No cat all the way! Whoever said you need backpressure

[eboy0]

I'm amazed to say how big a difference removing a cat converter makes. I read through the threads about back pressure and such, but decided to do it anyways. The car feels/sounds amazing, even with some exhaust(after turbo)/boost leaks. Now I don't have any dyno numbers to prove anything, but just by driving around on a 3" cat and swapping it in with a 3" straight pipe, makes a BIG difference.
If anyone else has anything to add, by all means please.

 

[silver bullit]

Yes it will help on turbo cars but back pressure is good for non turbo cars to help with torque at lower rpms, but a gutted cat is not likely legal in all states.

 

[DGajre777]

I run a long cat eliminator RRE downpipe, I know what you mean. If your CEL is on, make an o2 sensor simulator, check the tech guide for it, I made one for my car and the total cost for parts from Radioshack was $7.

 

[eboy0]

I run a long cat eliminator RRE downpipe, I know what you mean. If your CEL is on, make an o2 sensor simulator, check the tech guide for it, I made one for my car and the total cost for parts from Radioshack was $7.

Gatta love DSMLink

 

[romeen]

I'm amazed to say how big a difference removing a cat converter makes. I read through the threads about back pressure and such, but decided to do it anyways. The car feels/sounds amazing, even with some exhaust(after turbo)/boost leaks. Now I don't have any dyno numbers to prove anything, but just by driving around on a 3" cat and swapping it in with a 3" straight pipe, makes a BIG difference.
If anyone else has anything to add, by all means please.

Absolutely! Unlike a N/A car, a turbo car benefits from as little backpressure as possible. BTW, did you ever figure out what was causing your car to decelerate so suddenly when transitioning out of boost?

 

[eboy0]

Absolutely! Unlike a N/A car, a turbo car benefits from as little backpressure as possible. BTW, did you ever figure out what was causing your car to decelerate so suddenly when transitioning out of boost?

Its weird that you ask! I was just heading over to that thread, the hard bucking isn't as bad anymore, its just more loud now, but doesn't throw you forward anymore, yah know?
Right before I broke my bottom nipple on my Greddy Type-S, there was even less bucking, but now it has returned a little, this being due to the bov not opening fully at low rpm?

 

[romeen]

Since I don't wan't to "jack" this thread I'll post a suggestion in that other one.

 

[PieEyedPiper]

Backpressure is never good for any car.
Stop saying that.
http://dsmtuners.com/forums/showthread.php?t=168578

Glad you liked removing your cat though.

 

[Defiant]

I'm not sure that anyone competent ever did. WTF

 

[RedTurboEclipse]

You have dsmlink, you couldve logged the est. HP/torque it was giving you before and after you made the switch.

I want to remove my 3" cat a well, but im in cali, but too riskay.

 

[1999silverGSX]

You have dsmlink, you couldve logged the est. HP/torque it was giving you before and after you made the switch.

I want to remove my 3" cat a well, but im in cali, but too riskay.

That Sucks... overhere on the east coast my car passed with no cat... idk how but it made it through and i wasnt gonna say anything

 

[Dsm2nr89]

I plan on removing my cat as well next week for a welded-in straight pipe.Will the change be noticeable ? *my car is stock* for now...

 

[LordZed]

I'm not sure that anyone competent ever did. WTF

I did. I was going on what my drag racing step dad taught me. But apparently he was using old school knowledge in reference to back pressure.

 

[2GeNTSi]

You have dsmlink, you couldve logged the est. HP/torque it was giving you before and after you made the switch.

I want to remove my 3" cat a well, but im in cali, but too riskay.

I've been running my car without a cat for a while now, but then again my car isn't too blingy as opposed to yours. I haven't been pulled over once in San Jose yet, the cops don't seem to care about imports around here.

 

[JayRolla]

Backpressure is never good for any car.
Stop saying that.
http://dsmtuners.com/forums/showthread.php?t=168578

Glad you liked removing your cat though.

In a street car expecially a v8 you want some back pressure. It helps you keep your low end torque. If you free up the exhaust to much you gain a lot up top but lose a lot down low. And in a low rev v8 you dont need that power up top.

 

[Dsm2nr89]

Now that im on this topic,if i have a mechanic weld me in a straight pipe,where would my o2 have to go ? i havent really been under my car that well

 

[romeen]

Now that im on this topic,if i have a mechanic weld me in a straight pipe,where would my o2 have to go ? i havent really been under my car that well

A 1G has only one O2 sensor which goes into the O2 sensor housing. On a 2G the rear sensor goes into the first portion of the catback, so even then a test pipe would not affect it.

 

[xXaTaXx]

In a street car expecially a v8 you want some back pressure. It helps you keep your low end torque. If you free up the exhaust to much you gain a lot up top but lose a lot down low. And in a low rev v8 you dont need that power up top.

Again, I think you're referring to scavenging which is correlated to backpressure. With reference to an exhaust system, as each vavle opens and releases hot exhaust gas, a 'pulse' is created. This 'pulse' creates a momentary vacuum behind itself, which in turn helps scavenge remaining exhaust gasses from the cylinder. With large diameter pipes or dump/open pipe, the exhaust gasses loose velocity/temperature as they travel unrestricted throughout the pipes. N/A motors will loose noticible low-end torque because at the time when the intake valves open, there will still be small remaining amounts of exhaust gas in the cylinder--taking up very much needed space for the incoming A/F mixture. With a turbo'd machine, this is irrelevant because this mixture is being pounded into the cylinder with the help of your exhaust gasses anyway.

backpressure on the otherhand, is one of the most unwanted forces in any exhaust system. Backpressure is what PREVENTS said exhaust gasses from leaving the cylinder efficiently. Now, I concur that there is and will be backpressure in N/A, turbo, and most exhaust systems. However, because it is there, dosen't mean it's required. Natural Aspiration is very much reliant on scavenging. In in our cases, scavenging occurs better as the pipe diameter gets smaller, to a point. As the pipe gets smaller, the backpressure increases. OEM equipment is ususally up to spec right about in the median of backpressure vs. scavenging efficiency.

If you're trying to exhale through a coffee stirer, you'll find that usually you need to inhale again before you're finished exhaling. In effect, contaminating the incoming air with carbon dioxide. Now, If you use a regular straw. You'll find that it its not only much easier to breathe out, but also that you completely exhaled before having to inhale. Think about it, just in illustration of how backpressure is bad no matter the car--5.0, 2.0, 2.5, 1.8 or those monster diesel 7.2 (or whatnot). Not even a monster v8 wants backpressure. They want scavenging... Many cylinders of it. ;D

Chris

At work, trying to explain. Let me know if I was a bit incoherent. Sorry.

 

[flashburn02]

Again, I think you're referring to scavenging which is correlated to backpressure. With reference to an exhaust system, as each vavle opens and releases hot exhaust gas, a 'pulse' is created. This 'pulse' creates a momentary vacuum behind itself, which in turn helps scavenge remaining exhaust gasses from the cylinder. With large diameter pipes or dump/open pipe, the exhaust gasses loose velocity/temperature as they travel unrestricted throughout the pipes. N/A motors will loose noticible low-end torque because at the time when the intake valves open, there will still be small remaining amounts of exhaust gas in the cylinder--taking up very much needed space for the incoming A/F mixture. With a turbo'd machine, this is irrelevant because this mixture is being pounded into the cylinder with the help of your exhaust gasses anyway.

backpressure on the otherhand, is one of the most unwanted forces in any exhaust system. Backpressure is what PREVENTS said exhaust gasses from leaving the cylinder efficiently. Now, I concur that there is and will be backpressure in N/A, turbo, and most exhaust systems. However, because it is there, dosen't mean it's required. Natural Aspiration is very much reliant on scavenging. In in our cases, scavenging occurs better as the pipe diameter gets smaller, to a point. As the pipe gets smaller, the backpressure increases. OEM equipment is ususally up to spec right about in the median of backpressure vs. scavenging efficiency.

If you're trying to exhale through a coffee stirer, you'll find that usually you need to inhale again before you're finished exhaling. In effect, contaminating the incoming air with carbon dioxide. Now, If you use a regular straw. You'll find that it its not only much easier to breathe out, but also that you completely exhaled before having to inhale. Think about it, just in illustration of how backpressure is bad no matter the car--5.0, 2.0, 2.5, 1.8 or those monster diesel 7.2 (or whatnot). Not even a monster v8 wants backpressure. They want scavenging... Many cylinders of it. ;D

Chris

At work, trying to explain. Let me know if I was a bit incoherent. Sorry.

Clear as day to me, I have always wondered about this. Its sorta like cetripital motion and centripital force being used interchangeably. (SIDE NOTE: Centripital force doesn't exist).

 

[gdmbat85]

i put in a cat on my car the other day so i could pass inspection. i had a full 3" turboback exhaust with test pipe. i reluctantly cut the test pipe and put in a 3" high flow cat. i can honestly say i noticed absolutely NO difference. neither in power or noise, which i was kinda hoping it would atleast quite the exhaust for the 1200mile drive down to florida.

"Backpressure is never good for any car.
Stop saying that."

this is a little misleading at best. in order to get that vaccuum effect that the guy was talking about you need to use a smaller pipe to get maximum velocity thus creating backpressure. you dont need any backpressure or vaccuum (either way u wanna look at it) for turbo cars cause the turbo already baffles the exhaust gases but you will need atleast some for N/A cars.

 

[PieEyedPiper]

Again, I think you're referring to scavenging which is correlated to backpressure. With reference to an exhaust system, as each vavle opens and releases hot exhaust gas, a 'pulse' is created. This 'pulse' creates a momentary vacuum behind itself, which in turn helps scavenge remaining exhaust gasses from the cylinder. With large diameter pipes or dump/open pipe, the exhaust gasses loose velocity/temperature as they travel unrestricted throughout the pipes. N/A motors will loose noticible low-end torque because at the time when the intake valves open, there will still be small remaining amounts of exhaust gas in the cylinder--taking up very much needed space for the incoming A/F mixture. With a turbo'd machine, this is irrelevant because this mixture is being pounded into the cylinder with the help of your exhaust gasses anyway.

backpressure on the otherhand, is one of the most unwanted forces in any exhaust system. Backpressure is what PREVENTS said exhaust gasses from leaving the cylinder efficiently. Now, I concur that there is and will be backpressure in N/A, turbo, and most exhaust systems. However, because it is there, dosen't mean it's required. Natural Aspiration is very much reliant on scavenging. In in our cases, scavenging occurs better as the pipe diameter gets smaller, to a point. As the pipe gets smaller, the backpressure increases. OEM equipment is ususally up to spec right about in the median of backpressure vs. scavenging efficiency.

If you're trying to exhale through a coffee stirer, you'll find that usually you need to inhale again before you're finished exhaling. In effect, contaminating the incoming air with carbon dioxide. Now, If you use a regular straw. You'll find that it its not only much easier to breathe out, but also that you completely exhaled before having to inhale. Think about it, just in illustration of how backpressure is bad no matter the car--5.0, 2.0, 2.5, 1.8 or those monster diesel 7.2 (or whatnot). Not even a monster v8 wants backpressure. They want scavenging... Many cylinders of it. ;D

Chris

At work, trying to explain. Let me know if I was a bit incoherent. Sorry.

Thanks for handling that for me. I was just going to say "No."

Good post.

 

[PieEyedPiper]

i put in a cat on my car the other day so i could pass inspection. i had a full 3" turboback exhaust with test pipe. i reluctantly cut the test pipe and put in a 3" high flow cat. i can honestly say i noticed absolutely NO difference. neither in power or noise, which i was kinda hoping it would atleast quite the exhaust for the 1200mile drive down to florida.

"Backpressure is never good for any car.
Stop saying that."

this is a little misleading at best. in order to get that vaccuum effect that the guy was talking about you need to use a smaller pipe to get maximum velocity thus creating backpressure. you dont need any backpressure or vaccuum (either way u wanna look at it) for turbo cars cause the turbo already baffles the exhaust gases but you will need atleast some for N/A cars.

No.

You still seem hooked on the idea that backpressure is needed. The link I provided, and xXaTaXx's excellent post clearly describe how backpressure is merely an unwanted side effect of the smaller, usually stock, exhaust systems that cars can use. These exhaust systems are usually tuned very well for a balance in low end and high end power. As previously mentioned, the smaller the exhaust the higher the velocity and greater the scavenging effect, but you can only go so small before backpressure is too great and you've reached the point of diminishing returns.

So, when IS back pressure good? It's not, its just a side effect of a relatively small exhaust.

Also, as mentioned and should now be implicit, for an N/A engine we don't want backpressure and for a turbo engine we don't want back pressure.
Your post touched on how turbos don't need back pressure because the turbo itself "baffles" the air.

When in actuality, turbos don't "need" backpressure the same as N/A's don' t "need" it. Backpressure prevents the smooth unrestricted exhaust flow that turbo cars love.
The reason turbo cars like a large 3" 3.5" and even 4" exhaust is because our scavenging effect is almost conmpletely obstructed by the turbine in the exhaust housing of the turbo to begin with.
So by means of scavenging, a turbo car's low end has already been killed. No loss in going to a large diamter pipe - only a gain in the top end.

And as already stated once more, the only reason N/A's typically like smaller exhausts is because they can benefit more from scavenging than their turbo cousins. But go to small and backpressure is too high and performance is lost and go too big and the increased diameter piping will inhibit the scavenging effect, thus losing performance.

Now that everything has been said 10 times over hopefully you will see the light.

but you will need atleast some [backpressure] for N/A cars.

Just be extra extra clear, no one is saying go put a massive exhaust on your honda, v8 or 420a. We're saying don't go too big else you'll loose your scavenging effect.

 

[Dsm2nr89]

A 1G has only one O2 sensor which goes into the O2 sensor housing. On a 2G the rear sensor goes into the first portion of the catback, so even then a test pipe would not affect it.

Sweet,thanks

 

[1LE]

If anyone else has anything to add, by all means please.

My car originally had a 3" test pipe and I swapped in a 3" cat. Couldn't tell any difference at all. Maybe your old cat was partially clogged?

 

[eboy0]

My car originally had a 3" test pipe and I swapped in a 3" cat. Couldn't tell any difference at all. Maybe your old cat was partially clogged?

The exhaust was bought in like mid november iirc, there's a thread about it somewhere, looked through the cat when I took it off, could see the light pretty easily on the other side.