Air flow requirements

[Luckypabst]

Is there any reliable means to estimate the airflow requirements on a turbocharged engine?

So far for my 1.6l at 4k rpm, I'm coming up with 113cfm, assuming 100% VE. What does the turbo do to the VE at near 20 psi?

I'm looking to buy a canister air filter and there's lots of options...

Thanks
Chris

[cyrus #1]

Just guessing but I think at 20psi you're going to be higher than 100% VE.

Give this link a shot.  I've played with it before and it appears to be pretty accurate.  http://www.not2fast.com/turbo/glossary/turbo_calc.shtml

[Luckypabst]

Just guessing but I think at 20psi you're going to be higher than 100% VE.

That's my guess, too but just how much higher? I'll play with that link some - thanks!
Chris

[Luckypabst]

But what's the VE at zero boost? It's not 100% - and how does the static VE of less than 100% affect your equation after going to 2x atmospheric pressure?

Sorry - I know I'm totally overthinking this. I feel confident that an air filter with 3" outlet, roughly 5" x 9" element should be sufficient...

Chris

[Rabbit TD]

Is there any reliable means to estimate the airflow requirements on a turbocharged engine?

So far for my 1.6l at 4k rpm, I'm coming up with 113cfm, assuming 100% VE. What does the turbo do to the VE at near 20 psi?

I'm looking to buy a canister air filter and there's lots of options...

Thanks
Chris

I'm curious too about the amount of air going through these TD 1.6's as I am building one too for my 81 Rabbit.  I am just going to use the air filter housing from the 83 Jetta this motor came out of though, even though it will take up more room which as far as I am concerned is it's only drawback.  I've never really been sold on those K&N or some of those supposedly less restrictive filters myself.  In my mind the only way a K&N filter of the same size that you can just put in place of your stock filter on most cars can be less restrictive and therefore flow better is for it for it not to be filtering as good also.  I know figures show that they flow better and all that but I think the filter for these engines is big enough for anything we will ever need and then some {unless the motor is really extensively modified} and the main thing is they will also filter a lot better.  As far as the cone shaped ones go I think the only reason I would ever use one was in a situation where tere simply wasn't anywhere to put in the factory type.  Then there is the look factor, lots of people like the look of them and that is fine too if it is of importance to them.  These are just my opinions and such but when they do these dyno tests and show that a K&N or whatever of the same size is less restrictive it just tells me it isn't filtering as good period.  I would like to hear what some other people think about this also as I know it will probably open up a can of worms, but that's OK too, the filters will catch'em. :lol:

[TurboJ]

I think your opinion is shared by the majority of people who know anything about engines.

[Turbinepowered]

But what's the VE at zero boost? It's not 100% - and how does the static VE of less than 100% affect your equation after going to 2x atmospheric pressure?

Sorry - I know I'm totally overthinking this. I feel confident that an air filter with 3" outlet, roughly 5" x 9" element should be sufficient...

It's my understanding that your atmospheric pressure VE will continue to be your VE under boost. Assume for a second that you have 90% VE under no boost on a 2.0L engine, so every two rotations you're sucking down 1.8L of air through the intake.

Now, your engine is always going to draw that same 1.8L of air from its intake every two rotations, pressurized or no. What the turbo is going to do is make that 1.8L go from nonpressurized to pressurized air. Your engine's VE doesn't change, but your effective VE (as seen by the air filter) is going to skyrocket well over 100%.

Assuming 100% efficiency (all of the increased pressure is from compression, no expansion due to heat) on the turbo, a 14.5psi boost pressure would give your 90% VE engine an effective volumetric efficiency (airflow through the filter) of 180%. 29psi of boost, and you're at 270%.

Boost pressure isn't going to modify your effective VE so much as the turbo's efficiency at that pressure ratio, ie how much of your extra pressure is due to compression instead of heat.

[saurkraut]

I've always wondered about the dinky little air horn on the stock air box.  All the other plumbing on the intake is bigger.

I suspect that it was carefully sized to impart a slight vacuum on the crank case as that is supposed to contribute to piston ring seal.

But I could be wrong.

Thoughts?