Power Valve Selection: The Definitive Answer

Mattax said
An enrichment circuit like Carters use is another way to skin the cat.
As far as enrichment at 14" - the question becomes why.
If the engine has even half decent fuel distribution, half decent mid rpm combustion, etc etc then it should make more power with leaner mixes until at least 50% load.
If the engine cruises at 16"Hg, 65 mph, 3000 rpm on a flat interstate, then it should not need enrichment with a hair more throttle (14"Hg) to go up a slight incline.
Does that make sense?

The engine described above should accelerate without enrichment at 14, 12 and maybe even 10". But seeing it has somewhat poor vacuum at cruise, its probably not a great part throttle combo, so that's why 10" is a maybe.

Just looking throiugh some of the 60's Chrysler FSM, an 8.5 PV was used on many of the Chrysler Holleys.
The well known 3310 Holleys used on some of the Chebby hi perfromance engines had 10.5 PV in the primary side and 8.5 in the secondary.
The 2bbl Motorcraft 2150 on my Wagoneer's AMC's 360 has a PV that staged to partially open at 9"Hg and fully open at 5"Hg.
So there's some real life examples of what some different engine combos need.

Makes perfect sense, the 14" was just an arbitrary number. I guess maybe I've spent too much idle time thinking about power valves and tuning with this new to me AED Holley DP, and after extensive tuning on a Street Demon 750 with the metering rod step-up springs I previously was running. With the Demon, the tuning kit comes with 3-4-5 and 8" springs, the carb comes with 6" springs ootb. In the beginning it seemed I needed 7" to keep the rods down, and come up when i needed them to, so unable to find springs, I decided to do some improvising. Ended up using brass oil line ferrules that just so happens to position perfectly in the springs.I then used a file to whitle down the ferrule to a thickness that with it installed on the spring and checking the pressure (using a scale) to compress found it was exactly in between a 6" and and 8" spring, viola a 7"!. And it worked. Problem was, I did further tuning (and got better at it and went faster and quicker) and was last using an 8", but may have needed to go to a 9" as I was "so close" Basically, I've been racing with a street carb and tuning on it to the inth degree. Not the usual for carbs like that. Its a great carb, but it doesn't have the tunability of a Holley obviously. Id need to get into the idle bleeds etc and im just not gonna start drilling and tapping etc. So anyway, thats what got me thinking about pv's and the springs, likely too much anal obssession when one of the available valves will be all I'll need for my little van, Too much free time!! I'll see what happens with the current PV tuning, and see how much more I'll need to do.