Cats=torque?or no cats=topend?

v8only · Jan 31, 2008

ok...same combo, same dyno, same temps, NOT the same day

jimdrechsler · Feb 1, 2008

would hollowing out the cats be as effective? i have the h-pipe with cats. i would love 21 HP gain if i can hollow them out IF I gain that much!!!

v8only · Feb 1, 2008

still running the stock catted h pipe??

remember, that beast is only 2.25" thick, not 2.5 like the aftermarket pieces.

I'm sure you'll get a nice gain though, no doubt.

just be prepared, if it's your dd, it gets stinky and LOUD

cevtv · Feb 2, 2008

v8only said:
just be prepared, if it's your dd, it gets stinky and LOUD

I don't get it....everybody complains about the smell. I don't have any horrible gas stink.......

85_SS_302_Coupe · Feb 2, 2008

Unless you never run your engine past 3k RPM, any small little gain that cats gives you is going to be a restriction up top, i don't care how "high flow" the cats are supposed to be...anything inside a cat is more than nothing at all. The gains you'll get without them past 3k will more than offset the gains you might get with having them down low. Also we're talking 5hp/tq at the absolute most down low, probably a bigger gain up top though.

giddyup306 · Feb 2, 2008

As a rule of thumb you want to create some (a little not a lot) backpressure after the collector to make low-end torque. If you remove the cats you will gain horsepower up top from the more freely flowing exhaust gases but sacrafice torque down low. A good example of this is people that put 3" exhaust on a stock or mild 302 and loose torque.

According to "The Official Ford Mustang 5.0 Technical Reference and Performance Handbook":

"It is illegal to remove either the converters....

because the resulting reduction in back pressure confuses the computer and hurts the engine's low-end torque output. Although some sources have claimed an increase of 10 horsepower when the converters are removed..."

I have saw similar results. Removing the cats on a stock engine can result in a .2 quicker ET. As stated in this post it depends on what mods you have done to the car as well. If you have a healthy 302 it would probably be best (as far as a performance standpoint goes) to drop the converters. If it is a stock car you will probably see torque on down low before you see a few hp up top.

jimdrechsler · Feb 3, 2008

if i have an aftermarket highflow h-pipe with cats, is it worth hollowing those out? it is not stock. i dont think it willl make that much of a difference. someone told me that it could actually hurt me perfomarnce wise.

thanks

bloopbloob · Feb 3, 2008

jimdrechsler said:
if i have an aftermarket highflow h-pipe with cats, is it worth hollowing those out? it is not stock. i dont think it willl make that much of a difference. someone told me that it could actually hurt me perfomarnce wise.

thanks

it should help performance in my opinion. when i gutted mine it felt better when i got on the pedal. but i wouldn't gut them now. cut them out and replace that section with pipe. it'll flow better that way, and you may need them some day. or you could maybe sell them. they're worth a fair chunk of change.

ninety15.0 · Feb 4, 2008

dyno the car...thats the only way anyone would ever really know. Anything else is just speculation. I speculate that removing the cats is just going to shift your powerband up a couple hundred rpm.

Lets look at the torque output at say 2000rpm. With the stock h pipe you may see 200 ft-lbs at 2k rpm. Now lets look at an O/R H or X pipe. At 2k rpm you may only see 195 ft-lbs...but if you look at 2200rpm you would most likely see 200 or 205 ft-lbs. Its all about shifting the powerband up...the car is still going to make the same torque...just at a slightly higher rpm.

Thats my speculation anyways...but its well founded.

Fast63 · Feb 5, 2008

cevtv said:
I don't get it....everybody complains about the smell. I don't have any horrible gas stink.......

Massachusetts probably doesn't put STINK additives in their gas like some other states do. California gas is SO gross.....

Fast63 · Feb 5, 2008

v8only said:
on a stock motor, this is correct, you won't really gain anything.

However, on a full h/c/i bolt on 5.0, that's completely wrong. My friend runs a 5.0 with afr 165's, eddy 2 intake, custom cam, etc etc. He dyno'd it with the HI FLOW 2 catted h pipe, then switched it out for an offroad pipe and picked up 21 hp.

On an engine with mods, running no cats over hi flow cats make a BIG difference in power.

they do, however stink like hell with no cats...and imo is not worth running like that on a street car.

I think your buddies high flow cats had something wrong with them or somehow made his car run differently (were the 02 sensors placed in the same locations on both H-pipes?)

I ran with a stock 4 cat H-pipe from an 88 and an OR h-pipe at the track and had virtually NO difference in ET or MPH with a H/C/I and aftermarket carb combo. 13.6's at 104mph all day.

vristang · Feb 5, 2008

According to "The Official Ford Mustang 5.0 Technical Reference and Performance Handbook":

"It is illegal to remove either the converters....

because the resulting reduction in back pressure confuses the computer and hurts the engine's low-end torque output. Although some sources have claimed an increase of 10 horsepower when the converters are removed..."

confuses the ecu??? WTF?
That book has so many errors it isn't even funny....

Can someone explain how creating a restriction in the exhaust will improve performance???

*grabs popcorn*

giddyup306 · Feb 5, 2008

Having too big of a collector or exhaust system will reduce the scavenger effect in the exhaust and can KILL engine performance. An ideal exhaust system is one that has zero positive pressure will net the best performance.

And why do you think the book is incorrect?

vristang · Feb 5, 2008

giddyup306 said:
Having too big of a collector or exhaust system will reduce the scavenger effect in the exhaust and can KILL engine performance. An ideal exhaust system is one that has zero positive pressure will net the best performance.

And why do you think the book is incorrect?

In my opinion (and others will argue against this...)
scavenging is VERY different from having a restriction...

Every one will have their own opinion though

What does the ecu see that will tell it the cats are gone?
IF there is a drop in torque, I doubt it is due to the ecu....

giddyup306 · Feb 5, 2008

vristang said:
In my opinion (and others will argue against this...)
scavenging is VERY different from having a restriction...

Every one will have their own opinion though

What does the ecu see that will tell it the cats are gone?
IF there is a drop in torque, I doubt it is due to the ecu....

It doesn't know the cats are gone. Unlike OBD II vehicles the EEC IV systems do not have post-cat o2s. It doesn't go into detail why it confuses the ECM but my guess would be that it has something to do with the DPFE sensor which is probably calibrated with the cats on (remember the PFE sensor is a variable capacitance sensor that is sensitive to exhaust backpressure). The higher backpressure might force a higher % of exhaust through the EGR making it more effective (otherwise it would take the path of less resistance not going through the EGR). This in turn would lower combustion temperature (I know the EGR only operates under cruize conditions.. not at WOT or idle). This could confuse the computer by skewing the return voltage to the computer (let's say it is returning 2.0V instead of 2.5V). In this case it would not see the EGR working fully and not advance timing as much. This is probably what they are talking about and in all honesty probably just threw it in because they didn't want everyone taking off their cats. So in short the confuses the ECM part really doesn't fit into a track scenario but might be applicable during street use. I will agree that the drop in torque has probably nothing to do with the EEC.

If this isn't what they are trying to get at you have me.

vristang · Feb 5, 2008

EGR operation adds only .75* of advance....

As you said, EGR is disabled under wot... so it can't really have an impact....

Again, there have been other errors found in that text... I think this is just another error...

Not sure WHY the author thought this, but I don't see any way for it to be real...

criticman · Feb 5, 2008

I went no-cats. Mine were rusted and therefore inefficient. I had an exhaust shop custom make me an H-pipe the day before I dyno'd to connect my shorty headers to my magnaflow catback. In addition to the noise, there was a serious SOTP gain in power. Do I have the dyno #'s to back it up, no. But then again, if you replace an aging system that is cracked with a new part, even a stock replacement would likely have seen a power gain.

Mmm, that sound, oh how I miss it...

Cats=torque?or no cats=topend?

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