In reference to the air box modification a.k.a. "Randy's Air Box Mod", I was wondering if leaving the stock "ram air" snorkle in place somewhat negates the positive pressure at the cowling? In other performance cowl induction systems I have seen, there is no ram air intake tubes from the front. You either have one or the other. Would it be best eliminate the ram air intake tube if you were to go with the cowl induction method?

I Like the mod so far....Always looking for cheap HP!

 

A great question, and one that always seems to get danced around. I think it was momentarily discussed in a thread about the alternate vent options. Opinions have varied wildly as far as what the actual air pressures (positive or negative) in various areas were. It seems like there were two basic arguments:

--One school of thought claims that the OEM cowl hole just opens the system up a little better, like a breather on a bottle pourer in a bar, and that the system doesn't actually draw any air from this hole. In this way of thinking there is a direct flow of air from the grill duct straight through to the airbox. The cowl hole is possibly a negative pressure area, helping the grill duct to function better.

--The other school states that air is drawn from both grill duct and cowl hole, and that, especially with Randy's Air Box Mod--and even more so if you're using an aftermarket intake, more and more air can be drawn straight through the cowl wall, rather than flow through the grill duct/snorkel. So, for me at least, the logical conclusion of this train of thought would be that if you plugged up the snorkel, all air would be drawn from the windshield vent area.

But how much do we really know about the airflow through the system AND outside the car? Has anyone put this thing in a real wind tunnel to simulate the real world? Just thinking about the shape of things, I would tend to side with those who state that the windshield vent area would be a source of negative pressure (i.e. a vacuum) relative to the air under the hood. Air flows quickly over that area and would be inclined to reduce the pressure. It's the old Mr. Wizard trick. Stick a straw in a half filled glass of water and blow air directly over the top of the straw (perpendicular to its opening). The water level will rise inside the straw because the airflow reduces the pressure.

On the other hand, if you were to go get the aftermarket vent inserts that are out there now, it would DEFINITELY turn it into a positive pressure area and could provide a good rush of air into the airbox area, and from there through the perforated Randywall into the air intake. In that sort of a system, eliminating the snorkel would seem like the only logical thing to do.

More thoughts, PLEASE.

 

The base of the windshield is a high pressure area. If you imagine a several-foot thick layer of air flowing over car you see that both the front of the hood and top of the windshield are points of restriction or "venturis". The result is low pressure on those areas and high pressure at the widest passage -- the windshield / hood junction.

That said, the high pressure at the windshield cowl vent is only very slight and pales in comparison to the pressure at the front of the car. The factory box draws about 90% from the front and 10% from the cowl. That balance was no doubt based upon extensive wind tunnel tests or simulation.

The perfect balance would have both sources supplying air simultaneously but in practice that is seldom achieved due to the varying engine demand. You can imagine that if the car were coasting, engine-off, the high pressure air from the front ram would flow right past the air filter and out the cowl vent -- overcoming the slight cowl pressure. Imagine now that the engine is running and consuming exactly as much air as the front ram is delivering -- now the little cowl vent will provide an extra boost. The engineers likely tuned the small (3/4&quot cowl vent port for this perfect balance at say, 60Mph and 22% throttle @ 2300 RPM for optimum highway fuel economy. Hitting the gas from there would create a very high demand for air that would completely overwhelm the little cowl vent and the engine would not recieve quite as much air as the external aerodynamics were capable of providing.

Opening up the 3/4" port to 12" or so via the "airbox mod" will likely allow rammed air to escape out the cowl during many cruise conditions but at higher throttle settings and/or lower speeds the engine should receive extra air. Basically the balance point is shifted toward higher RPM and throttle settings (higher air consumption).

I would expect the "airbox mod" to sacrifice a small bit of highway economy and top speed for improved acceleration.

But to answer your question the front intake is dominant and critical.

_________________

2004 MCS-CR/W-Sport Pkg-Cold Pkg-17" R99's-bonnet stripes-PIAA

 

I did not notice change in performance with the airbox mod untill I installed the HVAC vent "scoops". No dyno on that statement, just my "butt" talking again, lol.

 

Vespa,
Your explanation seems logical. There will be a balance point in the induction systems. I agree that changing the available air flow by increasing the cowl vent openings should just move the "balance point" between the systems (cowl vs. snorkle) dependent upon the other variables you mentioned. I will definitely leave the snorkle in. After all, more is better, right? Thanks for your input.

 

https://www.northamericanmotoring.co...amp;topic=8220

https://www.northamericanmotoring.co...amp;topic=8631