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 Post subject: Exhaust pipes diameter
PostPosted: Sat Dec 01, 2018 9:22 am 
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Joined: Fri Aug 03, 2012 6:00 pm
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Hi, im running Kf-DE engine and i wonder what is best exhaust pipes diameter for it and what is the original one?

PostPosted: Sat Dec 01, 2018 9:17 pm 

Joined: Thu Sep 13, 2012 8:11 pm
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Location: Bowburn, Durham
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I had 2.5" on my ze. The difference at wot was massive, but the drone was unbearable at 40 to 60 mph. 2 or 2.25 would be much better, creating more flow than standard but not droning as much at low revs. I've currently got 2" on the ze now fitted in my xedos but its not road legal yet so I can't honestly say about the noise but it does sound much quieter on the drive than it did with the 2.5" fitted. Some parts (bends) of the original were only 1" diameter. (probably why they wore out 1st)

PostPosted: Sat Dec 01, 2018 10:19 pm 
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Bigger diameter than stock pipes takes power from low end and gives some in high rpms.
The downfall to this is that not always bigger is better.If you exceed some size there is counter-effect.
The exaust gasses travel with a certain max speed troughout the exhaust pipes, the pipes have certain max volume that they can get out, bigger exhaust pipes means that they can get out bigger amount of gasses at that speed, but does the engine produses them?
The best gasses flow is (i don't remember the number) but lets say it is "X".
This for a small engine is capable troughout a small pipes and the engine reaches the speed of the gasses quickly and because it is a small engine does not produce big amount of gasses and the small pipes are good enough for it troughout the whole rpms(well maybe killing some power at top end) but negligible.
This flow from a bigger engine will be acomplished troughout a bigger size pipes, meaning the same thing, reaching the "X" speed(best speed of exhaust gasses) sooner and hold it for long period(from maybe 2-2.5K to 4-5K RPMs) so there is peak power there. And again some restrictions at high end RPMs.Bigger pipes than stock means that you take the "X"speed of gasses to a bigger RPM range.If the stock "X"speed is reached at 2.5K, and you put a bigger pipes for exhaust, you move that to 3K RPMs or higher. Wich means that the peak power is moved also to the higher RPM.If it was at 5K, after the change to bigger size pipes it could go to 5.5-6K RPMs.
This is tricky, you have to know what you want, because when you take from low end you have to know how much and to know how much you will gain in high end.
If it's for daily driving to 3-4K max, DON'T even think about making upgrades to the exhaust,because it will only waste more fuel to drive like it should(in low revs) with the stock exhaust.
If you "drive it like you stole it" all day long, then upgrade to bigger pipes is something that will help the engine breathe and exhale easier wich leaves the power he made to the wheels.
If it's a "stressed" engine(wich all the EU market Japan cars ARE!!) then it will lose some power at top end and the engine will be "choked" and not revealing it's full potential(at high REVs), but will have desent Torque in low revs wich helps save fuel.
More or less, if you have good mathematical skills you can find out wich is the best size pipes for the engine,( i've found out that increase of 10-15% is the best for almoust all engines with internal combustion) above that you loose(at low end power) more than you gain(at high end power), talking this that the engine is without other serious mods!!
I think i've explained it, little messy, i know. :lol:

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