Inliners International Forums General Discussion Engines Low CFM 4 barrel carb options?

I'm a big Quadra-Jet fan too. I never ran one on an inline. The key to tuning them is the "step by step" part. Good MPG if you keep your foot out of it and hang on when the secondaries open.

I run a quick fuel 450 on my 261, good mileage and adjustable fuel and air jets.

I thought this thread ended in mid 2017 but it's back - great.

Moregrip - I reverted back to the three Weber 32/36's. They seem to work better on my car. Took car to drags in 2017 but car ran slower on the Holley. Don't know about the Weber 38's.

I have an old pre-electronic Q-Jet and intend to adapt it to my GMC. It sure sounds good.

The threads never end . Sometimes they rest for a while. That's the point of having a search feature.

Another vote for the QJ, tiny primaries for good tip-in and low speed, big secondaries for max air delivery.
Difference of opinion as to which way to face it on a 1 X 4 manifold: the factory Sprint faced the primaries away from the engine for best mixture distribution.

radar, thanks for the update. I bought the Clifford intake/Weber 38 combo so I'll run that as she sits. If I need more power I just might have to rebuild this 4 barrel Quadrajet I have. Come to think of it, I have a 2 barrel Varijet as well.

that's a nice piece!

IIRC it can be adapted to the Chevy 250 etc. (not a direct swap since the Pontiac-only ports are different, although I think the port positions match).
I did some searching, and no explanation as to why the huge volume difference between front and rear cylinder group plenums. Not visible but possible: the front leg (shown right, above) may be deeper, the rear leg is shallower for some reason and makes up the same X-section with width? The firing order (on this picture) is #1 (right), #5 (left), #3 (center), #6 (left), #2 (right), #4 (center), is this significant?
Beware: QJ to Holley base adapters notoriously have small gasket surfaces mating to your manifold, and are leak-prone.

I am running the front half of a Q-jet on my 1bbl intergrated head. I have the carb turned the same way that it would sit on that Pontiac manifold. Although it’s a little short on CFM the drivability is very good! I would like to try a WCFB on a little more aggressive engine. Apparently they were made in different throttle blade and Venturi sizes. Looking at the GM heritage center, some are shown with SMALLER secondary Venturis than the primary side. Jay

any detailed pics of this setup?

I can take pictures but can’t post them. Pretty sure I could e mail them. Not too great with tech stuff! Jay

cool, my username @ gmail

I wonder on the Pontiac if they put the primaries away to give those runners fed a little more length for better lower end performance.On the Big Ford's they recommend primaries close to the engines. Also I think the Aussies have carb orientation different aswell.

The explanation I read (Hot Rod?) 50 years ago: the primaries would be so close to the #3 & 4 that the vacuum difference with those ports only an inch or two from the manifold runners couldn't be fixed any other (cheaper) way.

Tom Lowe - How do I get written instructions on modifying a Q-jet to a 3 barrel? Plan to try one on my GMC 302.

I'm working on the intake manifold head brackets and tube runners now. Any suggestions cutting 3 vs. 4 holes in the carb plate and on hole orientation?

Thanks, Frank Hainey

Here are some pics of Jay's setup!

Thanks for posting the pictures. Sorry about the dirty engine compartment, a lot of it was sprayed with black rustproof when the car was new. Jay

modifying a Q-jet to a 3 barrel
Why?

To give it better street manners, off-idle throttle response, gas mileage etc.

Details here: Inliners Post circa '15

The Pontiac (Chevy) 230" used a QJ, why does it need to be smaller for a 24% smaller engine?
w/r/t cruising CFM: assuming the mixture matches, the CFM used will be the same regardless of how many throats. Depending on the venturi type the manifold vacuum could go either way.
Disabling 1 primary barrel will also have another effect: the operative throttle blade angle will now be higher (more rotation) in the throat, and the idle/transition slot relationship will change quite a bit. If the blade is above the transition slot the tip-in response relies on pump shot. The usual cure is a small hole in the disc right next to the slots to allow air to enter with a smaller angle.

I’m pretty sure that Tom made a 3 barrel out of a Holley 600 CFM carb not a Q-Jet. Please correct me if I’m wrong. According to Tom, Holley carbs perform better when
“Loaded” more with greater throttle blade opening. That and the high cost of the 390 cfm model is why he said he did it. Tom says it works well and I believe him. Jay

The hot idle position of the edge of the throttle disc (approximately mid-way between the "idle" and "transition" holes in the barrel) is universal among all carburetor brands, models and sizes. It's one of those factors that makes switching among non-OEM carbs a crap-shoot, and Holley has and will disagree that "loading" it more is an improvement (read their literature). If that were true, it would have occurred in the entire line 60 years ago.
My guess: it corrected an existing (bad) condition, such as the blades were too far closed (in or below the "idle" hole) with that small engine to compensate for lower idle CFM demand.

I think I got confused in reading the positive comments about the Q-Jet along with those for the 3 barrel carb. I'm in the process of making Q-Jet manifold for my GMC. Won't worry about the 3 barrel mod now. Sorry for the confusion.

looking forward to seeing that!

I am glad you are interested in my project. I hope you will participate in the design process with me me.

I have built GMC intake manifolds in the past. My best effort is my triple Weber 32/36 manifold for my GMC. This manifold provides a very smooth intake system. Please see the Harpor SP? intake manifold subject below.

I also built a manifold adapting a Holley 395 cfm 4 barrel to a GMC. It never really worked well. The biggest mistake I made was to use a smooth transition between the plenum and the runners.In the process I ignored the need to keep air/fuel mixture moving rapidly toward each intake port. The result was low speed bogging because the manifold runners were too big, especially at the point where plenum and long runners meet. I also made a very large plenum which did not help.

This time around, I plan to design a Q-Jet to GMC manifold where the the cross sectional area of the runners is constant and equal to the area of the head intake port. This is = 0.785 x(1.75)^2 = 2.404 sq. in. The runner will be made from 1 7/8 OD tubing which has an I.D. of 1 3/4 inches. The critical point of this design is the transition from the the long round 1.75 I.D. runner to the rectangular plenum.

The plenum volume will be made as small as possible, like the production Q-Jet manifolds shown above. The outlet opening on each side of the plenum will be wide enough the allow the smooth flow of the air/gas mixture from the Q-Jet's small and large bores simultaneously. This outlet opening bottom will match the bottom of the flat plenum.

I am thinking about a a rectangular opening into the plenum 1/2 inch high by 4.8 inches wide. This opening maintains the 2.4 sq. in. area like the long round intake runners and hopefully keeps the air/fuel flow rate up for good performance.

I have purchased the steel tubing and steel plate to mount the carb to the manifold and to bolt the manifold to the head. I have also roughly formed a rough a sheet metal transition piece for use between the 1 7/8 O.D. runners and the 1/2 x 4.8 in. openings on each side of the plenum. I have not done anything about the plenum wall material but think this will be a strip of 1/16 steel x 1 inch high and bent around the perimeter of the Q-jet carb bores.

Any comments or suggestions on my theory about keeping the air/fuel mixture flow rate up or on this design would be appreciated. I'm in the preliminary design stage so changes can still be made.

I know you might think I have too much time on my hands but I really enjoy building his kind of stuff. Thanks. Frank Hainey, (Radar).

Interested to see how the q-jet project turned out

Thanks for the interest in the Q-Jet manifold. I have had some setbacks in building this thing.
Two different attempts at welding the plenum to the carb mounting plate warped both plates. My plan is to increase the plate thickness from 5/16 to 3/8 and to bore the large holes after welding.
Secondly, the rounded plenum corners makes it very too difficult to build the tapered transition cones connecting the plenum to the intake runners. So now plan to make the plenum rectangular with straight sides. This should simplify the building the transition cones.
I hope to have something built by the end of the year.

Frank Hainey - Radar

Post up some photos once it's done, it will be cool to see.

Still following along!

I been running Toms 3 barrel for over 3 years now with no problems. That's on a 261 ci with a 700r4.