which ported source to use.

[68 Firebird]

Hello All,

I am running a 7043266 Q-jet Cliff setup a few years ago and has been running VERY well.
I am currently running the vacuum advance of the ported fitting above the throttle blades to the right
of the fuel inlet on drivers side.  Vacuum seems delayed slightly on that port coming in around
1600 rpms and full vacuum around 2300 for this port.  The vacuum advance is full deployed by
highway cruising rpm's.

Just for curiosity I put a vacuum gauge on the other port I have. Which is in the front baseplate on the
passenger side of car.  This line comes off base plate at a slight angle and has vacuum instantly
when throttle is moved and gets a higher reading across the board.  Just wondering if using
that source would provide any better efficiency for motor.  I read in Cliff's book  that vacuum
from a ported source should be present right of idle and just wondering if I have been using wrong spot.

Thanks
Gerry

[Cliff Ruggles]

The port with the lowest source location is going to be the best port to use.

Just make sure it doesn't have a bleed-off slot, and vacuum falls off with additional throttle angle.

This may not be apparent testing it in Park, you may have to hook up a vacuum gauge to it and do some driving.....Cliff

[68 Firebird]

Well finally got arounf to running car with a vacuum gauge hooked up.  Here is what I found.

Baseline was gauge reading direct vacuum source to see what the motor was pulling. vac can running of drivers side ported source.
idle 20" in park 17" in gear @ 650rpm, 20"@25mph ,  22" at 55mph and 25" coasting. 

on ported source on drivers side above fuel inlet I teed in vacuum gauge there and got these readings.
idle was 0, 17" at 25 mph, at 55 mph pulls to 17 and then fades to 8 then to 5 when just maintaining hwy speed.  if pedal is pushed slightly vacuum comes back but car is then accelerating.  Exhaust note also changes when vacuum fades lower.
coasting on hwy with no throttle goes to zero  as expected and will go to 5 or lower during WOT.
With 3:08 gears highway cruising requires very minimal throttle and run 2200rpms at 55mph.

here is interesting thing the ported source on passenger side base plate was then tried. same stretch of road.
idle 0", 15" at 25 mph, 20" at 55mph (steady), uphill at 35 mph was 12", and WOT was same @ 5"
car seems snappier with this source as vacuum comes up instantly, on normal  acceleration.

Ignition is set to 14 initial and 20 from mech at 3000. with 14 from vac can.

What do you think? Am I crazy, figured the port in baseplate would fade not the higher drivers side port.
not planning on going in carb to trace as car is running very very well.
just curious what you thought.

Gerry

[Cliff Ruggles]

A correct ported source to run the vacuum unit will provide FULL manifold vacuum right off idle.  In most cases this is located to the right of the fuel filter housing in the main casting.  If the baseplate source is doing a better job, then I'd use it. 

Might want to take a look at the baseplate to see why the other source is "weak"?.....Cliff

[68 Firebird]

Think I'm gonna leave it hooked up to ported source in baseplate and get some miles
on her to see if there is a difference in mpg.  Could the bleed off in the other port be caused
by size of added hole in primary throttle bore in the baseplate?

Thanks

Gerry

[Cliff Ruggles]

Not sure what you are asking?

Ported vacuum to correctly operate the vacuum advance MUST supply FULL MANIFOLD VACUUM right off idle, or it will not work well.  This can be verified by using a vacuum gauge (two of them work best), on one MVA, the other on the ported source to the advance.  Off idle they should show approximately the same reading.....Cliff

[68 Firebird]

Update

First off let me say, Cliff you we right.  I took your advice and tested the both vacuum ports
against a FMV source using two vacuum gauges.

 The lower left EGR port although it showed strong on takeoff it did bleed down some while cruising and never quite reached FMV.

The ported  port you added always had a strong signal off idle and remained steady at cruise.

Was interesting to watch how both gauges responded while driving.

Thanks

Gerry

[Cliff Ruggles]

Cool.  Engine vacuum is based on engine load, so it always helps to do some active testing to see which port is really doing what?.....Cliff