Power Valve Selection

carfreak6970 said
Now I do know that there are a couple ways to select the power valve based off of either the idle vacuum in gear, or the cruise vacuum halfed - 2. So I know the idle vacuum in gear is roughly 8-9 inHg. cruise at 50-70 mph has the vacuum at 18-19 inHg. So I think I am close with the pv, but it may be coming in a little soon. based off of the 18-19 cruise vacuum I should be having a power valve of 6.5-7.5. Is my line of thinking correct?

Using cruise vacuum as a parameter at least shows some understanding that enrichment relates to moderate versus heavy load for that engine and fuel distribution.

66Valiant528 said
From Holley.com

View attachment 1716288923

Well that's a set of "rules" which are not based in fact.
A. The main jetting typically has little to no effect on the "idle circuit". Watch Mark show that here on you tube.
There are exceptions - which is why a blanket absolute statement either way can be proven wrong.
B. Follow the fuel path from the idle discharge port (or the transfer slot) back to the bowl. Then tell me if the person who posted that "rule" has any claim to knowledge, nevermind expertise.

72bluNblu said
1/2 the idle vacuum in gear works. The PV should not be the first thing that comes in to add fuel, you have the accelerator pump and squirters that can be adjusted with the pump cam. The PV should come in after that.

The 750 double pumper on the 340 in my Duster came with a 6.5 PV, that’s a pretty standard off the shelf PV. If you have to double that, that should tell you something. My 340 runs 9-10 for idle vacuum, the 6.5 was too early. I run a 5.5 now, with 68 jets and 31 pump nozzles.

The fact that the OP his running really small primary jets and a PV that opens early should be an indication that things aren’t right.

It may work, but its not founded in engineering. I'm not sure of the origin of this one - but it goes back quite a ways. It probably relates to the belief that low number power valve must be for radical cams. Tuner gave his perspective on that a few years - see my post below.

Carnut68 said
You have to test to find out what pv works best. Find a spot where you have a flat spot on the road that slowly starts uphill. Gently roll into the throttle and if the engine hesitates go up a number on the pv so it adds fuel sooner. Whenever the hesitation goes away you found the right pv. Give the engine what it wants not what you think it wants.

^^^This^^^

mopowers said
To me, a power valve shouldn't be selected based on a vacuum reading. It should be selected based on when the engine wants the extra fuel like Carnut said in post #18. There's no magic number since it's dependent on so many external factors - vehicle weight, gearing, etc.

^^^This^^^
In addition to the engine's natural needs, the enrichment point can be thrown off by timing that doesn't match the engine's needs and the fueling.

72bluNblu said
There's a reason that most Holley's come with a 6.5 PV, because most gasoline engines will work well with that PV.

Yes. Holley thinks that 6.5 will be ballpark for most of the customers of those carbs. This is where Barry Grant's carbs had a big advantage. They sold a wider variety of calibrations with simple guidelines that got them closer to the engine's needs. Note that many of Holley's classic carbs had 8.5 and 10.5 PVs. Skim through Holley's carb list and check out the 4777 - 4780 double pumpers, the 1850s and of course the 3310 and related vac secondary carbs of yore.

72bluNblu said
If you have a more aggressive cam, then you need to LOWER the PV rating so it's not open more often than it should be.

This is the low number power valve myth I mentioned earlier.
Actually its generally the other way around. These engines are more radical tend also to have poor part throttle fuel distribution. They need enrchment well before 80 or 90% max load.