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Not saying it couldn't happen, and W30 and W31 QuadraJets had some "differences" , but 735 and 800 are the only cfm ratings I've ever heard for QuadraJets.
This should make you question everything else they said in that video...
All Qjets made in the 1970 model year were 750 CFM. The bores in the venturis in the main body determine the CFM and there are only two versions, 750 and 800 CFM. The 800 CFM versions first came out in the 1971 model year in very limited applications. Of course that's the max possible CFM. In many applications the actual CFM was reduced by limiting the travel of the secondary air valve. For example on the Olds 307 motors that air valve only opens to 70 deg vs 90 deg on other Qjet versions. Here's how to tell a 750 from an 800.
Edelbrock bought the tooling from Rochester when GM ended Q-Jet production. They release their own line of Q-Jets, the top dog is the 1910. It was rated at 850 CFM. I have no idea if that’s accurate or an inflated number. If it is accurate, I have no idea what they did to add the extra CFM I know for a fact it’s a kick *** carb. I ran one on my car for years, as did several friends.
For my ‘66’s I’m partial to the later qjet designs from the ‘70’s mainly for the primary “bump” design & reputed 800 CFM capacity which I’m inclined to believe.
Somewhere in the past 40 years I gained the impression that 800 CFM qjets were a bit quicker off the line as compared to the more widely-available 750’s. So I’ve stuck with them. I gave all my earlier qjet parts to my mechanic. Hopefully he is deploying good used parts to his other customers. I just didn’t want to take a chance on mixing early- and late- qjet parts to ill effect.
For reference, the 800 CFM versions I use are all of the 170 series designs (M4MC(?)), not the earlier 70 series design which topped out at 750 CFM owing to slightly (slightly) narrower primaries. It’s confusing since the earliest qjets were rolled out sort of in testing mode. There were a bunch of field-derived service bulletins in ‘66, ‘67 and ‘68. After that emissions came in shoving all the manufacturers away from power, then the gas crisis hit in ‘74, pushing performance yet further away in the name of fuel economy. By the time we got to ‘77 or ‘78, emissions was the primary target and the fine-tuning ability of the qjet allowed it to survive advancing emissions standards.
The really funny/weird part is that Rochester built a _bigger_ carb (800 vs 750 cfm) in the latter half of the ‘70’s. You’d think they would have reduced the CFM flow for marketing points. But no. Some unnamed engineer somewhere designed & put in production an emissions-legal carb that could be tweaked for performance. I don’t know who that is, but he/she is a hero of mine.
The other reason I use the later design is that from the 1966 introduction, by 77 or so, they’d had 10+ years to figure out how to make qjets great. The big advancement for me is the center fuel inlet on the Olds carbs, as opposed the more common passenger side inlet. The advantage, as I see it, especially in rebuilt carbs (like most carbs 40+ years later) is that the center inlet avoids a 90° turn for flammable fuel which most rebuilds fix with a “freeze plug”, often peened (pounded) in. These are subject to leaking gas on a bad day after a few hundred heating/cooling cycles. I had a fire due to a cheap side inlet rebuild in the 80’s, and have been scared ever since. I’m delighted Olds came up with a solution, albeit 10 or 11 years later. Factory derived safety? Sure, thanks!
It’s like disc brakes that can be retrofitted. If you’re into a factory-perfect restoration, go factory. But, if you can incorporate year-compatible improvements that came after your model year, especially for a driver, why wouldn’t you? At least that’s my view.
In my mind an 850 CFM qjet is entirely possible. There are a bunch of ways to get there - bigger primaries, hog out the secondaries, let the 2ndary air door go 90°+,etc.
It would be fun to put the question of whether these carbs _actually_ flow 850 CFM to Freiburger & Co. on Engine Masters. They’re not perfect, but are entertaining, and they’re doing A/B testing most of us would never do since we don’t own a dyno, nor have time and $ to burn to chasing claims & internet myths/assertions.
Enough backyard ranting. Hope this helps some of you along the way.
I'm guessing if I put that carb on my 1979 Hurst Olds 350 R motor I would drown it.....
The Beauty of Qjet is versatility and efficiency. Has been used from 305ci to 455ci by GM with pretty much the same basic casting. The internal calibration is mostly what governs it’s “output,” with various exceptions. Your 350 would be fine with nearly any Qjet as long as it has been calibrated correctly. A 350R doesn’t need an Edelbrock 1910 but would be happy with nearly any properly set up Qjet, even a 1910.
Edit - Joe got me (go figure), forgot about that one even though I know 2 people with OHC Firebirds.
….
Last edited by bccan; June 23rd, 2022 at 05:02 AM.
For my ‘66’s I’m partial to the later qjet designs from the ‘70’s mainly for the primary “bump” design & reputed 800 CFM capacity which I’m inclined to believe.
Somewhere in the past 40 years I gained the impression that 800 CFM qjets were a bit quicker off the line as compared to the more widely-available 750’s. So I’ve stuck with them. I gave all my earlier qjet parts to my mechanic. Hopefully he is deploying good used parts to his other customers. I just didn’t want to take a chance on mixing early- and late- qjet parts to ill effect.
For reference, the 800 CFM versions I use are all of the 170 series designs (M4MC(?)), not the earlier 70 series design which topped out at 750 CFM owing to slightly (slightly) narrower primaries. It’s confusing since the earliest qjets were rolled out sort of in testing mode. There were a bunch of field-derived service bulletins in ‘66, ‘67 and ‘68. After that emissions came in shoving all the manufacturers away from power, then the gas crisis hit in ‘74, pushing performance yet further away in the name of fuel economy. By the time we got to ‘77 or ‘78, emissions was the primary target and the fine-tuning ability of the qjet allowed it to survive advancing emissions standards.
The really funny/weird part is that Rochester built a _bigger_ carb (800 vs 750 cfm) in the latter half of the ‘70’s. You’d think they would have reduced the CFM flow for marketing points. But no. Some unnamed engineer somewhere designed & put in production an emissions-legal carb that could be tweaked for performance. I don’t know who that is, but he/she is a hero of mine.
The other reason I use the later design is that from the 1966 introduction, by 77 or so, they’d had 10+ years to figure out how to make qjets great. The big advancement for me is the center fuel inlet on the Olds carbs, as opposed the more common passenger side inlet. The advantage, as I see it, especially in rebuilt carbs (like most carbs 40+ years later) is that the center inlet avoids a 90° turn for flammable fuel which most rebuilds fix with a “freeze plug”, often peened (pounded) in. These are subject to leaking gas on a bad day after a few hundred heating/cooling cycles. I had a fire due to a cheap side inlet rebuild in the 80’s, and have been scared ever since. I’m delighted Olds came up with a solution, albeit 10 or 11 years later. Factory derived safety? Sure, thanks!
It’s like disc brakes that can be retrofitted. If you’re into a factory-perfect restoration, go factory. But, if you can incorporate year-compatible improvements that came after your model year, especially for a driver, why wouldn’t you? At least that’s my view.
In my mind an 850 CFM qjet is entirely possible. There are a bunch of ways to get there - bigger primaries, hog out the secondaries, let the 2ndary air door go 90°+,etc.
It would be fun to put the question of whether these carbs _actually_ flow 850 CFM to Freiburger & Co. on Engine Masters. They’re not perfect, but are entertaining, and they’re doing A/B testing most of us would never do since we don’t own a dyno, nor have time and $ to burn to chasing claims & internet myths/assertions.
Enough backyard ranting. Hope this helps some of you along the way.
Cheers
Chris
I agree. The 76 and later carbs are superior in many ways. I copied the Edelbrock 850 cfm air door secondary angle. The biggest fix needed is the super lean idle calibration. The goofy non CCC 80's Canadian carbs are so lean on idle calibration, they make those late 70's carbs look like performance carbs in idle and part throttle calibration! My 83 carb wouldn't idle properly on a stock 73 350 with probably 7.5 to 1 and shorty headers! I also thread the top of the air horn, add a plug and notch the APT adjustment like that Edelbrock Qjet. When I added the AEM wideband, it took 5 minutes to adjust from rich to right on. The Qjet is big part of why GM was better for years than the competition. I owned a Dodge with the Carter BBD, not great at all. The Ford Motorcraft carbs were all awful once the 70's hit. A 20 year Ford technician commented on how many less engine jobs the dealership I apprenticed at, did for motor jobs once Ford switched to EFI. Many carbed Ford's were towed in with stuck needles and seats. The Qjet and the reliable for the most part, HEI put on our great Olds V8, the 350 especially, in the mid 70's made for the best cars on the road at the time.
June 20th, 2022, 03:59 PM
