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#1471677 Wed Oct 19 2022 05:40 PM
Joined: Sep 2011
Posts: 2,726
L
Lugnutz Offline OP
'Bolter
Side-by-side, is a Rochester 4jet comparable to the performance of a Rochester Quadrajet?

I have heard good things about Rochester Quadrajet carburetors. However, I have not been able to locate an early Rochester Quadrajet with the correct CFM numbers for my 1965 SBC 283.
I think the early Chevy SBC 283 engines had a four barrel intake with a Rochester 4jet.

Joined: Feb 2004
Posts: 25,656
H
Kettle Custodian (pot stirrer)
The Rochester 4G carburetor was developed in the early 1950's. It's a square bore carb with some very tricky adjustments necessary for proper performance. The float chambers are so small that a minor error in the float level setting will result in a bad lean surge on hard acceleration caused by the float bowls getting sucked dry. The Buick and Oldsmobile V8's were some of the first engines to use it, along with Cadillac and Pontiac a bit later. Dad raised such a stink about not being able to make them run right that a Rochester field engineer started using his shop as a southeast area headquarters, and the modifications they developed working after hours were incorporated into the following year's production carbs.

The Q-Jet is a much better carburetor than the 4G series- - - -one calibrated for a 305 Chevy or a 307 Olds/Cadillac engine should get very close to being right for your 283, particularly with the cam you're running. Since the Q-Jet has a spring loaded air valve in the secondary side and very small primary bores, the CFM rating is irrelevant. It's impossible for the carb to open up and try to flow too much volume. The QJ also has 3-stage primary venturis, so there's a progressive transition between RPM ranges that happens long before the secondary barrels start to function. It's one of the best carburetor designs ever. The lower flow rate QJ carbs such as the ones designed for the 230 and 250 cubic inch Pontiac overhead cam sixes have a restricted travel of the secondary air valve which results in a reduced CFM capacity.
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

Love your enemies and drive 'em nuts!
Joined: Nov 2002
Posts: 2,465
C
Carburetion specialist
To add to Jerry's comment above:

The biggest hurdle in adding a Q-jet to a 283 is finding an intake manifold.

DO NOT EVER USE A SPREAD-BORE TO SQUARE-BORE ADAPTER PLATE (think NASCAR restrictor engines)

Up to and including 1974, virtually all Q-Jets were 750 CFM, a few were 800 CFM, and a very tiny few were 850 CFM (and currently worth a small fortune!).

But, as Jerry mentioned, the Q-Jet is an air-valve carburetor, so read the CFM as follows:

750 is 150 fixed plus 600 variable
800 is 200 fixed plus 600 variable
850 is 250 fixed plus 600 variable

And as Jerry said, some were equipped with a restrictor bracket on the air valve to reduce??? performance to comply with the GM equation for horsepower versus weight (exception - Corvettes).

The smaller Rochester 4G series carbs were about 350 CFM (read as 175 + 175) And 175 is pretty close to 150 or 200 wink

By 1955, the 4G was an excellent carburetor, and while the Q-Jet is somewhat more technologically advanced, the 4G is more reliable. The 1952~1954 4G's are best left to the numbers matching folks.

If you are running a basically stock 283, you will gain no "seat of the pants" performance increase by going to a Q-Jet; although a dynamometer MIGHT measure a tiny increase.

Two considerations when rebuilding a 4G carburetor:

(1) read the GM / Rochester bulletin on adding a bushing in the vacuum circuit to an airhorn mounted automatic choke, and add it if it has not already been added.
(2) Rochester (like Carter) used DIFFERENT FLOATS THAT LOOK IDENTICAL UNLESS ONE KNOWS WHAT TO LOOK FOR! As it was determined that on some units the primary fuel bowl should have a higher fuel level than the secondary bowl, the different floats were used. The difference is in the height of the pontoon where the arm is attached to the pontoon. The same arms and same pontoons were used, just soldered at a different height. DO NOT MIX THEM! The different floats were necessary as the maximum force created by the buoyancy of the float transferred to the valve occurs when the float arm is perpendicular to the valve.

There are more vendors supplying Q-Jet parts, but, with the exception of SOME of the 19 different floats used in the 4G, ALL calibration parts for the 4-G are available mail order.

Unless drag racing with a really modified engine, I would personally not make the change (and the Q-Jet is one of my top three 4-barrels).

Jon


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
[image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Joined: Nov 2002
Posts: 2,465
C
Carburetion specialist
And rereading Jerry's post, you will look long and longer to find one of the Q-Jets with the air valve restrictor still on the carburetor. Most were removed 3 nanoseconds after the warranty expired. The others??? were removed by independently wealthy buyers 1 block from the dealership after the delivery of the new car. wink

Jon


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
[image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Joined: Sep 2011
Posts: 2,726
L
Lugnutz Offline OP
'Bolter
Jon,
Since we spoke I have searched for a decent Qjet. I never bought one mainly because I didn’t have the correct intake with an oil fill tube up front. I could swap to a correct SBC intake, but then I’d need to change valve covers for the oil fill and then my 283 would look just like every other 350. I wanted to keep the vintage 283 look. I currently have a nice Rochester 2Jet. The engine is 0.060 over, putting it in the 302ci ballpark.

I have a vintage 283ci cast iron manifold correct for a 4Jet and have seen adapter plates to mount a QJet. The adapter has 4 holes. Two holes taper down to the size of the 4Jet size.

Last edited by Lugnutz; Wed Oct 19 2022 07:58 PM.
Joined: Nov 2002
Posts: 2,465
C
Carburetion specialist
The adapter plates are useful for five classes of folks:

(1) used car salesman
(2) 15-year-olds with more dollars than sense wink
(3) gardeners with a rabbit problem in the garden, and a strong right arm wink
(4) the manufacturer
(5) the seller

In 46 years of doing carburetors professionally, I have YET to see a spread-bore carb running through an adapter plate to a square-bore intake, that ran as well as the square-bore, let alone better!

Jon


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
[image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Joined: Dec 2008
Posts: 1,837
P
'Bolter
The 4G and QJ flange patterns have such small overlap in the adapter and gaskets that air leaks are common. The new placement of the throttle plates also affects cylinder-to-cylinder mixture distribution.
I've had excellent results tweaking the QJ with minimal time & expense (since 1970). There are also far more anecdotal reports on line for setting up the QJ for various engines, far more than the 4G.

Joined: Feb 2004
Posts: 25,656
H
Kettle Custodian (pot stirrer)
If you look at the adapter closely, there's probably a stamp somewhere that indicates "Made by P.T. Barnum"!
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

Love your enemies and drive 'em nuts!
Joined: Nov 2002
Posts: 2,465
C
Carburetion specialist
Originally Posted by Lugnutz
Side-by-side, is a Rochester 4jet comparable to the performance of a Rochester Quadrajet?

I think the early Chevy SBC 283 engines had a four barrel intake with a Rochester 4jet.

The 283 Chevy with 4-barrel came with both Rochester 4-GC and Carter WCFB. Either original carb would be an excellent choice for a basically stock or somewhat modified 283.

I personally prefer the Carter because of Carter's metering rod technology, which slightly improves A/F ratio AVERAGE over the complete range of RPM.

The metering rod technology can be easy / difficult to tune, based on the enthusiast's skill, and inventory of different metering rods.

Once tuned, the WCFB (opinion) is SLIGHTLY more efficient than the 4-GC.

As a general rule, the WCFB will cost more than the 4-GC, so this can be a consideration.

And Jerry, your Dad must have done some excellent work on the early 4-GC carbs, as after about a 3-year "teething" problem, they became excellent carbs.

Jon.


Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify
If you truly believe "one size fits all" try walking a mile in your spouse's shoes!
[image]http://www.thecarburetorshop.com/Avatar.jpg[/image]
Joined: Feb 2004
Posts: 25,656
H
Kettle Custodian (pot stirrer)
His 34 Ford dirt track race car ran a 300 cubic inch flathead V8 with a highly modified big Zenith 2 barrel for a Mack inline six, running on methanol and Hydrogen peroxide. They routinely outran the guys running three Stromberg 97's!
Jerry


"It is better to be silent and be thought a fool than to speak and eliminate all doubt!"
Abraham Lincoln

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self.
Ernest Hemingway

Love your enemies and drive 'em nuts!
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