Jetting for power valve

May 03, 2022, 07:40 PM

Consensus opinion shopping.
How many jet numbers (std. Holley jets) to add or subtract for adding or deleting a primary power valve?
Plain ol' 4150 Holley 850
I've heard and used everything from 6 to 10 over the years.
Maybe with the advent of modern tuning devices someone has nailed down a close starting number?

May 03, 2022, 10:38 PM

Eman

I'll take 8

May 03, 2022, 11:25 PM

TOP38

No magic needed, you just need the diameter of the main jet and the diameter of the hole in the metering plate behind the power valve. Calculate the areas of both and add them together then divide by 3.14 and then square root that number, that's the new radius of your new main jet, multiply by 2 to get the new main jet diameter. Use the holley jet chart to get you new holley jet number

May 04, 2022, 06:41 AM

CURTIS REED

If you believe your jetting to already be correct, then why would you not just square the jets? Or do you think the power valve is adding more fuel than squaring would give you?

May 04, 2022, 08:06 AM

Brktracer

Best answer is it depends on the size of the PV restrictions. 2 little holes under the PV are the PV "jets". Rarely are they the same size from metering block to metering block, especially if the blocks are aftermarket. Method Top38 describes is about the best starting point. Basically, compare the cross section area of the current jet vs. future jet (unknown) + PV "jet".

May 04, 2022, 10:23 AM

rusty

dont make it harder than it is, just a starting point,pick a number run it and check jetting by plugs and adjust accordingly.and top is correct

May 04, 2022, 12:07 PM

KEN GRAHAM

I have used 10 in the past on drag cars, and a few hotter street cars.
Better to be a little fat than lean...
good luck
Ken

May 04, 2022, 03:59 PM

Goob

quote:
Originally posted by CURTIS REED:
If you believe your jetting to already be correct, then why would you not just square the jets? Or do you think the power valve is adding more fuel than squaring would give you?

I've always run it squared with no PV's, and on the fat side.
It's a little dirty off idle (pit speed) now, so I'm thinking of running the PV to just keep things clean. I've also seen some data to suggest it might be better if I lean it out a touch.
New intake and a 7/4 swap cam have slightly altered the tune quality of my old carburetor (?).
It just had it's 20 year freshen up....

I think I'll take a gander at the PV orifices and start at about 6 numbers under.

Thanks for the scientific methodology as well Gentlemen!

May 04, 2022, 05:13 PM

CURTIS REED

Ah, I thought you were deleting, didn’t realize you were adding.

May 04, 2022, 05:39 PM

Bad News

6-7
But do a plug check after a clean wot shutoff coast around the corner.
Either carry 4 used replacement plugs in your pockets and remove 4 and replace them with the ones from your pocket or have someone tow you back so you can pull them all at the trailer.
50% of the cars out racing have too much fuel in the engine.

May 04, 2022, 08:10 PM

vetman

quote:
Originally posted by Goob:
quote:
Originally posted by CURTIS REED:
If you believe your jetting to already be correct, then why would you not just square the jets? Or do you think the power valve is adding more fuel than squaring would give you?

I've always run it squared with no PV's, and on the fat side.
It's a little dirty off idle (pit speed) now, so I'm thinking of running the PV to just keep things clean. I've also seen some data to suggest it might be better if I lean it out a touch.
New intake and a 7/4 swap cam have slightly altered the tune quality of my old carburetor (?).
It just had it's 20 year freshen up....

I think I'll take a gander at the PV orifices and start at about 6 numbers under.

Thanks for the scientific methodology as well Gentlemen!

no need to change much just try a little larger idle air bleed size should clean it up without changing full throttle tuneup

May 04, 2022, 10:06 PM

Goob

Roger on the idle air bleeds, I opened them up years ago.
Trying to zero in on a little leaner all weather tune....

May 21, 2022, 08:28 PM

jmarkaudio

The jet change depends on the PVCR size, but isn't a straight forward area calculation like suggested above. Restrictions in the passages thru the PV circuit cut down the actual fuel it supplies. A stock 850 had an 80 primary jet, typically 88 secondary. With a stock PVCR size figure 8 jets down to start. Jet the primary up first then down if needed to get/find best performance.

May 21, 2022, 08:48 PM

BG7X77

quote:
Originally posted by jmarkaudio:
The jet change depends on the PVCR size, but isn't a straight forward area calculation like suggested above. Restrictions in the passages thru the PV circuit cut down the actual fuel it supplies. A stock 850 had an 80 primary jet, typically 88 secondary. With a stock PVCR size figure 8 jets down to start. Jet the primary up first then down if needed to get/find best performance.

Thank you. Good info.

May 22, 2022, 09:52 AM

Curly1

quote:
Originally posted by CURTIS REED:
Ah, I thought you were deleting, didn’t realize you were adding.

True, if you are adding a power valve just look up the list number of the carb and go with original jetting to start.

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