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Old 12-18-2014, 03:45 PM   #1
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Back pressure, good or bad?

Hey guys,

Quick question on back pressure. I was watching a video about a new type of muffler and they were mentioning that it "didnt build up engine harming backpressure" or something to that effect.

But i've always known "some" back pressure to be desired, and therefore a good thing for the engine. Is too much back pressure really a bad thing?

I started to read up on it and it seems that for some turbo charged cars straight pipe and low to zero back pressure is desired since (according to what i've read) the exhaust pulses affect the turbo spinning and its bearing.

Granted this is for cars, and we ride motorcycles (although some bikes are turboed), but they are both internal combustion engines, although one is naturally aspirated and the other is not, im not quite understanding whether little back pressure is necessary for some or all engines, or whether back pressure is only desired for engines where the scavenging effect is also desired.. and for that matter how do turbo bikes fit into the equation where back pressure may be necessary for exhaust and what not

just curious if someone can help me fill in the gaps, and how it would relate to motorcycling and/or internal combustion engines as a whole.
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Old 12-18-2014, 04:07 PM   #2
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You don't want back pressure. Back pressure makes it harder for the piston to move upward and will lose power. But, you don't want too big of an exhaust because you will lose velocity and along with it, scavenging.

The perfect size exhaust would be the smallest pipe that wasn't a restriction.
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Old 12-18-2014, 04:18 PM   #3
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If you're talking about a turbo car then it is kinda apples to oranges. The turbo itself creates back pressure but the amount of back pressure is regulated by the wastegate.
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Old 12-18-2014, 05:12 PM   #4
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Some back pressure is required for scavenging exhaust gas from the cylinder and port, which is why you typically lose power if you're running just an open header, but no, you don't want excessive back pressure. Typically this will just entail having properly sized exhaust pipes at the header with a free-er flowing muffler. A turbo will create back pressure in the exhaust, which is why you can pretty much just run a dump off the exhaust side of a turbo. The exhaust manifold piping going to the turbo needs to be properly tuned (like you mentioned) to optimize the turbo due to exhaust pulsing.

At least that's my understanding. What is "optimum" is going to depend a lot on displacement, valve size, timing, and duration, port shape, and on and on.

That said, 2-stroke and 4-stroke engines also have much different requirements. On a 4-stroke, intake manifold design is more influential typically than exhaust, where 2-stroke is the opposite.

There's tons that goes into properly tuning an engine...
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Old 12-18-2014, 11:50 PM   #5
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The engineer side of me wanted to explain this in technical terms and very sciency then realized the question was answered. There is never a hard fast rule that will always be true. It will always be possible to be too little or too much.
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Old 12-19-2014, 07:13 AM   #6
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Quote:
Originally Posted by Hypnotik View Post
If you're talking about a turbo car then it is kinda apples to oranges. The turbo itself creates back pressure but the amount of back pressure is regulated by the wastegate.
But the primary function of the wastegate is to regulate boost
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Old 12-19-2014, 10:33 AM   #7
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Similar to what arkansasDave said, I was about to go on a rant from a engines builder perspective but I wont.

Plain and simple it doesn't matter how much BP you have don't have, scavenging blah blah blah. All that matters it that you make the optimum amount of power where you want it per your application.

Simply put... hours and hours and dozens upon dozens of dyno pulls must be used to R & D your app and optimize your numbers.
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Old 12-19-2014, 11:57 AM   #8
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discussing from a pure reliability stand point, what i've read and heard people discuss in the past is that you want atleast some back pressure because no back pressure will cause you to float a valve, but too much back pressure also destroys valves?

I was wondering how does back pressure factor into engine reliability.
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Old 12-19-2014, 10:27 PM   #9
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Quote:
Originally Posted by Bevo View Post
But the primary function of the wastegate is to regulate boost
And how does it regulate boost?


...by regulating the back pressure before the turbo and makes the turbo spin faster or slower.
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Old 12-19-2014, 11:37 PM   #10
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Quote:
Originally Posted by Hypnotik View Post
And how does it regulate boost?


...by regulating the back pressure before the turbo and makes the turbo spin faster or slower.
Or let's air bypass the turbine?
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Old 12-19-2014, 11:48 PM   #11
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Quote:
Originally Posted by Hypnotik View Post
And how does it regulate boost?


...by regulating the back pressure before the turbo and makes the turbo spin faster or slower.
Yeah, and you could also say that the purpose of a wheel at a grist mill is to give water a ride

The priority of the wastegate is to protect the turbine and the engine which by its operation changes the back pressure.
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Old 12-20-2014, 08:01 PM   #12
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I think you guys are missing the point. I'm not arguing the purpose of a wastegate. I simply stated that the turbo creates back pressure and the wastegate regulates that back pressure. I understand it is to regulate boost pressure but it regulates that boost pressure by regulating back pressure. You're trying to create a meaningless argument.
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Old 12-20-2014, 08:02 PM   #13
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Quote:
Originally Posted by Bevo View Post
...by its operation changes the back pressure.
Exactly. /thread
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Old 12-20-2014, 09:27 PM   #14
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Quote:
Originally Posted by Hypnotik View Post
I think you guys are missing the point. I'm not arguing the purpose of a wastegate. I simply stated that the turbo creates back pressure and the wastegate regulates that back pressure. I understand it is to regulate boost pressure but it regulates that boost pressure by regulating back pressure. You're trying to create a meaningless argument.
Would anyone in their right mind install a turbo just so that they can regulate back pressure?
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Old 12-20-2014, 09:33 PM   #15
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Quote:
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Would anyone in their right mind install a turbo just so that they can regulate back pressure?
How else are you going to have continuously adjusted/adjustable back pressure? Duh
Much easier adjusting this way than swapping headers.
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Old 12-21-2014, 12:47 AM   #16
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Quote:
Originally Posted by Bevo View Post
Would anyone in their right mind install a turbo just so that they can regulate back pressure?
Well, technically yes. If you have more back pressure you can spin the turbo faster and get MOAR BOOOOST!!!!

GIME DAT BACK PRESHA!!!
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Old 12-21-2014, 12:54 AM   #17
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Old 12-21-2014, 01:21 AM   #18
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You dont just pick a single variable and derive the result from it.
Ya can if ya want to.
"Hands up, don't shoot" or "I cant breathe" are examples.
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Old 12-21-2014, 02:14 AM   #19
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I had a discussion with my buddy about this when I drilled out my exhaust baffles. He was swearing that it would do more harm than good because I "needed" back pressure.

It makes no sense to need back pressure, but this guy explains it pretty well even though it's a semi-long read.
http://www.dsmtuners.com/threads/exh...essure.168578/
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Old 12-21-2014, 10:50 AM   #20
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