True cat delete

[281MERC]

So my plan is to bolt off the stock cat section and take it to a local muffler shop to get them to bend up a straight pipe with 02 bungs and flanges to take its place so i can just bolt it in place with no welding and i will still be able to switch back to the stock piece for emissions testing every 2 years. the rest of the exhaust system is stock. what kind hp/torque should i expect? Sound??? will i have to tune for air/fuel ratio's?? Any opinions?????????

[Stranger in the Black Sedan]

keep at least one set of cats, please... If your car is stock and you have no cats, you are just gonna be just as slow, and louder

[RR|Suki]

keep at least one set of cats, please... If your car is stock and you have no cats, you are just gonna be just as slow, and louder

Truth, nothing in the world worse than a loud slow car... ok maybe a couple things worse, but it's up there :lol:

[Glockafella]

Truth, nothing in the world worse than a loud slow car... ok maybe a couple things worse, but it's up there :lol:



This reminds me of the time I raced a teenager in an old Ford LTD. He sounded like he had HUGE power, I mean it was just idling along, loud as heck. We get to the light and it was a marage. I was 2 lengths in front of him quick and he was loud as heck, but I don't even think he broke the tires loose.

[FordNut]

You'll make more noise and make less power.

[ctrlraven]

I would just delete the rear cats only, that or remove the factory cats and install two high flow cats in place of the four.

[Stranger in the Black Sedan]

If he's stock though its gonna make like what, 1 tenth gain, at the expense of having lost federal emissions compliance. The car will vary a tenth just with temperature and how much you ate for lunch.

[RR|Suki]

If he's stock though its gonna make like what, 1 tenth gain, at the expense of having lost federal emissions compliance. The car will vary a tenth just with temperature and how much you ate for lunch.

Where I agree it's prob useless for a stock car. In non emissions states I don't see people getting pulled over by the feds for having no cats.

[Stranger in the Black Sedan]

Doesn't mean its legal or that its a good idea. People like me with asthma and allergies also don't appreciate driving behind cars that are not catalyzed. It really does make a difference

[ctrlraven]

I have my rear cats removed and been through emissions twice, pull up, hop out and let the guy plug in the computer do his test and I'm on my way again.

I picked up little over a tenth when I deleted mine and at the same time went from 245/55-18 (3.55 eff. rear) to 255/55-18 (3.50 eff. rear) rear tires.

On a bone stock car it's pointless unless you just want sound, if you have some mods, intake, tune, etc etc it's a cheap mod to do.


Doesn't mean its legal or that its a good idea. People like me with asthma and allergies also don't appreciate driving behind cars that are not catalyzed. It really does make a difference
If that's the case why move to Southern MD where they don't do emissions testing and lots of diesel trucks run around? lol

[Blackened300a]

This is my favorite cat delete. ;)

http://scienceblogs.com/omnibrain/upload/2007/05/CatGun.jpg

[RacerX]

Shhesh! It isn't THAT much louder. At least not with the stock manifolds and mufflers. I think it sounds great! I don't think you lose any HP and may gain a tiny bit on an NA car. Well, the tires boke loose at a lower rpm after removing the last two cats... MEOW!

[ctrlraven]

I could feel the difference in the mid rpm range, I might remove the front pair one day.

[MrBluGruv]

I think with the proper muffler setup and tuning, all cats being gone has been a very nice improvement for the vehicle. I will probably be looking to get hi-flow cats though, even though they aren't serious about emissions here I don't want to press my luck too far. On that note, and this may be pertinent to what's at hand somewhat, with a hi-flow cat, it's basically a straight-through pipe with the catalysts on the inside yes? I want to maintain my current exhaust tone as closely as possible if i do the switch.

[Phrog_gunner]

If that's the case why move to Southern MD where they don't do emissions testing and lots of diesel trucks run around? lol

Diesel emissions are actually much cleaner than gasoline, even though the color is black.

[ctrlraven]

Diesel emissions are actually much cleaner than gasoline, even though the color is black.
Yes that is true, black smoke is usually a signal that there's too much fuel, not enough air, most cases are when the exhaust system has been modified along with the intake.

[Big Black Beast]

Yes, diesel emissions are cleaner in harmful chemicals but the black smoke is carbon particulates which are a known carcinogen and still causes resperatory issues.

[RacerX]

Not trying to dis any backpressure guys!

http://www.horsepowerjunkies.com/forums/showthread.php?t=94346

There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.

[babbage]

Racerx you are my hero.. :)

On the exhaust stroke if engine has to push -- resistance energy is taken away from the crank. You do however need exhaust velocity, too big pipes like 3" will lose velocity and power

[RR|Suki]

Racerx you are my hero.. :)

On the exhaust stroke if engine has to push -- resistance energy is taken away from the crank. You do however need exhaust velocity, too big pipes like 3" will lose velocity and power

Maybe in an NA car

[Phrog_gunner]

Any dyno results or mathematics to backup the back pressure diatribe??

[281MERC]

Going by "RacerX's" long post which makes a lot of sense by the way so please read it if you havn't already: on our n/a cars, by completely removing the cat section and replacing with the stock diameter straight pipe you are going less restrictive/less back pressure(good) and keeping the same high velocity flow to maintain the scavenging effect(good). so theres no doubt that it should improve HP and torque EVEN at lower RPM, correct???????? I also don't see it being very loud at all if i'm sticking with the stock mufflers

[babbage]

Any dyno results or mathematics to backup the back pressure diatribe??


Mathematics? Are you kidding? On the exhaust stroke the exhaust valves in your engine open to release the burnt mixture of fuel and air (exhaust gasses) - If the cylinder encounters ANY resistance during the process of pushing out these fumes (aka back pressure) then energy is taken away from the engines crank. (the very thing that make the car move)

Can't say it any simpler than that. Needing backpressure to run right is like saying it's better to walk uphill than on level ground.

[RacerX]

It really makes it easy to understand the larger gains with LTs too though! Drafting in the tailpipe! :D

[ctrlraven]

Going by "RacerX's" long post which makes a lot of sense by the way so please read it if you havn't already: on our n/a cars, by completely removing the cat section and replacing with the stock diameter straight pipe you are going less restrictive/less back pressure(good) and keeping the same high velocity flow to maintain the scavenging effect(good). so theres no doubt that it should improve HP and torque EVEN at lower RPM, correct???????? I also don't see it being very loud at all if i'm sticking with the stock mufflers
Yes sticking with the same diameter and scavenging effect shouldn't give you any problems. You'll notice the car will be a little more throaty and throttle responsive because there won't be the initial exhaust choke that the dual pairs of cats causes.

Plus if you don't like it just put the stock cat pipes back on and figure something else out.

[MrBluGruv]

I personally believe that any loss of power or torque that may be seen from lack of restriction is meant to be fixed by re-tuning the engine. After all, the engine was originally tuned from the factory to make the power it does WITH all those cats on, so removing them will definately throw the engine's performance balance.