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Thread: The Quadrajet

  1. #1
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    Default The Quadrajet

    To Casitron.

    The old thread was closed. Apparently it seems some people on this forum are not interested in facts, just hang on to pre-conceived wrong info.

    To answer your question, why were the secs not open in the pic, there can be two reasons: hard to see if it is there in the pic, but there is link connecting the pri shaft to the sec shaft. It could be missing. Other reason is that there is a lockout on sec operation when the choke/fast idle is operating. Once the engine comes off fast idle, sec blades will open.

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    I realised I have made a mistake in the above post. On a QJ, it is the air valve that gets locked out [ not the sec throttle shaft ] with the choke actuated. I was thinking of Carter carbs that have the sec shaft locked out during choke operation.
    Sorry for any confusion.

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    Buy a Holley ...

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    There is a vacuum pull-off that limits the air flap travel, but there is a lever lockout on the secondary butterflies (assuming the carb hasn't been butchered as many have). If you open the linkage to full throttle on a cold carb the secondary throttle butterflies will not open, you will still get full lever travel but the secondary movement is taken up by a spring. If you move the lever attached to the choke assembly to release the lockout, the secondary throttle butterflies will open mechanically. However none of that changes the original point, which is that both fuel and air delivery is activated by vacuum via the air flap.

    The biggest problem I see with any carb is that so many people don't tune them at all, they expect to just bolt it on and have it magically work with their specific engine combination. This seems to be why a blunt instrument like the earlier Holley models are so popular, at the expense of fuel economy and optimum engine performance. Whatever carb you use, spend some time tuning it and you will unlock free horsepower and economy (well almost free, most will require some small parts to be changed). If you don't understand how it works, the info is all out there on the web. If you don't want to know and just want to jump in and drive, I think you're in the wrong forum...?

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    Quote Originally Posted by paulzig View Post
    Buy a Holley ...
    Bugger that, go EFI...

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    Quote Originally Posted by immortality View Post
    Bugger that, go EFI...
    Amen to that.

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    Lx hatch.

    Excellent comments.

    EFI. On those units that bolt to a 4bbl manifold in place of a carb, I have seen a few tests, & the carb nearly always makes more power than the EFI. The benefit of the EFI seems to be easier starting, better idle sometimes & that's about it. Oh....& a lighter wallet.

    All the runner length vagaries of the manifold remain with this type of EFI, which can & do cause distribution problems such as rich & lean running cylinders. Some people using carbs try stagger jetting to fix this.

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    Quote Originally Posted by paulzig View Post
    Buy a Holley ...
    Better still, buy some 48 IDF Webers.

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    Edelbrock (I think) have a simple EFI setup to compliment a carburettor, Maximum power from the carb, easy idle and low speed trims from the injector.

    Designed by Mike Laws, a man who knows more about carbys than everyone here has forgot.

    Or learn to tune. But not a quaddy, voodoo magik for 99% of gurus.

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    Quote Originally Posted by GtoGeoff View Post
    Lx hatch.

    Excellent comments.

    EFI. On those units that bolt to a 4bbl manifold in place of a carb, I have seen a few tests, & the carb nearly always makes more power than the EFI. The benefit of the EFI seems to be easier starting, better idle sometimes & that's about it. Oh....& a lighter wallet.

    All the runner length vagaries of the manifold remain with this type of EFI, which can & do cause distribution problems such as rich & lean running cylinders. Some people using carbs try stagger jetting to fix this.
    Didn't Engine Masters do some testing using the Fitech type EFI vs a carb and once tuned right there was nothing in it power wise?

    Maybe my understanding of carburetors is wanting but if I'm not mistaken the carb relies on the vacuum from the engine via the venturi in said carb to draw fuel from the booster and fuel atomisation is very much dependent on getting that all working well. With EFI fuel atomisation comes down to the injector and fuel pressure so regardless of the manifold under it (which doesn't care what the method of fuel delivery is) it comes down to the user getting the best AFR to achieve max power.

    I think you have to remember systems like Fitech have a self learn function that as long as the user puts in the right engine details will tune itself fairly well and make decent power but like any system out there, if you want max performance then some fine tuning is required.

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    Quote Originally Posted by immortality View Post
    Didn't Engine Masters do some testing using the Fitech type EFI vs a carb and once tuned right there was nothing in it power wise?

    Maybe my understanding of carburetors is wanting but if I'm not mistaken the carb relies on the vacuum from the engine via the venturi in said carb to draw fuel from the booster and fuel atomisation is very much dependent on getting that all working well. With EFI fuel atomisation comes down to the injector and fuel pressure so regardless of the manifold under it (which doesn't care what the method of fuel delivery is) it comes down to the user getting the best AFR to achieve max power.

    I think you have to remember systems like Fitech have a self learn function that as long as the user puts in the right engine details will tune itself fairly well and make decent power but like any system out there, if you want max performance then some fine tuning is required.
    There won't be anything in it at WOT on a dyno - they'll both make very similar power figures and this is where I feel people seem to only care about numbers on a dyno run and use this as gospel.

    Where EFI really shines is part throttle/cruising situations - big difference and EFI is much better in this situation, you know, where you drive your car to and from the drag strip, or to work, or to the shops etc.

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    Quote Originally Posted by Daymoe View Post
    There won't be anything in it at WOT on a dyno - they'll both make very similar power figures and this is where I feel people seem to only care about numbers on a dyno run and use this as gospel.

    Where EFI really shines is part throttle/cruising situations - big difference and EFI is much better in this situation, you know, where you drive your car to and from the drag strip, or to work, or to the shops etc.
    I completely agree

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    Yea Engine Masters did. But it was carb style throttle body with rail mounted injectors. Carb made more power. Only because fuel atomisation cooled the inlet charge.

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    Quote Originally Posted by conrod0 View Post
    Better still, buy some 48 IDF Webers.
    Now you’re getting into some carb wizardry!

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    Quote Originally Posted by Castiron355 View Post
    Now you’re getting into some carb wizardry!
    EFI is the new weber, its that magical dark art ...

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