Power Valve Tuning

[MD383]

Hello, I’m trying to select a power valve for my Holley 750 double pumper. I’m at the point where I have the transfer slot exposure set and I closed the primary idle speed screw and opened the secondary idle speed screw to keep the same idle speed. I leaned the primary main jets until it surged in 4th gear going up a slight hill and cruising on the highway. I went up 2 jet sizes once I got it to surge. I heard that you can plug the PV, take it out for a cruise, slowly get into the throttle and see at what vacuum it feels like it starts to surge. My question is that when should I be looking for it to surge? On the highway in high gear with light throttle or high gear going up a slight hill? Or just driving regularly in any gear? I currently have a 8.5 but I’m just trying to determine if the carb needs more or less. Thanks.

[MD383]

So if it surges when I open the throttle more at cruise at let’s say at 8” of vacuum then I would need a 8.5” power valve correct? The trickiest part is figuring out how I can route the vacuum house from the engine to inside the car because I don’t have any open holes in my firewall so it’ll have to be routed outside of the car.

[rgalajda]

Personally I have never heard of this method of choosing a power valve. Can you define " surging "? Hesitation , trailer hitching ?
I assume you are not having any issues that you are trying to correct. Some people take the cruise vacuum reading , divide the reading in half to choose the power valve. Example , cruise vacuum is 18" divided by 2 is 9. So 8.5 power valve would be acceptable.

[bigblockmark]

6.5 is the standard Holley power valve. Millions of them running around with that and doing fine. Vacuum at idle has nothing to do with power valve decision. Surge can also be a timing issue.

[MD383]

Jmarkaudio on a yellowbullet forum said “ I would plug the PV, take it out for a cruise, slowly get into the throttle and see at what vacuum it feels like it starts to surge. Or if you want to just throw a safe one in use an 8.5.”

[tbarb]

I have always thought to drive the car at 45 55 65 mph and note the lowest cruise vacuum then select a valve opening point approx 2" lower, the goal being that no power enrichment under low load cruise conditions.