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Old 01-02-2004, 10:26 AM   #46
riceizyummy
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so do you lose HP with backpressure?
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Old 04-01-2004, 11:27 PM   #47
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Re: Re: Backpressure-

Quote:
Originally Posted by Doubletap
Alright just want to run this by the resident geniuses, mechanical engineers and experts in fluid dynamics. For a turbo application could another plus to haveing a large downpipe be the pressure drop created when air moves from a small pipe o a larger one. I believe they use this on carburaters to speed up the velocity of the fuel air mixture by runnng it from a large opening through a smaller restriction then back to a large one. When the air exits the smaller restriction it expands to fill the larger diameter space lowering the pressure and pulling the air behind it in faster. Could this apply as well when the (relativly) small amount of air exits the turbo and expands into the large downpipe lowering the ambient pressure of the air and effectivly pulling more air through the turbo to try to equalize. Wow or maybe the fact that the turbo is shoving air out into the downpipe evens things out and this is way off bt if anyone has a though let me kow I am curious about this one.

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Ok i know this post/thread is really old, but i just thought i'd reply for the fun of it. yes, they do have this for turbo's.. it's not EXACTLY the same thing, but it's pretty darn close... all the high end tubular manifolds have what is called a merge collector which works on the same principles. it takes the exhaust from the cylinders and forces it through a smaller diameter (say 1.75 inches) then opens up to the outlet size of the manifold. it is commonly seen on the larger engines (v8's) and normally on n/a vehicles. just my
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Old 09-02-2004, 01:04 AM   #48
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Re: Backpressure-

Me thinks the confusion comes from guys converting to low backpressure exhaust system and lost the low-end torque... and therefore saying backpressure is good (for low-end torque at least).

Read from somewhere that this is due to the combustion chamber being able to expel the spent gases easier with the lowered backpressure, but then at the same time, at low rpm the intake charge isn't exactly charging into the chamber and filling it up well. So the net result is less air/fuel mixture (including exhaust gas) to compress, and therefore worse low-end torque.

Not sure whether this is the correct scenario but it makes sense to me.
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Old 09-16-2004, 05:52 PM   #49
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Re: Backpressure-

The scavenge effect of the exhaust manifold needs certian speeds to work. If you fit the largest pipes you can fit you will lose power, but it's not caused by exhaust backpressure but because of the loss of the scavenge effect.
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Old 12-24-2004, 11:14 AM   #50
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Re: Re: Backpressure-

SaabJohan is correct.

The lower the backpressure the better in every case. The problem comes in is determining the lowest back pressure at what RPM. No pipes at all would give the closest to absolute zero back pressure. However, with the pipes tuned correctly you can achieve scavange effects that in theory make the back pressure less than 0 at certain RPM/loads. Thus, cylinder efficiencies are gained.

That's why they are called "tuned" headers. The size, length and shape provide these gains only at certain RPM ranges.

We dyno test headers for a company here. Typical applications we test are 4:2:1 systems. Four cylinders are tuned with the primary 4 into 2 pipe for a particular RPM. Then the 2 into one pipe is tuned for a slightly different peak RPM. This broadens the range that the headers are most effective. Depending upon the application, these 2 RPM ranges vary. For a race engine that may operate at the same RPM most of its use, these points are the same. For one than runs from 4800 to 6600 the sections are tuned farther apart.

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Old 12-26-2004, 05:58 PM   #51
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Re: Backpressure-

Interesting...

Speaking of header systems, do you hv any experience with 4-1 shortie headers? Some claimed that they found this to be the best compromise between 4-2-1 & normal long 4-1 headers.
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Old 12-27-2004, 07:49 AM   #52
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Re: Re: Backpressure-

Quote:
Originally Posted by Esky
Interesting...

Speaking of header systems, do you hv any experience with 4-1 shortie headers? Some claimed that they found this to be the best compromise between 4-2-1 & normal long 4-1 headers.
Yes, but it's not quite so easy to make that generalization. All things being equal, the 4:2:1 are going to provide a wider HP band. Long pipes more HP at lower RPM's and short ones at higher RPM. But the size of the pipe and the merge point are going to determine the total performance.
BTW, I am not a header expert. My expertise is only in doing tests for header companies.

This much I have learned... In general, headers for street cars are typically of the worst quality product we come across. From ALL companies. They are typically poorly welded, ill designed and a wild tuning compromise. Part of this is due to the design of the cars they must fit into and the attmepts to make them installable. But for the most part the only headers that we have seen that fit well, are built well AND provide appreciable gains are those that are designed for full out race cars. All work to some degree, but are not optimum.

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Old 12-27-2004, 05:58 PM   #53
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Re: Backpressure-

Quote:
Originally Posted by sracing
Yes, but it's not quite so easy to make that generalization. All things being equal, the 4:2:1 are going to provide a wider HP band. Long pipes more HP at lower RPM's and short ones at higher RPM. But the size of the pipe and the merge point are going to determine the total performance.
BTW, I am not a header expert. My expertise is only in doing tests for header companies.
I thot it's the other way round, i.e. longer pipe for more top-end horses (as featured in most 4-1 systems, and virtually most, if not all, aftermarket 4-1 headers for Hondas)?
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Old 12-28-2004, 09:44 AM   #54
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Re: Re: Backpressure-

Quote:
Originally Posted by Esky
I thot it's the other way round, i.e. longer pipe for more top-end horses (as featured in most 4-1 systems, and virtually most, if not all, aftermarket 4-1 headers for Hondas)?
Nope. Everything else equal... It's longer for LOW RPM and shorter for HIGH RPM. THe same is true for induction runner length.

Do a search on the web. There are dozens of formula's for computing the peak HP range vs header length.

Jim
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Old 12-28-2004, 06:15 PM   #55
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Re: Backpressure-

Ok, so it's like intake pipe design- short & fat for good hp and long & slim for good torque.

So why aren't the VTECs using shortie 4-1, since most of the time the aim is for max bhp?
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Old 12-29-2004, 07:58 AM   #56
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Re: Re: Backpressure-

Quote:
Originally Posted by Esky
Ok, so it's like intake pipe design- short & fat for good hp and long & slim for good torque.

So why aren't the VTECs using shortie 4-1, since most of the time the aim is for max bhp?
Sort of.. You have to understand Torque and HP better than what your statement seems to indicate...

HP=Torque x RPM / 5252

Hp IS Torque across the element of time. Your statement should really say. " short & fat for PEAK hp at high RPM and long & slim for PEAK HP at low RPM."

You normally attempt to tune so that your engine produces it's peak HP at the RPM range you are going to operate at. NOT neccessarilly just peak HP.

IE. One could design the induction system so that the peak occurs at 8500 RPM and your HP would be very high. But if your engine can't live there or your drive train ratio aren't practical at that point it would make no sense. Plus at low RPM, the engines limited torque would make it less driviable.
If however, 6000 is where it is tuned for you would be able to operate the car in it most effective range. Your HP peak would be lower but the area under the curve that is utilized would contain more HP.

I don't know about the V-TEC shortie, but again remember, the aim is not for max HP, but for max HP under the curve in the area you will be operating the engine in normally.

Jim
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Old 01-16-2005, 04:47 AM   #57
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Re: Backpressure-

Adding Creatine in your fuel tank will boost your engine thanks to it's extra oxydation power... Just try it!
Note : Make sure the powder size do not exceed your thin fuel filter mesh size!
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Old 02-06-2006, 11:09 AM   #58
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Lightbulb Re: Backpressure-

here is some input from another mechanical engineer.

im don know if im just reiterating previous statement but, it needs to be explained in general terms for people to understand. at low RPMs , backpressure needs to be built for there to be any movement of the system. backpressure is built up until your pistons begin to move due to these pressure forces. if you have too large of an exhaust diameter and not enough air flow in, there will be no pressure build up at low RPM's resulting in less power in this range. a larger exhaust is only necessary when you are increasing the air flow into the head. for NA this would be adding a new intake manifold air intake cams. for turbo your exhaust size definately needs to be increased for max efficiency depending on the size of the turbo. and of course the other force that adds to cylinder pressure is air/fuel mixture. most often you need to change the air fuel mix in a car with exhuast and headers because combustion is not at peak efficiency, any more. backpressure is mainly a sign of how well your system is preforming with all the other components combined.


simply stated, if you are wondering what a backpressure exhaust is doing, it is allowing the exhaust to move more easily through the system without sacrificing the all important BACKPRESSURE.
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Old 02-06-2006, 11:21 AM   #59
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Re: Backpressure-

Backpressure: (BAK' preshur) noun; what it feels like to have my foot in your butt for reviving a thread from 2001

Its a good thread, but let's not revive the oldies.
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