[dpoltzer]

I am beginning my search for a Reasonably correct carb for my 1971 Formula 455. I believe that I need a 704126or7_. My question is: what was the original/correct CFM rating for this application and are all 704126or7_ carbs rated the same? I am not too concerned with date codes but want the right carb for the car for performance and correctness. Thanks much.

[74Grandville]

750cfm, 7041267

[Kenth]

Std 455 A/T engine code YC uses a 7041262, 750 cfm Q-jet.
455HO M/T engines codes WL, WC uses a 7041267, 800 cfm single booster Q-jet.
Oddly enough 400 M/T uses a 7041263, 800 cfm single booster Q-jet.

[dpoltzer]

Great info guys and many thanks.

[Cliff R]

The actual CFM from the Quadrajet carburetors are application specific. They basic castings come in two flavors, 750 and 800 cfm. As delivered they can vary considerably from those numbers depending on how long the stop is on the secondary airflap shaft to limit the opening angle of the airflaps in the airhorn.

The factory also limited to open angle of the huge secondary throttle plates in the baseplates on many applications to limit cfm as well.

Good examples of this are early Firebird carburetors that were delivered on the 400 engines. I've seen this quoted many times as done for "insurance reasons", but for whatever reason for sure they limited the open angle of the secondary throttle plates and had a longer stop to limit the air flap open angle as well. This is mentioned in several sources in print on early Pontiac stuff.

Anyhow, the factory also made some higher CFM models in 1971 as mentioned. They basically took the standard "750" cfm castings and removed the outer booster rings to increase cfm. Those castings are also "dubbed" 800cfm, but actually testing puts them at 827cfm.

At the same time Rochester released the first larger castings for the 1971 Buick 455 engines. They are also dubbed "800" cfm but use a short stop on the secondary airflap shaft and flow 849cfm.

The larger castings showed up for Pontiac in 1973 and were released on the 455 Super Duty engines, part numbers 7043270, 7043273, 7044270, and 7044273. Those were also produced as Service Replacement part number 17054910 and 17054911.

Edelbrock came out with a line of Q-jets and their 1904/1905 were dubbed "800" cfm and the 1910 were advertised at 850cfm. The 1910 is the EXACT same casting as the 1904/1905's just has a shorter stop to allow the secondary airflaps to open to 1.270" (just a little less than .100" when compared to the 1910's) as shown in my book.in my book on page 118.

There is also information on how to make a large cfm q-jet by removing the outer booster rings on page 116. I've done quite a few of this at this point and have supported over 700hp successfully with them in full race applications, these will flow apprx 897cfm.

I'll add here that Edelbrock patterned their 1904/1904 and 1910's after Chevy truck Q-jets produced in 1979-1980, and they are nearly identical to part numbers 17080212 and 17080213.

So when it comes to cfm there folks still "regurgitate" old information calling all q-jets either 750 or 800cfm. I typically stay out of these conversations as there is always some "resident expert" someplace that wants to argue the details. It's about the same as ridiculous comments we still see that the fuel bowl on the Q-jet is too small for high performance use. It wouldn't matter if it held 6 ounces of fuel or 60, you just need to take steps to keep it FULL on hard runs. The real troubled spot is that you are feeding the engine with only one needle/seat assembly, so ALL the fuel to feed the engine has to pass thru on small hole, not two of them. If we were arguing performance "potential" here the Q-jet design is EXCELLENT. The fuel bowl is in the center of the carb, jets on the bottom of the bowl and excellent venting (with few exceptions). I am constantly contacted by folks who are using carbs for off-road, rough terrain, rock climbing, etc wanting us to build them as they have all sort of issues with other designs and that doesn't surprise me in the least.

Anyhow, back to cfm. We also saw Rochester take the larger castings and install really long stops on them to limit cfm and use them on the Pontiac 301's and other small CID engines. I never spent the time/funds to CFM test any of those but I would estimate them around 500-550cfm or so. It's also interesting that Rochester also cross-drilled the fuel supply nozzles when the severely limited to opening angle of the flaps to insure adequate fuel delivery to the engine. So they actually made a wide variety of different CFM models if anyone is taking notes........Cliff

[dpoltzer]

Wow and thanks for all of that great info Cliff. I really appreciate it.

[Cliff R]

One additional note on CFM, and this one should save folks some time/funds when you are choosing and/or setting up a Q-jet for your application.

We have the smaller "750" CFM castings out there in full race applications well into the 9's in Super Stock, and deep into the 10's in Stock Eliminator.

Modified versions are also pushing FAST Class cars into the 9's on narrow bias ply tires. Some of these engines will make nearly 800hp to run those numbers. Coincidentally just got a call Friday from a customer who had us set up a spare 1971 Pontiac 7041263 carb (single booster same as the 455 HO castings) for his legal Stock Eliminator 400 CID Pontiac engine. His engine made 545hp on the dyno and he ran the carb right out of the box and no tuning anyplace. He said it was within 2hp of the Q-jet he uses on that engine and has "tweaked" for many years. I consider that a good results because I set them up just a tad "fat" when they leave here. No sense putting any pistons in the oil pan from our efforts. I'll leave that up to the customer if they decide to chase the power around some by leaning it up!

I've also done considerable dyno and track testing with different CFM Q-jets, and back to backed them at the track against Holley, Quick Fuel and Demon carbs to 1000cfm. My engine making about 550hp runs very close with all of them. I only saw about .02-.03 seconds between a 750 cfm Ram Air carb and a 1971 455 HO carb on back to back runs. The HO carb also ran a little faster in MPH. The ET doesn't change that much simply because my engine spends very little time up near the shift point on a full run. It ET's well because it leaves really hard, so running a little harder on top end isn't going to pick the numbers up much.

https://www.youtube.com/watch?v=6zVdoLR-VzM

Right on the dyno on a 455 topped with Edelbrock aluminum heads I back to back tested the same 750cfm Ram Air carb, an HO 455 carb, and my 1977 Pontiac carb (850cfm). The power numbers were dead even to about 4500rpm's and from there to the shift point (peak power at 5800) the larger carbs made a little more power at every RPM.

As far as removing the outer booster rings from a larger casting as we show in my book, those work very well in full race applications. One of the first ones we did and actually got number on it was about 20 years ago for Tommy Thompson's GTO running high 9's/low 10's. He tested it against his very expensive custom built Quick Fuel carb and they run about the same. He uses and prefers the Q-Jet because even with the outer booster rings removed it drives better in the pits and back the return road, and much smoother staging the car when he come up on the converter (foot braked).

Since then we've built a good many of them and a few are being used in FAST on the fastest GM entries, so they will support great HP and make excellent racing units. I still tend to shy away from them for street use. Loosing the outer booster ring is a trade of for smoothness and efficiency in the "normal" driving range. They do work very well for "spirited" driving on the primary side only, much like a large CFM Holley style carb used in the same application.........Cliff

['ol Pinion head]

Originally Posted by dpoltzer

I am beginning my search for a Reasonably correct carb for my 1971 Formula 455. I believe that I need a 704126or7_. My question is: what was the original/correct CFM rating for this application and are all 704126or7_ carbs rated the same? I am not too concerned with date codes but want the right carb for the car for performance and correctness. Thanks much.

71 Pontiac 455 & 455 HO Qjets:

'71 455 Dport, engine code YC took a 7041262, need one of these, feel free & note the date build of your Formula, have several.

455 HO, engine code WL & WC with dual snorkel dual heatriser aircleaner took a 7041267
455 HO, engine code YE, with dual snorkel dual heat riser aircleaner took a 7041268.
455 HO engines with factory installed Ram Air received 7041270 or 7041273's depending on whether automatic or manual trans engine application.

all of the original production run '71 & '72 455 HO application Qjets are expensive, have been for decades. Every now & then a SR version will pop up for sale for a fraction of a production run version.

[Cliff R]

I just sold a 7041273, it was the last 1971 HO carb we had in the inventory. I had an original 7041263, not SR, sold it about a year ago after sitting on it for over 10 years, so little demand for those.

I've purchased and sold at least a dozen 7042168 SR units, and nearly that many 7041263's, all to folks who no longer had their original 7041267, 7041270, or 7041273 carb.

At one point they allowed Pontiac powered SA and SS cars to use the 1971 single booster carbs even if they were not 1971 model years vehicles. That sparked quite a demand and drove the prices up a bit. To this day they are pretty rare unless SR's.......Cliff

[dpoltzer]

I did locate a 7041262 for my car late last week but have not installed it yet. Thanks again everyone! Dave

[unruhjonny]

I have tried to convey this exact thing to some people, but am generally scoffed at - so seeing the source, it bears repeating.

Originally Posted by Cliff R

... I've also done considerable dyno and track testing with different CFM Q-jets, ... I only saw about .02-.03 seconds between a 750 cfm Ram Air carb and a 1971 455 HO carb on back to back runs. The HO carb also ran a little faster in MPH. The ET doesn't change that much simply because my engine spends very little time up near the shift point on a full run. It ET's well because it leaves really hard, so running a little harder on top end isn't going to pick the numbers up much.

https://www.youtube.com/watch?v=6zVdoLR-VzM

Right on the dyno on a 455 topped with Edelbrock aluminum heads I back to back tested the same 750cfm Ram Air carb, an HO 455 carb, and my 1977 Pontiac carb (850cfm). The power numbers were dead even to about 4500rpm's and from there to the shift point (peak power at 5800) the larger carbs made a little more power at every RPM.

...

I am glad to read you sourced a 7041262 - quite frankly, if I were you, that's the exact carb I would have sourced unless I found a smoking deal on the correct one.

[meangene]

i have a 71 single ring booster carb will get rid of if i can get a 7041264 auto carb with august date code this is not a sr carb, other wise i will just leave it on the car till i get one, everything number matching except carb and wrong date code head

[meangene]

thank to everybody for the help getting the right carb and date code

[carbking]

The reason Rochester changed to the larger primary side was street driveability, not total CFM.

On a street engine that generally lives at much less than WOT, the larger primary side made for a smoother transition from primary to secondary.

Jon.

[Cliff R]

Exactly Jon, and it's advantageous to have MORE cfm available primary side only so many driving scenarios (climbing steep grades for example) can be accomplished without needing to get into the secondaries.

Many later model Q-jet also used a longer arm for the link to the mechanical secondaries for even greater cfm capabilities on the primary side without the secondaries coming in for this reason......Cliff