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Double pumber or not
I currently have a Holly street avenger 770 with electric choke and vacuum secondary's. I have it tuned well and have no bog or stumble and was wondering if switching to a double pumper would be worth the $500. I was looking at the
"HOT ROD" SERIES QFT 750 CFM DOUBLE PUMPER ELECTRIC CHOKE CARBURETOR So is it worth it and what should I expect to gain in performance The engine is a 408 stroker with an estimated 500hp. |
I have the mechanical Holley on my Cobra and the vacuum Holley on my Corvette. I wouldn't pay $500 to switch either one to the other.
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I have a double pumper and like it, but for $500 I do not see you gaining that much to justify the cost. Then again, it's you money...
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As no one else is rushing to answer the OP's question, I'll add this. My understanding is that a properly tuned vacuum Holley can run just as strong as a properly tuned mechanical Holley at full throttle. What little difference there is occurs during the transition from opening the primaries to opening the secondaries. On the mechanical, the transition is as fast as the throttle can move, so its up to the operator to set up the accelerator pump to dump additional fuel in accordance with the way the driver moves the throttle. On the vacuum Holley, the operator uses the throttle to tell the carb he wants the secondaries open, but they don't open until the vacuum signal is right. This helps assure the fuel-air mixture from the secondaries is right. The result is that the mechanical Holley may be very slightly quicker during the transition to full throttle, but only if the driver does it well. ... I've never noticed the difference.
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Don't forget about the different brands. The OP isn't just comparing vacuum vs. mechanical secondary. There is a noticeable difference between a base Holley and a high end carburetor like a QFT or a Holley Pro Series.
I am not familiar with that specific QFT carburetor, but I have a home-built carburetor made with a Proform main body and QFT internals. I put this together several years ago before you could buy a QFT carburetor. The QFT parts are top notch. For instance, the metering blocks have several emulsion channels with changeable "jets". It allows you to tune the carburetor better, plus it seems to be much smoother. I have removed a brand new carburetor and replaced it with my carburetor and the engine nearly always ran better. I have done this on a 351C, a 428FE, and 2 different 454s. So, I would also consider a brand new QFT vacuum secondary carburetor with or without an electric choke. |
I have swapped out two vacuum secondary carbs for mechanical secondary carbs on two different engines in two different cobras. Both engines seemed to come to life. Throttle response was noticeably better, fuel economy stayed the same and seat-of-the-pant's acceleration felt much better.
Worth the $$$ to me. And a well prepped mechanical carb will set you back about $750. |
I don't have a choke and that frees up a lot of room for more air and fuel to dump down the carb. I would venture to say to lose the choke and switch to a full bodied carb and you will see an improvement.
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I ran an 850 Holly double pumper built by a racing outfit in Los Angles with no choke or air horn and I thought it performed better than any other carb I have used. I have an 850 double pumper on my 69 Cobra and it has no choke, just the air horn and it works great. But if you aren't racing, I don't think changing what you have is worth the extra money..
Ron |
Anything is better than that Street Avenger.
I've sent many Quick Fuels out to replace them and the owners have all been nothing but ecstatic. If you need a choke, that HR series is good. If you don't need one, I would spring for the Q series. |
I ditched my regular double pumper and went to an annular booster style. Best money I ever spent. The throttle response is awesome even with pathetically low vacuum.
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Hopefully the OP is smart enough to get his car on a chassis (not engine) dyno before he makes any changes based on an estimate of what his engine is really putting out to the rear wheels. He can then make changes based on real world facts and figures so that he gets the right bang for his buck.
Bill S. |
In theory, they should be pretty close "if", the carburetor on the motor is sized properly. If the carburetor is "big" for the application, you run the risk of having a hesitant vacuum secondary. In this case you would benefit from a smaller carburetor of any shape or form.
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Getting choked by a choke
I would seriously consider dumping the choke.
I had a Holley double pumper on my 427...a fairly cold blooded motor...and was just fine without a choke. Not that you are looking for ultimate performance, but going chokeless will flow a bit better, IMHO. |
Years ago played with vacuum sec Holleys changed everything trying to get instant throttle response just not there. Finally picked up a dbl pmp dialed it in to the engine the throttle response was exactly what I was looking for.
A dbl pmp Holley has a slightly rich idle built in, which with adjustable bleeds can be tuned out. Even better have a pro carb person flow the carb. The difference between the two carbs for me a least was worth the extra money |
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