performance loss! help!

[CASH24]

i just installed my DC headers and a catback exhaust and i've noticed some power loss in the lower RPM. my car struggles when i first accelerate, but once I get to 3000 RPM it kicks it in the ass and takes off. Anyone have any suggestions on what I should do to regain that power in the lower RPM's.

[smufguy]

check ur vaccum lines and idle. Check for vaccum leaks and see if the header gasket is torqued to speck. and yo, where did u get the DC from?

[88LXi68]

less backpressure means less torque but more mid to high end. When my car was an auto and I did the header and exhaust I noticed that the low end was sluggish, but once over 2500 its moves well.

[Dutchboy]

How bad would it be to run without a muffler and just a straight pipe....????

I know someone who has a 97 civic and he did just that and says its alright for the car...Im like no!!

Isnt there a muffler for a reason..???? quieter and for the backpressure???? if you dont have backpressure it cant be good....

Ben

[CASH24]

i was thinking of getting a high flow cat or even punching the one i have out, but i dont know if that would help the cause or not? what about an intake, would that help any??

[A20A1]

Originally posted by Dutchboy

Isnt there a muffler for a reason..???? quieter and for the backpressure???? if you dont have backpressure it cant be good....

Ben

The muffler acts as a silencer for the exhaust and the resonator also has an effect on the tone.

I've droped the res and left my resonated/baffled muffler on with 2.5" cat back... the sound got a little louder but the tone stayed the same... if my muffler was straight through or it was just a plain pipe then it would be loud as hell. I'm sure the cat plays a part in the exhaust sound as well since it's restricting the flow.

__________________________________
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. "


Credit given to Texan and Fritz for their info on this topic.

[TeKKnoTeKK]

couldn't timing play a role also? I would think it may be worth adjusting your timing to get better performance.

[A20A1]

You can't have both unless you got a system like v-tec or some variable length intake runners to maximize performance down low and up high.

[startersgirl]

[1988starter]

OK after all the wrong answers here is the correct one switch your O2 sensor wires green to white and white to green. it will be fixed

[88LXi68]

Before I switched the o2 sensor wires my car actaully had some nice low end, but at higher rpms it was surging way too much. Once I switched the wires i got less down low, no more surging, but much better top end. now my car is a 5spd and it feels better down low.

[CASH24]

THE FUNNY THING IS MY CHECK ENGINE LIGHT CAME ON THE OTHER NIGHT, AND WHEN IT WAS ON MY CAR RAN 10 TIMES BETTER AND FASTER. I CHECKED MY CODES AND GOT 2 BLINKS "OXYGEN CONTENT B". THE LIGHT COMES ON ONCE MY CAR IS WARM AND EVERY TIME IT COMES ON MY CAR DRIVES BETTER. DOES IT HAVE SOMETHING TO DO WITH THE O2 SENSORS?

[iiviasterp]

switch your o2 wires..... i had the same problem..... i posted and got the same answer.... i was shocked that something so little like switchin the green to white and white to green would make that much of a differnce but it did. i had the exact same problems your havin.... just switch the wires and love the performance