more power with cats, why?

[GREYFOX4INCH]

ok i did a search and what im posting is the oposite of everything i have ever read or experienced. i have shorties,OR/H PIPE, & flowmaster 2 chambers. it was real nice sounding and loud but i just put on the stock h-pipe with 4 cats, the car is faster then is was without them.i have no traction at WOT in 2nd gear nomatter what the rpm's are, and its going sidways well into 3rd gear??? before it would just burn up 1st and second, with third chirps.
the only thing that i think would make the diff is maybe a more accurate o2 sensor reading,and a more correct air/fuel ratio. i think off road pipes can cause rich mixtures right? also i have not gone for emissions yet, i took it out for 2 drives with a total of about 20 miles on it, has my computer ajusted to the change of equipment? does it do it immediatly or do i have to drive it more?????
i dont have the air pump on so im still not sure if it will pass the test.

 

[larrendeuce]

ok i did a search and what im posting is the oposite of everything i have ever read or experienced. i have shorties,OR/H PIPE, & flowmaster 2 chambers. it was real nice sounding and loud but i just put on the stock h-pipe with 4 cats, the car is faster then is was without them.i have no traction at WOT in 2nd gear nomatter what the rpm's are, and its going sidways well into 3rd gear??? before it would just burn up 1st and second, with third chirps.
the only thing that i think would make the diff is maybe a more accurate o2 sensor reading,and a more correct air/fuel ratio. i think off road pipes can cause rich mixtures right? also i have not gone for emissions yet, i took it out for 2 drives with a total of about 20 miles on it, has my computer ajusted to the change of equipment? does it do it immediatly or do i have to drive it more?????
i dont have the air pump on so im still not sure if it will pass the test.

Cats shouldn't make a difference on your 02 sensor reading. The EEC IV used in the 87-93 Mustangs uses 2 sensors before the cats, so anything added after the cats shouldn't affect the readings. Is it possible the old pipe had a leak, or the crossover was improperly sized? Did you change anything when you swapped the pipes.

 

[10HoleLX]

Your engine needed the back pressure produced by the cats.

 

[vristang]

O2 sensor readings may be different if their placement is different. Their operation is dependant on temp so location will make a difference.Stock cats may need an air pump. They have been known to plug up without the air pump.

If the aftermarket H-pipe is poorly designed then I can see losing that much power there. But I don't think this is likely with any of the major brands out there. Not with all the dyno testing that the mags do.

 

[HISSIN50]

You are probably doing WOT comparisons (all the 'breaking it loose' comments), and O2's are not utilized at WOT.

I dont buy into backpressure stuff. There is definately wave and flow tuning with regard to scavenging, but that is different (I feel it is often misconstrued as 'more power/torque from backpressure).

I can think of no logic excuse if the old pipe was in good shape.

 

[v8only]

it doesn't make sense to me either. You really need either a before/after timeslip or a dyno run to really see what's going on. Seat of the pants feel can be very misleading.

 

[GREYFOX4INCH]

You are probably doing WOT comparisons (all the 'breaking it loose' comments), and O2's are not utilized at WOT.

I dont buy into backpressure stuff. There is definately wave and flow tuning with regard to scavenging, but that is different (I feel it is often misconstrued as 'more power/torque from backpressure).

I can think of no logic excuse if the old pipe was in good shape.

well yeh, im not real concerned with how much power im makeing at certain RPM'S
and to be honest i dont realy care how much HP im making at all. all i care about is how fast it will go in a strait line with my foot to the floor. i like test and tune and street racing on occasion, but other than that i just like to be pulled back and slammin gears, we all do.

"Seat of the pants feel can be very misleading"--- v8only

i totally agree i have riden in faster cars that feel slower just because of different rear end gears.its missleading. and maybe its not actualy faster or more powerfull than it was before, but it does feel it. i mean im driving the same and shifting the same but the car (to my standards) is performing better with the stock h-pipe.
"vristang" said that O2 readings depend on temp, could more back pressure create more heat infront of the first set of cats changing the readings even though you say they do not function at WOT?
this is not a verry important topic i just wanted to hear what other people had to say about it, maybe it should be in talk. i dunno
an updated 1/4 mile time wouldnt be much help because its gonna be faster this time around no matter what,i guess it would have to be run on a dyno with both pipes to compare. not that i care about a 10 hp difference that much anyway.
the sound of the off road pipe makes my decission on its own

 

[mr.b]

was it colder outside the day it got all sideways. It could have just been cold tires and asphalt.

 

[dead]

The stock cats are incredibly restrictive- unless you had some major exhaust leak or something, I think you're just crazy

 

[Paul Perreca]

it doesn't make sense to me either. You really need either a before/after timeslip or a dyno run to really see what's going on. Seat of the pants feel can be very misleading.

well said

 

[Daggar]

it doesn't make sense to me either. You really need either a before/after timeslip or a dyno run to really see what's going on. Seat of the pants feel can be very misleading.

Add to that, a retune for each pipe.

 

[5.0tbirdguy]

When I put shorty headers on mine with no cats and a mid mounted Magnaflow I dumped the exhaust at first. Lots of top end but seemed the same down low.

When I installed a complete system with rear mufflers it gained way more bottom end with the top seeming the same as dumped.

My stock cam, heads, and no custom tune need backpressure.

It could be just too lean down low with open exhaust. No telling what is actually happening. Maybe sucking the air/fuel mixture right out the tailpipe with open exhaust.

If you had time and money for a dyno tune with open exhaust there may be MORE bottom end than with the cats.

No matter what you should have more high rpm power with open exhaust unless its too lean.

 

[GREYFOX4INCH]

The stock cats are incredibly restrictive- unless you had some major exhaust leak or something, I think you're just crazy

yeh your prob right

was it colder outside the day it got all sideways. It could have just been cold tires and asphalt.

it was cold out the first time,and i discounted it as a surface temp/traction issue, but it is responding the same way in warm weather also. i made sure to remove all pavment/tire variables from the picture before accepting that it was a difference with the car, and not the condidtions.
like i said, on a dyno the car is most likely makeing more power without cats.i mean thats what an offroad is for,
but it seems that the power curve has made a big change with the swap.
this is powershifting without letting off the the gas,(stupid? yeh but tooo fun)
so i dont know what rpms the clutch is grabing, i shift fast enough that the rpms are not jumping between shifts.
oh yeh, i went for inspection today and they failed me for "the car being to loud"
with 40 series muffler cat back and stock h-pipe???????[massachusetts]
now im supposed to put stock mufflers and flow tubes on it.
this is shank, the things quiet!!!!! they would not even run emissions.

 

[7upstang]

The back pressure of the cats increases your bottom end. More noticable on some cars than others. You are probably making less horsepower after 2500RPM, but the car will launch harder and seem like it has a ton more torque when you put the cats on. MMFF magazine made a comment about this a few years ago when the went from an offroad HP to a catted Pipe on one of the cars they were building.. They mentioned that the torque was amazing after putting the cats back on. The car ended up losing horser power on the dyno though.

My 2 cents

 

[GREYFOX4INCH]

yeh that sounds like it makes sense considering what im experiencing.
it may depend alot on how the car is set up and the engine build. maybe some cars wont react the same way. the o/r h-pipe is gonna go back on the car, but its kinda cool that i got different results than i expected. i thought it would just feel slower plain and simple. i just wanted to know if there was a good reason why it seemed more ballsy. i bet the 4.10 gears are helping me notice this as well.
thanks
anything else?

 

[vristang]

The back pressure of the cats increases your bottom end. More noticable on some cars than others. You are probably making less horsepower after 2500RPM, but the car will launch harder and seem like it has a ton more torque when you put the cats on. MMFF magazine made a comment about this a few years ago when the went from an offroad HP to a catted Pipe on one of the cars they were building.. They mentioned that the torque was amazing after putting the cats back on. The car ended up losing horser power on the dyno though.

My 2 cents

I have heard this over and over again. I have also heard from University Professors that there is no logical reason for this phenomena, and that it is most likely due to other factors. I don't know which is true but I would be interested in seeing that article in MMFF. I don't suppose you could let me know what mo & year it was?

jason

 

[7upstang]

Oh Gawd. That article was years ago. I'll go through all of my back issues and see if I can find/scan it. I'll bet you the article was 5 or 6 years ago though. But i'll try to find it just so I have something to show I'm not talking through my hat.

Cheers

 

[blackstangt]

The back pressure of the cats increases your bottom end. More noticable on some cars than others. You are probably making less horsepower after 2500RPM, but the car will launch harder and seem like it has a ton more torque when you put the cats on. MMFF magazine made a comment about this a few years ago when the went from an offroad HP to a catted Pipe on one of the cars they were building.. They mentioned that the torque was amazing after putting the cats back on. The car ended up losing horser power on the dyno though.

My 2 cents

yea then they knowingly test a bunch of intakes, running no mufflers or mid-pipe

 

[vristang]

Oh Gawd. That article was years ago. I'll go through all of my back issues and see if I can find/scan it. I'll bet you the article was 5 or 6 years ago though. But i'll try to find it just so I have something to show I'm not talking through my hat.

Cheers

If it is going to be a major hassle to scan it in, then don't worry about that. I have issues from about 97.

Has anyone ever catalogued and indexed the MMFF and 5.0 Mustang mags? I have wanted to for along time, but it sounds incredibly tedious. I ask because maybe someone has and posted it somewhere.

 

[Seanie]

It's probably not a loss of "backpressure" per se, but a loss of exhaust velocity. If you aren't moving enough air to take advantage of the less restrictive exhaust you could be losing power. Basically something akin to the scavenging post made up a bit...