Engine Back Pressure

jakejacobson86 · 2005年03月09日

Back Pressure

Hey whats up guys. I just very recently discovered that everyone moved to vvt-i.net as opposed to the old vvti.net! so im excited that i finally caught on! anyways, I own a 99 corolla ce auto. I have a generic stainless steel muffler that a local shop welded on to my stock pipes. Ive heard from a few people that unless I got a full cat-back pipe set up or FI of some sort, then loosing my back-pressure my stock muffler gave would be degrading to my performance. is this true? help is appreciated as Im thinking about buying an OEM muffler for about 130 shipped. thanks.

polo708 · 2005年03月09日

You don't need back pressure period. Others will post otherwise, im sure of it... but you don't need it.

jakejacobson86 · 2005年03月09日

so even on my N/A 4 cylinder engine, you're saying i should go with less restrictive piping no matter what? my uncle used to race for ford and he's tuned cars since he was like 15, and he swears that even my 1/4 mile time would be cut down if i still had my stock muffler. i dont mean to put down what you said, but i consider him a pretty reliable source.

polo708 · 2005年03月09日

Here you go, I dug this up for everyone:

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.

Dr Tweak · 2005年03月09日

Let the Doc set this myth straight:

*edit* Damn, polo708 beat me to it. Oh well this is what I was typing:

An engine NEVER, under any circumstances, n/a, turbocharged, supercharged doesn't matter NEVER benefits from backpressure.

So why the saying that you shouldn't go too big or you'll lose backpressure and that will hurt performance?

By running exhaust piping that is too big, you will loose exhaust velocity. This will cause less power on the low end of the RPM band (less low end torque), however, the engine would still benefit when it gets to the upper RPMS.

Also note: the problem of losing exhaust velocity does not apply at ALL when you are using a turbocharger. With a turbo you want big piping after the turbo because you already have too much backpressure from the turbine.

jakejacobson86 · 2005年03月09日

wow. that was freggin awesome. like seriously, it feels like the clouds parted and a bright light shined before me... lol... seriously, thanks for that post polo. Which brings me to my next question. instead of investing in a pricey cat-back setup, would it be benneficial for me to find a shop that will replace my stock piping with say, 2.5" piping? which parts should I get them to replace? sorry if that is a newb q.

Dr Tweak · 2005年03月09日

jakejacobson86 said:

wow. that was freggin awesome. like seriously, it feels like the clouds parted and a bright light shined before me... lol... seriously, thanks for that post polo. Which brings me to my next question. instead of investing in a pricey cat-back setup, would it be benneficial for me to find a shop that will replace my stock piping with say, 2.5" piping? which parts should I get them to replace? sorry if that is a newb q.

Yeah, but two things to keep in mind: always use Mandrel bends (instead of crush bends that most shops use) and use a high flow catalitic converter.

If you use crush bends then at the bends the piping will only be like 2" instead of 2.5". Also, that's a little big IMHO, 2" would be closer to the number you want.

What is it stock? 1.5"?

jakejacobson86 · 2005年03月09日

sweet, sorry if i come across as inexperianced about this, but well, yeah i am! so if i found a shop that could do exhaust work, just tell them that i want my stock exhaust pipes replaced with mandrel bent 2" piping? where would i come about the high flow cat converter? i really appreciate the help by the way Dr Tweak!

Dr Tweak · 2005年03月09日

Yeah, and a decent shop should be able to hook you up with a high flow cat as well. Otherwise you could get one off eBay or whatever.

My pleasure

jakejacobson86 · 2005年03月09日

aight Dr Tweak, Im off in search for optimal exhaust flow! I have you to thank for the support. Thanks again. you're the bomb.

Vamp · 2005年03月09日

Stock is 1-7/8".

vortex · 2005年03月09日

2.5" is perfect for turbo applications. 2" seems to be sorta crappy. (i've experimented with both)

polo708 · 2005年03月09日

Yeah, I plan on doing a 2 1/4" custom job when I get back to the states in August.

1 7/8's seriously?? god thats small, haha.

DjFrOsT · 2005年03月09日

jakejacobson86 said:

aight Dr Tweak, Im off in search for optimal exhaust flow! I have you to thank for the support. Thanks again. you're the bomb.

you know...this post also helped me out alot...WOW...thanks Dr. tweak... :Nworthy

thats why i love these forums...neebies...(like me)...learn alot...

thanks!

Vamp · 2005年03月09日

You think 3" for turbo might be too much?

rainbow_star · 2005年03月09日

I thought turbo love 3" pipe?

vortex · 2005年03月09日

Vamp said:

You think 3" for turbo might be too much?

hehe i dont think it'll be too much, but ya might have issues routing that big of a pipe around. 2.5" was a pain enough for me.

Tamago · 2005年03月09日

just dump it behind the wheelwell

http://www.scionlife.com/forums/viewtopic.php?t=47351&highlight=

here's scionlife's take on backpressure

RollaRyder1103 · 2005年03月09日

i have a 3" mandrel bent exhaust system...on my 78 280z turbo...that thing sounds like a beast now...it had a 2.5" mandrel when i bought it w/ a stock muffler, but I upgraded the piping and threw on a nice magnaflow muffler w/ 3" inlet and voila, my turbo inline 6 now sounds like a v8. it's also nice my car doesn't require a cat, so absolutely no restrictions from the turbo back.

CarbonB · 2005年03月09日

Good lord that's some large piping.

killer2239 · 2005年03月11日

More like good lord thats a beast.

dw1986 · 2005年03月11日

nice car man

rice cracker · 2005年03月11日

I had some 3" piping on my Thunderbird Turbo Coupe and it sounded awesome. I got many people asking if it had a V8 302 in it when it only had an I4 140ci!

alby13 · 2005年03月11日

CarbonB said:

Good lord that's some large piping.

LMAO

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