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| | I have been using an old Carter 9625 AFB on my car for several years and have
finally decided to try a Rochester QuadraJet. As is often the case, I was
using the AFB because the price was right and will trying this particular Q-Jet for the same
reason. There are a number of technical advantages that the Q-Jet has over the AFB
but an important one is the small primary venturis from the spread bore design. The practical advantage is
that Q-jets were installed on thousands of cars in the past decades and should
still be relatively plentiful and cheap to acquire.
Upgrading to a 4bbl carb does make a large increase in performance.
Originally, the car did a 19.0 second 1/4 mile with the Carter BBS 1bbl carb,
automatic transmission, and 2.93 rear gears. Upgrading to headers, a
Direct Connection high lift, short duration cam (PN 4120243), 318 valves, 340
valve springs, and some mild porting, the 1/4 mile time improved to 18.5
seconds. Installing the AFB (jetted for a Chev 350) resulted in a best
time of 17.078 seconds @ 81.96 mph.
As I had already upgraded from the OEM Carter BBS one barrel carb to the AFB
using an Offenhauser 4bbl intake manifold, the manifold is still suitable for
the Q-Jet. However, with the intake being drilled for a square-bore carb,
an adapter will be required to make the new carb fit. Luckily, Edelbrock
makes one which allows the new carb to be a bolt-on upgrade.
At the time I installed the AFB, I was concerned about having a symmetrical
fuel distribution between the front 3 cylinders and the rear 3 cylinders so I
made a carburetor mounting plate for the intake manifold which turns the carb
90°. This allows the carb's throttles shafts to be parallel to the engine
rather than perpendicular as was supplied by Offenhauser. I think Offenhauser supplied it that way to simplify the fabrication of the throttle and
transmission linkages. I had to fabricate my own throttle linkage for the
new arrangement.
The following photos show the change from the AFB to the Q-Jet. This
page should take about 190 seconds to load with a 28.8 modem.
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Old Carter 9625 AFB that someone once gave me several years
ago for free. Notice there is no choke assembly, which makes it
tricky to start even during the summer. At least I didn't have to
worry about my wife sneaking off with it. |
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Spreadbore to squarebore adapter. This adapter is
made to mount spreadbore side down. Notice the bump on the gasket on
the bottom right-hand corner. It corresponds with a bump on the
adapter's bottom left-hand corner. I'm sure it was one of GM's
better ideas but the carb does seem to seal well in spite of it. |
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Spreadbore adapter mounted on the Offy intake. This
adds 3/4" to the height of the carburetor but keeps a separate
pathway to the intake manifold for each carburetor barrel. There are
open plenum adapters available too but I wanted to keep the plenum volume as
small as possible to minimize any chance of off-idle bog. I had intended to install a 1/2" insulator between the adapter and
the carb but found that I just don't have the head room for it with my
Corvette air cleaner. The Offy-supplied mounting plate is 7/16"
thick so you would gain an extra 1/16" clearance over the plate I am
using.
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I like the QuadraJet carb because of the small, double
booster primaries and
because they are so plentiful in the junkyards. I happened to have
this one surplus from a 1977 Pontiac I converted to propane a few years
ago. The other thing I like about this carb is the small central fuel
bowl. I found that the AFB was sensitive to fuel level in the bowl
and had a loss of power on hard left turns. Of course, after I
raised the fuel level, this problem seemed to disappear. The
as-installed QuadraJet has no sensitivity to turns at all in this
orientation.
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I was at a speed shop back in the 80's when some kid
replaced the stock air cleaner on his Corvette with one of those shiny
chrome ones. I gave him $10 (Canadian!) for it and have been waiting
for an opportunity to use it ever since.
With everything assembled, I have about 1/4" of clearance between
the hood and the top of the air cleaner lid. I knew the kids' play
dough would be useful for something other than a cat's birthday cake.
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I took the car out for a spin and it runs at least as well
as the AFB and this is with the carb set up for a Chev 350. Since
the carb has been sitting in my garage since 1997, a good rebuild probably
couldn't hurt. I know the sound the carb makes when the secondary
air valves open up on the 350 but I didn't hear it anything like it on the short drive I made to test
it. I got out onto the QEW and took it up to about 90 mph but ran
out of highway as I only went from one interchange to another.
Notice the two marks on the hood as the air cleaner was rubbing on
it. If you look at the previous photo, you can just see where the
paint was worn on the air cleaner lid. Looks like I'll have install
either a torque strap or a low profile air cleaner.
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I extended the air cleaner hold-down stud up to the hood and
marked the contact point. From that point, I measured a radius of
6¼" to the hood bracing which means a maximum allowable diameter of
12½". The height of the the Corvette air cleaner is
3-3/8". For aftermarket, I think you would be safe using a Mr
Gasket 9" diameter assembly which is only 2¾" high but I
haven't verified if this would work. There are smaller 6"
diameter by 2" high ones too but think these are more restrictive.
I took a walk through one of the local junkyards and found an extremely
short air cleaner in a 1984 Firebird. The lid diameter is over
14" so it hits the bracing. |
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The Firebird air filter diameter is less than 12" and
the height is 2½". After an afternoon with a saber saw and
some black paint, I got the following result. There is plenty of
hood clearance with the Firebird air cleaner so I will try adding some
heat dissipators later.
I got the electric choke conversion working but found that there isn't
a good ground from the thermostat housing. A short length of wire
fixed that problem pretty fast.
Notice the blue hoses going to the bottom of the intake manifold.
I got my prototype intake manifold heater installed at this time too. |
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A close-up of the intake manifold heater. This device
has completely eliminated the off-idle bog. The QuadraJet seems to
pull a lot harder than the AFB but this is my seat-the-pants
feeling. I'll have to go to the drag strip to know for sure.
With the open air cleaner, I can hear the air valves opening up
now. With a flow capacity of 750 CFM, the sound is not quite a loud
as I was used to with the Chevy. They do seem to open up quite
frequently while I'm cruising on the highway.
I also discovered that the dimensions of the heat riser base on the
Offy is bigger than that of the stock manifold. I machined a
depression in the plate help ensure a good seal under pressure. I
based the depression size on a stock 1bbl manifold and allowed for extra
clearance in my design. It still wasn't enough so I had to massage it a
bit with my die grinder. Back to the drawing board for some extra
clearance! |
The following is a list of parts to install the Rochester QuadraJet on your
six cylinder car.
| MANUFACTURER |
PART NUMBER |
DESCRIPTION |
| Offenhauser |
|
Intake Manifold |
| General Motors |
from 4bbl -V8 cars |
QuadraJet Carburetor, air cleaner, throttle linkage |
| General Motors |
from 4bbl -V8 cars |
1984 (third generation) Firebirds and Camaros
(verify dimensions first!) |
| Edelbrock |
2696 |
Squarebore to Spreadbore adapter |
| Edelbrock |
1932 |
Hot air to electric choke conversion kit (may
need to add ground) |
| Edelbrock |
1991 (74 & earlier)
1992 (75 & later) |
Race calibration kit (metering rods, jets, etc) |
| Mr Gasket |
1487 |
Chrome 9" diameter air cleaner,
2¾" high. (Clearance not verified!) |
| Mr Gasket |
97 or 86 (1/4")
98 or 86B (1/2") |
Heat dissipator (if you have the hood clearance and you want
to keep your carb cooler) |
| Mr Gasket |
3704 |
Vacuum Cap Assortment |
| Any |
(for fuel line) |
3/8" to 5/16" brake line bushing |
| Any |
(for fuel line) |
12" to 18" of 5/16" ready-made brake line |
| Any |
(for fuel line) |
2" of 5/16" rubber fuel line & 2 gear clamps |
| Any |
(for vacuum) |
1/4" NPT pipe plug (to plug vacuum line to GM
transmission) |
| Any |
fasteners |
assorted 5/16" nuts, bolts, washers, studs |
If anyone want to do a similar project, I've put a kit
containing the major components of this upgrade on the Mopar products page.
Since I have already had a transmission throttle linkage in place which was
working for the AFB, I didn't need to fabricate a new one. I researched
the installation of a Lokar throttle linkage
and they have the parts to make this carburetor work with the Quadrajet.
Check with first before buying any parts, just to be sure!
The following is a list of Lokar parts to install the Rochester QuadraJet on your
six cylinder car.
If you want to go instead with the stock GM throttle cable, brackets,
and gas pedal, you can easily fabricate a bracket to mount the kickdown cable by
itself.
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