1972 400 2bbl to 4bbl Conversion - Pontiac GTO Forum
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Old 02-27-2006, 05:11 PM   #1 (permalink)
 
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1972 400 2bbl to 4bbl Conversion

Hey Guys:

Just bought a 72 Lemans with the 400 2bbl. Everything is factory stock.

I am looking to upgrade to a quadrajet and need some advice on what to buy as far as manifold and carb.

I am looking to add some punch but nothing too crazy as I will probably not change the cam or headers anytime soon.

Ideas? Can I go with a "stock" manifold and 4 bbl?

Thanks,

jp
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Old 02-27-2006, 07:26 PM   #2 (permalink)
 
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the problem is that you still have the 2bbl heads with small valves
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Old 02-28-2006, 08:26 AM   #3 (permalink)
 
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Stock manifold and quadrajet will work fine. They should be pretty available. Might want to price out an aluminum manifold (Holley or Edelbrock) and something like a Holley carb to compare. I have both set-ups (on different cars) and both will run strong - also should help you burn more gas!
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Old 02-28-2006, 09:12 AM   #4 (permalink)
 
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I have a line on a Stock 1971 Quadrajet for a Pontiac 400. So, I can bolt this right on without getting a new manifold?

Thanks,

Jason
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Old 02-28-2006, 09:18 AM   #5 (permalink)
 
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The 4-bbl conversion is very straight forward, but you do have a couple of options to be aware of.

The factory cast iron Q-Jet manifolds are actually pretty good. And they're cheap. The manifolds used from 1967 through 1972 use a "divorced choke" system, utilizing a bimetallic spring mounted to the manifold exhaust crossover. In order to use this choke system, you need one of the early-type divorced choke Q-Jet carbs.

After 1973, the manifolds and the carbs used an integral hot air type system, utilizing 2 tubes sticking up out of the manifold crossover, feeding hot air to an integral choke on the carb. These carbs are nice, because you can install a 1980 electric choke into the carb and eliminate the hot air system (allowing you to block the exhaust crossover).

The best factory manifolds are the aluminum Ram Air and HO manifolds. These work great, but are getting pricey.

For a nice weight savings, you can use the Edelbrock Performer series manifolds. The Performer RPM is an outstanding manifold that will actually outperfrom the factory iron unit, and it's available at a modest cost. It is drilled for both the Q-Jet and the Holley/BG bolt patterns, so you can use any carb on it (a very nice benefit). Before running this manifold, be sure to go to the Edelebrock web site and check the height of the manifold against your available hood clearance.

Although your engine is a low-comrpession engine with the small valves, you'll still get a nice performance enhancement with the 4-barrel conversion. Next step after that will be large valves, a cam, and a pair of headers...
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Old 02-28-2006, 09:19 AM   #6 (permalink)
 
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Quote:
Originally Posted by jpdog357
I have a line on a Stock 1971 Quadrajet for a Pontiac 400. So, I can bolt this right on without getting a new manifold?

Thanks,

Jason
No. You need the 4-barrel manifold with it.
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Old 03-01-2006, 07:47 AM   #7 (permalink)
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I just sold my 68 firebird convertible that came from the factory with a 350-2bbl.

A few years ago I replaced the intake and carb with an edelbrock performer and a quadrajet for a 73 buick. At the same time i replaced the water pump, timing chain, installed summit racing cam and lifters, an electronic ignition from a mid 70s GM and replaced the generator with an alternator with built in voltage regulator. all nice tasteful updates.

Bottom line, the car was slower off the line, but at speed when i floored it and the secondaries opened up it was faster. I supposedly got better gas mileage. Why? because instead of being fed constantly with two big barrels, the car was fed with two small barrels, hence it was slower for everyday driving. Once floored, instead of haing two big barrels it was fed with two small barrels and two big barrels.

Just my personal experience.
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Old 03-03-2006, 09:14 PM   #8 (permalink)
 
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I had a 1970 Trans Am and worke on Pontiac BBs for a bit.
The new Edelbrock aluminum intake is patterned after the old 4bbl intake. It is an excellent intake. Otherwies any factory 4bbl intake from the performance years will do fine1968-1972. Q-jet 4bbl is a fine carb. Primaries are small for good throttle response and gas mileage.

You do have the small valves in a 2bbl. Now look for either 68-70 high compression heads or 71-72 low compression 4bbl heads. I do not know if you can open your heads for the 2.11/1.77 valves of the other heads.
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Old 03-04-2006, 10:37 AM   #9 (permalink)
 
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Quote:
Originally Posted by Felix C.
I do not know if you can open your heads for the 2.11/1.77 valves of the other heads.
Yes, the small-valve heads can be set up for the large valves, and they will flow great if you blend the over-bore back into the short-side radius and back into the inlet port about 1/2". SI and Ferrea make some great stainless valves for Pontiacs in 2.11/1.77 sizes - they're swirl polished, necked, and they really flow well. Problem is, that by the time you buy the valves, springs, keepers, retainers, screw-in studs, guideplates, and pay someone to do the conversion to the large valves and the screw-in studs, you're getting pretty close to the cost of a new set of Edelbrocks...
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Old 03-17-2006, 10:57 AM   #10 (permalink)
 
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Hi all! I'm new to this forum and have a question on this thread. I wanted to read through the archives before I post the question I came here for originally.

I am also thinking about replacing my 2bbl carb with a 4bbl Edelbrock. What I'd like to confirm is if all I need to get is the 4bbl carb and a new intake?
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