L88 Hood Fresh Air Chamber

[Michael M.]

On the underside of an original 68/69 (I think) L88 hood, there is a 3" or so hole in the fresh air chamber on the drivers side, just outside of the large opening for the air cleaner element. What is this hole for? It doesn't seem to mate to anything under the hood and it would seem to defeat the purpose of the fresh air chamber (which is to isolate the carburetor from hot underhood air). Thanks for any replies.

Most of the aftermarket hoods have an indentation in this spot but no hole.

[John C.]

Re: L88 Hood Fresh Air Chamber

Michael

I think the opening is to allow air to the engine if the cold air induction openings become clogged with say snow or ice. Now why you would be driving and L88 in snow or ice conditions is a whole other question.

John

[Henry J.]

Re: L88 Hood Fresh Air Chamber

Michael:

In the 67 AIM, page L88, sheet A1, there is shown a road draft tube. The hole in the plenum may be clearance for the upper end of the road draft tube, if one were used.

[Michael M.]

Re: L88 Hood Fresh Air Chamber

Henry, I think in 68/69 the L88 used a PCV system. Maybe they just didn't change the design of the fresh air chamber, but I'm not sure. John's observation could be correct as well. We'll see if anyone else chimes in. Thanks.

[John H.]

Re: L88 Hood Fresh Air Chamber

Michael:

In the 67 AIM, page L88, sheet A1, there is shown a road draft tube. The hole in the plenum may be clearance for the upper end of the road draft tube, if one were used.

Henry -

The '67 L-88 hood didn't have that secondary hole in the plenum chamber.


L88HoodInner.jpg

[Michael M.]

Re: L88 Hood Fresh Air Chamber

Any L88 experts know the answer for sure? I was thinking that possibly the speed of the air flow might be greater if the air had an escape route. Maybe it reduces turbulence as the air is entering the fresh air chamber/carburetor. Kind of like opening the front and back door of your home to create a cross breeze. I figure that the GM engineers must have had a real good reason to change the hood and add this hole.

[Michael M.]

Re: L88 Hood Fresh Air Chamber

Thought I'd resurrect this thread to see if anyone else has any ideas on this 3" hole in the L88 fresh air plenum/chamber. Thanks.

[Henry J.]

Re: L88 Hood Fresh Air Chamber

Michael:

I'm still of the view that the hole was provided, as contingency clearance for the previous year's road draft tube, to allow someone to retrofit one for some (unknown to me) race purpose.

From an examination of detailed pictures, it appears to me that the 67 L88 hood didn't need the hole for clearance but the 68/69 L88 hood did (if a road draft tube were to be fitted).

I point to pictures on pages 35 and 98 in Dobbins 1968-1972 Fact Book (4th Edition) in comparison with pictures on pages 270, 274 and 280 in Dobbins 1963-1967 Fact Book (9th Edition). With that road draft tube in place on a 68/69 L88, I suspect that the upper end of the tube would nestle nicely into that hole, thereby substantially sealing it.

I don't know anyone wealthy enough to own an L88 to test out my theory.

[Michael M.]

Re: L88 Hood Fresh Air Chamber

Henry, so you're saying that for racing purpose a road draft tube might be fitted to a 68/69 which would require the hole for clearance. I don't have access to your materials and I have to admit, I don't think I've ever seen a road draft tube, although I'm pretty sure I know what it does and that the PCV system took its place in 68. Thanks for your reply. Again. :-)

[Henry J.]

Re: L88 Hood Fresh Air Chamber

Michael:

Firstly, let me assure you that I know nothing about preparing a race car, so I can't tell you what benefit would arise from retrofitting a road draft tude on a 68/69 L88.

What the pictures reveal to me is that the undersides of the 67 and 68/69 L88 hoods are configured differently. The plenums are different. It appears that, with a road draft tube in place, the 67 L88 hood does not need a clearnace hole, as it would close nicely, whereas the 68/69 L88 hood does, as I think it would not close.

As the valve covers on the 67 and 68/69 are configured substantially identically, I can imagine the easy retrofit of a 67 road draft tube on the 68/69.

Therefore, I would say that if a road draft tube were in place on a 68/69 L88, and the hood closed nicely, the existence of the hole would not be by chance. Rather, it would be for that purpose.

I base all of this on an examination of my own (aftermarket) L88 hood which had the hole you describe, as well as on my examination of pictures. I wanted an explanation for the hole, just as you do. What I describe is what I concluded.


I'm just a humble engineer applying logic to explain an observation. Testing my theory with real parts on a real car would confirm or deny my thinking. It would be fun to try, but there is virtually no chance of my doing it. Oh, and by the way, I've been wrong before.

[Richard F.]

Re: L88 Hood Fresh Air Chamber

Don't you just need some way for rainwater or water from hosing the car to drain out of the airbox? Between the engine sucking air and the high pressure area right in front of the windshield, I could see rainwater easily getting into the airbox.

[Joe C.]

Re: L88 Hood Fresh Air Chamber

Michael:

Firstly, let me assure you that I know nothing about preparing a race car, so I can't tell you what benefit would arise from retrofitting a road draft tude on a 68/69 L88.

What the pictures reveal to me is that the undersides of the 67 and 68/69 L88 hoods are configured differently. The plenums are different. It appears that, with a road draft tube in place, the 67 L88 hood does not need a clearnace hole, as it would close nicely, whereas the 68/69 L88 hood does, as I think it would not close.

As the valve covers on the 67 and 68/69 are configured substantially identically, I can imagine the easy retrofit of a 67 road draft tube on the 68/69.

Therefore, I would say that if a road draft tube were in place on a 68/69 L88, and the hood closed nicely, the existence of the hole would not be by chance. Rather, it would be for that purpose.

I base all of this on an examination of my own (aftermarket) L88 hood which had the hole you describe, as well as on my examination of pictures. I wanted an explanation for the hole, just as you do. What I describe is what I concluded.


I'm just a humble engineer applying logic to explain an observation. Testing my theory with real parts on a real car would confirm or deny my thinking. It would be fun to try, but there is virtually no chance of my doing it. Oh, and by the way, I've been wrong before.

My view is that air/fuel ratio would be affected as the PCV valve bleeds off varying amounts of manifold vacuum as engine load changes. Road draft tube means more accurate fuel metering. Not critical for street driving or drag racing; more important for road racing.

[Joe C.]

Re: L88 Hood Fresh Air Chamber

Don't you just need some way for rainwater or water from hosing the car to drain out of the airbox? Between the engine sucking air and the high pressure area right in front of the windshield, I could see rainwater easily getting into the airbox.

The low pressure area at the cowl would draw in rainwater, which is something I never quite understood. I assume that the L88 plenum has neither baffles nor a diverter door. Since these cars were supposedly purpose built for racing, water hoses were certainly not a consideration.

[Richard F.]

Re: L88 Hood Fresh Air Chamber

There are no baffles or diverter. I suppose if you are racing in the 24 hours of LeMans you don't stop because it is raining.

[Michael M.]

Re: L88 Hood Fresh Air Chamber

I have to say that I Googled "L88 road draft tube" and saw a picture of a 67 L88 with the road draft tube installed. It certainly looks like that road draft tube would fit very nicely into that hole. It sticks up from the engine a lot more than I imagined it would and if installed on a 68/69 L88, it would definitely need a provision in the hood to clear it.