600 HP with Q-Jet

Discussion in 'The Venerable Q-Jet' started by Daves 71 GS, Dec 28, 2019.

  1. Jim Weise

    Jim Weise EFI/DIS 482

    Larry's difference is because his Q-jet is set up for mileage, and his 1000HP is jetted for all out WOT performance.

    We knew this going in... so no attempt was made with the Q-jet to maximize power on the dyno.. it was tuned for mileage and driveablity..

    I did this years ago, had one carb that we drove to the track with, and one tuned for the track.. both Q-jets.. difference between them was 2 tenths and a couple MPH. Got lazy once and did not put my street carb back on.. my 14mpg 12 second GS, all the sudden started getting 9.... on the freeway home.

    Brett Slater likes this.
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Yes, and the Q-jet runs great on the street. My AFR says it idles around mid 13's, about 14.8-15.2 at cruise, and 13.0 at WOT. I have gotten a high of 18 MPG (non ethanol fuel and 55 MPH). I wouldn't have it any other way.
    Brett Slater likes this.
  3. TORQUED455

    TORQUED455 Well-Known Member

    The a/f meter and engine doesn’t care what idle or part throttle numbers are when you’re at WOT going down the track. So the Qjet tune @ 13.0 is probably making the most power it can, and fattening it up any will make the tune “safer” but likely would result in higher ET’s. JMHO
    billiegoat likes this.
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Yeah, that's what I thought. I just bolt on the 1000 DP at the track.:)
    TORQUED455 likes this.
  5. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    I think those adapters were used almost exclusively on LG4 305s with 1406 "elderbrock" carbs.

    That'll be my last highjack post about those adapter plates..
  6. Cliff R

    Cliff R Well-Known Member

    "Cliff, any idea why Larry picked-up with a 1000HP carb?"

    If and when we see more than a few hundreds and more than .5 MPH or so between carburetors on back to back runs one of them isn't tuned quite as well someplace as the other more times than not. The 1971-74 Buick 455 Q-jets are among the best out there for high performance use. They are just a tad lean (emissions) but they really come around with a few tweaks in the right places.

    The later Q-jets have PLENTY of cfm to support big HP as well. Most side inlet and all front inlet automotive carbs 1975-up will be the larger castings. Then are continuously dubbed as "800" cfm but they actually flow 850 (848 measured) if you open up the air door as shown in my book on page 118.

    The somewhat famous Edelbrock 1910 "850" cfm carb is nothing more than a near exact copy of a 1980 Chevy 454 truck carb with much richer idle calibration and secondary airdoor stop shortened so the flaps open to 1.270". The same carbs with the outer booster rings removed gain another 50cfm.

    They make excellent race carbs on high HP set-ups even without removing the outer booster rings. Super Stock cars using them are well into the 9's these days and some have to use the smaller "750" cfm units.

    The biggest stumbling block with using a Q-jet as mentioned is keeping it full on hard runs thru one N/S assembly. As a race carb they actually are a very good design, float in the center of the carb and jets on the bottom of the bowl. Address a few potential venting issues that can spoil hard launches, take steps to keep one full and you'll very quickly find them running as good as and often better than anything else out there.

    I particularly love the fully adjustable secondary airflaps that can be dialed in for seamless transition. Combined with a correctly timed pull-off they can be set-up for nearly any combination without much effort, even those with not so great power to weight ratios. The design also works fantastic in the Marine environment where engines are almost always heavily loaded and slow to rev when power is applied.

    For a dedicated race car most folks run the big Holley's or Holley clones, no secret there, been that way since I got into this hobby. What we have shown folks is that IF you want to run a Q-jet, especially in a double-duty car that sees a lot of street use, you aren't really leaving anything on the table even on cars capable of running well into the 10's. Cripe we're seeing a few of the FAST Class GM entries go into the 9's with Q-jets, and they are completely stock appearing and run on narrow bias ply tires!.........Cliff
    Starc Traxler and Brett Slater like this.
  7. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    Dang Larry. I would think you would pick up a whole lot more with that bolt on 1000HP carburater.:D
  8. Cliff R

    Cliff R Well-Known Member

    Keep in mind that an engine spends very little time up near the shift point on the run. The ET is made on the starting line (hard launch). After that the engine combo applying the most power to the track for the rest of the run results in best ET. Thru apprx 4500-5000rpms power is usually about the same with smaller carburetors, so the advantages of a little upper mid-range and top end power doesn't typically result in BIG improvements in ET or MPH. I've back to back tested 750cfm Q-jets, 800's, and 850's at the track and rare to see much over about .5 to a tenth of a second and less than 2 MPH or so run to run from smallest to largest. Same goes for other parts that shift power or make more top end power without making more average power, intakes, spacers, etc.......FWIW......Cliff
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    That shows you just how good the Q-jet is, and as Jim said, it was set up for the street, not the track. What I like the most about the Q-jet is that you can get plenty of performance on the primaries alone, and that is great for stop and go on the street. The 4150 is very sensitive and seems to open all at once. It got even more so with the 9.5" converter.
  10. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    Ive never owned or drove anything with a holly. The qjet is so adjustable you can go outdide the box and set them for both. I guess in my case wide open freeway and milage. 17mpg is possible and ive gotten it with overdrive as you know.
  11. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    I like to run 76 jet with the fattest needle. 52n. I have a shitload of qjet parts
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The Holley has a shorter throttle throw than the Q-jet, it goes wide open a lot quicker, and with 2 accelerator pumps, the throttle response is much quicker. That is why most people are impressed thinking the car gained 50 HP, it didn't, but it feels that way.
  13. TORQUED455

    TORQUED455 Well-Known Member

    What does this mean?

    From a tuning and distribution standpoint, a single plane intake and a symmetrical carb will work better every time at the track once you get to a certain power level. The big, ugly secondaries on a Q-Jet are a tuner’s nightmare, when the big secondaries open and dump. Just like the Tri-power was a novel set-up, it was a air/fuel distribution nightmare.

    Keep in mind, like I’ve mentioned earlier in this thread, I use a Q-Jet on a heavy street GS convertible that runs high 10’s through a full exhaust, so like I like the Q-Jet but am mindful of its limitations as the engine build gets hotter.
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    I think it means it was set up somewhat leaner. At the track, wide open in 3rd gear, it was right around 13.0 according to my air fuel gauge. Might get more out of it if it was closer to 12.5. On the highway at steady cruise, it is right around 14.7- 15.2. It gets great gas mileage. It's seamless everywhere, I'm not going to complain.

    Bob, maybe Jim Weise could answer it better than me? He built the carburetor, I don't know what's in it calibration wise.
    Last edited: Apr 14, 2020
  15. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    So if you want balls to the walls with nothing inbetween a holly will take care of your balls needs.
    Their is a holly guy who has made a holly spreadbore with the metering electrinuc solonoids work very well.
    That would be David Vizard. Atomization, atomization and atomization
  16. TORQUED455

    TORQUED455 Well-Known Member

    Larry, .026” is .026”...

    Anson, your typical Holley HP carb or Dominator will be very difficult to be made to be as economical as a spreadbore or vacuum secondary 4 bbl carb. When all 4 barrels are flowing, all 4 barrels are flowing.
    sean Buick 76 and HotRodRivi like this.
  17. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Yes Bob, so then the main jet used comes into play. I have no idea what the jetting is. Weren't there .036" primary rods used in the later carburetors? If I ever have to take it apart, I guess I'll find out then.
  18. Cliff R

    Cliff R Well-Known Member

    Just some notes on intakes and carb designs.....

    Single plane and big-open intakes favor square flange carburetors. When you install a spread bore carb on an "open" intake the huge secondary throttle plates hang deeper into the plenum areas than a square flange carb. This causes distribution issues and can leave a lot of power on the table. It's an absolute MUST to use at least a 1" spacer if you try to use a Q-jet for big power levels on single plane open intakes.

    I found this out nearly 20 years ago on the dyno bolting a Q-jet directly to a modified Holley Street Dominator single plane intake. It didn't make power for chit on the big high compression CNC ported aluminum headed solid roller cammed engine we were testing. After a half dozen attempts to make decent power we removed the Street Dominator intake, installed a Victor and Dominator and made 102 MORE horsepower.

    While everyone in the dyno facility right down to the guy who took out the trash was "bashing" the Q-jet we took a 1" spacer and a high speed grinder with a sanding roll and custom blended it to provide smooth transition from the Q-jet into the single plane Street Dominator intake.

    We put the intake and Q-jet back on the engine and on the very first pull it made within a couple HP and Ft Lbs torque of the big Victor/Dominator set-up!

    Yes, not a typo, we picked up almost 100 HP just adding a well made 1" spacer to the Q-jet/single plane intake set-up.......FWIW.......Cliff
  19. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    What about dual plain intakes. Have you any info on them? With various set ups. Before and afters. Notching divider, qjets im thinking with first an open thin gasget, then a thin metal spacer, then a thick open bottom gasget, then an aluninum heat shield, then another thick gasget then the carb. Thats my baseplate combo samwich on. B4b dual plain
  20. Cliff R

    Cliff R Well-Known Member

    I've tested every intake that will fit under my hood. Have also tested a butt-load of 1" spacers and various gasket combinations.

    Keep in mind that my engine is 455cid, 11.3 to 1 compression and will EASILY push the car to low 7's in the 1/8th mile around 95-97mph.

    Tested 4 hole, fully open, fully divided, and semi-open spacers. Even added a "notch" to the divided one. Added a "notch" to the stock intake as well, but my intake really isn't "stock" it's opened up same as an Edelbrock RPM with two big openings and both bolt patterns to test square flange and spread bore carbs dyno and at the track.

    Now, on several different private track rentals where I had plenty of time to swap spacers, mess with gaskets, notches, and other combinations on the intake the only spacer that was worth much of anything on a dual plane intake was the semi-open design.

    This is basically solid under the primaries and open between the secondaries. However, as I removed more material the engine developed a "stumble/hesitation/bog" on the launch that would not tune out. I suspect allowing too much of one side to see the other started to cause some sort of distribution issue going from idle to full throttle as it was much less pronounced if I came up on the converter a bit.

    Anyhow, made run after run after run, logging all the data. The 4 hole and open spacer were HORRIBLE, lost ET and MPH at every point on the track and it was difficult to launch hard with the open spacer (huge stumble).

    At the end of the day the quickest run in ET was with NO SPACER AT ALL! Yes, no spacer, one 1/4" thick open gasket and a half a 50 cent piece notched from the divider.

    Best MPH was found with the semi-open spacer, but it shifted power and gave up just enough 60' and short times to not run as quick in ET as no spacer at all.

    I repeated this testing several times on different outings, even played with shift points some, launch RPM, and the results were still the same.

    I also found out during part of this testing as I was playing around on the street to "qualify" the spacers before wasting track time with them that IF I raised the carburetor then made any attempt to lower the air cleaner lid with a shorter filter putting it closer to the carb I LOST power and also induced some stumbling hitting it hard right off idle. Thinking I'm a pretty smart guy I obtained one of those K & N X-Treme lids, thinking that was the answer to a shorter air filter element to sandwich all that chit under the hood....NOT! My engine absolutely HATED that part and refused to run well with it in place.

    So reading all this think about how many folks with much smaller engines with less compression, less head flow, and smaller cams install all sorts of these parts and think they are improving vehicle performance. Kind of related but right on the dyno I tested my factory iron intake (modified), an Edelbrock RPM, and a factory aluminum HO (Reproduction) intake back to back w/o spacers. The engine was 440cid, 10.6 to 1 compression, hefty roller cam and aftermarket aluminum heads.

    The factory iron intake made 497hp. The RPM made 491hp, and the HO intake made 487hp. So much for a larger and taller intake helping an engine at the 500hp mark.........Cliff

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