power lost on new headers

[phess]

i recently installed new performance exhaust headers on my 2000 honda accord EX v6,well i lost power.i hollowed out the cat too.Does anyone know why?why i lost power .What can i do.help please

[Schister66]

mid range might die out a little, but the top should pick up. I cant believe you actually lost power!

[goingfor300k]

If the header is larger than your stock mainfold and you hollowed out your converter as well, you eliminated all your back pressure. What was your logic for hollowing out the converter? You need a certain level of backpressure to create power. Bigger is not always better. More than likely you're running a little lean due the mods. Anytime you make changes to an exhaust system you will need to make some fuel adjustments to compensate for this.

[TurboGSR96]

"Bigger is not always better"......Did your girl tell you that?, just kidding. You are exactly right with what you said

[knorwj]

Back pressure being good is a myth. Back pressure is always bad. When you say that you need back pressure you are over generalizing and what you mean to say is you need exhaust velocity.

[TurboGSR96]

Man im gonna stay out of this one.....think this could get ugly.....

[goingfor300k]

Originally Posted by knorwj

Back pressure being good is a myth. Back pressure is always bad. When you say that you need back pressure you are over generalizing and what you mean to say is you need exhaust velocity.

No, what I said is exactly what I meant. Although your intentions are good, I don't think you have thoroughly thought out a logical response. Too small a pipe increases resistance and backpressure. Not very efficient when trying to flow spent exhaust gases quickly. On the other hand, too large of an exhaust creates an environment where the velocity is unmatched, thus slowing down the flow of exhaust. Velocity is only one factor. If the diameter of the exhaust does not match the velocity the exhaust will escape at a slower rate. In laymans terms...no backpressure equates to very slow exhaust flow. Power is built from quick intake and quick exhaust. Get it in faster (blowers, turbo, etc) and get it out faster. A certain level of backpressure ( correct diamter pipe ) is required to maintain a quick flow of exhaust. Think of it as a garden hose. Same water volume and velocity between a 1 inch hose and a 2 inch hose. Which hose do you think will shoot water out faster? The 1 inch hose will win every time. Back pressure, in and of itself, is not a bad thing, it's required to build power. "Excessive" back pressure and "not enough" back pressure are bad things. By hollowing out and entire exhaust system, and then adding a bigger header, I'm convinced the poster has put himself in a position of "not enough" back pressure.

[MT-2500]

Well do not feel bad about it look at what happened to this person.
http://www.automotiveforums.com/vbul...50#post3647950

[knorwj]

Originally Posted by goingfor300k

No, what I said is exactly what I meant. Although your intentions are good, I don't think you have thoroughly thought out a logical response. Too small a pipe increases resistance and backpressure. Not very efficient when trying to flow spent exhaust gases quickly. On the other hand, too large of an exhaust creates an environment where the velocity is unmatched, thus slowing down the flow of exhaust. Velocity is only one factor. If the diameter of the exhaust does not match the velocity the exhaust will escape at a slower rate. In laymans terms...no backpressure equates to very slow exhaust flow. Power is built from quick intake and quick exhaust. Get it in faster (blowers, turbo, etc) and get it out faster. A certain level of backpressure ( correct diamter pipe ) is required to maintain a quick flow of exhaust. Think of it as a garden hose. Same water volume and velocity between a 1 inch hose and a 2 inch hose. Which hose do you think will shoot water out faster? The 1 inch hose will win every time. Back pressure, in and of itself, is not a bad thing, it's required to build power. "Excessive" back pressure and "not enough" back pressure are bad things. By hollowing out and entire exhaust system, and then adding a bigger header, I'm convinced the poster has put himself in a position of "not enough" back pressure.

just because your 1 inch hose can spray harder (more pressure) does not mean it is flowing more water. In fact its flowing less water.

You may be convinced that this guys problem is back pressure loss, I'm convinced that hollowing out his cat created a place for exhaust to slow and create turbulence hence Slowing down the flow of exhaust. Its just like having a straight pipe is better than bends because all those bends creates turbulence and slows down the flow of gases. Ideal flow is 240 ft/sec Anything less or anything more will hinder performance. Not knowing the piping size of his new headers or the exhaust system I can't do the math but i'm assuming it is slowed down significantly due to the headers and the swirling in the cat.

And another thing if back pressure is such a good thing then why do you run open header or short straight collector at the track? I'd assume those pros would have a nice 2 inch curvy exhaust on their cars if they wanted some back pressure.

The bottom line is back pressure is bad. When you go to the track you run the shortest straightest pipe allowed by the rules to increase velocity and decrease back pressure.



Honestly its just physics, Back pressure = bad. Delta pressure and gas velocity = good

And like I said originally you are confusing you're terms. I never said a large diameter is best. You are exactly correct in saying a too large pipe the velocity is "unmatched" meaning it won't flow quickly and efficiently outwards and can swirl or even go backwards. See you need low back pressure and high velocity and high delta pressure to help scoot the air along through the pipe.

If you have the time look up Fluid Dynamics and Bernoulli's equation it might help. It pretty much states that pressure (back not delta) and velocity are proportional, and that an increase in one will make the other decrease. So In "laymans terms" as you put it, If you increase velocity you decrease pressure and if you increase pressure you decrease velocity.

Its an honest mistake made all the time but it is a myth that back pressure is good. What you meant to say is that Delta pressure is good, and exhaust velocity is good.

[knorwj]

Originally Posted by MT-2500

Well do not feel bad about it look at what happened to this person.
http://www.automotiveforums.com/vbul...50#post3647950

those people are using the term back pressure in place of delta pressure. Most of them are just reiterating the myth of back pressure, however the one guy fierangero makes a very valid point but once again uses the wrong term. You need delta pressure and gas velocity to be high and back pressure to be low in order to get the scavenging effect he mentioned. Scavenging is what I was talking about when I said "help scoot the air along" You pretty much want the air to be sucked away from the valves to keep the gasses from being sucked back in from the valve overlap.


I know there are some very good and informative write ups on this topic around here somewhere, I'll see if I can find them. They're very old from like 2001. One of them was a sticky back when I joined.

[tofkrew]

most likely your car is running lean if you lost power through the entire rpm range. look into a chip upgrading an ecu or getting it remapped, or any type of piggy back system.

[stockprelude]

ahh the infamous backpressure...im quit positive that its extremely necessary, but im gonna know next semester when i take the air breathing engines class. for now im just gonna go with my assumptions.

are people serious? doesnt it just make sense?

[corning_d3]

I've got a '96 Monte Carlo 3100 V-6. I tried installing a set of headers with 2 1/2" exhaust and a hollowed out converter. My experience was also the same. After picking my instructors brain for a while, i learned exactly knorwj explained. I put the restrictive manifold back on, changed the valve timing and used the hollow converter as a expansion chamber to force some of the the raw fuel/air exiting the exhaust valve back into the engine. It took a very long time to get my cat pipe length tuned right. But now...WOOOO! The 3,000 - 4,000 RPM range will place you firmly in the seat. It's like having forced induction..

[knorwj]

Originally Posted by stockprelude

are people serious? doesnt it just make sense?

yes it makes sense that it would be bad.

I still don't understand for the life of me why people would still think that resticting the flow of your engine could be a good thing?

[knorwj]

here's the link to one of the threads I was talking about. It's explained much better here than I could ever do. You can also find tons of info on google regarding the physics of how this works.

http://www.automotiveforums.com/vbul...delta+pressure