Does a 3" catback really make any difference over a 2.5"?

[MalcolmV8]

Happy Thanksgiving guys.

Looking at picking up some long tubes and I see the X pipes have some options. 3 1/2" collector down to 3" or down to stock 2.5". Is it worth the money to get rid of my catback and get a 3" catback. Is the 2.5" really a restriction? Anyone made this change and have any HP numbers to report?

 

[me32]

only depends if the 2.5 is restrictive on your setup. if its not then no a 3in is not needed.

 

[SOVACobra03]

ive always wondered this as well. also wondered if it would be worth it to go from my 1 3/4 arh to 1 7/8

 

[MalcolmV8]

only depends if the 2.5 is restrictive on your setup. if its not then no a 3in is not needed.

LOL yes I got that much How does one know when the 2.5 becomes a restriction? What HP levels is it good to?

ive always wondered this as well. also wondered if it would be worth it to go from my 1 3/4 arh to 1 7/8

Yeah same here. I called Kooks and spoke to them about my setup and they recommend the 1 7/8th so I figured that's what I'll use. No point in undersizing as who knows what the future holds. Always upgrading.

 

[JuStAkId]

Op seeing that you are running 27psi I would say you would be able to benefit. That being said how much? you wont know exactly unless you did a side by side comparison on the dyno. I say go for the 3''. Anyways 3'' under the irs looks that much more meaner. Btw I am running full 3'' on my eaton car

 

[98 Saleen Cobra]

It's hard to say.. Most of the gains comes from the headers and midpipe I think.. The catback I don't believe matters much.. But I run a full 3" on my car..

 

[MalcolmV8]

Op seeing that you are running 27psi I would say you would be able to benefit. That being said how much? you wont know exactly unless you did a side by side comparison on the dyno. I say go for the 3''. Anyways 3'' under the irs looks that much more meaner. Btw I am running full 3'' on my eaton car

I would guess the same thing but hoping to find someone who's tested or knows. If I was on a stock cat back I'd do it in a heart beat but I hate tossing one expensive cat back for another "just in case" i get a benefit.

It's hard to say.. Most of the gains comes from the headers and midpipe I think.. The catback I don't believe matters much.. But I run a full 3" on my car..

Yeah probably so. At some point the 2 1/2" is bound to be a restriction but I don't know if that happens at 800 rwhp or 1500+ etc.

 

[PWORLDSTANG]

With the larger exhaust, the best we have to go off of is when rtusnake swapped to a full 1 7/8'' primary and 3'' diameter throughout exhaust and gained 80RWHP and 9MPH at the track. But he also replaced his clutch, swapped over to a tubular K-Member, and had much more favorable track conditions after the swap. It would have made of a hell of a comparison if testing was more controlled, but I don't think that was his goal as he just shared his experience.

The trend for a few years now has been that larger exhaust (larger diameter primaries on headers, and 3'' diameter throughout) is better for 'xxx' horsepower or 'xx' lbs of boost. However, I have yet to see any definitive testing proving so. Until I see something more concrete I've chosen to stick with 1 5/8'' and 2 1/2'' exhaust throughout. While it sounds small for how much power some of us is running through it, it's tried and true as far as I'm concerned. I picked up over 15RWHP with Mac headers and I remember a noticeable seat of the pants increase way back when I swapped out my stock mid-pipe and cat-back before I got it tuned.

 

[MalcolmV8]

With the larger exhaust, the best we have to go off of is when rtusnake swapped to a full 1 7/8'' primary and 3'' diameter throughout exhaust and gained 80RWHP and 9MPH at the track. But he also replaced his clutch, swapped over to a tubular K-Member, and had much more favorable track conditions after the swap. It would have made of a hell of a comparison if testing was more controlled, but I don't think that was his goal as he just shared his experience.

The trend for a few years now has been that larger exhaust (larger diameter primaries on headers, and 3'' diameter throughout) is better for 'xxx' horsepower or 'xx' lbs of boost. However, I have yet to see any definitive testing proving so. Until I see something more concrete I've chosen to stick with 1 5/8'' and 2 1/2'' exhaust throughout. While it sounds small for how much power some of us is running through it, it's tried and true as far as I'm concerned. I picked up over 15RWHP with Mac headers and I remember a noticeable seat of the pants increase way back when I swapped out my stock mid-pipe and cat-back before I got it tuned.

Thanks for the feedback. I did some searching around on the net and found some charts for HP vs pipe size and they seem to claim dual 2 1/2 pipe is good to 463 hp. That seems very conservative. They claim beyond that you're actually running into a restriction... humm.

Maybe to weigh my options I'll look at some 3" cat backs. Who makes those? Right now I'm running the common Borla Stingers. Rather like the sound but change can be a good thing.

 

[Bdubbs]

Interesting thread indeed. Since I'll be going with arh headers and 3 inch mid pipe, I'm now wondering if I should just buy a 3 inch catback or just have the mid pipe 3 inch then reduced to fit my 2 1/2 inch catback.

 

[JuStAkId]

Thanks for the feedback. I did some searching around on the net and found some charts for HP vs pipe size and they seem to claim dual 2 1/2 pipe is good to 463 hp. That seems very conservative. They claim beyond that you're actually running into a restriction... humm.

Maybe to weigh my options I'll look at some 3" cat backs. Who makes those? Right now I'm running the common Borla Stingers. Rather like the sound but change can be a good thing.

Is that just the cb being 2.5'' or the whole exhaust? Is that talking about a car with cats? 463hp seems awful low. I can tell you being an 500rwhp eaton car and having full 3'' exhaust I did not have out of the park dyno numbers. Mine are right about the same will all the other cars. You have to remember all the variables that come into play such as how must boost someone is running, also if the car is running cats or not, as well as if the heads are stock because there comes to a point where your heads will only move so much air as well, and even down to how restrictive of a muffler you are running. I would think it would start to play a role around the 800hp mark.

 

[MalcolmV8]

Is that just the cb being 2.5'' or the whole exhaust? Is that talking about a car with cats? 463hp seems awful low. I can tell you being an 500rwhp eaton car and having full 3'' exhaust I did not have out of the park dyno numbers. Mine are right about the same will all the other cars. You have to remember all the variables that come into play such as how must boost someone is running, also if the car is running cats or not, as well as if the heads are stock because there comes to a point where your heads will only move so much air as well, and even down to how restrictive of a muffler you are running. I would think it would start to play a role around the 800hp mark.

Yeah I don't even believe those charts after searching around more. I found one on hotrod magazine's website, magnaflow's website and a few others and none of them are even remotely close to each other which tells me they're all just guessing or marketing crap.

 

[spray'n mach 1]

I'm running 2.5 and made 658 rwhp through a stalled auto I wouldn't change my setup on the hope of picking up a little more power. Good luck op

 

[MalcolmV8]

I'm running 2.5 and made 658 rwhp through a stalled auto I wouldn't change my setup on the hope of picking up a little more power. Good luck op

Yeah I'm low 700s through my 2.5 and I'm sure I can make plenty more. But what if swapping to a 3", at these power levels, gave me another 20 whp on the top end? I don't know. Also I'm thinking for future gains. What about when I turn things up.

 

[spray'n mach 1]

Yeah I'm low 700s through my 2.5 and I'm sure I can make plenty more. But what if swapping to a 3", at these power levels, gave me another 20 whp on the top end? I don't know. Also I'm thinking for future gains. What about when I turn things up.

maybe someone will pull the trigger and can have some back to back comparisons.

 

[JetmechF16]

With all of the testing you've done on other areas of your car, why don't you test this theory too? A lot of us enjoy your technical threads, it would be nice to see back-to-back dynos after an exhaust change in a controlled environment. I'm no help in this area since turbos love big (or none at all) exhaust.

 

[MalcolmV8]

With all of the testing you've done on other areas of your car, why don't you test this theory too? A lot of us enjoy your technical threads, it would be nice to see back-to-back dynos after an exhaust change in a controlled environment. I'm no help in this area since turbos love big (or none at all) exhaust.

lol I was hoping to just change it out this winter and be ready in the spring. It also allows me to purchase the correct x pipe up front. If I end up leaving the 2.5" on and switching to a 3" later I'll definitely do before and after dyno tests.

 

[stangfreak]

I remember back in the day two of my buddies were building up there cobras. Both had stroker motors, same cams, same ported heads, everything. The only difference one car had the 2.8h kb the other had a 3.4 whipple. same shop same tuner.

The 2.8 kb car had regular mac headers, prochamber, and a 2.5 inch magnaflow catback. The 3.4 whipple car had a custom 3 inch exhaust system built by american racing headers. custom. 3 inch all the way even catback.

The 2.8 kb car was right on par with the 3.4 whipple cobra. The whipple car probably made a few more hp because he was running more boost than the 2.8. we saw nothing crazy. ARH does recommend 1 7/8 long tubes with a 3inch collector, to a 3 inch midpipe which tapers down to a 2.5 at the end. This is with high boosted big blowers cars.

so my guess is, the power is coming from the headers and midpipe.

 

[RubberDuck]

This is just something I copied and pasted from a truck website that I am on alot. I read through this and talked with the guy who wrote this alot before deciding on a system for my truck. I am in no way an exhaust guru, but this is an interesting read and I found it very helpful and informative.

Exhaust many times is compared to other fluids. However, it is not quite the same due to the temperatures created during combustion and the pulses an engine creates. For an exhaust system to function properly, it has to balance flow and velocity. I am writing this to cover a broad range of knowledge bases and ages that typically frequent forums and have questions about how exhaust really works. The operation and physics behind exhaust, and the common misconception that backpressure is necessary for proper engineoperation is the focus of this article. The hope is, with the knowledge gained here and from other sources, that an informed decision and discussion about exhaust can carry on.

Velocity, Flow, and Temperature:
Velocity is the speed at which matter is traveling. Velocity in an exhaust system has to be understood to select the correct pipe size. The straw example is a simple but effective method to explain velocity. If a person blows through a small diameter straw, there is a great amount of speed of the gases exiting the end. However, even as the person blows as hard as they can, they are restricted by the size of the straw to the volume of gas that can be moved. Conversely, if a large diameter straw is used, the person will be able to move large quantities of gas but the velocity of the gases will be greatly reduced.
Flow is the ease at which the gases travel through a set container. The walls of the container will always cause some resistance, but this force is usually negligible. The more things inhibiting flow in the path, such as baffles or chambers, the more force is required to flow at the same velocity. If a piece of paper is held next to the end of the large diameter straw, it causes a slight resistance to the gases escaping the straw. Gases will take the path of least resistance. Since gases also compress, the molecules are forced closer to one another for a short period because it is easier to compress than escape the end. This causes the pressure to increase. Once the pressure rises and overcomes the force the paper puts on the end of the straw, the gases begin to escape out the end. This is what many call backpressure because of the pressure that is built in the area necessary to escape the container. Backpressure only causes the origin to work harder to push more gases into and out of the container.
The temperature of the gases in a container greatly affects its behavior. The cooler the gas is, the closer the molecules are together in space. The warmer the gas, the larger volume the gas occupies. This is due to the force of the molecules as they come in contact with the container and other gas molecules.

Operation in the cylinder:
On the power stroke, the piston is moved down the cylinder from the combustion. As the piston begins to move upward, the valve on theexhaust side of the head opens allowing the hot products of the reaction out. When the piston reaches the top of the chamber, it begins the downward stroke and pulls air and fuel in to repeat the cycle. Simple really, but this is where the myth of backpressure comes in. My question all who claim they need back pressure to run properly is: What does it do? The cam controls valves so no “back pressure” is needed to close the valve, the piston returns to the bottom of the cylinder via the power produced, and the chamber needs fresh air and fuel to continue the cycle. Any gases that are trapped in the cylinder are detrimental to the next combustion reaction. The chamber needs to be fully evacuated so a pressure difference is created. As the piston moves on the intake stroke, a miniature vacuum (delta pressure) is created to draw fresh air and fuel into the chamber. The ability for the pulse to maintain the correct velocity and flow, interact and aid in the other cylinder’s operation, and exit the system are the key functions of the exhaust gases.
Modern vehicles do not rely on one cylinder in an engine so the interactions with the other cylinders are critical. This is extenuated in V6, V8, and larger cylinder engines. As one cylinder is firing, another is on the opposite stroke. This creates pulses throughout the system. Using these pulses to aid in the other cylinders operation is key. As a pulse is making its way into and out of the header/manifold it creates another pocket of delta pressure behind it. This drop in pressure is used to help “pull” the next cylinder’s exhaust gases into and out of the manifold and so on. This is referred to as scavenging. This is also why V6 and V8 engines enjoy the presence of a crossover of some sort. Due the firing order on each side, the pulses are not the same all of the time. Without some way for the pulses to equal the other side’s scavenging, the scavenging effect will be slightly different on either side. This is why many times a pop or poor operationof the engine is detected without a crossover. A pop may also be audible with a crossover system but this is usually due to a rapid acceleration/deceleration and the different rates of the pulses in the system for that moment. A crossover is recommended to be installed before the muffler, close to the headers/manifold to quickly maximize scavenging.
At any time there is approximately 14.6 psi pushing on all sides of you. The atmosphere exerts this pressure at sea level due to its mass. This is the only backpressure exerted on the exhaust system. The exhaust pulses upon startup must overcome this pressure to exit the system. After startup in a properly sized system however, the pulses continue to aid the next.

Adjusting the Power band:
The power band is described as: at X rpm produces Y horsepower/torque. This can be influenced greatly by the size and configuration of the exhaust piping. Exhaust is most effective at the point which flow and velocity are maximized. This point is when the pulse can expand to the size of the pipe without velocity being diminished. Too large of a pipe and the velocity slows, causing the pluses to create little scavenging behind them and the pulses weakly leave the system. Too small of a pipe and the flow is restricted, effectively “choking” the system.
The rpm range of operation is quite large on most motors. Idling at 700 rpm and redlining at 6000 rpm create very different environments in the pipe. At lower rpms, less volume in the system is needed. At higher rpms, more volume is needed. Since most drivers don’t do one or the other all the time, the median for normal operation is ideal. This is where the powerband comes in. By having a larger diameter system the higher rpm range may produce the best power because that is the point at which flow and velocity are optimized. However, in these systems a loss in low range power is felt due to the fact that velocity is not ideal. If the volume of the pipe is too large the pressure drop behind each pulse will not be as likely to help in the scavenging effect of the next cylinder. The same principle goes if the pipe is too short. The short pipe does not allow enough time for proper scavenging and leaves the system before it can occur. This is why things such as open manifolds and ending the system under the cab do not bode well. With a smaller system, low range power is optimized because lower rpms are most efficient at scavenging. At higher rpms, the system’s flow is not sufficient, thus less power is able to be produced.

More than just pipe:
Exhaust does more than simply get waste products out of the way. In vehicles that utilize oxygen sensors, the exhaust is the critical component to monitor air/fuel ratios. This measurement by the oxygen sensors is critical in maintaining correct a/f ratios and prevents from either lean or rich conditions. Changing the location of these sensors or how the gases interact with the sensors, needs to be accounted for by adjustments to the PCM.


Any modification to the system can have an impact on the performance of the engine. Understanding how the different segments interact with the whole is crucial when modifying a vehicle. With this knowledge, you should be able to understand how exhaust systems operate and be able to make quality decisions when choosing to modify your system.

 

[Riddick]

Thanks for the feedback. I did some searching around on the net and found some charts for HP vs pipe size and they seem to claim dual 2 1/2 pipe is good to 463 hp. That seems very conservative. They claim beyond that you're actually running into a restriction... humm.

Maybe to weigh my options I'll look at some 3" cat backs. Who makes those? Right now I'm running the common Borla Stingers. Rather like the sound but change can be a good thing.

You have to factor in you are running a dual 2.5 inch exhaust. I bet the numbers you looked up would be right if you had a single 2.5 exhaust (like a honda civic or something).