# Cold air feed to stock airbox?



## TurboJ (Jul 8, 2008)

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...


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## VWBugman00 (Mar 17, 2009)

I've heard both sides of the story on this. Colder air compressed, is colder air sent through the pipes. With a turbocharged car, diesel or no, I would think the heat on the turbo would negate any benefit of the cold air intake (especially at those boost levels). Personally, I wouldn't worry about the cold air intake, as much as having a good filter on the intake. Having an efficient intercooler, in my opinion, is much more important, and beneficial.


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## Big_Tom (Aug 19, 2007)

buy a real intake :thumbup:


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## TurboJ (Jul 8, 2008)

> buy a real intake


Never.

Or to be exact, that's exactly what I _will_ do - use the stock box, the only real inlet there is.

The stock air box and filter flow over 400 hp with ease. 
While protecting the engine as VW meant.

Not trying to be impolite, but I believe the performance air filters are the industry's biggest scam. While the power gain being minimal or non-existent, is not a big problem - no performance filter ever provides the same level of filtration as a stock air filter assembly.

Concerning my original question, though, I'd be interested if someone had made dyno tests or something.


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## Big_Tom (Aug 19, 2007)

TurboJ said:


> Never.
> 
> Or to be exact, that's exactly what I _will_ do - use the stock box, the only real inlet there is.
> 
> ...


oooo k, suit yourself :thumbup: I personally have not seen anyone make over 400whp w/ ease on the stock intake but ok... good luck


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## TurboJ (Jul 8, 2008)

Not wanting to offend anyone, I was just looking for experience on the kind of setup I plan to use.

95% of all big-hp people automatically assume a stock filter isn't going to be enough and never even try them. That's why the dyno tests are very rare.

Big_Tom, did you check at which point the stock intake became a limiting factor on one of your builds?

And you _have_ seen what the 'free flow' filters do to an engine, haven't you?

I know a stock air box is more than enough for my build, but I don't have any scientific information about the benefits/drawbacks of a cold air feed.

Anyone?


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## Big_Tom (Aug 19, 2007)

TurboJ said:


> Not wanting to offend anyone, I was just looking for experience on the kind of setup I plan to use.
> 
> 95% of all big-hp people automatically assume a stock filter isn't going to be enough and never even try them. That's why the dyno tests are very rare.
> 
> ...


seems like you just have something against after market intakes :thumbup: I have never had a problem using AEM, K&N, Spectre, etc cone filters. That's not even really a debate because what race car do you know of still running the factory airbox setup? it's not so much about the filter itself, it's the crappy design of our stock intake systems. I guess you've never seen a stock vw/audi 1.8t turbo inlet pipe collapese at WOT because they are not built for high boost. you prob aren't making enough power to be restricted by the stock airbox yet. If i went back to the stock setup now, i'm sure i'd lose HP. my current CAI is 3", larger diameter than stock. i wouldn't worry about the cold air feed i'd look into a better intercooler setup. that will do more for you than a cold air feed to stock intake. that's my 2 cents....


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## TurboJ (Jul 8, 2008)

You are absolutely right about the design weaknesses of the stock intake system.
That's why a substitute intake system is necessary when you have much over 400 hp..
At least the mk IV and mk II air boxes have been tested and retested with hp levels of 400.

The trouble is, you're going to have to build the 'real intake' yourself, because, to my knowledge, all aftermarket intake manufacturers use cotton, foam, or mesh filters.

What you ideally need is a better flowing intake system than stock, but *absolutely* with a paper filter element. I don't have time for such custom work right now, besides my engine will not improve with another air filter system, because my power of 230 hp is well within the capacity of the stock air box. Oh, and I'm doing the inlet pipework myself because the factory's design can't deal with much vacuum (for safety). But the air box itself has no big flaws.

Today I saw a measurement performed to a 1.8t engine. 200 hp and stock air box / filter:
At maximum power, the pressure loss of the air box/filter was 0,0018 bar.

The filter style is the problem. Flow characteristics of the stock air box are not perfect for a high-hp engine, but it's the only solution available that accepts a paper filter element.

Cotton, mesh, foam all have in common the very unpleasant feature of letting in many times the dirt and other unwanted particles compared to paper. Did you know that these materials were used as stock on many otto-type engines before paper filter elements were developed to a superior level?

Race cars often use cotton/foam/mesh filters because their engines need not last for 200k miles.
You know how often race engines are rebuilt. 

One performance parts salesman (who sells cotton filters BTW) did a test on his own daily car, and already at 35k miles the cotton filter had caused damage that a bare eye could see.

The reason for which performance intake manufacturers never use paper filter elements is not their poor performance - it's their poor profit margin. They could never justify 10x the price of an OE filter on a paper element.

-----

BUT; I would still like some research data on my original question... Makes me sad to have to argue whenever doing things differently to most people...


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## Big_Tom (Aug 19, 2007)

TurboJ said:


> You are absolutely right about the design weaknesses of the stock intake system.
> That's why a substitute intake system is necessary when you have much over 400 hp..
> At least the mk IV and mk II air boxes have been tested and retested with hp levels of 400.
> 
> ...


lol, this is not an arguement.... it's a structured debate, dont be sad :thumbup: Paper filters may filter a little better than cottton or mesh filters, but it's not that much better. you have to remember we are talking about machines/engines here, not hyper-allergenic infants. I've put hundreds of thousands of miles on cone filters, never once had one deteriorate or cause any type of damage to any of my engines. you can build a nice fancy setup off the stock airbox if you want to take advantage of the paper filter, but it more work than just buying a good aftermarket intake setup.


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