[RocktimusPryme]

Originally Posted by jonmachota78

Sumped tank. Return style regulator. Pressure gage i can see through the windshield, never drops below 6.5lbs

I didn't have the sumped tank or full-time visible pressure gauge, but I had all the rest of that and struggled with sticky tire traction until I went electric.

Two big fuel bowls and an extra needle and seat do seem to solve a lot of problems at the track though.

[70GS455]

The book has a issue date of Jan 1997 if that helps.

I did not see any conclusive tests of the fully modded carb. He alluded to a Dyno test of a partially modded carb on his Trans Am but did not present results

[Cliff R]

The information is both outdated and inaccurate.

Q-jets stopped using sintered bronze fuel filters in 1967.

4-6 psi fuel pressure???. Rochester moved the hinge pin forward in 1969. The later units will EASILY take at least DOUBLE that fuel pressure with a much smaller float and the larger N/S assemblies.

Thinning throttle shafts and knife-edging throttle plates is NEVER a good idea and a complete waste of time. The primary shaft is already 5/16" and the the throttle plates are helf the thickness of most other types of carburetors and delicate as butterfly wings as-is so best to leave them alone.

I'll stop being critical there, but will say that I've been supporting Stock, Super Stock, Pure Stock and FAST class racing for decades now and have build scores of Q-jets for them. Even the smaller "750" cfm carburetors are well into the 9's in Super Stock on small block 350 Chevy builds and also have some FAST "400" Pontiac RAII and RAIV builds into the 9's with the correct carbs on them. Unlikely your street or street/strip Pontiac builds needs the Q-jet hacked up for a little more cfm to get the combo up to full potential.......IMHO......

[steve25]

Two big fuel bowls and two needles and seats can at times solve the problem of not having a pump back at the tank purely due to the fuel volume on hand.

The root cause many times is the G force on the car at the starting line that will back out .3 PSI of fuel pressure per ft of fuel line.

This can take place so fast that a dampened gauge will not show the short time drop, but non the less the carb will respond to it.

[PAUL K]

Originally Posted by Cliff R

The information is both outdated and inaccurate

4-6 psi fuel pressure???. Rochester moved the hinge pin forward in 1969. The later units will EASILY take at least DOUBLE that fuel pressure with a much smaller float and the larger N/S assemblies.

Out of curiosity which applications are benefitting from 12 psi fuel pressure?

[steve25]

I recall a article in 91 where Don Kennedy’s 470 cid 600 Hp super stock bird needed 9 psi to keep up with the fuel volume usage.

[PAUL K]

Originally Posted by steve25

I recall a article in 91 where Don Kennedy’s 470 cid 600 Hp super stock bird needed 9 psi to keep up with the fuel volume usage.

And he was down 100 HP from today's engines. I'm guessing he wasn't using a well designed N&S assembly.

[70GS455]

Originally Posted by Cliff R

The information is both outdated and inaccurate.

Q-jets stopped using sintered bronze fuel filters in 1967.

4-6 psi fuel pressure???. Rochester moved the hinge pin forward in 1969. The later units will EASILY take at least DOUBLE that fuel pressure with a much smaller float and the larger N/S assemblies.

Thinning throttle shafts and knife-edging throttle plates is NEVER a good idea and a complete waste of time. The primary shaft is already 5/16" and the the throttle plates are helf the thickness of most other types of carburetors and delicate as butterfly wings as-is so best to leave them alone.

I'll stop being critical there, but will say that I've been supporting Stock, Super Stock, Pure Stock and FAST class racing for decades now and have build scores of Q-jets for them. Even the smaller "750" cfm carburetors are well into the 9's in Super Stock on small block 350 Chevy builds and also have some FAST "400" Pontiac RAII and RAIV builds into the 9's with the correct carbs on them. Unlikely your street or street/strip Pontiac builds needs the Q-jet hacked up for a little more cfm to get the combo up to full potential.......IMHO......

The mention of bronze filters was in the context of: if you should happen to have one then it should be replaced with a paper one.

[Tim Corcoran]

The small filter at the entrance of a Q-Jet is a restriction even with the paper filter. There just isn't enough surface area in that tiny filter. I never run a filter there I use an external aluminum body filter in front of the carb like the ones that RobMc sells. I was having a fuel delivery problem and took out the new clean paper filter in the carb and fuel delivery issue went away.

[Cliff R]

Fuel pressure and the Q-jet.

Right to start with anytime you see anyone telling readers a Q-jet can only handle 4-6psi they are simply regurgitating old/outdated information. Same thing with "leaking bottom plugs", or the "fuel bowl is too small"..

Early in the years of production the Q-jet went thru some major design changes and all of them for the better. Most of the rumors we read about come from the very early models.

For example, the leak rate for 1965-1968 units at the rear bottom plugs is right at 100 percent. Crappy design with thin brass cup plugs driven into a casting with not enough material around them. The front plugs weren't much better being not much more than a small piece of lead driven into a hole.

The factory stepped up in 1969 and made the castings thicker and pressed in large/solid aluminum plugs and swaged over them. That pretty much ended the leaky bottom plug issues on the rear of the castings. Shortly after that they started pressing in aluminum front plugs and swaging over them. You still might find a leaker on occasion but post 1974 it's extremely rare and you're not likely to find too many leakers between 1969 and 1974.

Nearly every single person on every Forum in existence or 99.9 percent of anyone who "builds" a Q-jet smears a ton of "monkey chit" over the bottom plugs and also tells anyone reading or trying to learn about these carbs to do the same thing. Do you know how much money I"ve made from folks doing that STOOPID move which in most cases prevents the main casting from effectively sealing the gasket to the baseplate when it's tightened down causing a vacuum leak or ever worse binds the secondary shaft up enough it doesn't fully close and causes a leak there as well.

Second STOOPID move is to install a regulator to keep the fuel pressure under 6 psi. Pressure is volume and even the early carbs with the poorly located hinge pin will take 6 psi without issue IF you use the correct factory float in them, even if you've installed "high flow" N/S assemblies

The later carbs with the relocated hinge pin are fine with much higher pressure with a much smaller float in them. Some "racers" run as high as 10-12 psi with smaller N/S assemblies where others run a little less PSI with larger inlet seats. Pretty much a wash there but I suspect the higher pressure deal simply came from the fact that rebuild kits dating back at least 25 years now did and do NOT come with the .135" seat. Matter of fact most these days are in the .110" to .125 range.

Using the WRONG parts and following POOR advice with fuel pressure is the BEST way I know of to get a nose bleed when you bounce it off the steering wheel at the top of first gear when you suck the bowl dry and the car noses over on a hard run......duh?

I have and continue to try to educate readers on all of these things but sadly it's a constant losing battle as boatloads of old/outdated/inaccurate information floods every Forum on the Internet when it comes to a Quadrajet carburetor.

The article in question here is no different and I suspect that if it were re-written today we'd some some MAJOR changes to it. That assumes the source has learned a few things in the last 30 years or so.

Anyhow I can tell anyone reading this that you do NOT need to thin shafts, grind, sanding roll, hack saw or take a chainsaw to any part of your Q-jet to support big power with it. You just need updated accurate information, the correct parts, and an adequate fuel delivery system for the power level you are using it on. At that point you aren't going to outrun it with anything else no matter how much money you spend or who tells you that you need to go another direction.......FWIW......

[Cliff R]

The article in question here suggests 4-6psi but doesn't mention what carb is being used or the size of the fuel inlet seat. I suspect that the author was either using a 1968 or older carb for the testing, didn't test at all, or regurgitating old/outdated information on the ability of a Qjet to work well with higher fuel pressure.

There are pics of later Q-jets in the article however. The later Q-jets have a revised hinge pin location and much smaller float. If you were using a later carb you could have easily used a .145 or even a .149" N/S assembly and 7-8psi fuel pressure and been fine at your power level.

I did some testing at the track with my 1977 Pontiac Q-jet and both fuel pressure and larger fuel inlet seats. WIth the standard .135" seat I didn't have any issues until I dropped the pressure to 6psi. I could feel the car start to surge slightly mid-track and if I went 5psi or lower it would suck the bowl completely empty and the car would nose over, then recover. If I turned the pressure up to 7 psi or higher no issues.

I went on to test .140, .145 and the .149" seat and no issues anyplace until I turned the pressure down below 5psi. I settled on using a .145" seat and 7.5psi fuel pressure and never had a single issue anyplace even when I made upgrades to make the car faster at the track or went to more powerful engines. So fuel pressure is your friend with these things. I need to mention here that NONE of the things mentioned above worked at all until I upgraded the fuel system for the power of the engine and performance capability of the vehicle.

I also tried to find the limits with fuel pressure and turned the regulator up to 10 and then on to 12psi and no issues anyplace. Since I was able to keep the carb completely full on a hard run with much less pressure I settled in on 7.5 psi and called it good.

As far as hacking up your carburetor or going nuts with a grinder and sanding roll I don't really see any problem with all that. To date I've never seen a rule book when it comes to modifying anything associated with your engine to help it out some. In the real World of high performance and making power however, very little of what I see done actually helps with these sort of things.

For example I've seen folk grind and polish the runners in factory iron heads to the brink of extinction and not do any work in the bowls and short turns or even worse screw that part of it all up. Then they call them "ported and polished" but never did a single air flow test to see if any of it actually helped anyplace and for sure no "back to back" dyno or drag strip testing of any sort. To this day I cringe when I hear the words "ported and polished" when it comes to cylinder heads. I truly believe that most of them were better before they touched them....FWIW.

Same with carburetor modifications. I've had carbs come in here with the entire choke areas machine clear off and venturi areas ground clear out of them leaving pretty much a straight shot past the boosters to the throttle body. Did any of it help? Who knows but for sure without a choke a cold start isn't going to be nearly as easy when you need to move the car out of the garage when it's 30 degrees outside.

[PAUL K]

FWIW many folks have found more power with increased air flow using a Quadrajet. Many times folks have found a power gain going from the "750" CFM unit to a large primary venturi "800" CFM unit. Many folks have gained power switching from the 800 CFM unit to the single ring primary booster (I hope I said that right) that flows more CFM. Some folks have gained power with radius inlets smoothing the air entrance of the Q-jet... It's not a carburetor designed for racing.

If your engine makes X power you only need enough air flow to feed that power level. At some point the air flow capabilities of the Q-jet becomes the limiting factor for power production. At that point the engine will respond to anything you do to increase air flow. If it's thinning the throttle shafts, putting a radius on the air entries, smoothing the rough edges or putting a big Holley on it the power levels will go up. The engine doesn't recognize where the increased air flow comes from. Now you can try all this on an engine that is happy with a 750 CFM unit you most likely won't find an improvement..... This is basic Horsepower building 101.


I posted a picture of a pile of carburetors we tested on high horsepower engines. There is a "reason" the pile on the right won the shoot out. Absolutely nothing wrong with the pile on the left. They just couldn't flow enough air to keep up with the engine they were mounted on.

[mchell]

https://youtu.be/huVFrjYMMQ0?si=t997xR7HFdonqyLT

[65 Lamnas]

Originally Posted by mchell

https://youtu.be/huVFrjYMMQ0?si=t997xR7HFdonqyLT

@jimbollinger8838
On the Edelbrock carb, did you guys verify that the throttle blades were open fully? I bet you DID.

Likewise, did you verify the secondary air valve on the Qjet was at its optimum opening angle? Same with the Qjet throttle blades? I bet you DIDN'T.

[mchell]

Originally Posted by 65 Lamnas

@jimbollinger8838
On the Edelbrock carb, did you guys verify that the throttle blades were open fully? I bet you DID.

Likewise, did you verify the secondary air valve on the Qjet was at its optimum opening angle? Same with the Qjet throttle blades? I bet you DIDN'T.

I wouldn’t know …something I just came across and found interesting …

I believe the YouTuber mentioned that the qjet was “race prepped”….by whom, I wouldn’t know..

The Edelbrock carb was of more interest as it seemed to perform very very well ….

[Formulajones]

That was an 800 cfm quadrajet built by Dean Oliver "Deano's Carburators"
"Performance carburators for stock and super stock"

[65 Lamnas]

Originally Posted by mchell

I wouldn’t know …something I just came across and found interesting …

I believe the YouTuber mentioned that the qjet was “race prepped”….by whom, I wouldn’t know..

The Edelbrock carb was of more interest as it seemed to perform very very well ….

My fault, Mike....wasn't a direct question to you, just was referencing the video to put context to my question.

[70GS455]

With the large amount of vacuum being pulled at the top end with the Qjet in the Dyno video, I don't believe it was optimized. My Qjet had 1.9" of vac, at 586 HP, the air door was set to 1.25"

[Cliff R]

A late model Q-jet or any that have the larger primaries are 850 cfm not 800, if you set the angle of the secondary air flaps as described in my book. I typically don't waste my time trying to explain all that even though the last time I looked it was 2024 and folks are still regurgitation old/outdated information with just about every topic we discuss on the Forums.

If your Q-jet gets outran by a 750cfm Holley or Holley clone on a 400 cid build at that power level for sure it was off tad someplace......FWIW....

[Stan Weiss]

Originally Posted by Cliff R

A late model Q-jet or any that have the larger primaries are 850 cfm not 800, if you set the angle of the secondary air flaps as described in my book. I typically don't waste my time trying to explain all that even though the last time I looked it was 2024 and folks are still regurgitation old/outdated information with just about every topic we discuss on the Forums.

If your Q-jet gets outran by a 750cfm Holley or Holley clone on a 400 cid build at that power level for sure it was off tad someplace......FWIW....

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