Alright, I've been looking on this board for awhile, and a lot of the posts I see say that "our engines need back-pressure" otherwise we lose low-end torque.
Not quite. It's true for larger exhaust size we lose low-end power, but it's not because the engine needs the back-pressure; far from it. Otto-cycle 4-stroke internal combustion engines do not need back-pressure--what they need is good scavenging--the ability for the exhaust system to suck the gasses out of the combustion chamber, and to do that, you need good exhaust velocity. Therein lies the problem. A lot of new systems on cars--like VVT (Variable Valve Timing), and CVT (Continuously Variable Transmission) are designed to be adaptable for driving in ALL conditions. your exhaust system is pretty static:
For example, using the two most common sizes of J-body engine (2.2 and 2.4), the difference in the amount of exhaust gases the engine pumps out varies considerably the RPM range of the engine. Since I'm not an expert in matters, I'll keep it simple, assuming that the engine will expell as much gas as it's displacement--2.2 and 2.4l.
Taking that into consideration, at a 800rpm idle, a 2000rpm "cruise" and at 6000rpm engine speed, the engines are expelling this much gas:
2.2l / 2.4l
31.08 / 33.90 <--800
77.69 / 84.76 <--2000
233.08 / 254.27 <--6000
(all numbers are in CFM--Cubic Feet/minute. Numbers figured out by Displacement multiplied1/2 of the RPM's (since exhaust stroke is every other revolution), and converted from litres/min to CFM).
The cross sectional areas of 2", 2.25", and 2.5" are 3.14, 3.98, and 4.91 square inches, respectively. Now, as you can see, each of those pipes has to be able to flow the aforementioned CFM of exhaust gasses per minute. Now, I'm I'm no expert on that math, but it would be near impossible to design a pipe that can meet idle, cruise, and at 6000rpm's. The smaller pipes give better low-end performance, because they can keep the exhaust speeds up at lower rpm's, but in the upper ends, they choke because the pipe pressurizess itself and can't flow enough, and you get back-pressure.
The bigger pipes flow well at higher RPM's, but, at the lower ones, there's too much area in the pipe, the exhaust loses pressure filling the pipe, and thus loses energy (it's got to push all the gasses in the pipe ahead of it), the exhaust speed slows down, it's not going to "draw" as much of the exhaust gases out the the combustion chamber--either some "stay" in the combustion chamber, and thus less air/fuel can get in, or restrictive elements in the exhaust *will* cause back-pressure, and the engine will be working against itself.
The best way to demonstrate this is to get a bunch of straws. Blow through one causing the air to flow out of it as fast as you can make it. Then try two, three, and so on. There will come a point where there's no more added resistance to you blowing into one more straw, but the speed of the air slows down. This is what happens, except your car reaches that point *somewhere* in the RPM range--unless you're using a ridiculously small--or ridiculously large pipe.
So, as you see, even though yes, a large pipe will kill your low-end power, it's not back-pressure.
So, what size pipe to choose? Well, first off, you have to think about your displacement, any charging of the incoming air (turbo/super/nitrous), and what your use is. Most stock cars have a restricted exhaust, because they are good to go around cruising speed--where the manufacture expects the engine will be most of the time. For those going N/A and drag racing, you spend little time at the lower ends of the RPM range, and as such, larger pipes will work for you, giving you better flow characteristics at the higher ends. AutoX and road racing will most likely want something that flows well in the mid-range RPMs to handle changes in engine speed.
As such, this size you choose for your exhaust will depend on the engine you have and the use of it. So, if you're driving a 2.4l, and you're dragging her and she's N/A. there are dyno results out there that prove that 2.5 is *not* too big. However, because of the lowered displacement of the Eco this may not apply to them.
So, please, DON'T SAY OUR ENGINES NEED BACK-PRESSURE It's erroneous at best. At worst, it shows you have little knowledge of fluid dynamics. A small exhaust pipe doesn't provide back-pressure at low RPM's, it provides high exhaust velocity at low RPM's. Unfortunately, were stuck here until someone can design a pipe for a car that changes it's cross-sectional area based upon the output of the engines, we're stuck trying to pick a size that will provide a "catch all" result for whatever we use our cars for the most.
Goodbye Callisto & Skađi, Hello Ishara:
2022 Kia Stinger GT2 AWD
The only thing every single person from every single walk of life on earth can truly say
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That's why I'm putting "The Tornado" in my exhaust!
just kidding!
I've always been curious about this. Very interesting write up, and a great read. I've always been told that if you run a very free flowing exhaust, you risk "burning your valves out". Since I'm no engine expert, can anyone explain IF this would happen, and what exactly does it mean?
Desert Tuners
“When you come across a big kettle of crazy, it’s best not to stir it.”
Good work. Ive been saying this for awhile but never really posted it. Too afraid to get flamed by some dude that talks to his dad from the american muscle era. Backpressure is not sweet.....
http://www.cardomain.com/ride/448390
Event used to post this all the time, its in the the Magnaflow article link he used to post.
'03 LS SPORT COUPE 5 SPEED black with teal/silver pinstripe
run open headers! lol right out behind the wheel well, no cats, no mufflers. the shortest amount of distance the air has to travil the better right?
Z24 FReQ (Jarett) wrote:I've always been curious about this. Very interesting write up, and a great read. I've always been told that if you run a very free flowing exhaust, you risk "burning your valves out". Since I'm no engine expert, can anyone explain IF this would happen, and what exactly does it mean?
If you are running too large of an exhaust, as KOTL said, the exhaust doesn't flow out as quick, hence, the still-burning exhaust gasses are burning closer to the valve then they otherwise would, thereby increasing the chances of burning an exhaust valve.
Think of it like this, take a stock 2.2 engine, utilizing a 1.75" pipe, and an identical one using a 2.75" pipe, the larger pipe will be colder at any given point on the exhaust system past the manifold than the smaller one will.
And KOTL: Get back in off-topic and WAR where you belong!!!
J/K, good write-up.
Currently #4 in Ecotec Forced Induction horsepower ratings. 505.8 WHP 414WTQ!!!
Currently 3rd quickest Ecotec on the .org - 10.949 @ 131.50 MPH!!!
Very very nice read, I do believe this is worthy of a sticky, at least to dispel the myth.
excellent writeup, i have been trying to tell people this for a long time, and im glad that you had the time to actually write it up. I say sticky, or at least amend this to the current performance one.
Ill make a sig someday
^^ that is a very good read eric, I'm not even done with it yet
great info, I also think its a sticky waiting to happen. *JBO
http://www.cardomain.com/ride/2187947
DOn't sticky it until the math has been fixed. You didn't account for engine VE. You're cfm numbers are not at all acurate. But the general principle is still good, I guess. It's the same thing I've been saying since I've been here though, and none of my crap ever got stickied.
meet my friends, hash, and rehash..........
KJRich wrote:Event used to post this all the time, its in the the Magnaflow article link he used to post.
the article
http://magnaflow.com/05news/magazine/05sportc.asp
it never fails. sport compacts dont run like EXACTLY older cars with big engines and alot of displacement, the technology is slightly different, even though the same simple concept of suck squish bang blow.
sad part is, the majority of the newer people never take the time to read, update, and educate themselves, considering alot of the posts on the first page alone...
its kinda like the kid who used to post a while back from MD with a 3 inch catback, fabbed to a stock downpipe setup, always complained about loss of low end, and many noise violation tickets....
but had the nerve to get pissed when people told him this very same thing....
some back pressure is good....bc in the combustion chamber if the air is flowing completely freely then the the combustion wont be right thats why when you port and polish heads the intake side you want to smooth as hell but the exhaust side you want to be somewhat rough. why do you tihnk dragsters have bends in their headers coming out the side...why dont they just run it straight pipe out the side from the motor? with the simple bends they have it creates some back pressure. losing backpressure completely could actually make your car slower then a car with some back pressure.
MY 2003 SUNFIRE 15.63@87.86mph
"A N/A ecotec is not gonna give Honda's and Mitsu's that much of a run for their
money unless their blown or bottle fed.GM is still smokin crack!"
~1QWKZ24
www.streetracing.org, 08/2001
Wow. Keeper came out of the War forum
There should be several more threads like this to dispel alot of misinfo.
Ohh3SunFire wrote:some back pressure is good....bc in the combustion chamber if the air is flowing completely freely then the the combustion wont be right thats why when you port and polish heads the intake side you want to smooth as hell but the exhaust side you want to be somewhat rough. why do you tihnk dragsters have bends in their headers coming out the side...why dont they just run it straight pipe out the side from the motor? with the simple bends they have it creates some back pressure. losing backpressure completely could actually make your car slower then a car with some back pressure.
I have read somewhere that top fuel dragsters create something like 800 pounds of pressure out of their pipes. if there were straight pipes from the exhaust ports, which are angled down anyways, it would seriously affect their traction. I would think they are turned up like that to aid in downforce.
I could be wrong though.
Desert Tuners
“When you come across a big kettle of crazy, it’s best not to stir it.”
you knwo I've been saying this about 2 years... you don't want backpresure, back presure is a good thingk and lack of back prusure has nothign to do with not making low end but noone ever believes me hahhaah
I'm new here and probly don't have much say but, somone mentioned valves burning due to open exhaust. This is not the case either, if you run open headers you run the risk of the extremely hot exhaust valves getting a blast of cold air and craking them, or essentially sucking them into the seat. But they won't burn even if you run with no manifold. Also, yes, some backpressure is created just from bends in the pipes but not enough to really hurt anything, In the short time I have been playing with my 2200 I have found that for what I do(mainly highway) the 2" exhaust is more than enough flow. I tried to see where the top speed limiter was set and thes car seems to run out of breath at about 175Km/H so for those at high rpms with the 2200, 2.25" is a pretty good bet for a street/strip car. Let me know if I am wrong on this one. I have dealt with alot of exhaust systems for a few years, mainly on V-8s and the good old 2.8 V-6. 4 bangers are new to me though. So yeah,Backpressure is bad, scavenging and tuning for max velocity in the range you will be using the car is most needed.
First off, i admit my numbers are inaccurate--at least i thought i did. i only took into account how much would be flowing throught the engine if it were simply moving air--a pump if you will. I would have to delve a bit more into some of my HVAC manuals about how much CFM could flow though what size of pipe, what bends do, etc. Plus, the amount of increase in pressure and gas volume for combustion, and even taking into account pressure losses in the combustion chamber, and possibly pre-pressurizing (forced induction).
So, no, i'm not looking for a sticky. All I'm saying is this--it's not backpressure that makes a small-pipe run better for lower RPM's, it's the fact that at that given RPM, the engine is putting out near the optimal amount to keep the gases flowing at the right speed. Any more, and the pipe becomes a restriction, any less, and it acts as an expansion chamber.
Hence, build your exhaust to perform optimally at whatever RPM your car will be at the most. If you build an ehxaust to flow well with your engine at, say 6000rpms, you're going to get a performace hit cruising at 2000rpms. If you build it to get good power at 2000rpms, it's oing to choke on 6000rpms. Unless there is a way that you can design an exhaust pipe that works like the "finger cuffs" that get smaller and larger in diameter, uniformly, with an electric signal, be abale to update reasonably quick, be ablt to withstand the heat of the exhaust, and be relaible and inexpensive enough, you're going to have to try and balance out where you want your gains based on what you want to build your car for, and sacrifice something in the process.
Some direct comments:
James: It seemed like a good topic to write....the response thus far proves me right.
Event: I have read that article, and aside from more pressing issues, i will be designing my own exhaust based on things out of that article.
Ohh3sunfire: Not quite, you can't compare a dragster to a normal car because unline our engines, they burn nitromethane. If you look a the design of their pipes, you'll notice that they are pointed up--to help with downforce, but also, that they increase in diameter throughout the length. This is because nitromethane burns very slow, and the pressure and volume of gas is increasing even AFTER it exists the combustion chamber. as such, using a fuel like that would complicate meattes...same as you would have to take into account burning, say LPG over gasoline if you were going for a top performing LPG car.
However, the myth of backpressure goes along the lines that the intake charge might flow straight from the intake valve to the exhaust valve. This is possible depending on the headers (if they are designed with lengths that improperly scavenge at a given rpm), but, with any properly tuned cam setup for a given RPM, headers tuned for scavenging at the same rpm, you can time it so that the intake charge will not only suck into and fill the combustion chamber, but will also push the exhaust gases out, and, if done properly, will ONLY push the exhaust gasses out--leaving nothing in the chamber other than a fresh charge.
the problem is, since the displacement in the combustion chamber fits a timable pattern, and can't change with respect to the engine speed, and because the ehxaust pipe/header can't change with respect to the engine speed, you can really only "tune" for one RPM, and as you go further from it, the effectiveness of your setup declines. Backpressure really doesn't enter into the equation unless you're overloading the pipe--the backpresure is as bad as, say, Tori Spelling's acting. It's not the fact that smaller diameter pipes have backpressure, and that makes it run well at lower RPM's, it's hat those smaller pipes work better with the amount of cfm flowing through them at lower RPM's, creating the high exhaust velocity needed for proper scavenging. Once you get to bigger pipes, at lower rpm's they act as an expansion chamber, slowing down the velocities and decreasing the overall pressure in the pipe, thus, the gas loses part of the energy it needs to push out the gas ahead of it.
It's all in how you tune it, and what you're tuning it for.
Goodbye Callisto & Skađi, Hello Ishara:
2022 Kia Stinger GT2 AWD
The only thing every single person from every single walk of life on earth can truly say
they have in common is that their country is run by a bunch of fargin iceholes.
Ohh3SunFire wrote:some back pressure is good....bc in the combustion chamber if the air is flowing completely freely then the the combustion wont be right thats why when you port and polish heads the intake side you want to smooth as hell but the exhaust side you want to be somewhat rough.. why do you tihnk dragsters have bends in their headers coming out the side...why dont they just run it straight pipe out the side from the motor? with the simple bends they have it creates some back pressure. losing backpressure completely could actually make your car slower then a car with some back pressure.
actually you got that backwards
you wantd the exhaust smooth , and the intake rough
but the intake rough was only for carb motors , to keep help keep the fuel in the air flow , and from puddling up
most fuel injection applications , you want the ports smoother now , but that also depends where the injectors are firing at in to the intake stream , the farther from the port the more towards the rough side you want
[quote=97cavie24ls(JDM&00s/c sedans™)]
Ohh3SunFire wrote:some back pressure is good....bc in the combustion chamber if the air is flowing completely freely then the the combustion wont be right thats why when you port and polish heads the intake side you want to smooth as hell but the exhaust side you want to be somewhat rough.. why do you tihnk dragsters have bends in their headers coming out the side...why dont they just run it straight pipe out the side from the motor? with the simple bends they have it creates some back pressure. losing backpressure completely could actually make your car slower then a car with some back pressure.
actually you got that backwards
you wantd the exhaust smooth , and the intake rough
but the intake rough was only for carb motors , to keep help keep the fuel in the air flow , and from puddling up
most fuel injection applications , you want the ports smoother now , but that also depends where the injectors are firing at in to the intake stream , the farther from the port the more towards the rough side you want
basically, from what I'm understanding you to say, is that you want it rough to help with atomization of the fuel into the airflow, right?
learning new things everyday.
Desert Tuners
“When you come across a big kettle of crazy, it’s best not to stir it.”
so who left keepers cage unlocked? He's supposed to be in the war forum
Nice post, it makes a lot more sense, I was always a bit skepticle of back pressure, hearing everything from it improves torque, to it keeps the fuel and air in the cylinder till it fires (absolutely knew that one was false, that's what the valves are for)
Great writeup. It makes a lot of sense.
It also explains why the plate based muffler systems(Supertrapp) work well for tuning. By adding or removing plates you decrease or increase the flow of the exhaust.
Awesome job keeper, hopefully we can look forward to more informative articles from you.
-Chris
No problem.
And I'm not trying to say I'm the end-all-beat-all expert at things like this. I just hate misinformation, and this seemed like a good time to post this one. So, no discredit to anyone that has said, wrote up, or posted similar things before. I'm just sick of people saying "our cars need backpressure." Our cars need backpressure like I need a septic tank to backflow into my basement.
Goodbye Callisto & Skađi, Hello Ishara:
2022 Kia Stinger GT2 AWD
The only thing every single person from every single walk of life on earth can truly say
they have in common is that their country is run by a bunch of fargin iceholes.
Ohh3SunFire wrote:some back pressure is good.....
MYTH.
basically the less backpressure and the higher exhaust velocity...the better.
in a perfect world, near zero backpressure and super high velocity would be ideal...aka the scavenging effect would be at its utmost, but thats in a near lab non variable environment, hence, nearly impossible in the real world.
so where as, there will ALWAYS be some backpressure as its inevitable, the less you have, the less work the engine has to do to pump and blow out exhaust. the less the engine has to struggle the higher the volumetric efficiency.
the statement, engines need some backpressure to work, or for it to be good.... completely FALSE. i thought the same till i started reading alot of tech and theory books on nothing but exhaust and intake.
