1968 Pontiac Firebird 455 Holley to Q-Jet Adapter

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So I had a problem. After the first 2 months of driving the car I thought everything was
great. The car started great, ran great, seemed very powerful. I took it to the strip for a
blast in early fall and it was very disappointing. I tried a few things but the car fell on it's
face every run that first night. The next day I looked at lots of things including the paperclip
trick on the vacuum secondary rod to see if the secondaries were opening properly. I had
always thought they were because of how well the car pulled. But I was surprised to find
out they were not opening at all. Not a bit. I took the brand new carb apart a couple of times
to see if there was something blocking the vacuum signal port, tried a lighter secondary spring,
and made sure there was no problem with the secondary diaphragm. all seemed fine. but
after a few more drives with eh clip I found out they were still not opening at all. I put a
bolt in the secondary linkage to force them to open along with the primaries under acceleration
and on the next strip test run I gained over 2 seconds. It actually did not "feel" much different
but obviously made a big difference. My next set of troubleshooting inspections led me to the
fix described below.

I put a Holley 770 Vac Secondary carb on my modified q-Jet stock manifold. In order to do this
one needs to use an adaptor. Not generally accepted as being the Hot ticket for performance,
I still thought it would work well for my application. I had one of the 4-port style adapters shown
farther down in the pictures below.

Here is the engine before I started taking the top end apart.

you can see the adapter here with the carb off. I have also drained the AF
and removed the upper rad hose, temp sensor, therm housing bolts ,etc..

when you look straight down the adapter you can see the sharp angle of the
primary side holes. from the holley holes down to the Q-jet manifold holes.

a little bit closer view of these sharp angled bevels in the adapter.

this is a view of the bottom of the adapter.

here you can see the placement of the 4 holes in my modified stock intake.

the pic below has some red lines drawn on it to indicate where the primary
and secondary vacuum ports are located. the air rushing past the primary
port activates the secondary diaphragm. Once it starts to open the secondaries
the additional air rushing past the secondary port also helps to fully open
the secondary blades.

My theory is that the sharp and short turns in the adapter positioned just below
the primary activation port are causing the air stream to not pass over the
port properly. In the picture below I have painted in red the material I plan to
remove from the adapter on the primary side.

and here is a shot from the top showing the adapter with that material removed.
the air should now be able to flow straight down past the vacuum port in the
primary and secondary venturis.

here are a few shots of the bottom of the adapter showing the material I
removed from the adapter. I am happy with the result and the finished shape
of the flow area.

here again is a shot of the intake before I started on it.

and with the finished modifications to the intake. the shape of the holes in the intake
exactly matches the shape in the bottom of the adapter. There was quite a bit of
material that needed to be removed from the drivers side edges(with the low runners)

and below is the finished shot of the two after the adapter has been attached
to the manifold. I used a small cartridge roll and burrs to match the two exactly
whole they were together. Then took them apart for a complete cleaning
before re-assembly on the car.

a close-up showing the nice fit of the two together before taking them apart
and installing the gasket.

after cleaning off all the old gasket material from the head sides and
vacuuming up all the debris.

I went to a few different auto supply places to try and find anther thin Q-jet
base gasket but all I could find were the big fat ones. I ended up making my
own from some sheet gasket material I still had. I do not have very much
air cleaner-to-hood clearance so the thick gasket would not work.

I applied a very thin layer of "the right stuff" gasket silicone to one side of
the new intake gaskets and placed those carefully in position. you can see the little steel
exhaust crossover plate in position in the middle of the gasket. I placed the
steel plate upside down so the steel "button" was facing in to the cyl head to keep my
aluminum block off plates from crushing the button and distorting the plate.

then applied a thin layer of the same gasket material to the intake surface.

and put it back in position. I placed the intake bolts in position and hand tightened
them down before snugging up the water pump to intake bolts to seat the
seal on the end of the intake to the water pump. I applied the same silicone
gasket material to the area where the water pump seal/gasket seats.

getting everything else put back together.

and all re-assembled. I started the car and checked everything for leaks. Started just
as good as it always did and no leaks so far. I tested the modification this spring
and it Works!. the secondaries open up just as they are supposed to now and I
am getting to feel what having a four barrel carb is like. Holy Mother of Pearl...
it goes like crazy. I have noticed the difference in how much gas I am burning
already but who cares... Once it gets real nice and warm, back to the strip I go.

 

 

Main Block
Cylinder Heads
Intake Manifold
Pistons
Installation
Rope Seal Tech Bulletin
Ignition
Rotating Assy
Valve train
Paint
Final Assembly
Fuel/Exhaust

 

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