Demus

Hrm. Can you elaborate on this? From my understanding of exhaust systems, as long as the primary tubes are of the correct length the exhaust pulses should have the exact same scavanging effect with a 1 foot collector as with a 5 foot collector. It also makes sense that you'd want to stick with the same size of pipe all the way through the system. If you go to a smaller pipe you're adding restriction to the system, and if you go with a larger pipe you will slow the exhaust gas, which will hamper any scavanging effect and will add restriction to the system.

93camaroLT1

alright i'm convinced now.. I'm just gonna run open headers and get it dynotuned with open headers. I got some glasspacks I can throw on for when I want to shut the car up a little (which will prolly be never anyway).

93camaroLT1

Although if this is all true about velocity and backpressure. Backpressure being bad since it is a restriction in the exhaust, but too much velocity is also bad which would be caused by running from a smaller collector to a larger pipe. Why is it that my civic si, which has somewhat small primaries (not sure what size) and comes to a 2 1/2" collector, does not move at 60 mph at WOT with a open header, which has no back pressure and shouldn't have too much velocity being that there isn't wouldnt be a bigger pipe attached to the header (there was no pipe attached to the header). What is the reasoning behind that? Don't get me wrong I already realize that running open headers on the camaro without O2s will be fine, but all this that i've read on this and other posts doesnt seem to make a lot of sense when applied to my civic si, being it won't move flooring it at 60 mph (even if I throw it in 2nd gear at 60 where it is at 7800 RPM), it just gets really really loud.

engineermike

I did alot of thinking about this since it goes against everything I've seen. I believe that your Civic was running extremely lean due to removal of ALL backpressure and subsequent drastic increase in airflow.

93camaroLT1

yeah I have no clue. I know honda exhaust are never bigger than 2 1/2" unless it is a "turbo" exhaust which is about 3", which would make sense if the collector of the header on a N/A car is 2 1/2" at most, so going to a 3" catback could probably too much velocity, therefore slowing down airflow. But didn't make a lot of sense why a open header cause that situation.

Demus

No. There is no such thing as too much velocity. What you're refering to is too much volume. The higher the velocity the better... always.

This is a common feeling most people have when running open headers. Basically what is happening is you're losing low end torque, so you're actually going to accelerate slower in the lower RPM range. You will however make more power and accelerate faster in the higher RPM range. Why? At the lower RPM range not enough exhaust gas is being produced to fill the volume of the pipe. The gas expands and slows down. This has two main results, first, the low pressure end of the exhaust pulse gains pressure as it equalizes with the rest of the pipe, and second the slower moving exhaust gas will stay in the pipe for longer, working as a restriction for future exhaust pulses. The low pressure end of an exhaust pulse is what "makes" torque. Exhaust pulses have two fronts, the first part of the exhaust pulse to leave the chamber is high pressure, in contrast to the last part to leave the chamber. The last part, the low pressure part, will attract the high pressure part of the next exhaust pulse, thereby "sucking" out more exhaust gas. This is called a scavanging effect. In essence, when the exhaust gas is forced out of the chamber at such a high velocity it creates a vacuum as it leaves. This key to understanding how headers work. The length of the primary tubes are tuned so exhaust pulses enter the collector one after the other, taking full advantage of the scavanging effect.

That's what happens in an open header situation. When you add a restriction, say a header reducer a more complicated scenario exists. When the exhaust gas reaches the reducer you force the gas to contract, and pick up speed as it exits the collector. At lower RPMs this means the exhaust velocity will be high. At higher RPMs this means the engine has to work harder to push the gas out of the chamber, because the gas must be forced through the smaller pipe. The loss is high end horsepower.

Ideally, you want the pipe volume to match exactly the volume of exhaust gas being produced. Naturally this isn't possible, since an engine produces different amounts of exhaust gas depending on RPM and load. Instead people tune exhaust systems to make power in a specific RPM range, depending on tons of different factors, like cam specs, transmission gearing, rear end gearing, application, etc.