Backpressure: The myth and why it's wrong.

I. Introduction

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?

I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

VI. Conclusion.

SO it turns out that Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

Dear god I hope you "cut and pasted" that. Thats a crap load of typing.

good article though, hopefully it will clear up some confusion, sticky anyone?:D

good post schmee

damn right i cut and pasted.

and its already a sticky.:D

good work fella, i dont know if you get them over there but MGF's also has a clever little system for back presure. they just use an actuator which works off the engine vacume to control a butterfly valve in one of the two exits on the back box. simple yet effective

the Evolution 8s have a valve in the backbox that closes at low pressure, then opens under full pressure as rpm rises. its for emissions though, as it SUCKS for off-throttle shifts. it slows the turbo, even with a BOV. but an aftermarket cat-back cures that up nicely. as well as adds like 20-30 horsepower.

speakin of emision how good are civics on them? as i wanna remove the cat from mine

dont remove the cat, get a high-flow one. it causes all sorts of problems, and it will hurt your emissions.

what is better for emissions, i high flow cat, or just a regular cat?

regular is a little better for emissions, but if the car is running well, a high-flow one is fine.

so what do u say to people that say not to buy a performance exhaust because it will kill all ur back pressure

Tell them they're looking at it the wrong way.

I live in California and emissions run my WHOLE LIFE [:@]! Sacicons, you feel me.

damn right. and dont feel bad if these posts all get deleted in the future. i dont want to bog down the stickies too much.

I printed it, it's gonna take reading it a few times to get it, but will be worth it.

thanks icons

yeah, ill leave the original, but in a couple of days, ill clean these comments up. its all good.

i dont have emissions tests la la la la la la!

you still have an ozone layer though, dont you?;) part of the reason i like imports instead of v8s, is the efficiency. you can have a beast that runs 11s, and still pass the sniffer test. whether you have to or not. thats another reason i dont like test pipes.

I think they actually might not have an ozone layer anymore if they don't have emissions [sm=icon_stickpoke.gif]

Yeah, Cali is the only state who's trying to save theirs, since it's already so hot down there:D. Everyone else is like, "Well, we already ****ed ours up. Why bother anymore?"

so what are u saying sacicons it is good just to cut your stock muffler off and just weld a performance muffler on because that is what i did but i was going to go to a muffler shop this summer and get a costom 2.5 cat-back i really did not know back then

no, its ideal to get mandrel bent piping in an appropriate size for your application, with a low restriction muffler. stock piping is "crush bent" and doesnt maintain its inside diameter through the bends, which can cause quite a bit of backpressure. so get the custom cat-back, but dont go over 2.5 inch. 2.25 would be a little better though.

Hi, what bad things happen when you use a de-cat pipe instead of high flow cat? I know it will fail the emissions test but i can get round that. you see I was looking for some power gains from my 1.6 vtec using a megan de-cat pipe as high flow ones can be pretty expensive over here.

megan makes civic parts? Hmmm I never knew that before today.


Anyway, for emissions Im just gonna get a new car before I have to smog again. Just not worth swapping it all back to stock

haha there is no emision anything where i am (canada/saskatchewan) so i could burn oil instead of gas if that worked and blow smoke like there is no tommorrow.. haha

megan makes civic parts? Hmmm I never knew that before today.

Hell yeah they do. I got the Drift Spec cat-back exaust on its way to my house right now. And i already installed their full set of strut bars.

ORIGINAL: sacicons

no, its ideal to get mandrel bent piping in an appropriate size for your application, with a low restriction muffler. stock piping is "crush bent" and doesnt maintain its inside diameter through the bends, which can cause quite a bit of backpressure. so get the custom cat-back, but dont go over 2.5 inch. 2.25 would be a little better though.


damn sacicons you had me scared when i was reading all this. i was sure i had bought 3" piping. i went and found the ad for the pipe i bought tho, thankfully its 2.5 mandrel bent pipe. PHEW! i also bought the high flow cat, so i feel a lil better. not much to the cat. looks pretty much like the muffler does on the inside. you can see straight through it. anyways, thanks for the info, i always did wonder about backpressure, you hear so many rumors.....

If you can see all the way through the cat I dont think its considered a cat anymore. a cat delete maybe.

ya i was wondering about that. its weird. i was giving my friend the "i need backpressure lecture" because he wanted to eliminate my cat and go straight exhaust from the headers. so he counters what packpressure? look at this and picks up the cat and looks at me through it like a telescope. so i dont know what good its doing me, but i spent 70 bones on it, so dammit i am bolting it up. (yes i was giving the backpressure lecture before sacicons edumucated me on this thread)

Whenever people start to argue with me about they need back pressure and cars need back pressure I just say, tell me why.



Best attempt at a response was cause cars are designed to need it, lol

i have a question on one point made in the article that has been bugging me. there was a comparison made to a garden hose. now i realize that if you narrow the opening on the end of the hose the water comes out with more pressure. but does it really come out faster? i dont think so. i think the water can come out as fast as the pressure of the source it originates from allows it to. in other words, if you fill 2 buckets with water one with a hose that has an unrestricted opening and one with a hose that has a narrow opening they should fill at the same rate if the pressure of each ones original source is the same. when u restrict the opening of the hose it builds pressure within the hose and struggles to relieve the same pressure it had with the unrestricted opening, thus appearing to be faster. isnt this in effect creating back pressure when you restrict this opening? and wouldnt this be the same effect when referring to exhaust? help me to understand better if you can please. thanks

Kinda, less will come out faster

its actually one of the basics of physics and fluid dynamics. if you have an equal amount of a fluid (air works as a fluid in these discussions) moving through 2 tubes, the fluid has to move faster to get through the smaller of the 2 tubes in the same amount of time. if you could measure the speed of the water coming out of the end of the hose, you would see that it is faster, thats why it sprays farther. also, one small problem with that analogy, it doesnt point out that the comparison is not the hose, its the opening. yes, it would just cause pressure to build up behind the end of the hose, but that is not where the comparison is. its only at that 1/8 of an inch of length that is the actual opening.

ok thanks, yeah i'm a lil clearer on that now. definitely not a physics major here so no argument from me

thanks for the post... cleared things up.

Thanks for finally clearing up the whole backpressure thing. Everybody I've talked to says you need it, but they have no idea why.

Now you guys have me worried I'm gonna have to check it.

I always thought it's because if there is no back pressure (i.e. your exhaust fell off at the header) the exhaust valves won't seat properly (as in slam shut harder) and will slowly mushroom up over time...

Impreza's explanation is what i was given by a few people. an amount of backpressure is needed for the valve to work properly. otherwise too little or no BP would cause them to become damaged over time.

look at the exhaust system on an F1 car. theres little to no BP there, and they can run at 14-18k rpms for hours on end. try and get someone to explain how backpressure could soften the blow of the valves.

they would just use softer springs if that were the case. and what about the intake valves?

Well, I recall a "seen it myself" example with a slant 6 Dodge Ram that was held together by improbability. The exhaust was gone, the only part being the header, and the engine spit fire wherever it went. Exhaust valves were not closing all the way anymore so the engine was barely making enough compression to run. This is the example I got. If there is a difference, or the problem is different, let me know!

for a split second the intake and exhaust valves are open during crankshaft rotation

or it had another problem. as far as i know (or care) there hasnt been a slant 6 made in a long time, and so I assume this was an old motor, and you sound as if it werent in the best of shape, so chances are it was mistreated in many other ways, and it could have been carbon buildup, bent valves from cam timing being off, or a bad valve job. or any number of other mechanical maladies. let alone that an engine with only a header will bend valves and crack valve seats easily because when you shut it off after a decent run, the head and valves are really hot, and cold air comes back up the header and shocks the metal enough to crack or warp it. you ever seen a frying pan that was put into cold water right after coming off of the stove? it messes them up bad, and itll do the same to the valves and heads.

Good explanation. That answers that.

I plan on making a custom exhaust for my Wagon, and I was wondering what size I should use. All the aftermarket mufflers I have found have only a 2.25" inlet. I believe my stock pipe is 1.75". Would I be ok if I went up to a 2.25" pipe, or should I stick with stock size? Thanks

Well in my honda i needed more backpressure, I turboed the car and when i did i took out the cat. I had a tial 38mm wastegate with a .6bar spring(8.7psi). So in all i had a 2.5" down pipe and staight through exhaust. When the boost came up, it shot to 9 psi, then creeped to 10,11,12,13. Nobody could figure out what was going on. Long story short, even with the wastegate wide open, the exhaust was travling through the turbo and out my exhaust causing the boost creep. So i put the baffle back in my exhaust tip and that gave me just enough backpressure to make the exhaust go out the wastegate instead through the turbo, no more boost creep.

Boost creep is a whole 'nother subject, it's a problem with big exhausts on turbo'd cars... I've experienced it a lot before but it's easily fixed.

Back in June of 2000 or so when I was in high school I remember reading a article called exhaust science in hot rod magazine and they stated that to efficiently scavenge your cylinders during valve overlap that the gas has to be traveling at .45 to .50 mock so about halve the speed of sound. I am not sure how to do the math for this equation but if some one could figure it out it would make picking out header pipe diameter easer. I would assume that you would want this to be at peek HP but I don’t remember that much just the speed stuck with me.

ive got a 94 hatch w/ the 16v 1.5, its got the header from the 1.6 v-tec on it and a catback system from there. i dont have hardly anypower until i hit 3000rpm (it basically hesitates), so i was wondering if putting the stock header back on would fix this problem.

If your exhaust is 3" wide, that is your problem. General consensus is 2.25-2.5" max. Also, 4-1 header makes less low end than 4-2-1 header.

Now I've got the D15B7 engine, and there is no power until you hit 5,000 RPM. Only upgrade is a short ram intake, which did not affect low end power.

what do you mean by '4-1' and '4-2-1' header?

So I would like a suggestion on a pipe size for my civic, when my ve-tech is cracking around 6000rpms.... I was thinking about getting a 3inch DC Sport cat-back exhaust.

First off its vtec. I have never seen somebody go so out of the way to spell it like you did :D. Second, a 3 inch exhaust is only good for a DOHC VTEC turbo beast. 2.25 will be the best. Anything more or less will cause you to actually lose power.

or you got a single slammer running 30pounds of boost with over sized valves...:D:D:D^^^

Well Technically speaking the Ozone Layer has slowly started to replenish itself...CFC's were the main cause of Ozone being depleted. The latest studies show that Ozone Replenishment is slower than thought and tied to cyclical temputures at the poles and wind direction...Also the CFC's hang around in the atmosphere much longer than originally thought.

So clean emissions out of the exhaust pipe will not save the Ozone...But they might help slow global warming(there is no stopping it short of covering the planet with a solar shield straight out of some sci-fi novel)...Smog might go away and the world might breathe a little easier....

So tailpipe responsibility should fall to all of us. At the same time we need to put more pressure on our governments to reduce emissions from power plants and to stop the world from using fuels like coal where there are not seriously advanced smoke stacks with scrubbers and carbon filters& convertors...etc

Automobiles are getting amazingly clean...The fuel we burn is getting very clean...The amount of Carbon released and nasty emissions is dropping fast worldwide from Automobiles. Power Generation is going to be our biggest problem when speaking of air quality in the future...The world is awfully power hungry and windfarms and solar panels are not capable of handling this power demand surge....So it likely means more oil, coal and natural gas will be burned to generate electricity

ORIGINAL: WhiteCivic15

what do you mean by '4-1' and '4-2-1' header?


4-1 means all 4 exhaust ports go immediately into 1 pipe
4-2-1 means 4 ports merge to 2 pipes then merge to 1 pipe


4-1 is like this

\\||||//
...| |

but 4-2-1 is like this

\\ || || //
...\ \/ /
.....| |

amen, amen.....so your saying to stay with the factory exhaust? or just give it straight 1? you lost me there bro?

I just learned about back pressures in my auto tech today. Not a good thing. This thread helped me get more knowledge on the backpressure...thanks.

That was a wicked right up... I totally didn't understand abckpressure before. Im still a little rough on it.. But it will be awhile till i can read again.

on the whole 4-1 vs 4-2-1 thing, it has to do with the scavenging effeciency.. meaning having the #1 cyl's exaust pulse help pull the #3's exaust out of the cyl... its kinds like a carb, where the venturi creats a low pressure and sucks the gas into the airstream.. the previous exaust helps the next bunch get moved along.. and where at lower rpm's for a given pipe size the velocity is lower than higher rpm's same pipe size cause your moving more exaust through there... in the 4-1 each exaust pulse dosent really help the next come out much, but with the 4-2-1 there is more helping going on...
just incase your still confused on what a 4-1 vs a 4-2-1 looks like, look at some big v8 headers.. most of thoes are 4-1.. all 4 tubes all come together at the same place, the collector.. now on a 4-2-1, the 4 tubes are broken into 2 pairs of 2, and joined.. now these new resulting larger tubes are joined down the line to form one... so it goes from 4 ports or tubes, to 2 tubes, to one tube.. the stock civic cast manifold is really more of a 4-1 configuration...

as for turbo runaway, i would love to read more about that one... so off to search i go....

wait if i get just some fart can and get it welded onto my civic will i lose power?? or will the muffler not even affect it... like if i get this muffer ...

http://cgi.ebay.com/ebaymotors/UNIVERSAL-4-5-N1-MUFFLER-Integra-RSX-Civic-CRX-Preulde_W0QQcmdZViewItemQQcategoryZ33636QQitemZ110 102414438QQrdZ1

It looks like the tip is 4.5. the actualy diameter of the pipe is a different story.

Wow, thanks

Hey buddy I read what you wrote about backpressure and I am that guy who thought putting a 3inch pipe would really impove my prefromance and what do you know I did loose power. thanks for the article

k i still dont get the first post about back pressure.if you think a about it you dont need it at all it will just perform better with the right amout.dosent it depend on your set up if your car is for top end you wantbigger pipingwith higher hp and if your more for bottom end you want 2in, or if you have turbo you want the best flow..if you have all motor non vtec like a ls witch you want the torque you keep the piping smaller but bigger than stock and if you got a fully built vtec for 9k rpm revs you want big pipeing but not as big as turbo cars?

Backpressure also creates a vaccum in your engine(which is not needed with forced induction) which helps to suck the exhaust gases out of the cylinder. You don't need alot of backpressure, but maintaining a small amount of backpressure is necessary...unless you have forced induction.

check out the second result, "reducing backpressure", in forumdig:honda civic backpressure (http://www.forumdig.com/search.aspx?q=honda+civic+backpressure)

I don't know if anyone can answer this question, how are OEM mufflers packed? I had a Borla free-flow exhaust on a VW I owned in the past, and you could see through the tail pipe and through the muffler and light coming in from on the other side (when not mounted). If you have narrow pipes like on an OEM exhaust, how much is the muffler packing slowing the exhaust flow? I think (and I might be wrong) that the OEM mufflers don't provide a direct path from the inlet to the tail pipe. So, there seems to be a "back-pressure creating" zone in the muffler.

This might sound like a nutty hypothesis (but I'll accept the Nobel Peace prize and the one mil in cash), but you could increase flow on a wide-diameter cat-back exhaust system by cutting out the 4" tail pipe and welding on apipe reducer that ends at 2.5" diameter.

high flow are better?

Anyone heard of Supertrapp mufflers? They have a funky appearance. There is a cap at the end of the muffler. The way the exhaust gas escapes is through the area between the cap and the muffler. You can adjust the amount of gas that escapes by adjusting the distance of the cap from the end of the muffler. The more spacerdiscs you put on, the more distance between the cap and the muffler and the more flow for high-rpm needs. The internals and design are free-flow like a cherry muffler,cylindrical tube and glass packed, with the exception of the cap and pressure control design.
Kind of an interesting thought, anyone every use a Supertrapp on a Honda? Too noisy or illegalfor road use?

what sort of problems does no cat cause?? I took mine out and i have 2 1/2 inch pipe all the way from my exhaust manifold, all straight through, is that stupid oh exhaust god?

Well in my honda i needed more backpressure, I turboed the car and when i did i took out the cat. I had a tial 38mm wastegate with a .6bar spring(8.7psi). So in all i had a 2.5" down pipe and staight through exhaust. When the boost came up, it shot to 9 psi, then creeped to 10,11,12,13. Nobody could figure out what was going on. Long story short, even with the wastegate wide open, the exhaust was travling through the turbo and out my exhaust causing the boost creep. So i put the baffle back in my exhaust tip and that gave me just enough backpressure to make the exhaust go out the wastegate instead through the turbo, no more boost creep.

Boost creep is a simple solution of your wastegate not being big enough. Just think about the mechanics behind it. The waste gate opens to reroute exhaust gases away from the exhaust wheel of the turbo keeping the turbo from spooling any higher. If the flow of exhaust coming from your ports exceeds the flow capacity of the wastegate then the gasses have nowhere else to go but through the exhaust housing of your turbo.

Also your manifold piping could have been too small and the exhaust gasses could have started flowing backwards before they even hit the wastegate.

Turbo's love the most free flowing exhaust system possible. Also how was the vacuum set up on the wastegate? It could all be something you installed incorrectly. Maybe the wastegate spring wasn't seeing vacuum... Who knows but the problem wasn't with you needing a smaller exhaust.