Well, backpressure does exist, so its not a myth. But what you are referring to is the myth that less backpressure will create less low end torque. And this is a myth. I have proven it on the dyno with doing back to back dyno runs, one with longtube headers going through the stock restrictive catback and then opening up the cutout and gaining nothing at all. They were identical graphs.

My understanding is that haveing less backpressure will only move your power up in the rpm range. But this is only a theory of mine, so please correct me if i'm wrong.

 

From what I understand it is a matter of volume (cfm), having a cutout closed is actually more restrictive than a stock exhaust due to the disruption in the exhaust flow.

Everyone has a rough idea of the boundry layer on the surface of a car but on the inside of an exhaust tube? I've never read a discussion or math equation yet. It would be different from just pumping air through due to the pulses of the engine so measuring it would be difficult at best.

I missed the artical somehow, is there a link to it we can read?


I apologize for my spelling

 

About backpressure,

What getting an aftermarket product, a high quality one, lower back pressure and increase power?

I'm asking cause I would like to know if I can reduce stress on the engine by adding er, lowering back pressure.

My theory is that the exhaust pipe has less pressure in it, the piston has to do less work to move that exhaust in the cylinder and in the pipes.

Exhaust is one of the few areas I have little understanding in, even a basic understanding of. People write more articles about nitrous than basic pewrfomance upgrades...>.>(off topic..)

To inform you more, I am really interested because the only thing I want to do to my mom's car when it becomes mine, is exhaust headers and possibly pipes, and intake. I want to reduce the stress on the engine from stock parts.

 

Exhaust systems become less of a concern the farther away from the engine. In other words the headers are more important than the tail pipes. If you want to get a deeper understanding read nateh's posts. I think he designs oem systems for a living.

Basicaly you want a system that's going to support your engine's cfm requirements, noise requirements and opperate most effeciently in the rpm range of your engine. Most of the setups on this site don't need to go that into it though.

-b

 

Thought you might find this useful:
http://www.broaderperformance.com/mu...flow_tests.htm

I think I'll go with hooker mufflers from now on .

Also, a question, what are "dumps", "over axle", "under axle", etc and what are the sound and performance differences, if any?

 

This has been an interesting read for sure as I'm still figuring out what diameter of piping, 2 cats vs one, etc.

Just a few questions though:

Were the initial numbers posted HP figures at the motor?
Would one Large High Flowing Cat equate to 2 smaller ones on each side?

What diameter exhaust would you guys recommend for a 383 Stroked LT1 with about a 200hp shot of nitrous (rwhp)?

 

from my understanding the myth about wanting backpressure for low end tourque is a farse, it isnt wat is actually happening, in a system like the internal combustion engine u want no restrictions on your engine to make the most power, but the reason that you can go to big of an exhaust pipe and lose power is the gases will expand, cool and therefore slow down if you have to much 'free room' in your exhaust system, people mistake this as 'needing back pressure' when in fact going to big will cause back pressure due to the gases slowing down and piling up. So at slow rpm that gases are going to slow, but as rpm picks up the amount of gases being pushed through the exhaust pipe is increased therefore to flow more exhaust in the same diameter pipe you must speed up the gases, making a bigger exhaust pipe more desirable for upper rpm. Please take this information with a grain of salt and If any of this information is wrong please correct me im no expert, and i hope this helped anyone intersted.
steven

 

Do the figures take into account what type of headers you have hooked up the the exhaust pipe? Does it matter?

 

I think we may be exagerating the importance of removing the exhaust pressure on our cars. There is alot more to designing an exhaust than cfm and pressure. Take for example turbo cars. They make tons of power with extremely high exhaust pressure. A properly designed exhaust will draw a vacuum near the exhaust port due to the speed of flow and Bernoulli's principle. We are not dealing with a one cylinder car. Oversized pipes and True duals IMHO are not necessarily the answer. Hence the X pipe on Corvettes and Y pipe on F bodies....

 

Is there any rough estimate for losses if your pipe diameter is too large? It seems, according to this thread, that a 2.25" dual exhaust system is more than adaquate for my needs; however, that limits me to a summit house brand with poor mufflers and no balance pipe. I'd like to use the flowmast 2.5 system, how much could I expect a 380hp pontiac 400 to suffer?

 

X pipes are the be all end all, y pipes are merely a packaging issue

Its all about sizing for your needs and the x pipe creates scavenging.

 

The Ypipe creates a scavenging effect as well. Given the choice of a y pipe and true duals I would take the Y pipe. If you don't think so try removing one of your smog/air fittings and feel the vacuum near the exhaust port.

 

More than just diameter- there are the number of bends, for example- each twist and turn adds "friction" to the flow. Also, as posted before, the gas is both expanding and cooling, and a well designed system will allow for this to keep the flow speeds up. Part of that expansion, or lack of, has to do with exhaust temps, basically you don't want the gasses cooling too quickly as they will loose energy- hence the benefits of coated systems that keep the heat in the exhaust rather than allowing it to radiate under the hood.

Lastly, there is tuning, where the entire system can be made to resonate at specific frequencies, and increase exhaust scanvenging. It borders on a black art, lots of trial-and-error as well a formulae and theory. Most systems have certain 'sweet spots', RPM's that they add the most effect. Car designers frequently will tune exhaust to add some benefits at one RPM range, tune the intake to help at a different RPM range, and maybe adjust valve timing for something else, so that, taken all together the engine will produce good power and torque across a broad range of speeds. This makes a car more driveable, though maybe at the cost of absolute peak numbers.

Rather than arguing H-pipes and X-pipes, maybe we should just run tuned stacks....... That would be impressive sitting at a traffic light belching flames! ;^)

 

I fully agree,Zoomies with no baffles are the only choice for traffic lights. I will argue with anyone H-pipes and x-pipes. X-pipes are better