from an article in SCC. Search for "back pressure exhaust gauge" in google to find the article.

You guys are diluted if you think backpressure is good. It's a byproduct and unavoidable in non-utopian situations. Let me repeat: You. Do. Not. WANT. Backpressure. It. Is. BAD.

Why not use straight exhaust ports or a huge pipe? Go back and read my post again, and I'll expand on it, again (this isn't the first time it's been discussed).

Benefits to small exhaust: Velocity is high, thermal expansion is low (keeping velocity from dropping as fast), and the pulses from each chamber actuall create a vacuum that pulls the gasses from the other cylinders. High RPM power is negatively impacted because the pipes aren't capable of conveying the volume at low enough pressure levels.

Benefits to big exhaust: High RPM power isn't restricted at all by the pipes. At high RPM the velocity is also high and scavenging is not important. At low RPM, however, velocity is really low and the scavenging effect suffers, causing poor mixture and lower power.

If you were to pull a ping pong through a tube with a string it would create a vacuum behind it, so if you put another ball in the tube behind it, it would travel along as well. If the tube is larger than the ball by X amount though, the ball won't from a seal and there won't be a vacuum, so you'll lose your scavenging effect. Same thing with exhaust pulses. One pulse pulls the others.

Backpressure simply put is the amount of pressure that is created by the engine trying to push the exhaust out the tailpipe. This number directly impacts the amount of power the engine can use for forward motion of the vehicle, just like any other parasitic loss - stock pulley/dampner, smog pumps, etc. If the engine has to work to overcome something, it takes that power away from the wheel power. By putting a CAI generally the amount of vacuum in the intake tract will go down, as the CAI will flow more air. Ideally you want 0 backpressure, but it's not going to happen. The engine will always consume some power in pushing the gasses out the end of the tailpipe. Get some different sized straws - you expend energy trying to blow through them in much the same way.

Lil - it IS all about velocity. What you're failing to understand though is that it's all about velocity at ALL RPMs, not just redline. If an engine flows 600cfm at 6000 rpm it'll need a big pipe to flow that much. At 2000 rpm though the engine only flows 200cfm, so it needs a lot smaller pipe to keep the velocity up. You have to compromise between the max rpm velocity and the bottom end velocity and find a happy medium that retains both decent high-rpm flow AND low-rpm velocity. And not to be too terribly offensive here, everyone has a brother's/uncle's/grandfather's best friend who is a (custodial) engineer at Ford. You tell me he got his masters in fluid dynamics and I'll be impressed. Being an "engineer" doesn't mean much though.


I'm not going to get into an arguing match here with you, but it's obvious to me that you don't have much experience in the mechanics of it all. I'm also going to guess that you're about 16 years old, even though iirc you didn't put it in your profile. Tell me, what is going to cause the valves to burn? I'm interested in knowing where you heard this and what legitimate sources have documented it.

For the record, in order to get close to 0 backpressure, you have to run huge pipes that are, for all intensive purposes, just an open atmosphere. Nowhere did I ever recommend doing so. In fact, I specifically said not to go too big.

Furthermore, "Flowmasters aren't loud enough" is an opinion, but based on what? Sure, I had a 70 series big block Flowmaster on my 99 Grand Cherokee with the 4.7 and it was pretty quiet, but still louder than stock. I've got a 30 series Flowmaster on my 360 powered Ram and it's loud enough to make me self consious when driving in residential areas late at night, but it's still well within the legal limits. I've got dual 40 series on my 5.0, and they're anything but quiet. Both my Ram and my 5.0 are catless though, so they'd be a lot quieter with an OEM cat in place. A high flow cat will increase sound levels significantly while retaining CARB legality.

 

Haha. You had alot of great points in your post, but I liked this one the best.

I don't think people will ever understand backpressure vs. exhaust velocity. I remember aruging this when I put the glasspack on my red Neon 5 years ago.

 

Wow I must have hit a nerve or what? I guess if you read it on the internet or in a magazine it must be true…..My older brother has a Master’s degree in Mechanical engineering from Stanford Univ. I didn’t catch where you went to school? Why so hostel? I don’t need a Psychiatric degree to see you have some deep issues…..Sorry I was not attempting to get into a pissing contest, you might have noticed if I did it just runs down my leg but from the sound of it you come up way short in that regard too.

 

Do some more research and get back to me. Your source obviously hasn't studied the material at hand, neither have you. I've done a fair bit of reading on the subject, both on and offline.

The only nerve hit here is that it's been hashed over countless times all over the place - people who don't know what they're talking about tell you that you need backpressure, people who know how to articulate what you really need tell you otherwise.

 

ok if you dont need back pressure lets all just run open headers and see how long the valves last in our engines so w/e i'm not gonna argue with you anymore its not worth it oh yeah and btw your garden hose vrs firehose remark makes absolutley no sence because a firehose is bigger and i know for a fact they spray a heck of a lot more water but anyways we all agree on one thing just run a single pipe and if u want duals that are decent size run a Y pipe

Oh yeah and you said you want to know about the whole burning out valves things well it is because when it lets too much exhaust out it also sucks much more air in causing your engine to run lean. now newer engines can handle this a lot more than older engines because the computers try to correct this but they cant make it perfect so it still cant handle this forever and it will burn the exhaust valves out in your engine prematurely

 

oh yeah and by the way read this quote

"Backpressure certainly can adversely affect your vehicle’s performance. But that doesn’t mean you want an exhaust system that completely eliminates backpressure.

“An engine needs a certain amount of backpressure, or resistance to flow, below the torque peak of the engine,� says Kevin McClelland of Flowmaster. “When you take away that restriction, ... the engine loses quite a bit of performance below torque peak, which is right where you drive for gas mileage and towing, and when you’re just driving around stoplight to stoplight, accelerating away from a stop.�

 

I've already told you, twice, why you don't want to run without an exhaust at all.

http://www.uucmotorwerks.com/html_pr...torquemyth.htm

http://www.proficientperformance.com...k_pressure.php

http://www.nsxprime.com/FAQ/Miscella...austtheory.htm


Try reading a little bit.

Don't discount facts because you dislike me telling you, read and learn. A LOT of people misunderstand the topic, including people who work for muffler manufacturers. It's not the backpressure you want, it's the exhaust velocity creating the scavenging effect. I can gladly post up more links later.

 

i'm not gonna argue with you anymore lil was right you do have serious issues and by the way i do not take kindly to being called a lier because i would not be lieing even if the information i gave was incorrect (which it was not) that would mean i was simply missinformed and you know what you contradicted yourself again because i read the information on the links of your post and it said the same thing i said about the engine burning valves due to it running lean from zero backpressure and the computer not being able to correct it. these are dodge dakota pickups not BMW's they have to have a certain amount unless you change the whole computer system they cant handle it like the BMW in the ariticle. if the guy that started this thread has an exhaust with zero backpressure its going mess things up. and low end torque will suffer. I'm done arguing with you its not worth it with people like you. so **** away i quit

 

Did I call you a liar?

And no, I don't have serious issues. I just know more about the subject than you do. Go ahead and believe what you want. Go ahead and believe that no backpressure will lead to burned valves. I'm not the ignorant one.



If you actually took the time to read and comprehend what those sites (and others) are saying, you'd understand that they're not backing your claims up. They're saying that backpressure is bad. It is caused by a restriction in the exhaust which robs power. Your claim that Dodge motors don't adjust the A/F ratio is also completely false. There are O2 sensors and they do compensate for changes to the A/F mixture. Furthermore it went on to say that the burned valves were typically found in carbureted vehicles where the carb wasn't retuned after improving the exhaust. Nothing earthshattering there.

So go ahead, ignorance is bliss right? I'll **** away knowing that I tried to help the uninformed.

 

ok i'll admit i misinterpreted the first article dakota's can compensate as far as A/F mixture goes. and i guess the reason i argued with you wasnt because you were completly false you are correct on newer vehicles. but like u said carborated ones do need it and i've been around a lot more of them than anything else. but we both agree that the best way to put duals on these trucks that arent a tiny pipe is to just Y pipe it correct? because after all i dont think the guy that started this thread really cares about backpressure and velocity as long as he gets power and loud exhaust lol one thing though i dont understand why you cant just take the entire exhaust manifold off if you want zero backpressure explain because i read all your posts several times and it never said.