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· Registered
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Discussion Starter · #1 ·
supposedly backpressure can be good on an NA motor...that makes no sense...

the engine is using energy to move the exhaust gas out of the exhaust system...the more backpressure there is, the more energy is lost exhausting it.

ive heard that it helps valves close but that seems ridiculous

can someone knowledgeable chime in and explain this
 

· 7M Fanatic
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3,614 Posts
No such thing as 'good' backpressure.
Forum follies.

You do want inertia, to help scavenge the exhaust gasses out of the combustion chamber, and when done correctly, it will help pull in fresh charge during valve overlap.

This can be achieved with header runner length, of the correct diameter, for what ever engine your doing.

What people are seeing with power loss when going to a larger free flowing exhaust,
is a lean condition, cause be less exhaust residue in the next cycle.
And they are too stupid, or too cheap to buy gauges to pick that up.
When ever going to a better intake, and/or exhaust you need to correct the fuel delivery to meet what your engine needs.
Most OEM systems are lean to begin with.
And by mearly dumping on a better intake and exhaust, you end up being way too lean.

I have tuned several customer cars that actually got slower after 'bolt ons'.
The easiest thing out there to do this with is an Apex Super-AFC piggy back.

Pick up any performance engine building book, this issue has been addressed since the early '60s.
Trouble is, many people today go by what rumors they see on some forum somwhere.
 

· Overlooked
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4,693 Posts
What are you talking about? NA motors DO need more backpressure in the exhaust but not the headers and vice versa for turbos. The reason for the backpressure in NA is that it creates exhaust velocity. Look at it this way, compressed gas, no matter how slight, is more dense than if it were not. More mass, the less likely it is to be stopped causing it to pull more gas. Valve timing has alot to do with this also. You want scavaging which is caused by slight retardation of the exhaust cam meaning the piston is on its way up before the valve actually opens. The pressure built up forces out, obviously, but also creates a slight vaccuum in the chamber which draws in more intake air/fuel. The headers have less backpressure than with a turbo to allow this "pop" of exhaust to be less turbulent in the rest of the exhaust piping to create the draw of exhaust gas. Make sense?

Turbo exhaust on the other hand requires backpressure BEFORE the turbo to create more pressure to turn the turbine. After the turbo you want as little back pressure as possible (not in excess though) to cause less turbulance to allow the turbo to spin freely.

Notice how properly setup NA cars have that plop plop sound? That's the overlap and scavaging at work. And turbo cars have a nice smooth purring sound? That's to allow the turbo to spin smoothly and not "jerk" it.

Hope this makes sense and helps you understand.

As far as the "lean condition" and all that... I have no idea where you got that. You may be talking about burning valve due to incorrect valve timing or gases passing too fast causing the same thing. Any time more air is allowed in, be it carb or EFI, fuel will make it in. The computer will recognize it and widen the pulse or the vaccuum created will draw more from the carb.

And you "tuning" a car and getting slower, sounds like you work for a dealership that has set programs that are crap. Anytime you get more air/fuel in and exhaust out you create horsepower and gains.
 

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Search for the thread someone explained this whole topic about backpressure, I'm pretty sure it was on this forum but if I remember correctly it was very extensive and pretty much answered any question you'll probably have.
 

· Overlooked
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The most you can get out of an NA is about 300whp which is very efficient, 100hp per liter. With a turbo you can virtually double your displacement with ~14.5psi of boost, depending on atmoshpere, by doubling the amount of air/fuel forced in. Since it's a smaller chamber it's actually just a bit more exponentially. That's why it's not too terribly hard to get twice the power (take into account of power loss through heat etc) you MIGHT have with less money than you spent to possibly get that power. Please read more and talk less, that's why you have two eyes and one mouth.

Last note, I feel like I might know what I'm talking about seeing as how I'm a physics major and all that. Again, study up a bit.
 

· 7M Fanatic
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I love people with a half of a degree, and absolutely no idea how to apply it......

I grauduated the top of my class, and have 35 years auto racing experience.
So don't talk physics to someone who not only pulled a 4.0 in it, and has been around Way longer than you.
You will just look stupid.

By the way, when F1 cars were still 3L, and NA, they were pulling 800+ HP, that that kind of kills your efficincy theory all to hell.
 

· Overlooked
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Ok, currently awaiting deployment, so sorry I haven't completed my degree. Also, F1 cars are a bit different than a 7M. Apples to apples buddy. Your 35 years off experience must be with model cars or you might know what's going on. Look, I'll admit your epenis is slightly bigger but from everything I have done, heard, read, and experienced in my 12 years, yes 12, leads me to differ from your idea, as well as many performance exhaust companies and physics itself. You don't have to have the degree to understand it. But do what you do and blow stuff up and I'll continue to build and repair cars to a dependable state.
 

· Im A Fuking Ninja!!!
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6,330 Posts
fuild dynamic/velocity, the right size and designe will increase flow in and out of the engine.
 

· Registered
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Sixpack is absolutely correct.

Backpressure being good is an old wives tale.

Someone needs to quantify the term "backpressure" and state where in a control engine that this "backpressure" would be measured because this topic cannot be discussed without knowing this important bit of data.

At the valve?

At the port exit?

At the end of of the exhaust primary tubes?

At the end of the collected exhaust?

Halfway down the exhaust system?

The very definition of "backpressure" in automotive usage is normally accepted as the pressure that is seen after the collected exhaust and refers to the restriction of the exhaust system.

Anything upstream of the collector will be hard to discuss since you now have to figure in temperature (which can affect reynolds number) and whether flow is supersonic (which throws in another variable that needs to be factored in).
 

· Overlooked
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I'm talking about at the collection point. That's the point of most signifigance, not the only though, since this is where the engine as a whole can first be effected. Low BP headers along with higher BP pipes to keep up the velocity. Temperature matters the entire time but that's a little deep for an MKIII. Either way, an NA engine does require a bit more BP in the exhaust system. Too much or too little, as with anything, will hinder. It just depends on where you want your curve.
 

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The best exhaust is a free flowing system with a little restriction, but it needs to be of the correct bends, and lengths that those bends happen.
 

· iSketch Master!
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If I am to understand correctly, it's not that NA cars NEED backpressure, it's just that they need exhaust velocity. That's not something you can just slap something on and keep. It has to be designed into the entire part.

That's how I've come to understand it, and why it seems some people think when you slap a huge exhaust on an NA car it loses power because it lost backpressure, when it's not directly related to the backpressure loss. That's my interpretation at least...

Alex
 

· 7m love... dont hate.
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2,728 Posts
take your exhaust off at the y pipe and run your car. its slow. put your restrictive backpressure stock exhaust on and its faster. n/a's do need back pressure but that does not mean that you need a 1/2 exit for the exhaust hence why they sell n/a exhausts and turbo exhausts. there designed differently for different types of engines.
 

· iSketch Master!
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Do you really believe that? Tell me why domestic cars run cutouts? Quit spewing shit you don't know about! If you removed your exhaust you'd lose low end torque and your car may be slower. But it's not because of backpressure that was needed. 4 stroke cars don't need backpressure, it's all about velocity really.

I don't see the point in explaining it. Go browse the internet and you'll see how flawed your thinking is. You do know that too large of an exhaust on an NA car can create backpressure? So your entire argument is moot.

Alex
 

· Overlooked
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You need it proportional to the amount of air/fuel in. To get real nitty gritty you need the EGT and cool down factor, gas partical speed, gas expansion due to burning fuel, velocity of excaping gases, sonic resonance, port size and shape, if any vortex factor is there, etc. Noone here is going to go through all that for a 7M lol. Seriously, just put a 2.5" and leave it alone.

Hitman, that's true of turbo engines.

ALxRSPTT, yep. Backpressure helps keep the velocity up. Problem is that with changing RPM the BP changes also. You would have to design an exhaust system to change sizes and bends constantly to maximize the entire RPM range. Again something that will not be done, ever.

Too large a pipe will create resistance of the exhaust velocity. Slightly different but same idea. Just one aids while the other hinders.
 
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