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1969 Slant Six 225 Air Cleaner Housing
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Author:  1969Valiant [ Sat Apr 09, 2011 4:35 pm ]
Post subject:  1969 Slant Six 225 Air Cleaner Housing
The design of my 1969 slant's air cleaner housing is almost totally sealed. Why did they not have more of an intake port? Would it benefit me to change the cleaner housing to a later design with an opening on the side? If so, would I have to adjust my carb to account for more air? It's a factory single barrel Holley.

I'm not looking to do any major modifications to the car, but I'd certainly look into an easy bolt-on that will help a little. I was just a little curious about the design. It seemed a little strange to me. Thanks for any replies.
Author:  SlantSixDan [ Sat Apr 09, 2011 5:12 pm ]
Post subject:  Re: 1969 Slant Six 225 Air Cleaner Housing
Quote:
The design of my 1969 slant's air cleaner housing is almost totally sealed.


Naw, it ain't. I don't have one of those air cleaners near my desk right now (subject to change without notice!), but I'm familiar enough with them to recall the gap between the lid and the base is about 3/8". Doesn't sound like much, but let's do some math: IIRC the lid is 11-1/8" diameter. Subtracting 3/8" from that gives us 10-3/4". The area of a circle 11-1/8" diameter is 97.2 square inches. The area of a circle 10-3/4" diameter is 90.7 square inches. The difference in area between the lid circle and the base circle is just shy of 6.5 square inches.

The inner diameter of the carburetor air horn is 2". The area of a circle 2" diameter is 3.14 square inches. So already our air cleaner's air inlet has more than double the area of the carb's inlet (and we're disregarding the choke, here, so the area advantage of the air cleaner is even larger than this calculation reveals) and we can clearly see the air cleaner housing is not restricting air entry -- it can't.

But wait, there's more! The throttle bore of the carburetor is either 1-9/16" or 1-11/16", depending on exactly which carburetor you have. That's an area of either 1.9 or 2.2 square inches, so the air cleaner has over three times the area of the throttle bore. The case against the air cleaner housing weakens.

But wait, we're still not done! The diameter of the carburetor venturi is between 1-1/4" and 1-3/8", depending on which exact carburetor you've got. A common one is 1-5/16". The area of that circle is 1.35 square inches; the air cleaner's got most of five times the area of the venturi. The air cleaner is exonerated, all charges are dropped, and jubilant throngs fill the streets in a ticker-tape parade.

(If I misremember and the gap is closer to 1/4", the numbers shift downward a little but the air cleaner inlet area is still way larger than the smallest part of the carburetor.)
Quote:
Would it benefit me to change the cleaner housing to a later design with an opening on the side?
The '70-up air cleaners do give a benefit, but it's not "more air". The '70-up air cleaners are thermostatic, so the carburetor gets warmer or cooler air fed to it depending on underhood temperature, to keep the air temp at the carburetor as consistent as possible. This improves driveability, warmup behaviour, and fuel economy—win/win/win. You can add this system to a '69-down car; doing so requires a small list of components: air cleaner with working vacuum motor and thermo sensor; heat stove and flex duct; vacuum tap, hoses. If you choose to do this, use a '74 or later air cleaner if you can find one; the oval snorkel causes less turbulence/less noise than the '70-'73 rectangular snorkel. Less turbulence means greater airflow, too, but here again, this doesn't matter because the air cleaner isn't the bottleneck in the system.
Quote:
would I have to adjust my carb to account for more air?
There wouldn't be any more air reaching the carburetor.
Quote:
I'd certainly look into an easy bolt-on that will help a little.
Tune-up parts and technique suggestions in this thread. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. The engine will need periodic valve adjustment.

If you want to make a mod that will help driveability, eliminate the 1969-only throttle plate heater. See here for picture and description. Current gasoline does not need this system, which boils the carb like a teakettle when the engine and/or weather is hot. To bypass it, simply connect a hose from the nipple on the underside of the air cleaner base to an invert-flare-to-hose-nipple adaptor you thread into the carb base where the heat tube currently lives.
Author:  1969Valiant [ Sat Apr 09, 2011 6:13 pm ]
Post subject: 
Cool. Thanks for the detailed reply Dan.
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