"Colder" air vs. larger intake area

[rd400guy]

Since this is an engineering forum, I'll pose the question this way: Is there more benefit to having a colder charge air source, with a smaller inlet area, or engine compartment temperature air (with some outside air) and a larger inlet area?

Here's the scenario, two modifications to a stock airbox (K&N type filter already in place):

1. Install ducting (~3"Ø) from front lower grille up to a hole cut into stock airbox.
- block off existing inlet to restrict intake path to nose inlet only? or
- keep existing inlet open?

2. Cut away "dirty side" of existing airbox to expose entire surface of air filter as the "inlet".
- use lower grille opening & minimal ducting to aim cooler air toward "inlet"?
- add heat shield by exhaust manifold?

Either situation will flow through unmodified throttle body and intake manifold.

 

[Fabrico]

Cool air will not make up for restricted breathing. A 3" duct is smallish for most engines. Without knowing anything else...I would opt for more air.

 

[rd400guy]

A little more info:

2771 cm³ water-cooled V6, bore 82.5 mm × 86.4 mm stroke, DOHC, 5 valves per cylinder (30 valves total), CR = 10.6:1, output: 194 hp (142 kW) @ 6000 rpm, 280 N·m (206 ft·lbf) @ 3200 rpm

 

[GregLocock]

On the contrary, I have never seen more than 0.5 kPa inlet depression measured ahead of the throttle, which would lose you maybe 1% of the charge, yet it would be easy to lose about 10% charge density by picking up hot air rather than cold.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Sorex]

Make sure the 3" pipe on the front of the car is fitted in directly into the air. Use a mesh as a filter.

Removing the "dirty" part of the airbox will increase the air intake surface at that point; however, after the airbox the surface of the intake is reduced again.. nothing to gain here (on a standard engine).

In autocross most of the time an intake pipe fitted to the standard airbox is used. (option 1). BTW, U won't need the standard air inlet of the airbox anymore.

Good luck with ur Audi..

grtz, Sorex

 

[spectreeng]

Focus on getting colder air. The dirty half of the airbox is likely not the main source of pressure loss. Why don't you just buy a cold air intake?

 

[rd400guy]

Why don't you just buy a cold air intake?

The "cold air intakes" currently commercially available for this vehicle aren't a true cold air intake, as they pull engine compartment air through a big K&N cone type filter (basically my option 2 in the original post) - but they add an aluminum heat shield. Or they use the same ducting as stock but instead of going into the stock airbox with a plate type filter, air goes into a can with a cone filter (where's the benefit?).

My option 1 (which is what I'm going to do when I get a free weekend) is a true cold air intake, as I'll be pulling in air from outisde the engine compartment throught a larger duct than the stock intake ducting. I managed to obtain some 3 1/2" diameter wire reinforced flex hose.

Plus it'll cost me about $300 less than anything you can *buy*.

 

[spectreeng]

"Or they use the same ducting as stock but instead of going into the stock airbox with a plate type filter, air goes into a can with a cone filter (where's the benefit?)."

Sometimes intakes will incorporate a air horn, or velocity stack, at the end of a pipe, and will attach a big cone filter to the large radius of the horn. This allows for less pressure loss at the pipe entrance and much higher flow rates. Designed correctly, they can make a significant difference.

 

[cammax]

Be aware of your new intake hieght, should you ever try to traverse a minorly flooded road. I know of at least 2
toasted motors from trying to pump water due to the cold air intake scooping up copious amounts of water

 

[DonRoss]

I have tested on my highly modified 300whp VW 1.8T a 3" K&N cone with inverted open top on a 3" Tube vs
Evoms V-Flow which features 6" to 4" velocity stack connected to 4" to 3" venturi based on Bernoulli's principle.

Here is a back to back Mass Air Flow Comparison.