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??