[Schurkey]

The 850 Edelbrock RPM Q-Jet used the same dimensions as the other "800 cfm" Q-jets.

A 750 Q-jet will support ~600 horsepower. If you need more primary throttle than the 750, there's still plenty of 800s ready-designed and ready to be rebuilt and calibrated. No point in reinventing the wheel. Yes, sometimes a bit more primary throttle is useful on certain applications.

I've got zero experience with the big Thermoquads; but they sound like fun.

Originally Posted by carbking

To compare CFM ratings, one needs TWO numbers.....CFM and rating vacuum.

...and whether or not the rating is for wet or dry flow. There's probably more still to be specified--intake air humidity and temperature, for example.

[ponyakr]

"The 850 Edelbrock RPM Q-Jet used the same dimensions as the other "800 cfm" Q-jets."


Edelbrock listed 750, 795, and 850cfm Q-jets. So what you are saying is that the 850 number was bogus--just an advertising gimick, to help sell 'em ? Sorta like overstating or understating horsepower ratings to serve the car builders purposes ?

And what about the SMI Quadzilla and others they say are over 800cfm. Are they using inflated numbers too ? There are lots of SS Pontiacs running in the 9's with Q-jets, which by the rules are supposed to be 750cfm. What cfm do most of them flow ?

And I've noticed that NHRA allows the 1901 E-Q for engines that came with a 750cfm Q. And they will allow a 1903 E-Q for engines that came with an 800cfm Q. If the 1910 is exactly the same cfm as a GM 800cfm, then I wonder why NHRA does not allow it, in place of a GM 800 ? Are they just going by the advertised numbers ? Or have they actually tested these carbs to see what they flow ? Just curious.

[MrWrestlingII]

FWIW- The primary throttle response on my 1977 800cfm qjet is unbelievable. Guys who run Holley's have commented how good the throttle response is on my car.

After a well planned combo, carb and distributor tuning is the key. Along with advice and parts from Cliff I spent allot of time playing with the vacuum advance, then after replacing the Morrosso dist weights for genuine GM parts part throttle response went from good to great.

[ta man]

Originally Posted by MrWrestlingII

FWIW- The primary throttle response on my 1977 800cfm qjet is unbelievable. Guys who run Holley's have commented how good the throttle response is on my car.

After a well planned combo, carb and distributor tuning is the key. Along with advice and parts from Cliff I spent allot of time playing with the vacuum advance, then after replacing the Morrosso dist weights for genuine GM parts part throttle response went from good to great.

X2 other than the Quad..stable timing at all rpms and correct acc pump settings and supply does wonders for throttle response.

[77 TRASHCAN]

I'm with Schurkey, trying to re-invent the wheel???, no need, my opinion...

An 800 cfm Q-Jet IS an awesome thing, properly tailored to your specific engine!!!

The last section of Cliff's Q-Jet carb book, he took a virgin 850 Edelbrock Q-Jet apart showed us "what" it's about. measured the air flap angle, and he has mentioned that he has worked on many of them. The cfm of an 800 cfm Q-Jet might be able to be taken over 800 by adjusting the air flap (grinding the stop arm some). I'm not the experienced expert that works on this stuff every day like Cliff, Jon and others. A 800 cfm Q-Jet w/ the air flap properly adjusted and the outer booster ring removed, may be close to 900 cfm.....just guessing mind you...anyone???

[Skip Fix]

A friend just got one of the new Demon Thermoquads for his Mopar 383 stroker(490ish) and so far has been really pleased with it. I think 750 is the max they come though.

[Skip Fix]

Primary have a shaved throttle shaft and a "103 blade that is 1.437 vs 1.370 for a "stock" 800 cfm Q jet throttle bore.

[torqjunki]

Just to be clear, primary throttle response and primary throttle power or torque are different animals. All the q-jets I have run have had excellent throttle response due to the small efficient primary bores. Blip the throttle and they are instant. A well built 455 with increased head flow, performance cam and intake, headers and optimum compression will be even more snappy with a properly tuned holley style carb because it can use the additional airflow. I have a shelf full of carbs and intakes and have swapped and tuned each so much it makes me dizzy. Even have a Demon Sizzler built TQuad that is supposed to flow over 1000 cfm. If you take your commonly built 455 with a q-jet, wire the secondaries shut, go out and hammer it, it will make you puke! Do the same with a Holley 750/850 and you will immediately feel the difference. Undeniable fact, the q-jet primaires are designed for economy and do an excellent job with that. No q-jet basher here, I love them! They are great for cruiser with economy in mind and also for drag car running at WOT. If you dig that part throttle torque that pushed you in the seat when you drive your Pontiac, the Holley style carb will put a big smile on our face. Tipping in the secondaries on the q-jet will work, but traction with street tires will be an issue.

[PAUL K]

Originally Posted by torqjunki

Just to be clear, primary throttle response and primary throttle power or torque are different animals. All the q-jets I have run have had excellent throttle response due to the small efficient primary bores. Blip the throttle and they are instant. A well built 455 with increased head flow, performance cam and intake, headers and optimum compression will be even more snappy with a properly tuned holley style carb because it can use the additional airflow. I have a shelf full of carbs and intakes and have swapped and tuned each so much it makes me dizzy. Even have a Demon Sizzler built TQuad that is supposed to flow over 1000 cfm. If you take your commonly built 455 with a q-jet, wire the secondaries shut, go out and hammer it, it will make you puke! Do the same with a Holley 750/850 and you will immediately feel the difference. Undeniable fact, the q-jet primaires are designed for economy and do an excellent job with that. No q-jet basher here, I love them! They are great for cruiser with economy in mind and also for drag car running at WOT. If you dig that part throttle torque that pushed you in the seat when you drive your Pontiac, the Holley style carb will put a big smile on our face. Tipping in the secondaries on the q-jet will work, but traction with street tires will be an issue.

Truth!

[carbking]

Originally Posted by torqjunki

Just to be clear, primary throttle response and primary throttle power or torque are different animals. All the q-jets I have run have had excellent throttle response due to the small efficient primary bores. Blip the throttle and they are instant. A well built 455 with increased head flow, performance cam and intake, headers and optimum compression will be even more snappy with a properly tuned holley style carb because it can use the additional airflow. I have a shelf full of carbs and intakes and have swapped and tuned each so much it makes me dizzy. Even have a Demon Sizzler built TQuad that is supposed to flow over 1000 cfm. If you take your commonly built 455 with a q-jet, wire the secondaries shut, go out and hammer it, it will make you puke! Do the same with a Holley 750/850 and you will immediately feel the difference. Undeniable fact, the q-jet primaires are designed for economy and do an excellent job with that. No q-jet basher here, I love them! They are great for cruiser with economy in mind and also for drag car running at WOT. If you dig that part throttle torque that pushed you in the seat when you drive your Pontiac, the Holley style carb will put a big smile on our face. Tipping in the secondaries on the q-jet will work, but traction with street tires will be an issue.

In a way, the Carter engineers said the same thing with the TQ, by increasing the airflow on the primary side from the 200 of the larger Q-Jet to 250 on the carbs used on the Chrysler 440's (which are also pretty torquey engines).

Jon.

[BILTIT]

Run two Q-jets!!

['ol Pinion head]

Originally Posted by carbking

Skip - somewhere (don't ask me where, I don't remember) I have a flow test of the single booster 1971 Q-Jet. Memory says 839 CFM (memory is not perfect ). Of course, these things are pretty scarce and not inexpensive....

Jon.

Over 20 years ago, the flow test on the single booster ring Qjet was done by the fellow in Alabama(?), his finding was indeed 838 cfm, always wondered if he tested at the time a known 800 cfm Qjet and it flowed 800 cfm.

Unfortunately for us 71 HO restorers that blurb and another short tech article in Smoke Signals drove the price of even the most common single booster ring Qjets through the roof. With all that's been learned about the signal from single booster ring qjets, my own belief is one is better off with a factory 800 cfm Qjet, unless have to have the single booster ring Qjet for numbers matching sake.

[carbking]

Originally Posted by 'ol Pinion head

Over 20 years ago, the flow test on the single booster ring Qjet was done by the fellow in Alabama(?), his finding was indeed 838 cfm, always wondered if he tested at the time a known 800 cfm Qjet and it flowed 800 cfm.

Unfortunately for us 71 HO restorers that blurb and another short tech article in Smoke Signals drove the price of even the most common single booster ring Qjets through the roof. With all that's been learned about the signal from single booster ring qjets, my own belief is one is better off with a factory 800 cfm Qjet, unless have to have the single booster ring Qjet for numbers matching sake.

OPH - for street use, I would definately agree.

As to the 800 flowing 800:

Something all of us should remember when tossing around CFM numbers.....virtually NO carburetors flow exactly their "rated" CFM. The engineers designed carburetors based on standard sizes (1 3/8, 1 1/2, etc., etc., etc.) and THEN the carburetors were flowed. There was no attempt to make a venturi of 1.37212548 (or some other silly number) in order to have an EVEN CFM rating.

Then someone (probably NOT an engineer) decided what to call the rating. The published rating may have been more, or less, than the actual rating, depending on what the "rater" was trying to accomplish.

In an earlier post, I mentioned that one of the later companies wanted a "600" to compete against the Holley 600, and just renumbered a 625 to a 600.

The flow test of the Holley RA V carb shows 812 CFM.

The flow test of the Carter 3636s was 939 CFM at 1 1/2 inches and 1128 at 3 inches.

Flow tests of most of the early Pontiac AFB's were in the 610 range, but that was sabotaged down to about 575 to allow the tripower engines to outperform the less expensive AFB engines. Of course, the knowledgeable mechanic could easily re-acquire those 35 CFM.

"Truth in advertising"??? - oxymoron

Jon.

[sdx455]

This has been siting on shelf 10 years , used 2 months while carb was at cliffs . drained and boxed