catback pipe size

[1s132c]

What would be the best size to get for a stock 90 KA24E. I would assume a 2.5, but I am not to sure. I seen a few ka24de's with 3in piping in my opinion thats to big for a N/A but it seems to work well with the car. so would a ka24e work better with a 2.5 or 3in striaght pipe no cats.

[InsanityInc]

well, your opinion is wrong. It's been dyno proven a few times that a 3" pipe on a KA24DE has no losses anywhere.

As for a KA-E, I'm not sure. The head differences probably have an effect on what you should use.

[navysnail]

i have a 2.25 on mine, its stock engine wise

[toptechracing]

can you provide a link or a sheet to show 3" on a DE was no loss?

[robbbby]

I wouldn't run 3" on a NA car. They need back pressure or else the car produces like no torque. Try driving around with just an open header when swapping exhausts. Car makes no power because there is no back pressure. FI cars are the opposite.

[InsanityInc]

I wouldn't run 3" on a NA car. They need back pressure or else the car produces like no torque. Try driving around with just an open header when swapping exhausts. Car makes no power because there is no back pressure. FI cars are the opposite.

must...resist... urge... to... stab...

Back pressure is never good. Ever Ever Ever. EVER. By using a properly sized pipe with minimal bends and restrictions, you create a low-backpressure condition. You also create a high-velocity condition, which creates a low pressure zone in the cylinder as the exhaust exits, which draws more air into the cylinder. The ideal situation is to get backpressure to zero and maximize velocity, this is for ANY engine.

[robbbby]

If backpressure is never ever ever EVER good why doesn't the same thing apply to driving around with an open header? That creates a low back pressure high velocity condition does it not? I'm not trying to be a smart ***, just curious.

[toptechracing]

SO I see one dyno sheet. Reading would indicate that the baseline was a stock exhaust and the power pull was a 3" catback system. I would expect to see a power increase from stock. I would like to see a comparison between 2.25,2.50,2.75 and 3.0...

I would be a little shocked to see any real gains between to 2.5 and 3.0 on a NA KA up to 12:1 other than noise.

[InsanityInc]

Quote »SO I see one dyno sheet. Reading would indicate that the baseline was a stock exhaust and the power pull was a 3" catback system. I would expect to see a power increase from stock. I would like to see a comparison between 2.25,2.50,2.75 and 3.0...

I would be a little shocked to see any real gains between to 2.5 and 3.0 on a NA KA up to 12:1 other than noise. [/quote]Stop being a tool and search around if you really want to find out. I doubt you'll find any 2.5in catbacks gaining 13+hp on a stock KA. I proved you wrong, deal with it nicely.

Quote »If backpressure is never ever ever EVER good why doesn't the same thing apply to driving around with an open header? That creates a low back pressure high velocity condition does it not? I'm not trying to be a smart ***, just curious.[/quote]Same reason why running without an exhaust manifold won't help you much. Also the same reason why a large number of parts in a car feature tapered fits for airflow. Keeping a smaller hole allows the gas to maintain a higher velocity. Remember how I told you that fast-moving gases create a vacuum in the cylinder? Well, it actually occurs everywhere in the pipe, so it can also help to suck along the other exhaust pulses, sort of like the exhaust pulses in your pipe are all drafting off of eachother (though, the principles at work are a bit different). Carburetors actually function off this principle as well(though they utilize it differently). The reason why large-displacement muscle cars can benefeit more from open headers is that having a large enough, long enough pipe is almost completely infeasable for engines that large due to the increased exhaust gas volume so it's easier to make a bit of a sacrifice for convenience's sake. Also, an open header will have better improvements on the high-end, but it would have worse performance down low. So if you were building a race car, you could weigh in that as a factor, since you probably won't be operating at low RPMs much.

[toptechracing]

Stop being a tool and search around if you really want to find out. I doubt you'll find any 2.5in catbacks gaining 13+hp on a stock KA. I proved you wrong, deal with it nicely.

Wow you really need to cut back on the coffee dude. You have proved nothing other than to show you have a bit of a chip. I think you will find there are a lot of ways to get to the end result and yours may not be the onlyone. Sometimes just opening your mind instead of your mouth can be a real benefit.

As the piston approaches top dead center, the spark plug fires igniting a fireball just as the piston rocks over into the power stroke. The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke. The gas pressure is still high (70 to 90 p.s.i.) causing a rapid escape of the gases (blowdown). A pressure wave is generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (and is dependent on gas temperature). Expanding exhaust gases rush into the port and down the primary header pipe. At the end of the pipe, the gases and waves converge at the collector. In the collector, the gases expand quickly as the waves propagate into all of the available orifices including the other primary tubes. The gases and some of the wave energy flow into the collector outlet and out the tail pipe.

Based on the above visualization, two basic phenomenon are at work in the exhaust system: gas particle movement and pressure wave activity. The absolute pressure differential between the cylinder and the atmosphere determines gas particle speed. As the gases travel down the pipe and expand, the speed decreases. The pressure waves, on the other hand, base their speed on the speed of sound. While the wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. At all times, the speed of the wave action is much greater than the speed of the gas particles. Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together, as in a collector for example, the waves travel into all of the available pipes - backwards as well as forwards. Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

This reflecting, negative pulse energy is the basis of wave action tuning. The basic idea is to time the negative wave pulse reflection to coincide with the period of overlap - this low pressure helps to pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. Typically this phenomenon is controlled by the length of the primary header pipe. Due to the 'critical timing' aspect of this tuning technique, there may be parts of the power curve where more harm than good is done.

Gas speed is a double edged sword as well, too much gas speed indicates that that the system may be too restrictive hurting top end power, while too little gas speed tends to make the power curve excessively 'peaky' hurting low end torque. Larger diameter tubes allow the gases to expand; this cools the gases, slowing down both the gases and the waves.Exhaust system design is a balancing act between all of these complex events and their timing. Even with the best compromise of exhaust pipe diameter and length, the collector outlet sizing can make or break the best design. The bottom line on any exhaust system design is to create the best, most useful power curve. All theory aside, the final judgement is how the engine likes the exhaust tuning on the dyno and on the track.

My guess is what works on a race motor will be different than a stock street motor. In planning my exhaust I would plan for what future mods I was going to make.

Modified by toptechracing at 10:16 PM 11/14/2004
Modified by toptechracing at 10:18 PM 11/14/2004