Cats or no Cats why do one or the other?

[Ovenmitt]

Of course. You address that in your tune

[5GSS]

Originally Posted by lionssfan

what about fuel mangament does the ecm still look at 02 sensors with cats removed and tuned? if you know.

Rear O2s are not used for fuel management, they only exist to verify properly functioning catalytic converters.

[clg_98ta]

Originally Posted by 5GSS

Rear O2s are not used for fuel management, they only exist to verify properly functioning catalytic converters.

True. You can either edit the rear o2 tests out of the ECM, or plug o2 sims into the wiring harness where the removed o2 sensors were. If your modding (tuning) the ECM anyway, edit out the tests. Otherwise, you can preserve your powertrain warranty by using o2 sims.

[SUX2BU]

I did no cats with full bolt ons because of weight, cost and a little more power. If i ever do a cam though, I will put some high flow cats on to not get the gas smell.

[lionssfan]

Originally Posted by clg_98ta

True. You can either edit the rear o2 tests out of the ECM, or plug o2 sims into the wiring harness where the removed o2 sensors were. If your modding (tuning) the ECM anyway, edit out the tests. Otherwise, you can preserve your powertrain warranty by using o2 sims.

so if you go with no cats, you only need one O2's sensor for each bank instead of the two for each bank with cats, makes sense....... thanks for the clarification

[lionssfan]

Originally Posted by 5GSS

Rear O2s are not used for fuel management, they only exist to verify properly functioning catalytic converters.

thanks 5gss.

[lionssfan]

Can't get enough of the technical stuff......so is not having enough back pressure a concern at some point with no cats? if anyone knows

[SUX2BU]

Originally Posted by lionssfan

Can't get enough of the technical stuff......so is not having enough back pressure a concern at some point with no cats? if anyone knows

No, these motors LOVE to breath.

[5GSS]

Originally Posted by lionssfan

Can't get enough of the technical stuff......so is not having enough back pressure a concern at some point with no cats? if anyone knows

People often confuse the relationship between back pressure, exhaust velocity, and scavenging. Check out these articles for some detailed technical stuff:

http://www.superchevy.com/technical/...exh/index.html

http://www.boyleworks.com/ta400/psp/exhaust.html

[5GSS]

Cliff notes version...

Engines care about 1 thing: flow. The single most important part of designing a performance exhaust is properly sized header primaries. Too small, and you restrict flow; too large and you lose the scavenging effect. Scavenging is the effect of the negative pressure (vacuum) created by each exhaust pulse as it travels down the header primary. The majority of the pressure differential between the intake and the cylinder is created by this negative pressure during the overlap of valves, and not by the actual intake stroke of the piston. Exhaust leaving the combustion cylinder effectively sucks intake in behind it.

The best analogy I can think of is a simple syringe. When the plunger of a syringe is pulled, it creates suction that draws in liquid behind it. If the cylinder of the syringe is too large, no suction is created, and nothing is drawn into the cylinder.

As long as the scavenging effect is properly promoted by correctly sized headers, the rest of the intake is mostly for sound, or in the case of mufflers, the dampening of sound. The more free flowing, the better.

[SUX2BU]

Honestly on LSx motors I have yet to see primaries that are too big. Back in the 4th gen days people thought 1 7/8" were too big for an LS1 and you would lose power. I dynoed 1 3/4" and 1 7/8" back to back on a bolt on only car (no power adders, no cam) and lost zero power down low and gained a few hp up top. The same exact thing has been proven on our LS3 motors with the 1 7/8" vs 2.0" primaries.

[5GSS]

Originally Posted by SUX2BU

Honestly on LSx motors I have yet to see primaries that are too big. Back in the 4th gen days people thought 1 7/8" were too big for an LS1 and you would lose power. I dynoed 1 3/4" and 1 7/8" back to back on a bolt on only car (no power adders, no cam) and lost zero power down low and gained a few hp up top. The same exact thing has been proven on our LS3 motors with the 1 7/8" vs 2.0" primaries.

You're absolutely right, and really just goes to show you that these exhaust manufacturers know what they are doing. I was really just speaking conceptually. Bottom line is, if you buy from a reputable manufacturer, you can't go wrong. Buy mufflers for the sound you prefer, cats if you need to pass emissions, and whichever headers you think are prettiest and have the best fit and finish, as long as they are from a reputable company, and in the end, you'll be happy.

[lionssfan]

Originally Posted by 5GSS

Cliff notes version...

Engines care about 1 thing: flow. The single most important part of designing a performance exhaust is properly sized header primaries. Too small, and you restrict flow; too large and you lose the scavenging effect. Scavenging is the effect of the negative pressure (vacuum) created by each exhaust pulse as it travels down the header primary. The majority of the pressure differential between the intake and the cylinder is created by this negative pressure during the overlap of valves, and not by the actual intake stroke of the piston. Exhaust leaving the combustion cylinder effectively sucks intake in behind it.

The best analogy I can think of is a simple syringe. When the plunger of a syringe is pulled, it creates suction that draws in liquid behind it. If the cylinder of the syringe is too large, no suction is created, and nothing is drawn into the cylinder.

As long as the scavenging effect is properly promoted by correctly sized headers, the rest of the intake is mostly for sound, or in the case of mufflers, the dampening of sound. The more free flowing, the better.

Originally Posted by SUX2BU

Honestly on LSx motors I have yet to see primaries that are too big. Back in the 4th gen days people thought 1 7/8" were too big for an LS1 and you would lose power. I dynoed 1 3/4" and 1 7/8" back to back on a bolt on only car (no power adders, no cam) and lost zero power down low and gained a few hp up top. The same exact thing has been proven on our LS3 motors with the 1 7/8" vs 2.0" primaries.

so then it comes down to noise level cats being quieter and no cats louder. by the way I have corsa cat-back exhaust with no resonators. so with no cats and if its to loud I can aways get a resonators? thanks 5gss and sux2bu

[ARK Performance]

The OEM cats do a very good job in reducing noise due to its function. However, this is where the most power is made. This also results in a much more aggressive sounding exhaust tone. Even with resonators, it will be significantly louder.