Exhaust System

[MyGarbageLXi]

So this is what I'm thinking, Ima spurt for that Borla Hemi Muffler soon, i wanna get new 2.5' piping i believe it is but i wanna know, will i really notice a difference in the sound and the HP if i keep the stock headers and the Cat?? thanks in advance

[Jims 86LXI HB]

If someone is telling you to get 2.5" piping they are doing you a disservice. Use 2.25". Unless you plan on installing some heavy duty mods that would create the need for 2.5", don't go that route. You need some sort of backpressure to maintain your lowend torque. You will notice a big difference in sound, it will make listening to the stereo difficult. You will notice a hp gain, it is not huge, I won't play it as a big gain, cause it isn't. The most hp gain you could hope for would be around 6hp. The biggest gain I felt was when I added my DC Sports header onto the custom exhaust system. I got a good hp gain when I did that.

[1988starter]

I think it is not back presure but exhaust velocity that is needed the idea it to find where the two meet low presure with still good velovity. Kinda like a calc based physics problem.

[GreenMachine]

Anyone ever run just straight pipe with just a muffler and no CAT, with 21/4 piping. I was gonna do this because can't afford to get a cat right now but would this be really loud and would it be not enough back pressure - what if a through in a bottle muffler as well.

[TeKKnoTeKK]

First of all you don't need backpressure. And as long as you don't have to pass emissions that should be ok, I don't think the CAT has anything to do with the sound. The resonator and muffler are what factors how loud the exhaust is.

[Jims 86LXI HB]

Originally posted by TeKKnoTeKK
First of all you don't need backpressure. And as long as you don't have to pass emissions that should be ok, I don't think the CAT has anything to do with the sound. The resonator and muffler are what factors how loud the exhaust is.

And I guess you don't need torque either. Their are way to many tech articles around to NOT support what your saying about not needing ANY backpressure.

[TeKKnoTeKK]

Originally posted by Jims 86LXI HB


And I guess you don't need torque either. Their are way to many tech articles around to NOT support what your saying about not needing ANY backpressure.

This is an article I got from the FAQ's

I think you are misunderstood about backpressure and what it is Jim, maybe this will help.

Backpressure-
A lot of people have different thoughts on backpressure, and often confuse it with Velocity and Delta Pressure...
I will now post a colaboration of posts from Purehonda.com

"THE MYTH OF BACKPRESSURE"

…is probably the most widely misunderstood concept in engine tuning. IMO, the reason this concept is so hard to get around lies in the engineering terms surrounding gas flow. Here's the most impotant ones you need to be aware of to understand the things I'm about to say:

BACKPRESSURE: Resistance to air flow; usually stated in inches H2O or PSI.
DELTA PRESSURE (aka delta P): Describes the pressure drop through a component and is the difference in pressure between two points.

One other concept needs to be covered too, and that's the idea of air pressure vs. velocity. When a moving air column picks up speed, one of the weird things that happens is it’s pressure drops. So remember through all this that the higher the air velocity for a given volume of gas, the lower it's internal pressure becomes. And remember throughout all of this that I’m no mechanical engineer, simply an enthusiast who done all the reading he can. I don’t claim that this information is the absolute truth, just that it makes sense in my eyes.

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things). The larger the delta P measurement is, the higher this pressure drop becomes. And as earlier stated, you can understand that this pressure drop means the exhaust gas velocity is increasing as it travels from the cylinder to the exhaust system. Put simply, the higher the delta P value, the faster the exhaust gasses end up traveling. So what does all this mean? It means that it's important to have gas velocity reach a certain point in order to have good power production at any RPM (traditional engine techs sited 240 ft/sec as the magic number, but this is likely outdated by now).

The effect of having larger exhaust pipe diameters (in the primary, secondary, collector and cat-back exhaust tubes) has a direct effect on gas velocity and therefore delta P (as well as backpressure levels). The larger the exhaust diameter, the slower the exhaust gasses end up going for a given amount of airflow. Now the ***** of all this tech is that one exhaust size will not work over a large RPM range, so we are left with trying to find the best compromise in sizing for good low RPM velocity without hindering higher RPM flow ability. It doesn't take a rocket scientist to understand that an engine flows a whole lot more air at 6000 RPM than at 1000 RPM, and so it also makes sense that one single pipe diameter isn't going to acheive optiaml gas velocity and pressure at both these RPM points, given the need to flow such varying volumes.

These concepts are why larger exhaust piping works well for high RPM power but hurts low RPM power; becuase is hurts gas velocity and therefore delta P at low RPM. At higher RPM however, the larger piping lets the engine breath well without having the exhuast gasses get bundled up in the system, which would produce high levels of backpressure and therefore hurt flow. Remember, managing airflow in engines is mainly about three things; maintaining laminar flow and good charge velocity, and doing both of those with varying volumes of air. Ok, so now that all this has been explained, let's cover one last concept (sorry this is getting so long, but it takes time to explain things in straight text!).

This last concept is why low velocity gas flow and backpressure hurt power production. Understand that during the exhaust stroke of a 4 stroke engine, it's not only important to get as much of the spent air/fuel mixture out of the chamber (to make room for the unburnt mixture in the intake system), it's also important that these exhaust gasses never turn around and start flowing back into the cylinder. Why would this happen? Because of valve overlap, that's why. At the end of the exhaust stroke, not only does the piston start moving back down the bore to ingest the fresh mixture, but the intake valve also opens to expose the fresh air charge to this event. In modern automotive 4 stroke engines valve overlap occurs at all RPM, so for a short period of time the exhaust system is open to these low pressure influences which can suck things back towards the cylinder. if the exhaust gas velocity is low and pressure is high in the system, this will make everything turn around and go the opposite direction it's supposed to. If these gasses reach the cylinder they will dilute the incoming mixture with unburnable gasses and take up valuable space within the combustion chamber, thus lowering power output (and potentially pushing the intake charge temp beyond the fuel’s knock resistance). So having good velocity and therefore low pressure in the system is absolutely imperative to good power production at any RPM, you just have to remember that these concepts are also dependent on total flow volume. The overall volume of flow is important because it is entirely possible to have both high velocity and high pressure in the system, if there is simply not enough exhaust piping to handle the needed airflow.

It’s all about finding a compromise to work at both high and low RPM on most cars, but that’s a bit beyond the scope of this post. All I am trying to show here is how the term backpressure is in reference to a bad exhaust system, not one that creates good low RPM torque. You can just as easily have backpressure at low RPM too, which would also hurt low RPM cylinder scavenging and increase the potential for gas reversion. And understand that these tuning concepts will also affect cam timing, though that is again probably beyond the scope of this post. At any rate, hope this helps, peace. "

-here's a reply to the above post-

"I've been seeing a resurgence of the backpressure misnomer, but didn't have the time or inclination to write it up. So, again, thanks.

There is one thing I'd like to add to texan's work:
Exhaust Scavenging
In essence, this is the opposite of the exhaust reversion that texan describes.

Reversion: at the beginning of the intake stroke during cam overlap, exaust gas in the header is under high pressure (negative delta P) and is pushed back into the cylinder, diluting the new air/fuel charge.

Scavenging: at the beginning of the intake stroke during cam overlap, the momentum of the exiting exhaust gasses creates a brief vacuum (positive delta P) in the header, pulling out the remaining exhaust gases from the combustion chamber, and allowing the new air/fuel charge to be full-strength.

Scavenging is also the reason for differently shaped headers (4-2-1, 4-1) and collectors. We use the momentum of exiting exhaust from one cylinder to scavenge exhaust from another that is next in the firing order! The different shapes allow for this to happen at different airflow velocities thus at different RPM bands.

Scavenging takes advantage of the momentum of the exiting gasses. In essence, the fast moving exhaust pulse pulls a vacuum behind it. Momentum is mass times velocity. So not only do we need to keep the velocity high to prevent reversion - but it greatly improves the scavenging effect.

Thus we have a balancing act (as others have pointed out). We want to minimize friction to lower the backpressure as much as possible - larger pipes have less friction because they have less surface area per unit volume. But we want to increase the delta P as much as possible to prevent reversion and increase scavenging effects - smaller pipes increase delta P because they increase velocity.

There are lots of tricks to try to widen the useful RPM band (stepped headers) or to increase the overall effiency (ceramic coated exhausts), but it's still subject to this basic tradeoff:
Friction vs. Velocity
AKA: Backpressure vs. Delta Pressure
You want low friction and high velocity.
You want low backpressure and high positive delta pressure. "

Touché

[GreenMachine]

I couldn't saty interested for that long but in short term could I run 21/4 piping with no CAT ? Please any responses of people who have done this

[Jims 86LXI HB]

I don't get it. If you read the article. It supports that I'm saying and goes against what your saying.

[TeKKnoTeKK]

What? lol. It says that backpressure is bad and you don't need it, of course it is impossible to get rid off ALL the backpressure. But to say you need it to produce torque is wrong.

[MyGarbageLXi]

backpressure, wahhhhhhhh?? watever, thanks for all the input, i meant to say 2.25, didnt mean to start any controversy, but its been a learning experience, thanks

[GreenMachine]

I realize that you need the perfect amount of back pressure/Delta pressure to obtain peak performance but I was just wonderig if this setup would not be enough back pressure - cause i don't have my rocket scientist handy, to work out a formula and figure the perfect setup. lol :lol (just kiddin)

anyways anyone know if I could just use a bottle muffler ( cherry bomb) in place of a CAT - should that be enough backpress.
thanx yall

[TeKKnoTeKK]

Ok people.....read CAREFULLY.

Originally posted by TeKKnoTeKK

Ok, so as you can see, backpressure is actually defined as the resistance to flow. So how can backpressure help power production at any RPM? IT CAN'T. I think the reason people began to think that pressure was in important thing to have at low RPM is because of the term delta pressure. Delta pressure is what you need to produce good power at any RPM, which means that you need to have a pressure DROP when measuring pressures from the cylinder to the exhaust tract (the term "pressure" is what I think continually confuses things).

Can it be any clearer?

It's all good Garbage, you didn't start any controversy, I was just trying to educate everyone here based on information posted elsewhere on the board. It doesn't seem to be working tho. Lets put it this way man, taking off the CAT is not going to hurt your performance, if anthing it will help.......the CAT is just there for emission purposes.