There should be some practical way to tap into the aneroid chamber and provide a considerable amount of extra fuel to the bowl capacity of 1975 and later style 800 q jets. These are great carbs and this would make them even better. Is Cliff out there?
When using a Q-jet on an engine that makes good power or excellerates hard, you need to run a larger fuel pump because of the small capacity bowl and single feed line.
Adding bowl capacity is not needed, and will show no gains anyplace if the fuel supply system is not keeping up. Does it hurt anything....probably not, but you will see absolutely NO GAIN ANYPLACE if the fuel delivery system is inadequate for the vehicles ET and MPH. Holley and Holley clones experience the same trouble as q-jets, but it doesn't show up quite as soon in the rpm range. Just like any other carburetor, IF you don't keep them full, they do NOT respond predictably to metering changes. The q-jet simply has one needle/seat assembly to feed the engine, so an excellent system is required, or at least a system capable of the power level. *One important item to add here, is that the q-jet requires "low" fuel pressure. This is complete nonsense, and passed down from the piss-poor fulcrum position and giant float used in very early models. For Buick carbs after 1968, they will easily run with 6-7 psi and the .135 fuel inlet seat. Some of my customers run 8-9 psi and even larger seats. We use and recomend electric pumps for really fast cars, 12's or quicker. Not that you can't do OK with mechanical pump set-ups, but a superior system will make other modifications to the car a positive experience. Keep in mind, when reading this, that when you leave hard, all of the fuel is up on the bumper, and typically not in the front of the stock tank where the pick-up is typically located. Some tanks are shaped different than others, and simply work better on harder leaving cars than others. This is why we see some folks running into the 10's with stock tanks and mechanical pumps, and others that can't get out of the 13's trying to do the same thing. For my own car, mid 12's was the limit without sumping the tank, installing an electric pump behind the tank, and 8AN lines/fittings everywhere. When I did this, my car was using a 455hp engine and running 12.40's to 12.60's around 108-109mph. I noticed that the power seemed to go "flat" above 5000rpm's, but it never nosed clear over so I just figured the "tiny" cam I was using wasn't making it. The very first runs on the car after the sump was installed was 12.11, 12.12 and 12.05 right at 112mph. The first two runs were slower than the 3rd simply becaused the engine bounced off the rev-limiter at 5800rpm's. It pulled so hard and fast past 5000rpm's, that I didn't move the shifter quick enough! (For those of you that are wondering if I used/tested a Holley before sumping the tank, the answer is yes, it ran no quicker anyplace than the q-jet, and picked up exactly the same ET and MPH after we made the upgrades) Anyhow, after the sumping the tank deal, I made a few other mods (better heads and HR cam) and went clear down to 11.50's, same engine and drivetrain parts. What we recomend here, is to install a fuel delivery system capable of a full second FASTER than you think the car is capable of, then go on and worry about the other items in the "recipe" to make the vehicle really fast.:TU: .....Cliff
Cliff, Thankyou so much for the thorough info on the fuel systems. I'm a big fan of q-jets and try to learn what I can. Love your book too. The .250 float level is a real hard one to go for, because I've seen so many get wet or leak around the horn gasket. That is the highest level I have ever heard of, and I know of no one else prescribing it. How did you come up with that? Thanks for all the help you provide to me and so many others.
The level of the fuel in the bowl is determined by the float height, size/bouancy of the float, fuel inlet seat diameter, and fuel pressure. Also keep in mind that there are three different float/fulcrum designs and at least a dozen different float sizes. As part of our testing over the years, I've removed the top from various carburetors and installed different floats and fuel inlet seats into them. With each one the fulcrum was held down and an electric fuel pump turned on. The fuel level was recorded with each set-up, and the fuel pressure regulator adjusted and changes in the level of fuel noted. I followed up the testing with street, dyno and dragstrip runs with various units. We settled on 1/4" as the baseline setting. Of course some applications use high pressure fuel pumps, most later units, and will require lower settings if the stock parts (OEM equivalent) are still in place. As far as "wetting" gaskets, this occurs with any float setting if the airhorn is not effectively sealing off the gasket. Since so many castings have some warpage across the front, that would not be an effective way to evaluate whether you had the float level correct or not. None of them will leak if a good gasket is used and the main casting and airhorn are effectively sealed off across the front when the screws and retaining bolts are tightened. Anyhow, fuel pressure is a big contributor to fuel level, more than one would think. At any particular float setting, running the pressure up and down with the regulator changes the level of fuel in the bowl considerably. We simply came up with a "standard" or baseline setting that will cover all units, based on various testing with different applications. As with anything else, one may find a custom setting to work better for their particular set-up. Even with that said, we have carburetors powering some of the fastest cars in the country, and the vast majority of them have the float set at exactly 1/4". What we have seen in recent years, that contributes to fuel level and vehicle performance with really fast cars, is the trend to run smaller fuel inlet seats and high fuel pressures. Very, very few applications we've worked with these days use the .149" needle/seat assembly. Many have found that smaller seats and higher pressure(s) are more effective. One customer in particular runs low 9's with a Super Stock Camaro, using a .125" solid fuel inlet seat, over 10 psi fuel pressure and two 250gph pumps to feed it! It's not uncommon at all these days to see racers, especially stock and super stockers running high fuel pressures, and the regulator mounted really close to the nose of the caruburetor. For my own vehicle, I've ran fuel inlet seats from .125 to .160", with every possible float level and fuel pressure in between. It runs exactly the same from about .130" and larger as long as I have at least 3.5 psi at the nose of the carburetor. I do however, use a very good fuel delivery system. I settled on running a .145" seat and 6.5psi pressure, small float set at 1/4", been that way since 2001, and the carburetor has never faltered once for any reason, street or at the track. We made 4 runs last Sunday at Norwalk, and the spread for all for runs was less than .008 seconds!......Cliff
Cliff, Thanks again.Reading your information is more fun than a birthday card with a $100 bill inside an will last a whole lot longer. If you don't mind, I have a real puzzler I,m bewildered with. I see some Pontiac guys running 450-500 HP engines with 1975-79, 800CFM (APT) carbs with .073-.074 jets and .050m or bigger rods. this flies in the face of the countless 1971-1974 (non-APT) 800CFM carbs that run .076 jets with .046B rods. Those ..050M-.054M rods have .036 tips too! The Pontiac APT 800 carb guys also use more modified skinny tipped SECONDARY rods such as .018, .028, .030 V.S. the older 800carbs usually have .039 to .053's. Jim Weise, on this site, used an APT with .073 jets with .050 rods (and AU secondary's) on a 500HP eng! Yet nearly every street engine dyno'ed in the GSXTRA has .076 jet and .046B rods just like some of our carb builders do. So I'm refering two types of 800 cfm carbs that seem to require very different flow areas at wide open throttle as well as cruise.
Interesting, as I use a 1977 Pontiac carburetor on my own 455, making up near 600hp and over 600ft lbs torque. The carburetor is a completely stock casting, part number 17057274. Although it is recalibrated with custom performance modifications, the castings have never been touched with a grinder or sanding roll anyplace. I run it with .065 main jets, no metering rods (primary), combined with "recipe" #3 from our book. I ran it with .073" main jets and 44L metering rods for years, and also tried .077" main jets and 54M rods in it for a while, with about the same overall results. Once I started racing the car more than street driving it, we went straight off the jets, but gained no measurable performance anyplace in doing so. Most of the year we will run .041" secondary metering rods, then switch to .048" metering rods in really hot/humid weather. I've tested metering rods at the track on back to back runs dozens of times, and it will not tolerate a secondary metering rod beyond apprx .055". Keep in mind when reading this that my metering rod selection have the same tip length, taper on the upper section, and included angles leading to the tips, as they are machined on a lathe from larger cores. This is the ONLY way to accurately assess metering rod changes. You can actually install a metering rod with a lean tapered rod with a small tip, and come up with a leaner overall result that using a rich tapered rod with longer tips, on back to back dyno pulls. The metering rods with the short tips and/or fatter tapered section may need to be much higher than a set of long tipped rods with a smaller tapered section in order to show a richer A/F ratio. We discovered this years ago, so started cutting metering rods from the same cores, with a custom cutter that leaves the same included angle and blend from the large section to the tips. When these custom rods are used for testing, they will show a change in the A/F ratio that is consistant and predictable. *Once again, and as it relates to this thread, you will NOT see predictable or desirable results anyplace unless you are keeping the carb's fuel bowl full on hard runs. As far as the large cfm 71-74 Buick units go, we use a LOT of them for custom units. They are simply the only early divorced choke large cfm units available, so we end up building a lot of them for high HP large cid Pontiac engines where the owner wants the "stock" look. We typically end up with .073-.074 main jets in them, but use different metering rods in those jets, and free up the baseplate located APT screw and replace it with an externally adjustable screw. Perhaps a lot of folks who work with them don't address the APT system in the baseplate. They typically used large primary metering rods and .073" main jets from the factory, with the APT set relatively low provided a pretty lean part throttle A/F ratio. To get them on par for higher HP applications, it becomes necessary to increase the jet size quite a bit unless the APT is used along with metering rods that taper gently down to .026" tips, or early style tapered second section metering rods. Totally confused now?......Cliff
You did'nt confuse me, I just have more questions to finish the scope of this metering stuff. Here is the wide open throttle view of all this. A typical street car on this board with 450 HP is running a .076 jet with an .026 rod tip and .004000 Sq in. fuel passage at wide open throttle An .075 jet with an .026 rod tip metering rod is stock for a 1971 Stage 1 GS 455. The area of space for fuel to pass through is .003650 Square inches An .073 jet with an .036 rod tip (APT type M rod) is being used in a 500-600 HP 3600 lb. car and has .003230 sq. in. for fuel. I have heard lean is mean but should we stay with stock jetting and leaner with a 600 horse build? THIS is what confuses me. Thanks so much for your patience with me. Bless You. Gary B.
Gary, I have only observed a few "M" series metering rods being used with the mid to late 70's carburetors that had two pairs of upper main airbleeds. There were used in all of the single main late style models that started showing up around 1979 and continued to be used thru the 1980's. Since those carburetors used only one pair on "indirect" main airbleeds, they provide a clear A/F curve using the later .036" tipped "M" series metering rods. Any and all early or late style carburetors prepared here leave with .026" tipped metering rods. As far as calculated A/F ratio or the change to the A/F ratio using mathmatical formulas, inaccurate to say the least. The main airbleeds simply have too great an effect on the amount of fuel that ends up getting delivered to the engine. Just as an example, I can install .069" main jets, pound in two pairs of .040" main airbleeds into your Buick carburetor, and delivery as much if not more fuel to the engine across the load/speed range than you can with the stock airbleeds and .076" main jets. Nothing wrong with doing math, but I've seen boatloads of great information posted over the years about calculating fuel delivery with q-jets, and not in one single instance were the main airbleeds used factored in anyplace. The airbleeds actually have a greater impact than the size of the jets, pound for pound...FWIW.......Cliff
You just "made my day" with that .44 magnum statement about air bleeds. I think I need to check my 1979 350 carb and see if I drilled them to .070! I might need some .040 air bleeds, then reinstall the stock .073 jets and .048k rods with .026 tips. Woops, you meant use .040 air bleeds with .069 jets, and graduated steps upward. Then keep meetering rods matching the jets as well. My stock 79 Buick 350 800 ATP qjet had .073 jets, 48k tapered rods and .070 air bleeds. I used the .077 jets out of my John Osborne 1972 carb in the 79 carb. I will now drop down to .074 jets and .045k tapered rods with .026 tips. I was convinced all 800 qjets needed .076 or .077 jets with .046 rods with good street cams because so many others were doing it. I'll see how close I get to 90mph in the 1/8 and go from there.
The numbers I used were mostly as an example. The airbleed to jet requirement "ratio" is not a linear curve. When you start going really small with the airbleeds, the 30 something difference between jets and metering rods on cruise curcuit goes out to lunch. Same deal going to really large MAB's. As another example, the later Chevy carburetors with the .120" bleeds may require jets as large as .077", and metering rods as small as .036". These are nothing more than "basic" examples. I just wanted to point out that the vast amounts of jet-rod relationship information we have been reading for years, means little to nothing in the "big picture" of things unless you are providing the specifics of all MAB's used for each example. Sort of getting off topic here, but very few builders of these carburetors really understand how the MAB's and other items will effect the A/F ratio across the speed/load range. There is a somewhat "famous" shop that builds circle track q-jets. They have came up with a calibration using one part number, pretty sure it's a late 70's Cadillac unit. Anyhow, they will only build that one part number, and over the years we've had numerous inquiries to our shop from folks searching for them. Kind of interesting to me, is that any similiar casting q-jet, Chevy, Old's, Buick, Pontiac, etc, could easily be built to the exact same specifications with a few minutes of extra work......FWIW.....Cliff
So,that's why you have .070 main air bleeds as a sort of standard in the recipes, so a range of jets and rods work without modifying , drilling or just getting out of the useful range of jet & metering rod efficency. Wish it were that simple with secondaries. A gentleman on the web documented his experiments with a pontiac carb and said that after 80 degrees of air valve opening he got "deminishing returns" (less engine performance or HP). He also noted Pontiac performance carbs used 76-80 degrees back in the early 70,s. This raises some questions with hangars. If an AU .052 long tip rod is too lean , and you install a DA .044 medium tip rod expecting to increase fuel flow, and the air valve is set at 76-80 degrees max, the AU will be richer than the DA at that air valve position. This a very likely senerio on many rod combos. There are charts on the web that show the rod diameters in 10 degree increments up to 90 degrees. Checking at 70 degree opening, or 80 which may be the stop point on your carb, a DA is larger in diameter (leaner) at that point, than an AU, and the skinny DA tip may never reach the jet opening. Now if you use a hanger with more lift, and your using two airhorn gaskets or thick one, on a warped carb, now the rods will lift out of the jets. I read these rod charts and my reaction is WHAT THE HELL WAS GM THINKING. You said you cut your own rods. I can see why. Now you can match your hanger to the gasket thicknes,air valve position,and know exactly what the rod is doing all the time. I sure don't know what mine is doing. How about cone tapered rods that will be maxed at 70 degree air valve opening with .030 thick air horn gasket, and G hanger... in sets of .015 thru .055 tips? Are there any out there??
