Rear air door opening vs CFM

[70GS455]

So I decided to do a little experiment. I rigged up my leaf blower as a makeshift flow bench. I attached my carb and added a pressure meter and cfm meter.I wanted to determine if there were any surprises in flow measurement vs how much the rear air door was allowed to open. And...no surprises. The more the door was opened the more it flows. My leaf blower is limited to about 450 cfm so that's as high as the reading would go.

[77cruiser]

That's a worthy difference.

[novadude]

How is air door opening measured?  From leading edge to back wall, or??

[Kenth]

This:

[novadude]

That's what I thought (air door opening drawing), however, the graph of results would indicate that the CFM is less with larger openings if measured that way (which doesn't make sense).

[70GS455]

How is air door opening measured?  From leading edge to back wall, or??

Actually I used the procedure in Cliffs book, that is to measure from the flap leading edge to the rear wall. So 1.2" means almost wide open

[Pav8427]

If memory serves me right, arent specs for secondary rods shown in degrees of opening.
Would be interesting to rig up a degree wheel and compare degrees to degrees. 

[Cliff Ruggles]

I measure to the rear as it's the same on all models.   Airflow is not the only thing to consider here.  When you go too far with the airflaps you are going to loose some signal to, or fuel flow from the nozzles. 

When I did airflow and dyno testing back when we wrote the book there is no gain past the numbers given in the book in terms of CFM or making more power on dyno pulls.

I backed up that testing at the track. 

We also found that at the largest opening recommended in my book the carburetor flowed 849cfm, not 800 like most folks regurgitate on the Forums and such.

Removing the outer booster rings on the larger models combined with the maximum open point yielded 897cfm.......

[Frank400]

This:

that is one way of measuring, but is it me or this is NOT the way Cliff's numbers relate to in his book ? Just want to make sure I measured correctly when I followed Cliff's recipes.............

[Kenth]

Following Cliff´s recipe measure like Cliff does.

I prefer and have always used Rochester instructions in my documentation of Quadrajets.

Both ways works if you know what you are using.

FWIW

[Frank400]

excuse my ignorance, but where does rochester instructs to measure secondary air door opening ?

[Kenth]

They did that in 1965 and 1968 bulletin 9D-5 among others because the 1965-66 Quadrajets had the adjustable stop on the dash lever for air valve opening.

Later units have shafts with a fixed stop for the degree of opening of the air valves depending on the application.

[Frank400]

ok I see, wow thanks for the info, never worked on any rochester older than 67 so that is a new one for me.  Cool info.

[Cliff Ruggles]

Your are lucky.  The 1965-66 units are HORRIBLE and gave the entire line of carbs a BAD reputation that still exists to this day.

The secondary dampening system is HOPELESS.

The plunger type fuel inlet valves with the bypass port are HOPELESS.  Even the "conversions" that come in the rebuild kits with a push-in or retainer held in fuel inlet seat and bypass plug are horrible.  99 percent of the ones I've had in here leaked at the "O" ring on the bypass plug.

The leak rate for bottom plugs is 100 percent, and 90 percent will be leaking at just about every plug in them including in the very front where the fuel filter housing screws in.

Coincidentally I have an early 1967 Chevy small block Q-jet in here right now that still used the crappy fuel inlet valve set-up.  It is leaking at all four bottom plugs.  The plug just below and to the left of the fuel filter housing is leaking.  The bypass plug driven in during the last rebuild is leaking.  I only took it in for a heli-coil install, but it was fully assembled so now I've inherited a MESS!.....FWIW......