Cowl Plenum VS. Cowl Induction Air Cleaner Systems

[bcmiller]

I would like to see a pic or two as well.

[X33RS]

Posting pics here has been hit or miss for me but if Gary doesn't mind, (If I still have his email I'll try to find it), I can send a picture his way if he wants to post it that's fine by me.

[X33RS]

Sent you an email Gary, not sure it's yours but I think it is.  If you don't have an email from me shoot me a pm and I'll get this to you.  Thanks.

[69Z28-RS]

I got your photos Larry!   Thanks the car looks great.. 

and here they are after a bit of 'resizing' to fit requirements...

[X33RS]

Oh good I had the right guy then 

Thanks for doing that Gary.

[hgger69]

My 70 formula is a ram air car, however it grabs air front and center just over the bumper of the car.  Said to be one of the more effective setups from the muscle car era, but seldom seen today (293 made in 1970)  On a setup that grabs fresh air at the windshield, it might work, but I'm sure it won't be as effective as the design on my Pontiac.  I'm going to test the theory on my Z though, at the track with the ZL2 setup and without.  I'm predicting I won't see the differences I see with my Pontiac.  Especially since the Z is such a small engine anyway.

That's interesting cause I always read that's it the complete opposite due to the stagnation pressure that you get when having a scoop pointing towards the front...unless you have something that sucks air in like a blower or turbo!? Found this interesting regarding this issue:

The basic aerodynamics is that you cannot increase air pressure above static air pressure until you start to approach compressible fluid flow - near the speed of sound.

A "Ram Air" scoop (or any other scoop) can increase the volume of the air flow if it is otherwise restricted, but it cannot increase air pressure to your engine, or other wise provide a "supercharging" effect.

Some basics:

Static (non-moving) air pressure at seal level under standard atmospheric conditions (a "standardized" temperature and humidity) is about 14.7 PSI. Without otherwise compressing air, that's the most you can get. Period. As air moves over a surface, it often speeds up - an example is an airfoil (wing) of an airplane, or more simply, your car. Ignoring altitude, temperature and humidity, typically, moving air is of lower pressure than 14.7 PSI. Non-moving (static) air can be no higher than 14.7.

As air moves over a surface, it creates a boundary layer. The air on the surface is at zero velocity relative to the surface (or your car for instance). The air some (very) small distance above the surface is moving rapidly relative to the surface of your car (i.e. to you the air feels that it is moving at 70 MPH when you are driving at 70 MPH). This air is lower than 14.7 PSI, relative to your car, because it is moving (to get out of the way of your car). The air flow between the top and the bottom of the boundary layer is moving at speeds in between the two, proportional to their distance from the surface...

Some areas of your car have relatively static non-moving air - the tip of your bumper for instance. Other parts of your car have rapidly moving air (the top of your hood, the sides of the door, etc.), that are at a relatively low pressure (ever open the side window a bit while driving fast and feel the air rushing out of the car at the opening? It's the "high pressure" air rushing into the "low pressure" air outside of the car.

Sticking a scoop or "Ram Air" intake on a car cannot provide more than static air pressure. Air entering the scoop, at best, can "slow down" to low velocity and relatively static air pressure, but that's it. The scoop can increase the volume of air that can be "sucked" into the engine, but it can't increase the pressure unless you are approaching the speed of sound - and I doubt that many of us have to worry about that. If the car has a very restrictive air source, the intake can be "starved" for air, and not getting enough. Adding a scoop to provide more available air can help things, but so would putting a bigger less restrictive air box on the end of the intake (that what the cone filters are supposed to be doing). In either case, 14.7 PSI is the maximum air pressure that is available. Period.

Besides the front of the bumper, a very good source of static (relatively high) air pressure is the base of the windshield. As air flows over the hood of your car, the air "smoothes" itself out, and leaves an area at the base of the windshield that is relatively static - non-moving. If you arms are long enough (and the road empty enough), try an experiment. Reach your hand around the side (or over the top) of the windshield while traveling at say 60 MPH. As you reach around the windshield, you will feel a strong "wind". But as you get your hand to the area near where the wipers park, it will be relatively calm. This is a "high" pressure zone. It's where most every car made has the intake for the cars ventilation system - "high" pressure, slow moving air that can pass thru to the cars interior and out the windows/vents. This also makes a good place to suck in the air for the engine's intake. Some will remember back in the early '70s when Chevy used to sell cars with "Cowl Induction" - it was a backwards facing scoop on the hood that pulled "high" pressure air from the base of the windshield. They worked quite well. Some company (I don't remember which) sells a set up that feeds air from the Miata cowl to the air box. It should also work well, but it still can only produce 14.7 PSI. And the area under the hood near the air box on a Miata is most likely as close to 14.7 PSI as possible.

However, the real advantage of the Cowl Induction, Ram Air, or just a plain air scoop, is that it feeds cooler air to the engine. The air box in a Miata pulls air from under the hood that has been warmed - heated by the radiator, and the exhaust - and warm air is not as dense as cold air. With cold air, more actual air gets into the cylinders, which in turn makes more power. Cold air is good. My old Honda used to pull air from behind the head light. My '96 Ford Ranger pulls air from in front of the right front wheel liner, behind the head lights - nice cool air in both cases, and protected from road debris and water...

If a Ram Air, Scoop, or cowl induction makes any more power for an engine, it is simply that it is allowing cooler air into the engine. Period. Or to be generous, it might be allowing a less restrictive air flow to get to the intake. But none of them can add more pressure above static pressure that is what's all around us.

Then there is subject of NACA Ducts ("scoops"). These are nothing new, and have been around for many, many years on aircraft (NACA was the predecessor to NASA). NACA ducts are designed/shaped such that they bleed off the boundary layer into the ducting. The air is diverted into the duct at relatively static pressures, and a new boundary layer forms on the rear of the duct. It allows air to be drawn in, without any noticeable increase in drag - that's why they are used on aircraft (and high end race cars). For most cars, they are just for looks...

Tim Mullen - Yea, I studied (and used) all this aerodynamics (subsonic and supersonic), fluid flow, heat transfer, etc....

[JKZ27]

I think its relative. Increasing volume to maintain pressure where it's consumed. Draw air from the area with the highest pressure, not necessarily compressed.....or something like that.?

Fabulous looking car BTW! 70' T/A, Formula may be in my top 5 best looking ever.

[X33RS]

Obviously a lot of testing was done in this area eons ago.  From what I've found and read, there were 2 areas that worked fairly well.  One of course was the base of the windshield.  What was found is that it had to be right at the base, such as the ZL2 or cowl plenum.  The Cowl Induction on chevelles with the flap about 12 inches away from the base of the windshield was not all that effective because it's out in the "dead zone" of air.  I can attest to that because I've played with this on my chevelle as well.

What was also found is that forward facing scoops in the middle of the hood as so many muscle cars had were not that effective.  That's basically a dead zone when the scoops are down low.  However what they also found was that raising the scoop off the base of the hood one or two inches worked.

On the front nose deal, when the scoops are low they don't work that well.  (Think 69 TA)  So in 1970 GM took that another step on the Formula and raised those scoops off the surface about 2 inches and also pushed the scoops a little further forward.  Now they had a system that worked effectively.  I can attest to that playing with this car at the track.  Even driving this car in very cold weather you can see the scoops of the hood frost over while moving through the air.  Pretty neat actually.  There is a ton of air being pushed through those things and this car does in fact see a slight change in mph and ET when I don't use it.  There was an article written a couple decades ago I have somewhere that stated these were one of the more effective ram air setups on a production vehicle.  From what I've played with I tend to lean towards that statement.   Sadly they were overshadowed by the Trans Am package and not many were made.
  I'm planning to load the Z in the trailer, take a flat hood and stock open element with me, and swap the ZL2 setup just to see what happens.  But I'm betting that being a very small air pump under there I probably won't see any changes in how the car runs.

[X33RS]

Thanks John,  I'm a 2nd gen fan as well.    TA's are very nice too, but I've always been a more subtle Formula guy.  With the Ram Air engine, you have an identical TA drivetrain in disguise.  Supposed to be lighter too.  However I'm not completely confident in those claims, since mine scales at 3690 without a driver.  Still seems kind of heavy to me.  You would think a fiberglass hood would be lighter but that thing sure seems to weigh a ton, lol.

[KurtS]

Tim uses "high pressure" in quotes because it isn't. The sides of the doors aren't seeing low pressure, they are seeing high velocity.
Putting your hand at the base of the windshield is showing (lack of) air velocity. Cabin air intake is there because it's clean air (not engine air).
If you think there's a significant pressure difference, just put a gauge with a rubber hose - place the end of the hose whereever and measure it. It won't be much...

[camaroman1969]

Kurt, Hi!  I started this discussion on the "cowl plenum" set-up vs. "the cowl induction" set-up.  You've been around these cars along time and probably have seen it all, talked to different guys with success stories and there failures.  What is your take on the "cowl plenum" set-up on a 1969 Camaro SS 396?  Would this move be a no-no?  Or would it be a way to create some cheap HP?  Please share your thought with me on this cause I'm on the fence about doing this change to my 69.  I currently have the ZL-2 set-up  Thanks Kurt!    Don

[KurtS]

I'm not the expert, but my opinion is there is more hp to be gained from other details than air intake.
And there is no notable hp difference between ZL2 and cowl plenum, especially for a street car.

[70z28lt1]

Since most of this was born from the Trans Am racing series it may also be possible that the pit stop serviceability with a hood mounted (i.e. cowl induction) induction was better with hood setup as compared to the firewall plenum setup.  Opening the hood and then staring directly at the carb is faster than fussing with removing the air cleaner, plenum etc.  I believe the real Trans Am racers didn't use all of the cowl air cleaner setup.  Maybe just a baseplate that sealed to the hood. 
The best as I remember is the Corvette L88 hood where the air cleaner was in the hood.

[camaroman1969]

I want to thank everyone who replied to my ad request.  A lot of good info came out that I wasn't aware of.  I just enjoy posting a issue or question here because this site is where all the "Camaro Gurus"  hang.  Again, thank to all.