Picked up a 1973 QJet off a 472 Caddy which has notches cut out of the secondary air doors. Why would they do this? A look online and I see some other 472/500 Caddy QJets that have the same cut air doors from the factory. Can anyone tell me why they would do this? Pic attached.

 

Unfortunately we do not have privy to factory testing data. The cutouts were use on other late model versions. Only back to back testing would determine if the mod did any good. Pontiacs notched the front of the plate. Some have distribution tabs welded to the underside, usually just one side. It would take deep pockets to duplicate the test equipment the factories had. It wasn't just GM. The 428 Cobra Jet Holleys had the secondary fuel boosters notched differently for the left and right. The 426 street hemi aft carb secondaries had an 8 jet difference between the left and right. Good post but the secrets are hermetically sealed on Funk and Wagnalls front porch, only Carnac The Magnificent can read them.

 

That is very interesting stuff. I'd never seen this before. Thanks for the great Input.

 

The slots on the leading edge of the air flaps, like dragline shows, are somewhat common. Ruggles and Roe both recommend making notches in the air flaps. The notches line up with the tip-in bleed holes. Supposed to help smooth the transition as the secondaries open by getting air to flow past the bleed holes first.
I don't believe I've seen notches on the back outside corners like that first picture. Not sure any good would come of that. Actually, I think that would reduce the opening rate of the air flap. Less surface area on the back which is the part that forces the flap to rotate, thereby lifting the secondary rods. So maybe for the big cars, and especially the caddy 500, this helped avoid the Quadrajet Bog without setting the spring real tight?

 

Exactly the thought I had. The 472/500 motors move a lot of air. Reducing the size of the air valve on that side of the pivot shaft would reduce the torque on the shaft as a result of mass airflow. This would allow the wrap spring to be adjusted less tightly, which would minimize "snapping shut" if the throttle was released quickly. That's just a theory - I have no data to back that up.

 

Great discussion, I think you guys got it. I had never seen it before, but clearly it is meant to mute the opening and closing of those air doors. Interesting the only QJets I saw the little triangular cutouts was always the huge Caddy motors. Great stuff. Yhanks to all of you for the education. Good stuff.

 

I’ve seen the Pontiac front slots. The factory used them on a number of hi-po engines. I’ve notched mine and think the notches help.

I have never seen rear side cutouts like that.

Just my opinion, but I wonder if it’s factory at all. Looks like a Sunday afternoon dremel experiment.

But hey it ain’t stupid if it works. And there’s plenty of room for mechanical creativity in my view. Would be fun to test.

cheers
cf

 

I can say these little side cutouts are perfect, not a hint of DIY. Also, I did find another couple Caddy QJets with the same little triangular cutouts. Seems factory. As others have pointed out, I think the size of the Caddy motors (472/500), and the related airflow does create a unique situation, and perhaps a unique (undesirable) effect on the rate those doors open and close. Those Caddy's are huge motors. Interesting stuff.