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PostPosted: Tue 03 Feb 2004, 7:57 pm 
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you've won tihs argument astaroth, dyno proven results can't be argued. but, ... but.....I'd like to learn from you and your large experience. what i'm saying though, is that you are right buuuut you haven't explained WHY you are right yet.
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blow into the end of a garden hose and feel how fast the air comes out the end...

yes it does, because you are pushing the air really really hard, applying a lot of pressure. now...
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blow into a piece of 3" pipe and you will require MUCH more effort to get it to move at the same speed ... why is this?

because the big pipe poses a restriction to flow about 1000 times smaller that that of the garden hose you need to blow a huge volume of air into it to make it move fast, volume is NOT effort. if the pipe is very efficient (as a 3" pipe is) is won't take much effort at all to get the gas velocity up really high into that pipe. it will also take a hell of a lot LESS effort to move a given volume of air through the pipe than the garden hose.
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Because when you blow into the garden hose, there is less stationary air for you to blow down the tube... whereas you need to move more air in a 3" tube to get the same results... in short, there is more resistance to the efficiently fast flow of gas in the BIGGER tube than there is in the smaller tube,

I see what you mean here, it's a lot harder to get a high gas velocity in the big pipe than it is in the garden hose, that's true. totally agree, but this part...
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in otherwords, MORE backpressure.

what?, it's because there's NO back pressure that it's so hard to get a good gas velocity in the big pipe. it because the small pipe has HUGE back pressure that it's gas velocity is so high. that point's critical, it's the sudden closingof the exhaust valve that will create a vacuum in the garden hose, but the big pipe will just absorb the wave with it's volume and flow and it's lost. now that garden hose now has a vacuum in it, what you want is for the valve to open again while the vacuum is still about so it'll suck the exhaust gas out when it opens right? that brings me back to my quote in your last post.
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so you're suggesting that it doesn't matter in terms of how much power you're making how much pressure it takes to force the exhaust gasses out of the cylinder. It only matters if it effects scavenging or not?

what i mean there is that even though by puting a small ass pipe after that valve and forcing it to push really hard against the gas to shove it through that small pipe, where a big pipe wouldn't restrict it at all. it's critical that it pushes on it so the gas velocity increases and with correct pipe tuning can have a proper extraction effect. sooo you're saying it doesn't matter in terms of how much power you're making how much pressure it takes to force the exhaust gasses out of the cylinder. but the critical point is the scavenging effect created by the vacuum AFTER the valve closes again and in the next cycle will actually PULL the gas out rather than the big pipe which neither restricts flow nor aids it?. my wrists hurt, :wink:

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PostPosted: Fri 06 Feb 2004, 1:25 pm 
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mmkay, one last try... I can't get this to sound the way I want LOL

The larger pipe causes more backpressure because you are relying on the engine to push the gas out of the valve and into the pipe, instead of the low pressure area around the valve (which is caused by having fast air speed) to suck it out.

With a pipe that is too big, the exhaust gas in the pipe will be pushed down the pipe while the valve is open, but will stagnate in the pipe when it isn't. With a pipe that is too small, it will move down the pipe quickly but it will be restricted. This means that when a pipe is too big, the gas has to be PUSHED from the cylinder into the dump, instead of it being SUCKED from the cylinder, which in turn helps to suck intake gases in.

You need to balance the need to reduce restriction with the need to keep the gas moving at a constant speed... so while it is necessary to have an exhaust big enough so as not to restrict too much, it's also necessary to keep the exhaust gas moving instead of moving down the pipe in pulses.

I'm going to throw a cat amongst the pigeons here... I saw some dyno graphs while I was getting my 3" piping quoted today (and no, I couldn't get copies of them to post, sorry :P )... the local exhaust centre here put a 3" turbo-back exhaust on a slightly modded WRX (boost increase + cooler and it already had a 2.5" turbo-back system) and LOST 18kw @ 3000rpm. :shock: They gained 22kw @ 7500rpm though, but it's hardly worth it.

They then replaced the 3" dump with a 2.5" dump (retaining the rest of the 3" system) and got that 18kw@3000rpm back (plus more), but only lost 2kw of the 22kw they gained at 7500 with the 3" dump... So at the end of the day the 2.5" dump + 3" cat-back gave about a 15kw increase @ 3000rpm and a 20kw increase @ 7500rpm, whereas the 3" dump + 3" system gave an 18kw LOSS @ 3000rpm and a 22kw gain @ 7500rpm. They also found that boost came on about 800rpm LATER with the 3" dump than it did with the 2.5"!

Which would you prefer? :)

What I'm getting at here is (and yes, I know I have also said 'there is no such thing as too big on a turbo' but it seems that theory may also be flawed) is that it appears that turbo's are almost as non-receptive to too big exhausts as n/a's.... I'm keen on doing more research into this, but it seems as though it's just as important to keep good air speed for a couple of feet after the turbo, which in turn helps the gas to be sucked through the turbo and hence creates lower, quicker spoolup. The theory is sound - having your exhaust gas move through the turbo wheel faster could never be a bad thing :)

Interesting at least, I'll be keen to test some 2.5" and 3" dumps and see what the results are ;)

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PostPosted: Sat 07 Feb 2004, 4:10 pm 
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the high velocity is created by the high pressure yes, but when the valves are lapping (cant rember the term when both intake and exhaust valve are open at the same time) the high velocity of the exhast gasses suck all the gasses out because of the momentum


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PostPosted: Sat 07 Feb 2004, 6:46 pm 
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thumbs up to your reasoning finally there astaroth. very very interesting thing you mention about the exhaust on the turbo there...i just had the front pipes remade to raise them a bit for emmissions and while doing it, the exhaust shop removed the short length of 2" pipe coming straight off the turbo and replaced it with a full 3" piece. i was like sweet!. but now when it hits about 10psi on partial throttle it seems to missfire a little. but at full boost (13psi) it's smooth as hell and feels more powerful!. interesting stuff..

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PostPosted: Sat 07 Feb 2004, 6:58 pm 
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ox - LOL, I knew what I was saying but couldn't get it to sound right hehe.

Interestingly, that's EXACTLY what they said the WRX did. It was good on high boost (and the dyno graph showed that, shoulda seen the spike!), but anything else it was undriveable.

I'm keen on testing this further... anyone wanna make me a 2, 2.5 and 3" dump pipe (just rough) for testing? :)

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PostPosted: Sat 07 Feb 2004, 9:47 pm 
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lol lots of discussion
You know the big pipes you cn buy form auto barn or repco that sit right on the end of the exhaust do they do anyhting good or are they just looks?

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PostPosted: Sun 08 Feb 2004, 4:50 am 
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Looks, mainly. They might make it sound a *little* louder if you're lucky... but not much more than that.

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PostPosted: Sun 08 Feb 2004, 10:53 am 
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yeah, most of them are just to increase the wank factor. be careful because some of them are really dodgy, so as soon as u put it on u could get defected (too loud, legal limit is 90 decibles i think)
if your not sure get them to put the tip over your pipe. this will give you a rough indication. it will be louder when installed because fumes will be gettin out before your tip even starts when testing it cos it is not sealed around your tip.
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PostPosted: Fri 18 Jun 2004, 1:24 pm 
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moved to FAQ

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