https://www.youtube.com/embed/IpzqpjmOSB8
Trying to push the performance of your turbocharged car? You might have some questions about exhaust back pressure.
Thankfully, this video helps explain what exhaust back pressure is, why it happens, and how to manage it–plus what happens when you have too much.
Back pressure, you don't want it NA or turbo.
Unfortunately over the years people have confused poor exhaust scavenging through the use of exhaust pipes that are too large (which slows velocity) and think back pressure is good.
A wise man once said, "exhaust backpressure is fine when the valve is closed."
z31maniac said:Back pressure, you don't want it NA or turbo.
Unfortunately over the years people have confused poor exhaust scavenging through the use of exhaust pipes that are too large (which slows velocity) and think back pressure is good.
THANK YOU.
Now let's work on the myth of coolant moving too fast to transfer heat 
Turbos work just fine with bigger = better, at least after the turbo, because exhaust energy should all be used to spin the turbine, or at least any kinetic energy left in the exhaust after the turbine means nothing to the 20-50-80psi of pressure pre-turbine.
Paul_VR6 (Forum Supporter) said:A wise man once said, "exhaust backpressure is fine when the valve is closed."
Not if it blows the valve open!
In reply to z31maniac :
Dont take this as an argument because Im genuinely curios. Has "exhaust too large" been proven? Ive looked for examples in the past and havent found anything, and on the platforms im familiar with, they guys with the biggest exhaust generally go faster/make more power. Internet wisdom is always you cant run the bigger headers/or 4 inch exhaust on a stock cube or close to stock motor, but when tested it always seems the larger guys end up making the HP.
Ive never seen anyone do the obvious extreme, which would be interesting.
Opti said:In reply to z31maniac :
Dont take this as an argument because Im generally curios. Has "exhaust too large" been proven? Ive looked for examples in the past and havent found anything, and on the platforms im familiar with, they guys with the biggest exhaust generally go faster/make more power. Internet wisdom is always you cant run the bigger headers/or 4 inch exhaust on a stock cube or close to stock motor, but when tested it always seems the larger guys end up making the HP.
Ive never seen anyone do the obvious extreme, which would be interesting.
Of course not, I always want to learn as well!
I honestly don't know if anyone has done something like that. Keith, any insight?
I have never seen a bigger exhaust lose power, maybe not gain any. Totally different for header primary/secondary sizing as that is crucial. After the turbo or collector, infinity diameter is the answer 
other than packaging.
In reply to Paul_VR6 (Forum Supporter) :
I've seen collector quality and primary length make a difference, but everything equal I haven't seen larger primaries lose power, but like I said I haven't seen anyone go to the extremes to test it.
So I don't know if the platforms I'm familiar with run primaries too small commonly so when people upsize, they are actually still headed in the right direction or not.
If you've got some testing or info where only primary size goes up and power goes down I'd be real interested in reading about it.
Are we talking peak power, or overall powerband?
In reply to Pete. (l33t FS) :
I'm interested in either
I have heard people say that the exhaust I have on my M3 (bimmerworld 3.5") will reduce lower-end torque, and it's true that my dyno plot shows less torque below 4000 than some of the stock ones I've looked at on the Internet. I never dyno'd my car in stock trim though, so I can't say for sure. Also, it makes a ton more power up top, on track it's never below 4K, and it's a caged race car, so even if that is a tradeoff it's absolutely the right one to make.
Sorry for the book here......
Today there is software that will quickly answer these questions. Burns stainless will design an optimal system for you if you buy the material to build it from them....you give them the numbers they ask for and they tell you what the very best exhaust design for you application is. Done.
I have dynomation6 engine simulation software that can "test" about any design but you need to tell it what you're testing so is does require some knowledge to get an optimal design but if you want to know Design A or Design B it does that very well.
I quick look through screen shots I had this is a Tri-y vs a step here are a couple headers designs
Step header vs long tube
Step header vs standard
Tri-y header vs itself....a tri-Y is really 2 different headers mashed together, there is a high rpm design and a lower rpm design, this it those separated. The effect on a running engine is you basically get which ever line is higher at any given RPM and on this engine it fills a pretty big hole in the torque curve from 4-6K....Burns designed this header for me.
Back pressure, as other have said is bad, it just lowers everything in a well designed system. Turbos, nothing after the turbo does anything but cost hp.
On an NA engine that is also true about anything after the collector...but you need to be very careful about how you define "the collector". The exhaust if full or pressure waves and any place there is a change in cross-sectional area there will be a reflection. The lower graph below show intake(green) and exhaust(red) pressures at the valve....and there is a lot going on.
Focus on the middle between the line IVO and EVC, this is the valve overlap period and and shows why headers add hp....the pressure at the exhaust valve is dipping to -7psi...that is a heck of a vacuum being pulled on the intake while the piston is still going up. any back pressure you add to the system reduces that vacuum. No question back pressure is bad.
The hard part as I said can be in understanding what is the collector and I'm pretty sure this is what leads to the back pressure is good myth living on. If you have a set of headers with say a 3" collector with a flange at the back and you bolt on say a 2.5" exhaust the header works as it should, there is a wave reflected at the flange what the area changes and that wave is normally tuned to boost midrange.
Now lets say you bolt on a 3" pipe to it....there is no area change and the whole pipe you just bolted on IS the collector and it may or may not be tuned to anything. If you had back pressure, the now reduced back pressure should add some top end HP but you will almost certainly loss some midrange because you lost your collector.
Solution? There are 2. The most common is a choke. Here's a header with just about everything. The choke is quite a small Dia and for the most part terminates the waves. The actual collector is very short here, and probably tuned to the 6th reflection which means it is providing a helpful pulse at 6 rpms, one of which will be peak hp. After the choke no back pressure is the goal so bigger is better until its just adding weight. Its odd to see a megaphone like this but in this particular example the the functional choke is at the V-band.
This is mine, the choke is the narrow section toward the right, then after that you can expand to whatever size tail pipe is needed to minimize back pressure. My engine is transverse and towards the back giving me 1 pipe about 1 ft long and 1 about 5 ft long, so I use 3" and 3.5". Thisd is a Tr-y so effectively has 2 collectors. 1 very short on 6th reflection which is generally best for peak hp
Then a second tuned to the 5th which is generally best for midrange along with primaries that are also tuned to a lower rpm
The other option is kind of the opposite and rather than a chock, add something that forces an area increase. This is called a wave termination box and many race type mufflers can serve this purpose. Here, if there was no choke up stream the collector ends inside the muffler so exactly how far the muffler from the header matters as does the diameter of the pipe connecting the muffler to the header so while this setup works fine it can be very challenging to install correctly . At the back of the muffler, 0 back pressure is the goal and the only hard to big casues is extra weight.
You'll need to log in to post.
