Cowl Plenum VS. Cowl Induction Air Cleaner Systems

[camaroman1969]

Hello everyone.  Hope everybody had a nice Christmas.  Question to anyone who has knowledge on the rare "cowl plenum" air cleaner set-up.  Is it work the cost and time to convert to "cowl plenum" system?  I've been told this system brings in more cooler air to carb verses the popular cowl induction set-up.  Would appreciate anyones thought or experience with this type of air delivery to a 396 big block 69 Camaro.  Looking to squeeze every bit of horsepower from this engine.  Thanks

[68camaroz28]

If your trans-am racing or something similar possibly but according to Jerry McNeish his 67 Z/28 NHRA Stocker drag car runs the same times with or without the air cowl plenum set-up.

[BULLITT65]

Interesting. I thought I had read the ZL-2 hood had about a 5+ add on horsepower, but a 100 plus adder in the cool points in the 70's

[bcmiller]

Not really fair to use Jerry's car for determination. His car is far from "stock".

There will be some small gain, but hard to quantify. And it depends on the difference in temperature under the hood compared to the outside air.

[camaroman1969]

Thanks guys for your input. It all sounds logical to me.  This guy told me the big gain on these "plenums" is the larger cowl ducting diameter going to the Holley carb, verses the tapered snorkel on the ZL-2 set-up.  More cool air, more power they say.  Keep the comments coming.  I'm really trying to get a better understanding of these two so different systems.  Thanks again.....

[BULLITT65]

tapered snorkel?
Not sure that makes a difference but the plenum is large and feeds into a good size opening on the side of the air cleaner, Vs the hood duct feeding the top of the air cleaner. May be slight differences with air flow from the top of the cowl vs through the grate and cowl.
If you could run each set up on the same car, I am not sure it would be a huge difference. So my thinking goes back to GM. Was it cheaper for them to re-tool and develop the hood to be functional, vs more work on the line to have the plenum functional? Did marketing play more of a roll than the function of the hood?
Did the ZL-2 set really perform better on Z cars as well as everything else so it was available on SS paccar, COPO?

One of my questions would be why didn't SS and other performance models of 67-68 get the cowl plenum option? (I know 66 Chevelle SS-396 cars had it as a little known option as well)

[68camaroz28]

Not really fair to use Jerry's car for determination. His car is far from "stock".

I would hope most people know a NHRA Stocker drag car does do not have a stock motor. The point was he's hitting some good trap speeds and it is not making a difference in his situation which I thought might be interesting to note. Jon might have some good insight on comparisons from the Trans-Am engines and the two air induction set-ups. I believe the main reason 67/68 Z/28's had the option was purely due to Trans-Am.

[cook_dw]

Back to the original question...  You could use the $1-3K in other areas than an air cleaner to get power out of a big block...  Its not worth the time, money or effort.. 

[BULLITT65]

I would concur. I would think you could spend your money in other paces to gain more HP, or drop weight.

[hgger69]

You should never underestimate the coolness factor with a cowl plenum air system, and that adds a bunch of hp:s, guys! 

[bcmiller]

Not really fair to use Jerry's car for determination. His car is far from "stock".

I would hope most people know a NHRA Stocker drag car does do not have a stock motor. The point was he's hitting some good trap speeds and it is not making a difference in his situation which I thought might be interesting to note. Jon might have some good insight on comparisons from the Trans-Am engines and the two air induction set-ups. I believe the main reason 67/68 Z/28's had the option was purely due to Trans-Am.

My point on Jerry's car is that everything else (heads, cam, rods, oil, etc.) is already optimized for making horsepower. It gets plenty of air when the front wheels are a foot off the ground.

Making the heads flow well and getting compression as high as you can - based on the fuel you are going to run - are the best ways to spend your money if you want to make gains with the lowest cost per horsepower.

[rare396bronze]

Easer to put a cowl hood than cut a hole in firewall on a 69 Camaro that a cowl plenum that was not offered that year. Met a lot people that messed up a good car that years later said why did I do that.

[JoeC]

Cowl Plenum and Cowl Induction Air Cleaner Systems use the high pressure area at the windshield to provide a very small "supercharger" effect but more then that is the effect of air temperature.
Cooler intake air makes more power.
Chevy used cool air intake systems on many cars and trucks in the 1980s with the Q-jet 4BB and other induction systems. It was just an air hose from the grill to the air cleaner but it works.

here is some info found online I thought was pretty good.........


Air scoops of various kinds are a common feature on cars with performance pretensions. Many scoops are purely cosmetic, but those that aren’t typically serve one or more of the following functions:

COLD AIR INTAKE

Burning fuel requires oxygen. Unless an engine carries its own oxygen supply (as with a rocket engine), that oxygen must come from the surrounding air. The amount of oxygen available to burn — and thus the engine’s maximum power output — depends on ambient temperature and local static atmospheric pressure. As a rule, cooler, denser air will yield more power while warmer, thinner air (such as on a hot day or at high altitudes) yields less.

The engine compartment of the average automobile tends to be very warm indeed. The normal operating temperature of the typical water-cooled passenger car engine is well above 160 degrees Fahrenheit (71°C) and an air-cooled engine may be considerably hotter. The heat radiated by a running engine quickly heats the air around it. Since most automotive engine compartments are enclosed and rather cramped, with few opportunities for the heat to escape, the air in the engine compartment is usually significantly hotter than the outside air. If the engine draws its intake air from under the hood, the high temperatures will reduce the density of the intake charge and thus reduce the engine’s net power output.

An obvious solution to this problem is to add a cold air intake channel that allows the engine to draw its intake air from the cooler, denser air outside the engine compartment. An effective cold air system can counteract much of the power loss caused by high under-hood temperatures, potentially improving engine output by 5% or more.

Simply cutting a hole in the hood does not a functional cold air scoop make. To be effective, a cold air intake (a) must be located in a high-pressure area of the hood; (b) must be designed in such a way that it actually allows outside air to pass through the inlet; and (c) must have a tightly sealed connection to the air cleaner and intake manifold so that the engine will breathe through the scoop rather than drawing some of its air from under the hood. The distance from the scoop to the air cleaner must also be as short as possible — the greater the distance the incoming air has to travel, the hotter it will get, both through friction and through absorbing engine compartment heat. A poorly designed or badly placed cold air scoop can be worse than useless, costing power by restricting the flow of engine air.

[X33RS]

Paul, a good friend of mine, bought his 66 L78 chevelle brand new with a cowl plenum setup, he still owns the car today, unrestored with about 30,000 original miles on it. 
  I've always thought the setups were cool looking, and neat to see on 67-68 Camaros, but I've always had mixed feelings putting the setup on any car after 68.  Since 69 Camaros had the benefit of the ZL2 option the cowl plenum deal sort of went away as far as production cars are concerned.
  My 69Z being an original flat hood car, the thought of a cowl plenum setup is appealing, but if I wanted a fresh air setup, a ZL2 hood would go on it.  I couldn't bear to hack up the firewall.  If it were a 67 or 68 however, I'd have a cowl plenum on it no question, whether it added power or not.

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)  I've raced the car in PS form for years, and found that if I blocked or even restricted the fresh air the car slowed down nearly 2 tenths and would lose about 1 mph.   I found there is A LOT of air being forced in the air cleaner base when at speed.  It would blow my PCV filter out of it's plastic container and would be laying in the back of the air cleaner base after a pass.  I ended up wire tying the little filter into the base so it would stay put.   I even tried removing the air filter once looking for more ET, and that absolutely killed the performance, slowing the car more than 3 tenths.  My theory was that so much air turbulence was being forced in there it really screwed up the carbs  metering ability, the filter helped to slow and straighten the air.  Replaced filter and the car went right back to normal ET and MPH.   This is just to provide one example of a fresh air setup on a bone stock 400/335 hp car that runs low 13's at 106 mph on stock F70-14's.   On this particular car, with this particular setup, it's effective. 
    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.

[69Z28-RS]

A 70 Formula 400?  I think I liked those even better than the '70 TA cars (which were the nicest of the TA cars IMO)... 

Is this a section you could post a photo?? 

[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.