Lose The Rear Cats, Lose Torque.

vld · Sep 26, 2005

Just bought an 03 gt. Looks like the previous owner cut out the back cats. She seems to lag through first then really kick it in second. A friend told me the the loss of rear cats is what is causing the sluggish #1 gear performance. Any truth to this?

Dragstr05 · Sep 26, 2005

Sure there is truth to it. An engine actually does need 'some' backpressure, but its not like the whole exhaust was unbolted. Thing is, people put midpipes with only 2 cats all the time and they dont complain.

Good example though; I unbolted the exhaust on my protege just for the heck of it when I was bored at the shop one day and took it for a drive in the lot. The loss in torque was insane...not that it has much anyway, but it was REALLY bad. Bad to the point that I could hold it full throttle and it would just make noise and go nowhere. Kinda like a honda...

WillDuhhBeast · Sep 26, 2005

Dragstr05 said:
Sure there is truth to it. An engine actually does need 'some' backpressure, but its not like the whole exhaust was unbolted. Thing is, people put midpipes with only 2 cats all the time and they dont complain.

Good example though; I unbolted the exhaust on my protege just for the heck of it when I was bored at the shop one day and took it for a drive in the lot. The loss in torque was insane...not that it has much anyway, but it was REALLY bad. Bad to the point that I could hold it full throttle and it would just make noise and go nowhere. Kinda like a honda...

With my brothers Saturn the expericance was totally opposite. With cats it seemed to drag and be restricted, he went to the super loud and stupid no cats and a high flow muffler and you could FEEL the improvement.. eventually he welded in 2 high flow cats and it was toned down and didnt lose too much of that feeling. Still quick for a saturn, he runs 10.4 n/a on the 8th. It's a 93 sl2.

jimfitzgerald · Sep 26, 2005

BS. One of the first mods most of us do is an O/R x-pipe. How many cats do you think they have?

Ste · Sep 26, 2005

losing torque to loss of backpressure isnt BS, its a known fact. The O/R X-Pipe is a good mod, it does improve HP and improve flow, but it does decrease torque in the low rpms due to loss of backpressure. But cutting just 2 cats shouldnt drop it enough to say 'what the hell?' haha...jm .02

Kilgore Trout · Sep 26, 2005

gutted cats = increased exhaust gas flow = less backpressure = loss of lower torque but more top end power

Moves the powerband up into higher rpms...

No one ever recommends gutting cats on 99+ Mustangs

Best to get a nice o/r x and whatever catback you prefer for sound

BLUE03GT · Sep 26, 2005

first of all... its not back pressure... back pressure is bad... i forget the correct term but someone should else should know why...

second of all... removing the 2 rear cats will only help performance... i did it and i felt a power gain throughout the whole powerband... more in the higher rpms of course.. the sound with my flows = .... besides its not the main cats.. i didnt even set off my check engine light by removing them

i recommend it highly if youre looking for a bit more sound and a bit more hp up top...

Kilgore Trout · Sep 26, 2005

Ok well, you have actually done it and I have just read about doing it so I am going to change my answer to; "what blue03gt said"

BLUE03GT · Sep 26, 2005

Kilgore Trout said:
Ok well, you have actually done it and I have just read about doing it so I am going to change my answer to; "what blue03gt said"

im not saying anyone is wrong... im just speaking from personal experience

then again... i have gears... so i may be bypassing that low end sluggishness that 3.27's might have

all in all... im happy about the removal of the cats... im low on mod money so i cant really buy a catted-x... plus i dont want it any louder than it is right now with Flows and 2 removed cats.. i cant stress enough the EXHAUST TONE

jimfitzgerald · Sep 26, 2005

BLUE03GT said:
im not saying anyone is wrong... im just speaking from personal experience

then again... i have gears... so i may be bypassing that low end sluggishness that 3.27's might have

all in all... im happy about the removal of the cats... im low on mod money so i cant really buy a catted-x... plus i dont want it any louder than it is right now with Flows and 2 removed cats.. i cant stress enough the EXHAUST TONE

Don't waste you money on a catted X. When and if you get one, get an O/R X. Plus, they are cheap.

tomustang · Sep 26, 2005

you ever think about how the pressure is when the cats are gutted? It seems like a gas popping effect, small diameter to large back to small. I never felt a loss in anything when i gutted my cats on the GT

03ghoststang · Sep 26, 2005

jimfitzgerald said:
BS. One of the first mods most of us do is an O/R x-pipe. How many cats do you think they have?

thats funny........................ but i dont see where you will lose power by removing two cats

Stang|ess · Sep 26, 2005

I felt a loss of power when moving from a stop.

AdRock · Sep 26, 2005

03ghoststang said:
thats funny........................ but i dont see where you will lose power by removing two cats

Ok, you're not losing power. HOWEVER, you are moving the power. Therefore, technically, the torque will drop a little in the lower rpms. But it isn't gone, it has just moved to a higher rpm.

Hopefully that explanation will make it a little easier to visualize.

ryant430 · Sep 26, 2005

Any time you cut back on any amount of back presure from a stock setup you will lose torque. It works the same as increasing/decreasing valve overlap, increasing back pressure is the same as decreasing the amount of valve overlap, because less compression is lost through the exhaust valve while a new intake charge is entering the cylinder.
So, at low RPMs, backpressure actually increases torque, because the least amount of compression is lost through the exhaust valve ( due to back pressure ) during the intake stroke, and the ratio of intake to exhaust gases is high. The balance that needs to be tuned through back pressure and/or cam profiling is important because as you rev up through the range, assuming a higher back pressure, there is increasingly less time to evacuate the exhaust gases during the exhaust stroke, and more and more spent air-fuel stays in the cylinder when the exhaust valve closes, doesn't make for a good charge. However, if its too free flowing the scavenging can be overbounds. Sometimes the flow out the tail pipe is so high it creates too much vacuum in the cylinder, this low pressure sucks the intake charge into the cylinder and out the exhaust port before the bang. I have had several vehicles in which I could physically feel the loss of low end torque from opening up my exhaust to much. EX. before the new exhaust I got on my old 96 Firebird I could squeal the tires from a stop, after the exhaust the squeal went away. Back pressure is something nobody ever takes into proper consideration when increasing their vehicles performance.

Justin03GT · Sep 26, 2005

i wish i had my dyno sheets available online...i did some pulls with a catted X and then again with an O/R x...and guess what...

it showed zero difference anywhere on the dyno...made the same, hp and tq, and the curves didnt change at all...and when compared to my stock midpipe dyno sheet, it showed the same thing...no loss in tq but the peak HP was higher and power stayed on longer, but it didnt move it upward...

i think what people are feeling is the extra power and how the power lasts longer through the powerband, giving the illusion of loss of tq down low...

sgarlic · Sep 26, 2005

My experience is this:

Before I did the midpipe swap, I could be in 1st, floor it going around 10 mph, and burn off the tires. With the mac o/r h (and no other mods,) it just took off, but wouldn't spin the tires. That's not an illusion, that's torque loss. (And no, I don't have any exhaust leaks.)

BLUE03GT · Sep 26, 2005

i really doubt that anyone would see any torque loss from removing the 2 rear cats(not main cats)... the piping size is still the same and your not adding high flow cats

like i said.. i just removed the 2 rear cats... and i felt a power increase

csledd · Sep 27, 2005

I dunno what to tell you because I did my whole exhaust swap at once. Needless to say though after the LTs, o/r x-pipe and the cat-back it felt very sluggish down low, but once I got past 4k you could tell a huge difference. When it was cold I could launch the car fine but would lose traction after 4k. When I had my car dyno tuned I was running 20% lean, I dunno if this was from the exhaust because I added a 75mm tb and plenum, but once we adjusted my a/f ratio I gained 40-50 rwtq back in my lower rpms around 3000-3500. Why are you gonna need all that power down low anyways? Just put some gears in there

mrvax · Sep 27, 2005

jimfitzgerald said:
Don't waste you money on a catted X. When and if you get one, get an O/R X. Plus, they are cheap.

Those of us who live in smog land have no choice. I had 6 cats on my car and I now have 2. They are hi-flows and still pass emissions. Big difference it made in performance. Pulls harder in the upper rpms and especially noticable in 3rd & 4th gear. Of course, if we didn't have to battle emissions (and the fine for no cats wasn't several thousand dollars) I would gladly swap to O/R X pipe.

Lose The Rear Cats, Lose Torque.

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