another exaust question...LOW-END???

[sporkHSP]

i was wondering what would be the best exaust set up to boost some highend and keep the low end close to where it is... any thoughts?

[Mike's89AccordLX]

I think a 2.5" is what you want. 3" you might lose your low end.

[88accordhb]

lots of people say 2.25 is better than 2.5, but i got me some stocks and i'm lovin it.ehehe

[AccordEpicenter]

the bigger exhaust you use the more torque youll lose n/a... 2.25 is the nicest combo for these cars...

[bobafett]

from experience driving my car with 2.25" and ambers car with 2.5" i would say go with 2.5"

both of us had intakes, headers, and high flow cats, and her lowend is still reallly good...

so if i had it to do over i would get 2.5"

**** oh wait im going 3" heheheh ****

[A20A1]

you certainly want a header with 2.5

[Rendon LX-i]

yep 2.5 thats what i have works good i have low end but want more. Right now my header getting done by a shop so i can bolt on to my 2.5 piping

[sporkHSP]

i want 2.5headers or headers and 2.5in piping? will I lose any lowend by going with 2.5

[shepherd79]

going with 2.5 pipe you will lose some low end for sure. go with 2.25 insted.
and if you get header, make sure it is 4-2-1 header and not 4-1. 4-1 is for top end, you will lose big chunk of torque with that headers.
pacessetter makes 4-2-1 header. i have one and i love it.

[blahblahblah]

2"all the way , no cat I love it. I had 2.25"woth a flowmaster d-50 ... 2" better with that muffler(or similar) and no cat has best Tq..

[pixiesfan78]

its uncool when current members sighn up on another screenname jus to post something stupid.jmho

[AccordEpicenter]

its uncool when current members sighn up on another screenname jus to post something stupid.jmho

WTF are u talking about?

[pixiesfan78]

wtf im talking about is blah blah blah obviously an older member mocking people trying to have conversation and why should my comment provoke you to get annoyed with me.whats the matter short man complex their napolian.

[zero.counter]

"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 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 to unknown...

* * *

Cliff notes:
1) Backpressure is your enemy.
2) High flow velocity and positive delta pressure are your friends.

[smufguy]

2.25" for a NA with headers, cat (or no cat) and a muffler and a CAi should work good. Its just the preferences besides your car is also different from others. i got 2.25" all the way thru including the cat and the back. The headers are 2" if i am right, i think thats how the DC was made. Also, i am running at 43psi of fuel pressure with 5-10 degrees of ignition advancement and a CAI. it works for me. To me it seems like the car does not 'boost' till 2.5K even tho i dont have a turbo . If u know what i mean. But on a roll, or if ur 3K and above on the move, shit just pulls hard.

[88accordalltheway]

with all that saide zero counter, wtf would be a good size? So far ive heard either 2.25 or 2.5, WHAT IS BETTER? Or is it just personal prefrence?

[zero.counter]

with all that saide zero counter, wtf would be a good size? So far ive heard either 2.25 or 2.5, WHAT IS BETTER? Or is it just personal prefrence?

That is all up to you and how you interpret the given data. My opinion is 2.5, with no noticeable loss in power (trivial if any was to be).

[AccordEpicenter]

idk for sure... i still run a stock exhaust except for no muffler... pulling 16.2

[Magny]

2.25" for a NA with headers, cat (or no cat) and a muffler and a CAi should work good. Its just the preferences besides your car is also different from others. i got 2.25" all the way thru including the cat and the back. The headers are 2" if i am right, i think thats how the DC was made. Also, i am running at 43psi of fuel pressure with 5-10 degrees of ignition advancement and a CAI. it works for me. To me it seems like the car does not 'boost' till 2.5K even tho i dont have a turbo . If u know what i mean. But on a roll, or if ur 3K and above on the move, shit just pulls hard.

wow 43psi huh, damn I thought the fuel pressure is supposed to be around 3psi, I think my car needs the extra pressure instead of the stock 3psi. I only get a few seconds of good power until the pressure in the line drops, anyone have any ideas to boost the pressure up a bit to say like 6psi or 10psi. if I can get my fuel pressure that high, I think it will give me better power for a longer duration. the guys that went to the bay area meet know this cause a crx blasted by us, so I decided to punch it since we were cruzing at a mild 65-70mph. when I punched it I pulled out pretty quick to blow past him even though we were blocked by traffic a bit. but when I tried to blow by again, my car lacked the power to even move. so I am thinkin since I didnt have the power the second time, my system wasnt fully pressurized to do it again. I need something to give me a few more psi or a higher one that wont chug me out. I'm lookin at around 8-10psi to give me the extra safety of duration time. what do you guys recommend???

[smufguy]

i dont know where u been magny, but the stock pressure is about 37psi, or atleast thats what i was told.

[k-roy]

Smurfguy and Magny you are both correct.
Magny has a carb and 3 psi is correct.
Smurfguy has PGMFI and 37 pis is correct.

[smufguy]

Smurfguy and Magny you are both correct.
Magny has a carb and 3 psi is correct.
Smurfguy has PGMFI and 37 pis is correct.

oh yeah, i forgot his shit was a carb. man...... no wonder u were chasing after crxs hehehe, just kidding magny.

[sporkHSP]

can the carb pressure be increased tho?...
thanks for the info on backpressure and delta pressure, i do have a 3inch intake on a civic airbox, and it has given me a boost in power so i figured id investigate exaust options next to give my motor extra room to suck and blow in the system...

[smufguy]

i think to increase the actual fuel pressure, u need a different pump and change the primary jets in the carb. I aint no carb god, so i dont really know shit about carbs. Mike and andy should be able to help u with that.

[HostileJava]

I have 2.5 on the hatch and 2.25 on the sei, one of these days I'm going to get to pulling the 2.5 of the hatch and before I ship it off I should dyno with both and see what happens.