As most people here are bound to know, cotton/mesh/whatever "performance" filters aren't always the safest choice, and don't usually make big power gains on moderately tuned engines, so I have a question. I'm using a stock air box on my '91 mk2 Jetta Turbodiesel. Power numbers are expected to be around 220-230 hp at ~35 psi boost. (the engine is thoroughly built, so please no BS about this) Turbo inlet is 60mm and airbox has a 70 mm pipe connector on it - the intake tube will be made out of mandrel-bent aluminium tubing with a 60->70mm adapter. Air box itself is the biggest one on all mk2s, an Eco diesel one. So I'm thinking about building a cold air feed pipe for the airbox. No the question: How beneficial a cold air feed is on a turbocharged engine? There is a big intercooler after all. I'm thinking the cold air feed has less relevance than on a N/A engine? Still, there is bound to be some benefit with using a cold air feed - but how does its length and shape affect its performance? For example, if I have to do a two-feet long pipe with three bends in it to achieve a proper cold air feed, will that help power output or impede it? I was thinking the diameter should be 70mm as that's the max diameter of the airbox-to-turbo pipe as well. Any dyno-proven experience, or true physics about this? Also, will heat on the airbox itself have any effect? I have seen people cover them with heat-resistant foil, but that seems a little superstitious to me...
from my limited experience playing with turbo diesels i would say there is not much point in putting in a cold air feed if your using a turbo as air passing through a turbo is going to heat up. as the more pressure the hotter the air anyway
