nib1980
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| posted on 14/4/09 at 10:07 AM |
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Which Exhaut System?
Hi All,
Not that it really affects me, but which is better and what are the differances between a 4-2-1 system and a 4-1 system?
Many thanks
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blakep82
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| posted on 14/4/09 at 10:10 AM |
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think about everyone trying to leave a football match through 1 small door.
then think about if they went through 4 doors, then, 2, then 1. its still the same number of people leaving just as quick, but they're a little
more organised.
thats how i see it anyway, but i don't know if thats right
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Mr Whippy
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| posted on 14/4/09 at 10:12 AM |
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if the pipes are all the same length in the 4 - 1 then both are just as good
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philw
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| posted on 14/4/09 at 10:18 AM |
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I know the Zetec prefers 4-1 and the XE 4-2-1.
Blake i will sort your photos out today
Must try harder
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mr_pr
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| posted on 14/4/09 at 10:19 AM |
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If an exhaust system is properly designed for a vehicle then either system (4-2-1/4-1) will effect the characteristics of the engine performance, for
example the torque curve of the engine.
However, this is normally done by fluid engineers using CFD, so I wouldn't worry too much. Just make sure the system you pick/make follows the
basic rules.
The two biggest problems usually seen are, unequal pipe lengths (from the manifold to the joins) and very restrictive collectors.
.JPG)
My Build Progress
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omega0684
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| posted on 14/4/09 at 10:47 AM |
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if you get a good 4-2-1 system this will give you more power in the mid range torque curve where are a good 4-1 system will get you better performance
at in the higher rev area of the power band.
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craig1410
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| posted on 14/4/09 at 11:47 AM |
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Exhaust system tuning is a vast subject and very complex. To visualise, you need to be thinking in terms of pulses of high pressure gas coming out of
each cylinder in firing order. Each exhaust header has gas at atmospheric pressure or at least a much lower pressure than the gas coming out of the
cylinder. The high pressure gas will expand into the header and on through the exhaust system to the tailpipe.
At a basic level you want to make sure that high pressure in one header doesn't restrict the flow in a neighbouring header as this will cause
the latter cylinder not to empty completely during the exhaust stroke which will affect the next intake stroke and ultimately the power stroke. You
can do this by ensuring that the headers are long and that adjacent cylinders in terms of firing order are not adjacent in terms of 4-2-1 exhaust
layout. For example, if the firing order is 3 1 4 2 then you would want headers for cylinders 3 & 4 and 1 & 2 to join first and then those in
turn would join into a single pipe further downstream.
However, having said all that, it is possible by tuning the lengths of the headers and connecting them in a certain way to make use of the momentum of
the high speed exhaust gases to scavenge exhaust gas from adjacent cylinders. Think of a 2 cylinder engine for a moment to keep it simple and think of
a simple pair of headers which combine into a single pipe before going to silencers and ultimately the tailpipe...
Cylinder 1 fires and sends high speed gas out into the header. This gas then combines at the 2 -> 1 collector and continues into the main exhaust
pipe. At this point the Cyl 1 exhaust valve closes and the momentum of the gas in the single pipe starts to create a vacuum behind it in both headers
1 and 2. This vacuum builds up to a point where the high speed gas has slowed to a standstill just as the Cylinder 2 exhaust port opens. Now Cyl 2 is
sending high pressure gas into a low pressure header which helps to get the gas out more quickly and will result in the Cyl 2 gases achieving a lot of
speed and hence momentum which will in turn provide the same vacuum effect to Cylinder 1 when it opens again.
Now ideally the best time to apply this scavenge effect is when the exhausting cylinder is almost empty because the high pressure gases can exit by
themselves just fine, it's the dregs which need some help. So by adjusting the length of the headers you can adjust the timing of the scavenge
effect. The complication is that this timing is altered by different RPM so you can tune the exhaust for high speed scavenge or low speed scavenge but
not easily both. I think in general, short headers are good for low speed torque and long headers are good for high speed power.
This is my understanding of exhaust tuning but I stand willing to be corrected as I am by no means an expert.
Cheers,
Craig.
[Edited on 14/4/2009 by craig1410]
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mr_pr
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| posted on 14/4/09 at 11:15 PM |
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I agree with the above, it pretty much reduces one aspect of exhaust design down to fairly simple terms.
However, I would say for yourself, as I doubt you will be going to calculate header lengths to tune engine performance across your RPM range then the
other aspect is the flow characteristics of your chosen system.
This will most likely effect your engine performance much more noticeably.
The importance of smooth bends, well designed collectors and good quality workmanship.
A tight bend will cause huge flow issues, sharp angles at the collectors will result in a lot of turbulence and also rough welding inside piping will
cause all-sorts of turbulence too.
My Build Progress
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craig1410
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| posted on 14/4/09 at 11:23 PM |
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Yep, sorry I got so hung up on pipe length tuning I missed out that important bit...
I remember a while back seeing a program about formula 1 technology and it showed an F1 engine on a dyno with the red hot exhausts clearly visible.
These things are absolute works of art. It also showed someone carefully welding one together using a Tig welder and it was serious craftsmanship.
If you can pick up a good tuning book for your engine you will probably find recommendations for header & secondary pipe diameters for different
states of tune and different engine characteristics. I have a book on the Rover V8 engine by David Hardcastle which does just that.
Cheers,
Craig.
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