulled the trigger on a new exhaust system. I went from dual 2-in pipes to a single 2.5 in pipe with a big flowmaster muffler. In the process I also had a 2.5 in aftermarket cat installed in hopes of obtaining the cleanest exhaust smell possible. My EFI runs at 14.7 to 1 at Cruise so I thought this would be compatible.
What I find is that I have replaced one smell with another, now the exhaust smells like burning matches on deceleration and sometimes at idle.
Worse yet, I find that my performance is drastically decreased at highway speeds. Pulling hills like Ash fork to Flagstaff or Verde valley to Flagstaff, I used to be able to stand on it and accelerate at any of those approximately 6% grades while going 75 mph. Now I can't even hold 75 miles an hour while going up those grades.
I am preparing to have the cat removed but my question is, is it the cat or is single 2.5 tube not enough? BTW, it is a roller 5.0 with Edelbrock proflo 4 and wild horses shorty headers.

THX,
JB

 

[Wild horse 75]

We always ran dual 2.5” on 5.0L mustangs. Your single may be too small but I have a feeling you put a cheap cat on it and that’s your big restriction. There are huge differences in cats. Cheap ones make up for lack of precious metals with lots of substrate to slow the exhaust down to give the cat time to do its thing. There’s nothing wrong with running a cat on an older vehicle especially one with a more modern engine and fuel injection but you don’t want to cheap out. Personally I would have done 2.5” off the headers into a single 3” ”high performance” cat and single 3” exhaust.

 

[1969]

Pulled this from one of the Mustang forums.

 

[Johnnyb]

Pulled this from one of the Mustang forums.

View attachment 960875

That is very useful. So with 2 X 2" pipes, I was at 288 on the chart and I estimate that the motor should produce around 250-300 HP in present configuration (1995 Mustang GT block, ProFlo 4, Shorty headers). With a single 2.5, I have gone down to 232. Looks like I need to have a dual re-installed.

THX,
-John

 

[Bruners4]

Or up the single pipe from 2.5" to 3".
I run 2.5" from the mid-length headers to a 2 into 1 into 3" to a ProFlow muffler and out the rear without a CAT. I don't have your inclines to test on, but I don't feel a lack of power.

 

[DirtDonk]

While I can’t say it’s not a problem, it doesn’t seem like a single 2 1/2 would be much of a restriction.
At least not enough to do what you’re describing.
Most of us with single 2 1/2’s don’t complain about such. I can’t really compare apples to apples with my 5.0 because I have ultra low gearing and that would kind of overcompensate for any restriction up to a point.

Can’t argue that a single three would certainly open it up. Or duals! But I would first do what was suggested and at least temporarily remove the cat to see if that makes a big enough difference to prove the theory.

 

[DirtDonk]

Oh, and a generic semi-modern Chevy Silverado pumps approximately 370 hp and no lack of power going up any hill, through a single 2 1/2.

 

[serial car restorer]

I doubt the "lack of flow" of the single 2 ½" is a serious issue below 5000 RPM...

I'd suspect the cat.

 

[Johnnyb]

CAT is coming off tomorrow. This is what I installed: MagnaFlow 59956 - MagnaFlow Universal Spun Catalytic Converters

 

[Broncobowsher]

2.5 should be fine. I have 375HP in a cargo van through a single 2.5 exhaust.
As mentioned, I think the can just isn't high quality in the flow department. Good cats don't have any noticable backpressure. I'm guessing you got a bum cat. If you were running duals I doubt you would notice it. Now I need to look at a couple of aftermarket cats I have sitting on the shelf.

 

[DirtDonk]

Surprised that a Magnaflow wouldn’t be a high-quality though. Should be more than up to the task.
That’s what I have online in fact. Maybe even the same part number.

If it turns out to be the culprit, I wouldn’t hesitate to call and notify them.

 

[ntsqd]

Flange the system so that you can put another one back in, and have a "test pipe" made.

The Flowmaster guy at SEMA a number of years ago warned me not to go 3" with a 302. That was after I'd already done exactly that. The exhaust can move too slow thru a system and when it does it misbehaves. In my case it droned so badly that you simply did not go that small range of vehicle speeds. You've maybe heard of the "Brown Note" myth? This was almost that bad, but real.

 

[Johnnyb]

I had the CAT cut out last week and it immediately seemed a little bit happier. I had no real baseline until I pulled Ash Fork hill (which I just did 20 minutes ago), and I can safely say that I am almost back to where I was with dual 2" pipes. I was almost able to hold 75 and my foot was on the floor the whole way, whereas previously I had a little room to spare.
I got curious and ran my motor specs through AI and it crapped back out that I should be making about 345 horsepower. That seems a little high to me, but maybe I have always been exhaust limited based on the chart posted by @1969 .
I guess the question is now is whether it's worth it to have the y-pipe and everything back swapped out for 3"?

THX,
JB

 

[Broncobowsher]

I call that 'paper horsepower'
On paper, computer simulations, it's going to be great. Actually in a vehicle, so-so

If it thinks you are going to make 345HP, you have enough speed parts that it will want some RPM to be making that HP number. My guess is the gearing isn't matched to the engine and tire size. I've run those hills with a 351 that actually dynod 250HP (at the tire) and could pull those speeds with a trailer and plenty of reserve. It took some RPM to pull that off, but that is where the HP is at.

torque is what moves you. horsepower is just torque at speed. If your 345 HP is at 5000 RPM, but you are cruising at 2500 RPM, even if the engine is making the exact same amount of torque, your HP is half. For that paper HP number, I'm guessing the cam makes more high RPM HP/torque than low, at 2500 RPM you might be lucky to make 150 HP.

 

[Johnnyb]

I have 4.11's and 35's. I am turning about 3150 at 75 MPH. Compression is 9.0 (151 PSI compression), cam about 210 Degrees, roller block, shorty headers, y-pipe, 2.5" all the way back, BIG Flowmaster (FX) muffler. I am pleased with the response up to about 2500. Seems at least equal to what I had before. It is just the freeway hill climb where I seem a little bit lacking. Would also hurt in big sand. Since I run a NP435, there's no overdrive and no real possibility of downshifting unless I bog to about 50 MPH.
I am thinking about doing a back pressure test on the road. Right now the EGO is right at the Y, but I have bungs in the headers. Maybe I could temporarily move it to one of the headers and get pressure off the Y-bung while climbing a hill? I know unloaded, it's not supposed to be over 3 PSI, but what would I expect wide open at 3K RPM?

THX,
-JB

 

[ntsqd]

No way would I go to 3" on a 302 in a Bronco. BT, DT in the Ranchero, it wasn't pretty. Maybe 3" with a Boss 302 in a Vintage Trans-Am car. I'd call 2.75" the max that I'd go.
Now if you have a 347 or a 351, then a 3" tube will work.

 

[Grey owl]

I have 10 to 1, with 280 .490 lift cam aliminum heads larger valves, aluminum intake, roller rockers, 650 double pumper, long tube headers dual exhaust. I am guessing 300 hp range.

I wouldn't use 3 inch exhaust. A 302 would not push the exhaust at a high enough speed. You may make it even worse.
Going to single exhaust is goning to make a difference