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Alan,
"Still needs drain hoses though or plenums would drain into the rear compartment."
There are no drain hoses on the convertibles. Just the round holes open to see the ground. No way for any water to get from them into the rear storage area on the convertible.
I'm pretty sure the plenum assembly is back farther under the decklid on the convertibles, making extra room for the top decklid and then top stowage?? Just seems to me, the rear vents on the coupe are up closer toward the back window and further back on the convertible. JMO!
Tooch
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I just editted to add some pics of my 68 vert when I saw your post. They are the same as Alans. Maybe not in later years.
Im trying to uploaD more pics but the cloud isnt cooperative
Last edited by Rescue Rogers; 03-19-2017 at 09:07 AM.
Hi tooch,
That's true for cars (what years?) in which the plenum is on the EXTERIOR side of the rear bulkhead.
My photo, GB's drawing, an RR's photos shows a convertible with the drain tubes, and it clear that the plenums would drain into the interior if the tubes weren't there.
Look closely.
Regards,
Alan
Hi RR,
It IS a bit of a hassle to replace the drain hose.
It extends through the round hole and the hoses rubber flange is held in place by that 'diverter' you show in your photos.
So the diverter needs to come off to replace the hose.
Not an easy job.
Regards,
Alan
Hi tooch,
That's true for cars (what years?) in which the plenum is on the EXTERIOR side of the rear bulkhead.
When did the bulkhead move?
Yes, I believe that is what happened, they moved the bulkhead in 71.
I think the 68-70 bulkhead is straight across, like in Rescue Ranger's picture above, but not the 71-75. Maybe some convertible owner can confirm this, but I think the 71-75 bulkhead was pulled forward, to be flush with the position of the deck hinge springs. I'm having a hard time trying to describe this, but basically I think the 68-70 deck springs are in front of the rear bulkhead, while 71-75 deck springs "nestle" in alongside of it?
As I said in my earlier post, 68-70 used the drain hoses, while on 71-75's, water drains from the plenum holes, straight to the ground. Without the hoses in place, water will drain into the rear storage compartment of 68-70 convertibles.
Below is the rear bulkhead and plenum area as seen in the 74 AIM. Unfortunately, I could not find a similar illustration in either my 69 or 70 AIM's.
I have 2, 69 convertibles they both have the drain plenum enclosed with drain tubes.
1 on them is factory A/C car, the other is Not, both with Enclosed rear drain plenum.
Hi tooch,
That's true for cars (what years?) in which the plenum is on the EXTERIOR side of the rear bulkhead.
My photo, GB's drawing, an RR's photos shows a convertible with the drain tubes, and it clear that the plenums would drain into the interior if the tubes weren't there.
Look closely.
Regards,
Alan
When did the bulkhead move?
Hi Alan,
I hope these pictures from my 73 convertible offer up some clarification. I tried to get a few pics from my rear deck hinge areas and rear bulkhead above my carpet. Clearly there isn't anything that looks like a drain tube etc. I also took a couple lying on my back under the car, up thru to show where the open drain hole is located. You can also see how close the plenum box runs parallel to the gas tank.. I've also included some pics from the top down with my vents removed to show the bottom of the box, and drain hole looking straight down past the gas tank. The holes on the 73 convertible are not over the spare tire carrier.
Since we know what the cars up to your 71 are like, and what the 73 convertible has, it would be nice to hear from a 72 owner, then we could determine for sure when the plenum moved back. 72' or 73'?
Hope these help!
Tooch
my 73 convertible............... I tried to get a few pics from my rear deck hinge areas and rear bulkhead above my carpet. Clearly there isn't anything that looks like a drain tube etc.
Originally Posted by Rescue Rogers
Compare the position of the rear bulkhead and carpet on your car, to the position of the rear bulkhead and carpet, in Rescue Rogers picture. His bulkhead appears to be set back under the rear deck, while yours is almost flush with the deck lid opening. Look at the relationship between his bulkhead and the deck lid hinge and springs, compared to yours. His deck lid hinge and springs are forward of the rear bulkhead, while yours are flush with the bulkhead. You can also clearly see the drain hose, on the right side of Rescue Rogers' picture.
The curved convertible drain hoses (coupes use a different, straight hose) were GM part number 3928849. The convertible drain shields are part number 3938107. The Corvette Parts Books only show the hoses and drain shields for use in 68-70.
As I said in my earlier post, I'm pretty sure GM moved the rear bulkhead forward in 71, effectively moving the rear vent plenum out from inside of the rear storage compartment, and eliminating the need for the drain hoses. If the plenum was still positioned inside the rear compartment in 71, then the drain hoses would still have been needed, and the parts books would have shown them as 68-71, not 68-70.
Originally Posted by Tooch1
Since we know what the cars up to your 71 are like, and what the 73 convertible has, it would be nice to hear from a 72 owner, then we could determine for sure when the plenum moved back. 72' or 73'?
Hope these help!
Tooch
Alan's car is a coupe, and the drains and drain hoses, are completely different on a coupe. The picture in post number 18 by Alan, is not his car, and he does not mention what year car it is. Since the picture shows a car with the drains, I'm pretty sure it's either a 68, a 69 or a 70.
On 1968 - 1970, the rear bulkhead was straight across the back. It was only about an inch in front of the gas tank. This maximized luggage space inside.
The vents are forward of the bulkhead. From outside, you should see a couple of deflector plates in there and you will see the hole in the outside end for the drain.
About this same time, GM was battling a couple of lawsuits on passengers getting burned from gas tanks shoved through and dousing the passengers. This was very quiet, and most have never heard of that. It was in a couple of magazines, questioning Chevrolet's design of placing the tank in the same location as the Sting Ray after several accidents had created horrible deaths.
In 1971, the bulkhead was redesigned to give a couple of inches for the tank to move and not penetrate the passenger compartment. Unfortunately, the spring position for the deck lid could not move, so the bulkhead was no longer flat, but had two setbacks for the spring perches and hinges.
Now the grill openings are behind the bulkhead, and a plenum inserted to get the air out of the inside. A different vent system was set up, partially due to the change in location and partial because the '68 system really didn't work that well. The air flow is very little for all the work involved. This is why they finally disappeared in 1976 even though all cars weren't AC.
I have to agree with the others. The "overspray" on the spare tub is not from the vents. Someone either painted it or were very sloppy when painting the lower quarter panels.
Yes, I believe that is what happened, they moved the bulkhead in 71.
I think the 68-70 bulkhead is straight across, like in Rescue Ranger's picture above, but not the 71-75. Maybe some convertible owner can confirm this, but I think the 71-75 bulkhead was pulled forward, to be flush with the position of the deck hinge springs. I'm having a hard time trying to describe this, but basically I think the 68-70 deck springs are in front of the rear bulkhead, while 71-75 deck springs "nestle" in alongside of it?
As I said in my earlier post, 68-70 used the drain hoses, while on 71-75's, water drains from the plenum holes, straight to the ground. Without the hoses in place, water will drain into the rear storage compartment of 68-70 convertibles.
Below is the rear bulkhead and plenum area as seen in the 74 AIM. Unfortunately, I could not find a similar illustration in either my 69 or 70 AIM's
In 71 the bulkhead moved 4" forward. Don't know why, but have heard was to do with crash protection and the gas tank. So on 71 convertibles and later, the vents are behind the bulkhead so on those with a/c you can look through and see the ground.
In 71 the bulkhead moved 4" forward. Don't know why, but have heard was to do with crash protection and the gas tank. So on 71 convertibles and later, the vents are behind the bulkhead so on those with a/c you can look through and see the ground.
Tom...
True,
My '71's vent plenums are outside of the rear compartment and without drain tubes. I'll post some pictures later when I take my hardtop off in a few weeks if you guys want.
Hi,
So, from looking at the 71 AIM and the 74 AIM it appears on convertibles, (467), the plenum was STILL being installed during that period… but observed cars do not bear that out.
When in production might someone expect to begin to see convertibles without the plenum? When the configuration of the "u/b closing panel" was changed?
Does, or how does, the a/c option enter into this?
Regards,
Alan
I'm not saying from any facts I've read anywhere, but it seems to me from what I've seen, the convertibles with air conditioning did not have the plenums. What years involved I do not know.
Off the thread topic, but since several of you have lots of detailed info. On the side gill slits, should the color of the car be in the backing of the posterior 2 slits and also on the bracket in the engine bay? If not, should I paint them with JD Blitz Black and tape off during the paint application?
03-19-2017, 08:36 AM
