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Author Topic: Cowl plenum air cleaner, cold air induction, etc  (Read 7444 times)
Jon Mello
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« on: August 12, 2012, 05:31:12 PM »

An NOS cowl plenum duct, real GM part, fresh from the box, courtesy of Jim Webb.
This duct attaches to a hole cut in the RH top of the firewall either by the dealership
or car owner and directs cold air from outside the car at the base of the windshield
to the metal cowl plenum air cleaner which mates to the duct via a rubber sleeve.
The duct is made of injection molded plastic, not fiberglass, and is smooth inside and
out. There is a top half and a bottom half which are glued together and 6 screws hold
it to the firewall. Many thanks to Jim Webb for sharing this very rare part of his with us!

























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Jon Mello
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« Reply #1 on: August 13, 2012, 10:48:11 AM »

Here are the cowl plenum duct installation instructions. Two styles have been seen.
A single sheet or a two-sided tag, both with the same information. (Jon Mello Collection)






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Jon Mello
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« Reply #2 on: August 27, 2012, 12:27:23 AM »

Here's an NOS '68 cowl plenum air cleaner from the collection of Jim Webb. The '68 version of this
air cleaner has an indentation at the front of the housing to clear the smog pump used that year.
The indentation at the back is to provide clearance for the distributor. Take the time to notice how
original versions of these air cleaners were painted. The coverage is hit-and-miss on the inside and
even in a few small areas on the outside. Also note that the rubber boot is stapled to the air cleaner
before it is painted so the rubber actually got painted along with the air cleaner itself. The stapling
of the boot to the metal is done by hand and varies from unit to unit. Some have been seen with
staples more neatly aligned than as seen on this example. Notice the taped seam on the bottomside
of the rubber sleeve. These cowl plenum air cleaners never came with any engine ID decals or decals
of any kind on them either as a service replacement part or as delivered with the car.





















The four square openings indicate a small block Camaro air cleaner. Five square openings are found on the
cowl plenum air cleaners which were used on big blocks, typically Chevelles and full-size cars.




Here are some of the labels on the original box that this air cleaner was packaged in.




« Last Edit: August 28, 2012, 10:33:41 PM by Jon Mello » Logged

Jon Mello
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« Reply #3 on: August 27, 2012, 12:19:30 PM »

Here's the '67 style cowl plenum air cleaner as seen on the special order paint '67 Z-28 RS owned by Paul Lasiter.
Paul's car was built at the Van Nuys plant and sold new in California which is why it has the K19 smog pump on it.
As you can see in the photos, the size and design of the smog pump still allows for plenty of clearance between
the pump and the air cleaner housing. As such, the '67 air cleaner (p/n 6424495) did not have the the indentation
at the front for clearance as seen in the previous photos of Jim Webb's '68 air cleaner. It should be noted that the
air cleaner on Paul's car has had the rubber sleeve replaced which is why the staples and rubber are a little bit
different in appearance than what is seen on Jim Webb's NOS unit.












Here's an NOS '67 air cleaner still in the box. This is from a car that was ordered new with the cowl plenum
air cleaner and the factory headers, both of which were put in the trunk and left to be installed by either the
dealership or the new owner. This car was built on the last day of '67 production. (Jackie Haney Collection)
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Jon Mello
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« Reply #4 on: August 27, 2012, 12:31:12 PM »

Missing from the previous air cleaner photos is the flame arrestor screen which sits atop the tube inside the air
cleaner housing. The tube and screen are part of the positive crankcase ventilation system. (Jon Mello Collection)



« Last Edit: August 27, 2012, 03:36:41 PM by Jon Mello » Logged

Jon Mello
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« Reply #5 on: August 27, 2012, 12:53:46 PM »

The original '67-'68 AC air filter has a brownish colored paper in it which makes it appear dirty, but it isn't. The
AC part number is embossed into the rubber on the top of the filter and it looks a bit different than later years.
Note that the screen mesh is square but at a 45 degree angle and has little balls of solder where each bit of wire
intersects. The AC part number for the filter is A226CW but there are subtle changes as the years progress.






The '69 version of this A226CW still has the brownish paper and the wording has changed on the rubber top.
It now has an expanded metal mesh which is no longer square but is essentially a diamond shape.



By 1971, there was white paper in the filter and the part number was simply ink stamped in white on the rubber.




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Jon Mello
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« Reply #6 on: August 27, 2012, 05:28:50 PM »

Outstanding photos and detail on these great parts, Jon.  Thanks a bunch for posting. 

Do you have any individual photos of the six original screws that came with these kits, and whether or not they were used elsewhere, for mounting anything else on Camaros... Or any other GM cars?

Thanks again.

-Chad
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« Reply #7 on: August 27, 2012, 10:07:51 PM »

Fantastic post and great documentation Jon!
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« Reply #8 on: August 27, 2012, 10:25:20 PM »

Neat VIP label too!
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Jon Mello
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« Reply #9 on: August 27, 2012, 10:59:28 PM »

Thanks for the compliments! I asked Jim Webb about the screws. He said he has had three NOS plastic ducts new in the box and none have come with screws. That may have been left up to the owner or dealership to supply. I can say that what is seen on Paul Lasiter's car is the most common style of screw that I have seen to mount the duct. Whether that screw is 3/8" long like the screws used to attach the plastic wire gutter to the top of the firewall or whether the screws are 1/2" long, I will have a better idea tomorrow when Paul removes one to have a look.
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« Reply #10 on: August 28, 2012, 09:08:12 PM »

Paul Lasiter removed one of the screws from his duct and it is 1/2" long and the head of the screw takes a 5/16" socket.



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Jon Mello
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« Reply #11 on: August 28, 2012, 10:02:07 PM »

Very nice.  Thanks for the follow up, Jon.

And thanks to Mr. Lasiter for sharing the detail.

-Chad

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Jon Mello
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« Reply #12 on: August 29, 2012, 08:31:13 AM »

You're welcome, Chad. I'm glad this is helpful to you and others as well. It's great to have this forum as a resource to be able to do this.
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Jon Mello
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« Reply #13 on: August 29, 2012, 12:52:12 PM »

Years ago when I first installed factory heater delete plates on the firewall I also went looking for the "correct" screws. The 67 & 68 Camaro assembly manual showed a part number. I searched the rest of the pages and found the same number listed as holding the resister on the top of the heater box that is under the dash. That was easy since I had just removed it. Naturally they were easy to find but not all were identical as to length and head markings. They were also all 5/16 headed self tapping style screws as I recall.
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Robert Lodewyk
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« Reply #14 on: August 29, 2012, 05:13:50 PM »

Great additional information, Robert.  Thank you very much.

-Chad
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Jon Mello
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« Reply #15 on: September 24, 2012, 12:02:35 AM »

I had Jim Webb check the cowl plenum duct screws in his red '67 rally sport Z-28. His screws look pretty
much the same as Paul Lasiter's although Paul's have four tiny bumps in a line across the head of the screw
and it looks as though Jim's might only have two bumps. There are no other markings on the screws.



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« Reply #16 on: June 16, 2013, 12:36:35 AM »

The diameter of an original, authentic cowl plenum air cleaner lid is 16.75".
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« Reply #17 on: June 16, 2013, 12:39:11 AM »

The cowl plenum air cleaner base is 16" across, from the outer edge of the top lip on one side to the same point on the other side.
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« Reply #18 on: June 16, 2013, 12:40:23 AM »

Here's a look at the underside of the correct lid.
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« Reply #19 on: June 16, 2013, 12:44:44 AM »

Here's a closer look at the groove that the top lip of the air cleaner base rides in. There is a very similar
looking lid that you will see at swap meets that is close to the same size (around 17" dia) that has rubber
in the groove. Don't be fooled into buying it. It won't fit on an original GM cowl plenum air cleaner base.
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Jon Mello
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« Reply #20 on: June 29, 2013, 07:16:26 PM »

this type air cleaner was originally supplied for the Z-11 impala drag cars.
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new Camaros owned 68 and 69 Z-28. new Corvettes owned 59,62,63,64,65,66,97,99 02,05 and 08.
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« Reply #21 on: June 30, 2013, 07:27:01 AM »

this type air cleaner was originally supplied for the Z-11 impala drag cars.
also was used on the 63 NASCAR impalas with the "mystery" engine at daytona
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« Reply #22 on: June 30, 2013, 05:06:36 PM »

Yes, you're right. 1963 was the first use by Chevrolet for this style of air induction. The W-series (409 style) 427 Z-11
engine used a modestly different looking cowl plenum air cleaner than the MKII "Mystery Motor" Impalas used at Daytona.
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« Reply #23 on: June 30, 2013, 11:20:48 PM »

Has anyone tested the power gain from an open element filter to a cowl induction setup? Just curious if it actually was a benefit.
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Jon Mello
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« Reply #24 on: July 01, 2013, 12:09:10 AM »

There's no doubt it's a benefit but I haven't seen published figures that show what the horsepower gain was.
There would be more benefit on a course with longer straights where a car sustained higher speeds for longer
time (Daytona and Sebring) than there would be for shorter tracks with slower speeds.
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« Reply #25 on: July 01, 2013, 09:48:18 PM »

If it didn't work better, they wouldn't still be using it in NASCAR.
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« Reply #26 on: July 02, 2013, 02:20:36 PM »

Has anyone tested the power gain from an open element filter to a cowl induction setup? Just curious if it actually was a benefit.
you gain 1% in HP for every 10 degrees you lower the intake air temp
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« Reply #27 on: July 02, 2013, 02:24:05 PM »

also those systems cause air turbulence around the carb air bleeds unless you use the L-88 screen on top of the carb to straighten out the air flow if you run without a air filter
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« Reply #28 on: July 02, 2013, 09:00:09 PM »

also those systems cause air turbulence around the carb air bleeds unless you use the L-88 screen on top of the carb to straighten out the air flow if you run without a air filter

That's a good tip. Thanks.
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Jon Mello
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« Reply #29 on: September 01, 2013, 01:15:28 AM »

If someone is needing a cowl plenum air cleaner for their Trans-Am Camaro or street car, there's one for sale on ebay right now with a buy-it-now of $3,000.

http://www.ebay.com/itm/1967-1968-Z28-Camaro-Trans-Am-Cowl-Plenum-Air-Cleaner-SCCA-Smokey-Yunick-/121168600120
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« Reply #30 on: September 04, 2013, 02:35:28 PM »

If someone is needing a cowl plenum air cleaner for their Trans-Am Camaro or street car, there's one for sale on ebay right now with a buy-it-now of $3,000.

http://www.ebay.com/itm/1967-1968-Z28-Camaro-Trans-Am-Cowl-Plenum-Air-Cleaner-SCCA-Smokey-Yunick-/121168600120

Sold 9-4-13.

Paul
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Jon Mello
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« Reply #31 on: September 04, 2013, 07:18:47 PM »

Looks like a best offer was accepted, not even the full $3,000.
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« Reply #32 on: September 07, 2013, 09:23:48 PM »

Is the chrome lid correct? My repop has a black lid which I thought was correct.
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Jon Mello
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« Reply #33 on: September 07, 2013, 11:22:53 PM »

As supplied by GM, the lid was black. I have seen maybe 4 or 5 where an owner has chosen to chrome plate the lid.
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