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Thread: power valve tuning for best fuel economy

  1. #1
    Robbie Racer
    I have some questions on power valves. I want to try and get a little better fuel economy while at cruising speeds by keeping the power valves closed until the motor goes into a boost condition. I have a 604" blower motor with Holley 850's (boost referencing disconnected) with mechanical actuated secondaries. The latest activating power valves that I can find are 1.0 HG but that should still allow them to stay closed longer than the current 2.5's in the secondaries and 4.5's in the primaries that I have now. I have the following questions.
    When in a cruising condition on the primary jets only (around 3000 RPM), will the secondary power valves open once the motor gets to 2.5 HG vacuum even though the secondary butterflys aren't open and add extra fuel? Or would I have to have the throttle open enough to open up the secondary butterflys for the secondary power valves to open once I passed the 2.5 HG vacuum zone?
    With the 4.5's on the primary and 2.5's on the secondary side, I think that means that the primary power valves will almost always be open at even a very low cruising speed and then the secondary power valves will open up around 50 mph. I'm thinking of replacing the secondary power valves with the 1.0's so hopefully, I can get up to around 60 (or maybe 70) or so before they open. I'm also thinking of then moving the 2.5's over to the primary side and replacing the 4.5's. Do you have any thoughts on trying this to see if I can improve fuel economy at cruising speeds? I currently run jet sizes of 85 on the primary and 90 on the secondary side. If I do change the primary side power valves from 4.5 to 2.5 will I have to increase the jet size to compensate?
    Thanks in advance for any input. RR

  2. #2
    Blown 472
    Have you put a vac gauge at the base of the carbs to see what you are pulling? you could map your vacuum to rpm and figure out where you want the pv's to open.

  3. #3
    Moneypitt
    Have you put a vac gauge at the base of the carbs to see what you are pulling? you could map your vacuum to rpm and figure out where you want the pv's to open.
    This is the "key" to P/V tuning. You have to know what kind of vacuum the motor is pulling throughout the rpm range. I would plug the seconday P/Vs, and dial in the primary to "driveability". The P/V function is all about transistion as the throttle is applied to pervent the stumble created by the additional air and lack of vacuum, kind of a crutch for the accelerator pump, another area that you can modify for the desired effect. Remember, the secondaries opening should be an extension of the acceleration, not the "bog" and recovery usually experienced with out of sync fuel systems. Most, if not all, performance carbs come with no P/V in the secondary circuit, which assumes the motor is "happy" enough to blend into WOT without needing the additional fuel from a P/V..........Put a temporary vacuum gauge where you can see it while driving and record some data as it relates to "driveability"....MP

  4. #4
    Robbie Racer
    The drivability is fine. I can't mash the throttle from an idle anyway in my boat or I will break the drive. I have to roll the throttle slowly on from about 60 mph on to prevent that from happening. I do have a hand held vacuum gauge and it reads around 2 inches of HG around 55 mph and around zero vacuum around 60 mph (depending on prop and blower over drive). I was trying to figure out how to improve fuel economy by keeping the power valves closed at 60 mph at zero vacuum but then open up when the motor goes into a boost condition. I think now that the only way to do that is to back off on the blower overdrive to try and see if that will keep the motor in vacuum at 60 mph to keep the power valves closed.

  5. #5
    Blown 472
    The drivability is fine. I can't mash the throttle from an idle anyway in my boat or I will break the drive. I have to roll the throttle slowly on from about 60 mph on to prevent that from happening. I do have a hand held vacuum gauge and it reads around 2 inches of HG around 55 mph and around zero vacuum around 60 mph (depending on prop and blower over drive). I was trying to figure out how to improve fuel economy by keeping the power valves closed at 60 mph at zero vacuum but then open up when the motor goes into a boost condition. I think now that the only way to do that is to back off on the blower overdrive to try and see if that will keep the motor in vacuum at 60 mph to keep the power valves closed.
    Zero vac wont keep them closed, they use vac to hold them shut, how is your jetting? and fuel psi?

  6. #6
    Moneypitt
    Zero vac wont keep them closed, they use vac to hold them shut, how is your jetting? and fuel psi?
    It would seem that "boost" under the carbs would create vacuum at the carbs? ANY chance the airflow to the carbs is too restricted to keep the vacuum below the venturi? Once the blower starts pulling air/fuel, it wants ALOT.....MP

  7. #7
    Infomaniac
    Yep your vacuum will need to be measured between the blower and carbs.
    AND... If your making boost at 60 MPH I would not recommend trying to keep the PV's closed.
    But unless those carbs are just horribly tuned, I would'nt expect much of a fuel savings tuning the PV's

  8. #8
    Robbie Racer
    Thanks for the input guys.
    Blown 472, my jet sizes are currently 85's on the primary sides and 90's on the secondary side. My fuel pressure is around 8 psi.
    MP, I don't think there is any chance of an air flow restriction. The blower and carbs stick out of the engine hatch and I only have a screen in the "air filter" to keep the big debris out of the carbs.
    Info, at 60 it's not making boost but it's at zero vacuum. Blown 472 is correct that at zero vacuum it won't hold the PV's closed. My question is at that condition with the secondary side throttle blades closed, will the power valves still add fuel at zero vacuum? Or do the throttle blades need to be slightly open for the power valves to add fuel?

  9. #9
    Infomaniac
    The secondary powervalves will be open but wont be delivering any fuel.
    The powervalves add fuel to the main metering well that the jets feed. if the throttles are closed then no fuel will be flowing.
    You might consider blocking the primary PV's off and jetting up the secondary jets to keep the "TOTAL" amount of fuel at WOT the same. Blocking off the secondary PV's is useless at cruise if you do not have the secondary throttle blades open.
    But keep a close eye on everything.PV's open at cruise defeats the purpose of them. They are supposed to keep it economical at cruise and open at high power settings.
    But... every engine is different. Just make sure you are not sucking them shut an WOT.

  10. #10
    Robbie Racer
    Info, thanks for the information and the suggestions. I think I might try swapping the 4.5's in the primary side to the 1.0's to see if that helps fuel economy at cruise. Hopefully I can keep the vacuum to at least 1.0 to prevent them from opening. If that doesn't work, I will try your suggestion of blocking off the primary side power valves and increasing the secondary side jetting to compensate. Is it still about 7 jet sizes per power valve?

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