[Cliff R]

It works just like I said it did. All the Q-jets with the larger primaries are NOT "800" cfm, and never have been. The factory used quite a few different stops on the secondary shaft to control CFM potential on the smaller and larger castings. So anytime you see someone post that they are using or working on an "800" cfm Q-jet it could be anywhere from around 500 cfm if it came of a 301 Pontiac clear up to "850" cfm if it came off a 1971-74 Buick 455 or 73-74 Pontiac Super Duty.......FWIW....

[Charlie Brengun]

Originally Posted by Stan Weiss

Just how does that work? You mean the next time someone say run an 041 cam. Is or isn't that regurgitation old/outdated information. For me good information is good information no matter how old it is.

Stan

He's talking about people saying the quadrajet can't handle more then 6 psi of fuel pressure as one example.. or the other old wives tales that keep repeating itself when a quadrajet is mentioned.

[Ram Air IV Jack]

Originally Posted by Stan Weiss

Just how does that work? You mean the next time someone say run an 041 cam. Is or isn't that regurgitation old/outdated information. For me good information is good information no matter how old it is.

Stan

That sounds like a politician! Anything they say, expects everyone to believe it!!! Cliff is correct here and tries hard to wade through all the BS on the internet. BS information is not good, no matter how old/new it is!!!

[PAUL K]

Maybe this will help explain things


https://forums.maxperformanceinc.com...errerid=134156


Pontiac found the 850 Holley to make more power than the Q-jet on their SD-455 developmental engine that made more than 600 horsepower.

I'm sure as as heck not going to argue CFM rating but will say this as first hand knowledge and in no particular order

The Q-jet is a fantastic carburetor.

The 850 CFM Edelbrock carb flows more that an SD-455 carb on my flow bench.

The carb mentioned by the OP flows a ton more air than the 850 Edelbrock on my flow bench

The single ring booster and SD455 carb flow the same on my flow bench.

The 750 units flow less that the Buick Stage 1 carburetors

You increase the flow on a Q-jet and put it on a 700 HP Pontiac and it makes more power than it did before the additional air flow modifications.

You take an 800 horsepower engine and run a back to back test with a Dominator and a Q-jet and the Q-jet is going home black, blue and bloody.


You can twist numbers around all you want but at the end of the day you can't change the facts.

[Half-Inch Stud]

We should run the 3/4 Race Cam in the Q-JET Secondary Air Flaps right?

Seriously though, i kept my Q-JET activity at or below 6 PSI fuel pressure and would have benefitted to know an unbridled 8-9 psi was going to be okayee.

[Stan Weiss]

Originally Posted by Ram Air IV Jack

That sounds like a politician! Anything they say, expects everyone to believe it!!! Cliff is correct here and tries hard to wade through all the BS on the internet. BS information is not good, no matter how old/new it is!!!

Absolutely!

Stan

[Stan Weiss]

I was just looking a draft of an article that was sent to me to review a few years ago by the late Harold Betts. It is on Quadra Jet Carbs. I don't know who owns the copyright so I will not post the article. But there is a table which I will show a small piece of. Note how Harold is using 5.00 PSIG flowing.

Stan

[Formulajones]

I'm only stating what they said the carb was in the video, an 800 cfm built by Deano. No idea what has been done to it they don't go into any details

[Cliff R]

"The 850 CFM Edelbrock carb flows more that an SD-455 carb on my flow bench."

The Edelbrock 1910 is a copy of a 1980 Chevy truck carburetor part number 17080213. They set the full open angle of the secondary air flaps to open further and put two large round holes in them and bent up two taps and the trailing edge of them as well. It got a richer idle calibration but otherwise that's what it is, a copy of a Chevy truck carb.

The 1903-1906's and the exact same carb with the open angle of the flaps set to less distance. Edelbrock rated those at 795cfm which is probably about right.

I had several carburetors flow tested when I wrote the Q-jet book.

A 1969 RAIV carburetor was right at 750cfm.

A 1971 HO 455 carb 827cfm.

All the larger castings tested flowed right at 850cfm when set up in the same manner as the 1910's but no holes drilled in the plates or tabs added to them.

Removing the outer booster ring from a late model "800" cfm carb and using the same open angle as an Edelbrock 1910 we got 897cfm.

NONE of the carbs were touched anyplace with a grinder or sanding roll. I adjusted the full open angle of all throttle plates to exactly 90 degrees. The secondary throttle plate angle was also adjustable by grinding down the stop and adding an adjustmemt screw. That's how I came up with the dimensions in my book to set them. There is a poinit of no return with that deal and even worse when you go to far you start blocking fuel flow from the nozzles.

I also did some dyno and drag strip testing with several of them.

On a 455 with Edelbrock round port heads and RAIV camshaft the 1969 RAIV carb was down about 12hp to my 1977 Pontiac carb set to flow 850cfm. The power numbers were exactly the same thru 4500rpm's, from there on up the big 455 liked the 850cfm carb better.

At the track I tested the 750cfm Ram Air carb back to back against the 1971 455 HO carb and the 1977 Pontiac carb. Remarkably they ran very close in both ET and MPH. The 1971 HO carb for some reason ran more MPH than the larger 850cfm carburetor but we are talking about ..2-.3 so it may have just liked the tune on that one a tad better instead of the added airflow from the larger carburetor. The smaller 750cfm Ram Air carb was nearly as quick as the larger carburetors running within .03-.04 iin ET and giving up less than 1mph on top end.

At first that suprised me until you think about where ET is made at the track and the fact that the engine spends very little rime right up at the shift point in each gear especially if your car has a really tight/efficient converter and rear gears more suited to street driving that drag racing.

I've done a LOT more direct testing and mentioned some of it over the years on this Forum.

My 1977 Pontiac Q-jet which has never had a grinder or sanding roll touch it has NEVER once been outran by a Holley, Holley clone, or an Edelbrock AFB or AVS clone at any power level unless it was on a single plane intake WITHOUT a spacer on it. SIngle plane intakes don't like the huge secondary throttle plates from a spread bore carb sticking deep into the plenum areas. They aren't really all that happy with a square flange carb bolted directly to them and why I ALWAYS use at least a 1" spacer on a single plane intake, no exceptions.

I did some of that testing with witnesses and documented for the Popular Hot Rodding Engine Masters articles back in 2004. They folks writing the articles asked for the testing on the dyno and we carried the big Holley carbs to the track and the Q-jet outran them there as well. Of course you aren't seeing all that in print, one of my early lessons with that sort of thing.

I will mention that right after writing my book I gained a LOT of attention from the magazines and was asked to contribute. However they ended up NOT liking my carburetor, distributor and camshaft testing so very quickly brushed me under the rug. With that sort of thing you are NOT allowed to make MORE power than a "high performance" aftermarket part or you'll very quickly fall out of their graces. That's just the way it is with that sort of thing. I discussed this recently with Lars (the Corvette guy) and his testing with carburetors, distributors and such mimics my results almost exactly. I found it both remarkable and interesting that two dedicated tuners and carburetor builders from completley DIFFERENT backgrounds working 100 percent independent from each other came up with the same conclusions when it comes to these things........FWIW.......

[rtanner]

thank you all for the GREAT info, esp Cliff,
may be a stupid question, but here goes, on the later model 77-78, carbs, would there be any improvement by cutting off the choke horn and blending/radiusing the air entry there?
Also i use one of the ancient plastic flow smoother outer thingama jigs from the late 70-80s, any comment on those? thanks guys

[70GS455]

Interestingly enough, Vizard's personal car that he used for testing was a TA

[Cliff R]

"may be a stupid question, but here goes, on the later model 77-78, carbs, would there be any improvement by cutting off the choke horn and blending/radiusing the air entry there?"

Not sure as I've never removed one.

I've also done some testing with and without the choke flap and shaft installed and saw NOTHING, at least at my power level (well over 500hp and near 600ft lbs torque). I also supply a LOT of racing carburetors, pure stock FAST, and have done a number of truck pulling carbs (well known for that deal), and some Stock and Super Stock units. We've acheived some excellent results with the Q-jets and never touched them anyplace with a grinder or sanding roll. No "thinning" shafts or "knife edging" throttle plates, etc.

The two biggest gains I've seen in power are optimizing the angle of the throttle blades in the baseplate and the full open position of the airflaps in the airhorn. For really big power levels you'll see some improvement removing the outer booster rings to get the larger CFM units up to 900cfm.

The other MAJOR improvements in all cases are fuel delivery and installing the correct fuel inlet seat/float and getting the fuel pressure up to par. For some reason with these things, and the last time I looked it was 2024, folks are still regurgitating and following old/outdated information on many things. Top of the list is that a Q-jet doesn't like, want, or need high fuel pressure in a performance application. Nothing could be further from the truth, IF you are using a later design made in and after 1969 when they moved the hinge pin location to improve leverage to keep the needle on the seat with much smaller floats.

On my 1977 Pontiac Q-jet I've tried N/S assemblies from .135" to .149" and fuel pressure to and past 10psi and never had a single issue once I moved to a superior fuel delivery system with an electric pump, regulator, return line, and 8AN lines/fittings everywhere. I did see some issues when I tried to get a high pressure/high flow mechanical pump to make the grade there and quickly moved away from it. I suspect, but not sure that the pulsations from the mechanical pump stopping/starting fuel flow with each stroke may "push" the needle off the seat past about 9psi as I had some minor flooding issues during that testing. :Even then I only saw momentary flooding on really hard braking, but no issues on hard runs on the street or at the track aside from the pump not being able to keep in in low gear past about 4500rpm's with good traction.

That deal always puzzled me as well as you'd think a 120gph mechanical pump would easily feed 500hp and a car running into the mid-11's. Even so every single time I've tried that deal I've came up short, and learned that it is just better to "push" most of or all of the fuel with a rear mounted electric pump than to "pull" on it with an engine mounted mechanical pump........FWIW......

[PAUL K]

Originally Posted by Cliff R

That deal always puzzled me as well as you'd think a 120gph mechanical pump would easily feed 500hp and a car running into the mid-11's. Even so every single time I've tried that deal I've came up short, and learned that it is just better to "push" most of or all of the fuel with a rear mounted electric pump than to "pull" on it with an engine mounted mechanical pump....

X 2!!!

[Stan Weiss]

If you think those vacuum gauges Pontiac installed were just good for fuel economy, well think again.

Also HP is only a very rough gauge of air flow needed. While not shown on most dyno sheets BSAC tells as interesting story as BSFC does.

Stan

[JSchmitz]

Originally Posted by PAUL K

X 2!!!

X3

When I got into about mid 12's the puller became an apparent impossibility. The fuel pressure gauge also told an interesting story after I switched to an electric pusher pump. I initially installed a Mallory 110. It's the same pump as the 140 except it has a lower pressure internal bypass spring (7 psi?). The 110 was initially set up with a deadhead regulator. The pressure was steady 6.5# during idle and moderate driving conditions. Upon hard acceleration, it would dip to 4#. Upon braking it would climb to 9#. So, I blocked the pump internal bypass and installed a bypass regulator and return line. Then it was rock steady 6.5# under all conditions. The inertia acting on the column of fuel is significant. Oversizing a supply line, in a deadhead system, can have diminishing returns. In a return system the inertial affect is offsetting. The suction line on a puller should be minimum to suffice.

[scott70]

When they dynod my 70 455 the #'s were 491 hp at 5200 rpms with a iron intake and a 7040267 carb gone over by cliff. They started the dynos with the shop holley 850 DP. Both carbs had good AF ratios. Only difference was they had to run a adapter to run the holley. The Qjet made about 3-4 HP more than the holley.