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Topic Review (Newest First)
03-09-2008 01:37 AM
pelotonracer2
Quote:
Originally Posted by CaysE View Post
OK you went waaaay overboard on the explanation. LOL...

My question wasn't "are you getting air into the vents" which I'm sure you are. My question is how you're able to determine that air is going in via cowl effect, or just engine vacuum. Obviously air is going in... but you have to get pretty close to the windshield to actually get a cowl induction going.
I tend to go overboard sometimes...

Yes and you'd be correct, not much cowl action is happening, but at higher speeds it does a little. If a car is using plastic covers underneath (especially the front half of the car) "cowl" type pressures are nearly non existant.

I just figured a little info on aerodynamics might be beneficial.
03-09-2008 12:54 AM
FrozenFoci
Quote:
Originally Posted by pelotonracer2 View Post
Testing is easy. I used a plastic bag taped underneath the open squares where the vents were going to be. Depending on suction, the bag moves up or down... With a mostly sealed box, the filter was pulling the bag down (especially at higher throttle openings). It's an easy and effective way to test for high and low pressure areas.
......
......
NACA ducts are better "air scoops" in low pressure areas and that's why they are used on racecars in odd areas. NACA function involves a venturi effect which I'm not going to discuss in this thread (it would be TMI at this point).. hehehe. Some "production" cars (Viper and Z06 Corvettes) use NACA style ducts for ram-air but they are mostly for styling than actual function (they look sweet)
I feel like I just got a lot smarter. Or at least better informed...
03-08-2008 10:36 PM
CaysE OK you went waaaay overboard on the explanation. LOL...

My question wasn't "are you getting air into the vents" which I'm sure you are. My question is how you're able to determine that air is going in via cowl effect, or just engine vacuum. Obviously air is going in... but you have to get pretty close to the windshield to actually get a cowl induction going.
03-08-2008 10:18 PM
pelotonracer2
Quote:
Originally Posted by MVent View Post
How do you determine what direction the plugs are facing?
Take a magic marker and make a mark on the socket. Make the same mark in exactly the same place at the very top of the extension so you can see it as you are tightening down the plug. Insert the plug with the ground electrode facing out (aligned with both marks). Wherever the "mark" on the top of the extension ends up facing once the plug is snugged down, is the direction of the opening of the ground electrode.
03-08-2008 07:21 PM
pelotonracer2
Aerodynamics

Quote:
Originally Posted by CaysE View Post
How did you test the cowl induction? It seems unlikely as they are still a good distance from the windshield.

The vacuum is always there at speed... air is flowing over the hood and vents, pulling air out of the vents. With the vents facing the opposite direction, you'll have less resistance into the intake. As you said, it's not sealed though, so you're likely just getting a portion of air from the vents and the rest from whatever openings are around the airbox.
Testing is easy. I used a plastic bag taped underneath the open squares where the vents were going to be. Depending on suction, the bag moves up or down... With a mostly sealed box, the filter was pulling the bag down (especially at higher throttle openings). It's an easy and effective way to test for high and low pressure areas.

My airbox is sealed more than that, just not a "pressurized" system such as a ram air. I'd say 90% or more of the air has to be coming IN from the vent. I did this mainly to isolate the filter from underhood air. But then there's really not enough area to "pull" air from under or around the filter and there is not a steady stream flowing under the hood and up through the vents (thanks to the mostly sealed box).

The further back on the hood the less pressure there is until you get to the cowl area. This has more to do with the aerodynamics of this particular front end design and the shape of the hood than anything else. The part of the hood where the vents are is a low pressure area (outside of hood). Also, the cowl area creates a swirling effect and there is very little suction unless there is an actual opening or bowl area. The cowl pressure is effected by the shape of the hood and the rake and angle of the windshield. The lower angle of the windshield, the less cowl disturbence there is. Cowl turbulence or using it as a form of induction might be ok for drag racing but it actually can cause more CD at higher speeds such as oval racing or flat out top speed runs.

The frontal area of the car pushes the air outwards until it hits the upper half of the windshield or the roofline. The rear half the hood up until the cowl area there is not much movement of air, at least not enough that would adversely effect performance. Air coming over the top of windshield and roof is what creates the actual "cowl" phenomenom. This only works *IF* there is a place for the air to escape (ie: an opening). If you've ever seen a car inside a wind tunnel you be able to see the actual stream line of the air as it goes over the entire body of the car. There is very little pressure that far back on the hood. This is the biggest reason a properly designed ram-air system is placed in a high pressure area (usually at the front and radius of the hood).

There is less wind resistance the further downstream you get from the initial (offending) frontal area (the area actually cutting or splitting the wind) until a convergence point. A more streamlined rear portion of a car is much more aeodynamic than a wedge (like our hatchback is). For instance, stick your hand outside your window 6" from where the glass would be (like where your side mirrors are for instance) and then stick your hand out 2-3 feet. If you could do it further back on the car, the more it would make drag. The faster you go, the bigger the difference but you get the idea. This is because the air does NOT hug the curvatures and surfaces as you suggest. Air hits a frontal area (high pressure) and there is a certain amount of "deflection". This is normal.

This is one reason you need a BIG rear spoiler (v.s. a smaller one) to catch and apply downforce to the rear of an automobile.

Suspension heigth also plays a very important role as well. Keeping air from going UNDER the car adds more down force and makes it stick better at high speeds. Doing so also creates more frontal area and LESS pressure closest to the actual surface of the car. That's why you have a fan shroud on most cars (it creates more frontal area to catch more air and literally force it through the radiator which of course then there is a heat transfer/exchange). Just ask any NASCAR driver how much of an effect taping the front air inlet areas and how it effects aerodynamics, downforce and drag.

Again, if you study the slipstream of a car in a windtunnel, you'll see that the more aerodynamic the body shape, the less frontal area there is and the less coefficient of drag you will have. This also effectively moves the slipstream closer to the body (as it isn't pushed outward as much as a squared shape would). On most production cars (and especially hatchback designs), you will see that most of the air moving over the body of the vehicle is several inches to a foot or more from the actual surface depending on the area of the car. These "high slipstream" areas are what are called low pressure areas below the "line". If my vents were closer to the front of the hood (where a high pressure area exists at higher road speeds) or at the second half of the windshield/roofline for instance, then there would definately be some issues, especially if air was able to be dragged from under the car and through the vents.

Usually cars with good coefficient of drag, utilize lower body panels to funnel the air BACK smoothly instead of up through the engine bay and over suspension parts which create significant drag. As you get to the rear of the car, you want to straighten the air as much as possible as otherwise it will cause turbulence, LIFT and subsequent drag. A properly designed under spoiler, rear apron or diffuser is will straighten the flow of air as it exits the underbody.

NACA ducts are better "air scoops" in low pressure areas and that's why they are used on racecars in odd areas. NACA function involves a venturi effect which I'm not going to discuss in this thread (it would be TMI at this point).. hehehe. Some "production" cars (Viper and Z06 Corvettes) use NACA style ducts for ram-air but they are mostly for styling than actual function (they look sweet)
03-08-2008 01:07 PM
CaysE How did you test the cowl induction? It seems unlikely as they are still a good distance from the windshield.

The vacuum is always there at speed... air is flowing over the hood and vents, pulling air out of the vents. With the vents facing the opposite direction, you'll have less resistance into the intake. As you said, it's not sealed though, so you're likely just getting a portion of air from the vents and the rest from whatever openings are around the airbox.
03-08-2008 01:00 PM
pelotonracer2
Quote:
Originally Posted by CaysE View Post
Do you have the airbox sealed up against the hood for that driverside vent? If so, you'll want to turn the vent around. At speed, the vacuum on the hood will want to pull air out of the vents, and the intake will be fighting it to pull air in.


It's not a perfect seal like a ram-air should be, so there shouldn't be a vacuum situation. There are small pressure relief holes at the bottom rear part of the box so there is no chance of the incoming air competing with outgoing air. Also, the vents actually do get some cowl induction benefit being so far up on the hood (I've tested this already... air is definatelty only going in, not out). Besides that, the filter actually sits only 2mm from the vent so it pulls in plenty of air. Needless to say it is *VERY* effective. Soon though I will be going to a completely sealed box with a ram-air scoop. The vents will just let hot air out on both sides then. Also, the direction of the vents wouldn't make much difference anyways as only the front one has any frontal area to actually pick up air.
03-08-2008 11:42 AM
CaysE
Quote:
Originally Posted by pelotonracer2 View Post
I removed the header heat shield and cat heat shield. I may end up cutting the stock heavy bracket out since I'll never use the stock cat again.

There are more pics of my SRI posted somewhere on this forum, minus the box. I've added one below but it doesn't show the box that the filter sits in. The hood is vented and as you can see it is functional. I'll take some more pictures after all this SNOW goes away...
Do you have the airbox sealed up against the hood for that driverside vent? If so, you'll want to turn the vent around. At speed, the vacuum on the hood will want to pull air out of the vents, and the intake will be fighting it to pull air in.
03-08-2008 10:49 AM
MVent How do you determine what direction the plugs are facing?
03-07-2008 07:39 PM
SVT-SAGE Here's a vid of the CPS mods location.
http://videos.streetfire.net/video/H...-SVT_22150.htm
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