Rear air door opening vs secondary nozzle discharge vaccum

[70GS455]

Another experiment I decided to conduct was to check how much the rear air door opening had an effect on how much draw was on the secondary discharge nozzles. This took a little more effort as the secondary emulsion tubes had to be soldered closed and I had to fabricate some tubes to seal to the sec discs in the fuel floor. The attached chart shows a slight increase in vaccum at one opening but no real significant trends otherwise. The vac readings were referenced to the pressure under the carb as the door opening also changed that.

[qjetsrule]

Excellent tool, maybe you could experiment with different throttle blade angles effect on vacuum, designs of flappers, putting a window on the side of carb & below carb & do a smoke check. Vortex generators & profiling the shaft. Good luck.

[novadude]

Just so I understand....

You were applying constant vacuum at the base of the carb and manually varying the positon of the air door with no spring tension on the air door?   Pressure measurements taken at the secondary jets (discs in bottom of fuel bowl)? 

It would probably require one hell of a pump, but I'd love to see the test with different levels of tension on the spring, as the vacuum pump pulls the door from closed to wide open.  I suspect that is outside what is possible in a home set-up, due to the CFM required to get that door open.

[70GS455]

Yes, and yes.

That 2nd idea would take a lot of doing, I doubt I have those resources.

[70GS455]

Excellent tool, maybe you could experiment with different throttle blade angles effect on vacuum, designs of flappers, putting a window on the side of carb & below carb & do a smoke check. Vortex generators & profiling the shaft. Good luck.

Maybe when I retire someday I'll have that much time 😜

[Cliff Ruggles]

Keep in mind here that as you open the air flaps further you will reach a point of no return.  This simply happens because you start to loose too much area under the flaps which starts to reduce "pull" from the fuel nozzles. 

I did all of this testing over 20 years ago when I wrote the Q-jet book and backed it up with dyno and drag strip testing to find the most idea open angle for the airflaps.

I also had the carbs flowed for CFM.  The later castings with the air doors at the maximum useable open point flowed 849cfm. 

Removing the outer booster rings increased flow to 897cfm........