Silvia Club of NSW

Why drive when you can drift?
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PostPosted: Thu Jul 14, 2005 9:31 pm 
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Quad T66
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Risking wrote:
A well designed cat will speed up the gas flow even though it is a minor restriction, in that case you can keep the gas flowing quickly.


this is where i have to disagree, as dumhead mentioned a forced increase in velocity through a restriction will reduce the mass the pipe can flow, and this is the case with a cat, it is a restriction and any increase in velocity it creates is due to a constriction forcing velocity to go up.

in n/a extractors after all the gasses have merged together after the collector, my view is that the quicker you can move the mass out the quicker you create more vacuum which gives the suction effect.

for example at the tailpipe of the muffler, a 4" tip will give more velocity then a 5" tip, but the 5" will flow more gasses through it for any given amount of time... say for example 500cfm for the 5" a minute and 400cfm for the 4" a minute, which will mean the 5" will create 500cfm of vacuum behind it every minute over the 400cfm of the 4" - this is disregarding the principle we have established that increases and decreases in piping diameter throughout the piping will cause turbulance and actually decrease flow, but more so a model of say a 4" straight through exhaust compared to a 4" straight through.

i have the view that the same effect occurs after the collectors of n/a extractors. the diameter and length of extractors arent all about flow though (which is why they arent huge, along with the fact that you want to match the exhaust port size so you dont have increases and decreases of piping diameter), they are mostly to do with timing of the pulses so that the scavenging affect is happening at the right time on the exhaust stroke of the next firing cylinder.


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PostPosted: Thu Jul 14, 2005 10:03 pm 
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Quad T88
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You are right I should have been clearer in what I meant.

The cat matrix naturally increases velocity thats the pyhsics of gas flow but the inside design of the cats shape can also help to increase velocity in the same way a pipe diameter change can.

You are right about the NA extractors they are not about capacity they are designed with velocity and scavenging in mind. The idea is to keep the waste gas away from the head of the valve (so when negative pressure wants to draw spent gas into the chamber on valve overlap) it can't because the gas has already moved away from the valve and is doing so at a rate at which it wont be reversed.

Thats why extractors are designed 4-2-1 normally in firing order fassion so the pulse from one "extracts" the next.

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PostPosted: Fri Jul 15, 2005 11:02 am 
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Token Old Guy
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Risking wrote:
The idea is to keep the waste gas away from the head of the valve (so when negative pressure wants to draw spent gas into the chamber on valve overlap) it can't because the gas has already moved away from the valve and is doing so at a rate at which it wont be reversed.


I wouldn't say it "can't" because it actually can and does. remember that the exhaust valve closes after TDC so the piston going down creates negative pressure in the cylinder which in turn will pull back in the last bit of exhaust gas that has just past the exhaust valve. If the valve was closed at TDC I would agree but engine scavenging is just not that efficient.

Your theory though is totally correct the higher the velocity past the valve and out the runner the less gas will return back pass the valve. Remember as the valve closes the volume will decrease causing a reduction in velocity. With a lowered velocity once the piston passes TDC it then creates a negative pressure in the cylinder the velocity will come to a stop and actually start to flow back into the cylinder.

The higher the velocity (at the valve) the less gas will flow back, so you are right on the money.

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PostPosted: Fri Jul 15, 2005 11:43 am 
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I might disagree with my own post… What I said holds true for N/A but may or may not be true for turbo cars with the exhaust gases being aided by the exhaust turbine to maintain velocity.

When the negative effect caused by the falling piston takes place the valve is almost shut. In a N/A it will flow back marginally but with a turbo engine the pressure differential (college teacher hated the word suck) of the exhaust turbine may have sufficient effect on the gas flow to keep it going out the exhaust.

I guess this would be subject to the head / exhaust manifold design and engine load.

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PostPosted: Fri Jul 15, 2005 12:27 pm 
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Takumi
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nah I think it'll be the opposite.

That's why turbo engines can't run really massive cams.
Generally there is more pressure in the exhaust manifold than there is boost being made by the turbo.

So, with the small volume (and mass) of gas in the exhaust manifold runners, under high pressure it's very easy for reversion to occur during the time the valve is open but there's very little cylinder pressure.

That's where stepped port exits and other anti reversion designs can help, but generally it's a case of matching cam specs with turbo size and the desired rpm range.

Basically the turbo does no favours at all for exhaust scavenging, pulse definition, or gas velocity.
It's a massive restriction, and the gases leave it at funny angles, spinning at high speed, and then having to make a sharp turn down the dump pipe straight away.
You won't get reversion back through the turbo, but that's simple because there's always heaps of pressure before it.
That pressure differential is where the turbine gets its energy, which is why the least restrictive exhaust possible is needed.

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PostPosted: Fri Jul 15, 2005 1:19 pm 
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What if the runner length was longer??? I recall seeing a bug eyed sprite at Willowbank about 10-15 years ago that had a turbo four in it. He basically had the turbo sitting at the end of a set of extractors.

Hypothetically speaking what would happen if you went somewhere in between like mini extractors and put the turbo at the bottom and then instead of having a dump pipe going down, have it coming straight out the end of the turbo and going straight down the exhaust pipe?

Might need to re-route oil lines but might work… and wrap the pipes with tape to keep in the heat.

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PostPosted: Fri Jul 15, 2005 1:59 pm 
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heaps of turbo setups are done like that (mainly race cars), and they probably help with scavenging a bit at very high rpm, but only if the turbo is quite large.

It tends to result in a very laggy turbo, and you still can't get away from the header pipes running at much higher pressure than they do in an n/a engine.

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