600 HP with Q-Jet

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

  1. Daves 71 GS

    Daves 71 GS Well-Known Member

    I see the Quadrajet master is here often,Cliff R..What are your thought on a Quadrajet being able to make 600-625 on a 464 bbb?Ported TA heads,Hydro.roller.I have b4b,performer,sp1 all Q-jet intakes can use any 1 of them.
  2. Brandon Cocola

    Brandon Cocola Well-Known Member

    As long as you use a 800 cfm carb cliff's book lists them good to atleast 650 hp.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I use a 7042240 800 Q-jet on my current engine. At the track, the 1000 DP carburetor is about 1 1/2 -2 tenths and 2 MPH faster though.
  4. 436'd Skylark

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

    That question has been asked 1000 times.. the answer is yep.
  5. 436'd Skylark

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

    I'd bet a cold beer with some tuning the times would be the same.. q-jet tuning is nothing to sneeze at though.
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    While I'm sure there is room for improvement (My AFR gauge said 13.0 at WOT in high gear), JW seemed to think it was just a matter of air flow.
  7. 436'd Skylark

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

    I went 12.05@114 with my q-jet. That was with a 465 and 3.42s.

    I couldn't get the afr better than 11.5s with mine at WOT in high gear. That was with the robb mc 550 pump with no regulator. In hindsight I think the needle and seat couldn't handle the pressure. I was leaning out both the secondary and primary side with no real improvement.
  8. TORQUED455

    TORQUED455 Well-Known Member

    Here's a 10.90's pass with a Qjet:
    Daves 71 GS and UticaGeoff like this.
  9. sean Buick 76

    sean Buick 76 Buick Nut Staff Member

    Anything over 550 HP I would switch to a holley. Just my opinion.
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    There is a distinct difference in engine response between a Q-jet and Holley DP. The throttle throw is longer in the Q-jet. You can be just on the primaries of a Q-jet for more throttle rotation than the Holley DP. In comparison, the Holley DP opens much faster and has a shorter throttle throw. That's why the Holley makes the engine feel more powerful. This year, at Cecil, I was running the 9.5" converter from TSP. It was a full 11-12 lbs. lighter than the 10" I had before. After I put the 1000 DP on I went to move the car, and I almost hit the big garbage can in front of the car.:D. The response was night and day, the difference due to the quicker opening and lighter converter. I think my Q-jet is much better for the street in typical congested light to light driving, and the Q-jet gets much better gas mileage. I like having the choice to run what I want. My fuel line and throttle connection lets me swap carburetors in 15 minutes or less.
    Daves 71 GS and Brett Slater like this.
  11. TORQUED455

    TORQUED455 Well-Known Member

    It all depends on what your goal is. If getting your lowest ET is your goal, then at 600 HP with a Qjet, a Holley is a better choice because it will make more power. Many people think that if an engine makes a certain HP a Qjet somehow stops working or something. There is certainly more power to be had with a Holley, but you can certainly make 600+ with a Qjet. The engine in my convertible makes 600 with a Qjet, and full-weight, full exhaust, etc runs 10’s. Pretty surprised about that myself!
  12. TORQUED455

    TORQUED455 Well-Known Member

    You have a spread-bore intake I presume?
  13. 436'd Skylark

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

    Nah, he has one of those godawful Mr. Gasket intake adapters........
  14. Cliff R

    Cliff R Well-Known Member

    It's not nearly as much about the carb you have chosen but more about how it is set-up for what you are doing.

    I've been going to the same dyno for many years and even did the cylinder head testing for KRE's new heads back in 2004 which included dyno testing then a private track rental to see what the heads did at the track. Those articles made it into HPP and Popular Hot Rodding's "Engine Masters" editions.

    Right in the dyno room we back to back tested my 1977 Q-jet against a Holley 850 DP carb. Everyone wanted to see "how much power the engine will really make".......it LOST a couple of HP going to the Holley....LOL.

    We installed the Holley a few days later at the track, once again to see how fast the car really was.........it SLOWED down .03 seconds and LOST .5 MPH.

    One of my customers has a 3400' Pontiac powered car with an engine making 735HP. It runs high 9's over 135mph. He has a Q-jet we prepared for him and several other carbs including a Quick Fuel 1000cfm full race carb. He uses the Q-jet instead of the Holley because they both run about the same ET/MPH but the Q-jet is "cleaner" on the return road, never "loads up" anyplace, and a chit-ton better when staging the car and coming up on the converter (foot braked)....and yes the big Holley just about runs over his trash can too as it's super-sensitive to light throttle openings with the 5000 stall 8" converter he uses!........FWIW.

    Using a Q-jet for high HP applications requires superb fuel delivery. It's got NOTHING to do with the small fuel bowl, that's just regurgitated chit thrown on the websites from folks who need to keep their day jobs.

    All the fuel to the engine has to go past ONE needle/seat assembly instead of two, so keeping the bowl full requires that fuel delivery be up to the task based on the power level and capabilities of the vehicle AND that you have the correct parts inside the carburetor including a N/S assembly with a big enough hole in it for the demands of the engine. How many times have you seen someone put together some high power big CID engine then install some sort of "high performance" mechanical pump on it, then try to feed it thru stock 3/8" fuel lines sucking thru a stock in tank pick-up.....then wondering why they are having all sorts of issues when they get to the track and not running nearly as quick as they should be? 8 out of 10 times the same guy trying to use a Q-jet just "rebuilt" it with some POS rebuild kit he Google'd up on-line and it's got a tiny .110" hole in the fuel inlet seat. Not for nothing but a Holley uses two fuel bowls with .110" inlet seats and you can even suck those bowls low or empty with high powered set-ups and inadequate fuel delivery....DUH!

    Folks still think huge fuel bowls are the answer to fuel delivery issues. That's another myth with this hobby dating back at least 40 years as ALL carbs need to stay full on hard runs or the metering to the engine can and will be adversely effected.

    Another thing seldom mentioned is the intake. IF you have a square flange intake like a big single plane deal putting a Q-jet straight down on it without at least a 1" spacer can and will kill off some power. The throttle plates in a spread bore carb are open in the plenums at different levels which causes distribution issues.

    On a dual plane spread bore intake a square flange carb is at a disadvantage as the center of the throttle bores are not lined up with the center of the plenum areas. This is why every single time we go to the dyno and back to back a big Holley style carb against a spread bore carb on a dual plane intake they get outran by the ugly Q-jet. Really got nothing to do with the brand of carb it is the type of carb.

    I'd also add here that to date we have ALWAYS made more power and ran quicker at the track using a single plane intake once a spacer was added to it. This kind of throws a monkey wrench into things as I see LOTS of folks buying these big single plane intakes then trying to stuff them under the hood and cobble up the air cleaner, cowl induction, Shaker assemblies and Ram Air parts to get things to fit. So they add a spacer to get the power then put a drop base air cleaner on the carb and move the lid too close to the carb and loose some power and vehicle performance.

    I've drag strip tested all that stuff and remarkably surprised at how much power can be gained or lost messing around with some of these parts.

    I also see folks with little tiny 350's and low powered bigger CID engines stuffing 1" spacers under them thinking they are helping things out.

    On at least three private track rentals we tested 4 different 1" spacers with two different vehicles. 4 hole, fully open, fully divided and semi-open (between the secondaries), and no spacer at all.

    On my engine making well over 500 (at that time it now makes closer to 580hp) it ran quicker in ET with NO SPACER AT ALL. It did run the highest MPH with the semi-open spacer (killed 60' some), and didn't work worth two squirts of duck poop with the 4 hole or fully open spacer (horrible stumble on the launch with the open spacer and WAY down on mid-range and top end power with the 4-hole).

    Hope this helps to clear up a few things when it comes to this topic........Cliff
    Last edited: Dec 31, 2019
  15. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Yes, just use the thin plate to square off the opening. SP1, I would try a spacer as Cliff suggested, but it would not fit under the hood.
    Last edited: Dec 31, 2019
  16. TORQUED455

    TORQUED455 Well-Known Member

    I remember those
  17. 436'd Skylark

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

    That adapter is probably what's costing him the tenth! Ha!
  18. TORQUED455

    TORQUED455 Well-Known Member

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

    I’d like to try a 1000HP in place of my Qjet too. Who knows, maybe ET’s would be in the 10.70’s? Seems like most people pick-up power with a well tuned 1000 Holley vs a well-tuned (fuel system and all) Qjet once the engine demands more air.
  19. BQUICK

    BQUICK Well-Known Member

    I've run fully prepped Q-jets, 925 Thermoquad, 1000 CS Thermoquads, 950 Holley and 1050 Dominator.....1050 Dominator (9375) ended up quickest on single plane Wildcat intake.
    On dual plane B4B 1000 Thermoquad was quickest. Best all around manners was my 925 TQ (modded 850 TQ).
    This was on a 675 hp 464.

    Just relaying my experience.....
  20. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    :DNo, I know the adapter you are talking about. I'm only using this thin plate to square off the bore and avoid any chance of a vacuum leak.

    Some guys skip it on the spread bore opening and just use the square bore gasket.

    The one you are talking about is 3/4" thick.

    Buicks like bigger carburetors. The Chevy guys have been claiming for years that our big blocks don't need anymore than 750 CFM. Put a bigger carburetor on a Buick Big Block and it makes more HP on the Dyno, and goes faster at the track. 1000 CFM is more than 800 CFM. I'm sure you can modify the Q-jet on the primary side to flow more air by eliminating the venturi? That would compromise some street ability though?
    Last edited: Dec 31, 2019

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