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Engine Specific Info and Questions => IDI Engine => Topic started by: Master ACiD on December 06, 2005, 07:36:57 am
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on a 1.7 gas rabbit no one would use a 2.5 or even a 2 inch exhaust because it would be considered way too big and you would lose some of that pressure wave scavenging effect that happens when 1 exhaust pulse created a small area of lesser pressure which pulls the next high pressure pulse along. a 2.25 inch exhaust would be considered way too big. somthing more like a 1 7/8 inch or 1 3/4 would yeild better results.
on a 1.6 diesel on the other hand, i see people recomending very large exhausts, up to 2 1/2 inch for a normally aspirated engine! now i am i am not doubting anyone here but i am rather curious as to why a diesel needs such a large exhaust even though its smaler engine.
i have also heard that the 1.6 idi has no overlap in the cam. if this is true, i can see where someone would think you would lose all scavenging between the intake and exhaust valve, but scavenging happens elsewhere and should still happen even with no overlap. you just wouldnt be able to actually draw in unburned air through the intake and draft it through to the exhaust port?
i guess i just dont understand. can anyone shed some light on this?
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2.5" exhaust would only be recommended for a turbo diesel application. i can't see why someone would suggest that large a diameter for a naturally aspirated diesel motor... at least not anything VW offers.