General Category > Quadrajet Carb Talk and Tips
TV cable to early Quadrajet
old cars:
The question is : Were these these these flow ratings determined with wet or dry flow. Since there are very few facilities that can provide wet flows but many can provide dry flow testing I can presume these to be dry flow. We know these are not Rochester wet flow testing numbers. Also flow testing can be done one barrel at a time which will skew results. Dry flow testing is approximately 8% higher than wet flow.
850 cfm less 8% = 782CFM ratings
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Sometime during the 1950’s, engineers found that a passenger engine with a four-barrel carburetor would not maintain a vacuum of 3 inches of mercury at wide-open throttle; and by some convention 1 ½ inches of mercury was chosen for rating 4-barrel carburetors. The ratings for 1-barrel and 2-barrel carburetors were left unchanged.
To convert from one system to another (with a very small percentage of error) is relatively simple. Simply use the square root of 2 (1.414). Thus to convert a two-barrel rating into a four-barrel rating, divide the two-barrel rating by 1.414. To convert the four-barrel rating to a two-barrel rating, multiply the four-barrel rating by 1.414.
This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).
As the general public was unaware of the “wet” versus “dry”, this system worked fairly well until the mid-1980’s when it seems that other scales were “needed”. No attempt will be made to explain ratings of carburetors produced after 1980. The best way to compare these units would be throttle area (which was a measurement used back in the 1950’s and 1960’s); or more accurately, the comparitive area of the main venturi. On the later carburetors, best to write to the company, and ask at what vacuum the carburetor was tested, and whether it was tested wet or dry. If this information cannot be obtained, then the rating is suspect.
So, to give an example of the 4 ratings scales prior to the mid-1980’s:
Using a Carter 4-barrel AFB carburetor that is rated wet and rated 500 CFM for an example.
To convert this rating to the 4-barrel dry scale, add 8 percent (multiply by 1.08). 500 CFM multiplied by 1.08 is 540 CFM.
To convert this rating to the 2-barrel wet scale, multiply the rating by the square root of 2. 500 CFM multiplied by 1.414 is 707 CFM.
To convert this rating to the 2-barrel dry scale, first convert to the 2-barrel wet rating as above, then add 8 percent. 707 CFM multiplied by 1.08 is 764 CFM.
Going the other direction, if we take a Rochester 2-GV carburetor that is rated wet and rated 435 CFM.
To convert this rating to the 2-barrel dry scale, add 8 percent. 435 CFM multiplied by 1.08 is 470 CFM.
To convert the rating to the 4-barrel wet scale, divide the rating by the square root of 2. 435 CFM divided by 1.414 is 308 CFM
To convert the rating to the 4-barrel dry scale, first convert to the 4-barrel wet rating as above, then add 8 percent. 308 CFM multiplied by 1.08 is 333 CFM.
Cliff Ruggles:
I'm not sure what method Edelbrock used to test their 1910 "850" cfm units. The 1910 is nothing more than a 1904, 1905 or 1906 with a shorter stop on the secondary shaft allowing them to open a tad further at WOT. The added holes and bent up "notched" flaps do NOTHING at all to improve cfm over the standard units. All of those units are based on a 1980-81 Chevy truck Q-jet.
I had a good friend with a flow bench make comparisons using one half of the carb to verify the numbers. He also used mathematical calculations to compare the results to carburetors (Holley) that were also tested.
He came up with "849" cfm for the later q-jets (larger castings) when the air door stop was ground as shown in my book.
The smaller castings were right at "750" cfm, the 1971 Pontiac 455 HO carb "827". He even tested a later carb with the outer booster rings removed at "897" cfm.
Matters not in the big scheme of things. What does matter is the power levels they will support.
Right on the dyno I've back to back tested a Pontiac 750cfm Ram Air carb, a 1971 455 HO carb, and my 1977 Pontiac carb set-up same as an Edelbrock 1910.
The engine was a 455 making right at 514hp. All carbs tested were equal to 4500rpm's then the two larger units showed a little more HP to peak power. There was almost no difference between the HO carb and my 1977 Q-jet.
I finished the testing at a private track rental. Remarkably the smaller carb wasn't much off the larger ones for ET or MPH, and the 455 HO carb ran the highest MPH for all runs even though it flows slightly less than the 1977 unit.
The entire spread of ET for all test runs was less than .05 seconds and just less than 2 MPH. The problem with track testing for cfm is that the car leaves just as hard with each carburetor and spends very little time up neat the shift point for the entire run. Folks who drag race will understand this and see why CFM isn't as big a player with these things as most would think.
I would also mention that I supply carburetors for vehicles use in FAST and Pure Stock racing. We've got a few into the high 9's on 8" wide bias ply tires. This clearly shows that a well set-up Q-jet is very capable as a "high performance" carburetor as the vast majority of folks reading this have cars that are much, much slower.........FWIW........
77cruiser:
Since they were based off the truck carb, did they have truck secondary linkage or car?
Cliff Ruggles:
Edelbrock pretty much duplicated the 17080212/213 truck units when they designed the 850 cfm 1910. They gave it a "fat" idle calibration and did some dumb work to the secondary air flaps. They also used a front pull-off with link up to the secondaries instead of a rear P/O with two links on it.
Nothing really "special" about a 1910 and if you know what you are doing you cam build one yourself from any 1979 or later truck carburetor.......
old cars:
"Since they were based off the truck carb, did they have truck secondary linkage or car?"
17080213 is a truck secondary linkage . I don't have a 1910 Edelbrock here
All of these truck linkage carbs have a late opening secondary and can benefit from manipulating this linkage to quicker opening secondary. The early quadrajets with the 3 point opening secondary linkage are the best, but all can be manipulated. Getting the secondary to open sooner is what works good because of the very small primaries. It will not effect cruise conditions if done right. This is where square bore carbs have the advantage. I adjust all quadrajets to have an earlier opening secondary, except for very few applications . It makes for smoother feeling driving experience and more responsive at lower speeds.
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