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Thread: jet extensions

  1. #1
    Junior Member JOURNEYMAN
    Join Date
    May 2007
    Pfafftown , NC

    jet extensions

    If I put jet extensions in my 750 demon and remove the power valve then how much do I need to fatten up my rear jets.

  2. #2
    Senior Member DYNO OPERATOR
    Join Date
    Apr 2008

    never used a demon before but

    7 to 10 on a holley is a common enrichment for blocking the PV circuit.

  3. #3
    Senior Member DYNO OPERATOR
    Join Date
    Aug 2007
    Lost Creek, WV

    Re: never used a demon before but

    Quote Originally Posted by TopspeedLowet
    7 to 10 on a holley is a common enrichment for blocking the PV circuit.
    There is a lot of misunderstanding concerning power valves in Holley carburetors. Many 4-barrels come with a particular power valve depending on the carb list # and application. Some carburetors have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" of vacuum and will remain closed above. One of the misconceptions is that they can't be trusted to work because an engine backfire can "blow out" the power valve. Many of the newer Holley performance carburetors now come with a built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install Holley kit #PN - 125-500 that will protect the power valves in case of an engine backfire in the carb. I use the CENTEK, "Power Valve Shield". See their website at , which takes about two minutes to install and does not require any drilling.

    Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. When the power valve circuit is plugged, part throttle fuel economy is lost and may become overly rich. Plug fouling may become a problem at part throttle.

    Stock engines have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work. Longer duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lower. The power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve. To determine this, you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "DRIVE" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.

    Holley makes a performance style "standard" flow or the "high" flow power valve. The latter has larger openings for more fuel flow with bigger engines. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". Holley also makes a "two stage" power valve that is more for "economy" minded users rather than "performance" enthusiasts.


    Bruce, you are correct


  4. #4
    Senior Member RACING JUNKIE jmarksdragster's Avatar
    Join Date
    Jul 2007
    For a 750 Demon start with 75 front with a PV, 85 secondary with extensions. Might be on the rich side to start and that's OK, go up first then down if the MPH slows, jet for best MPH. Nothing wrong with blocking the PV on the rears on a drag motor, unless you run 1 to 1 linkage you won't likely be cruising on the secondaries.
    Mark Whitener

    Good work isn't cheap, and cheap work can't be good.

  5. #5
    Senior Member MASTER BUILDER
    Join Date
    Mar 2008

    power valves

    i understand bruces reply and the nice informative info by ts1955, and i go by that info. i thought i had once read that as long as the vacum reading is anywhere above 10 inches, then just go with a 6.5 power valve and it would be fine. is there any truth in that statement thanks art

  6. #6
    Senior Member RACING JUNKIE
    Join Date
    Mar 2006
    Never heard that one. The report TS posted is what I have used. I had to deal with altitude issues too; buying a car @4000+ down to 300 ft of elevation once. I remove the secondary PV too; ran a jet extension.
    "I would walk through hell on Sunday before I fear the enemy"

  7. #7
    Senior Member RACING JUNKIE
    Join Date
    May 2006
    Depends on the ap, in a drag car power valve(s) are all but useless.

    Oh and I don't jet up like 10 sizes either, more like 4-5.
    ***IN GOD WE TRUST***

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