Re: Power valve selection?
[Re: ireland383]
#459196
09/04/09 05:07 PM
09/04/09 05:07 PM
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Joined: Mar 2005
Posts: 1,373
Rancho Cucamonga, CA
D_C
pro stock
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pro stock
Joined: Mar 2005
Posts: 1,373
Rancho Cucamonga, CA
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I have read about Two methods of selection based on Idle manifold vacuum. The Holley Factory Method is 1/2 the idle vacuum, so 125-70 or 7.0 PV. Here is the Holley Factory Link with explanation: Holley Power Valves
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Re: Power valve selection?
[Re: ireland383]
#459202
09/04/09 05:34 PM
09/04/09 05:34 PM
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Joined: Mar 2005
Posts: 1,373
Rancho Cucamonga, CA
D_C
pro stock
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pro stock
Joined: Mar 2005
Posts: 1,373
Rancho Cucamonga, CA
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Sorry, I thought earlier you said you had 14 In/Hg, but that was your Neutral Idle. So, Yes, if you have 7 In/Hg at "In-Gear Idle, 1/2 that would indicate a 3.5 or Holley P/N: 125-35 Power Valve. Here is the Exact Quote from the Holley Website: Quote:
Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number “65” indicates that the power valve will open when the engine vacuum drops to 6.5" or below. An accurate vacuum gauge, such as Holley P/N
26-501, should be used when determining the correct power valve to use.
A competition or race engine which has a long duration high overlap camshaft will have low manifold vacuum at idle speeds. If the vehicle has a manual transmission, take the vacuum reading with the engine thoroughly warmed up and at idle. If the vehicle is equipped with an automatic transmission, take the vacuum reading with the engine thoroughly warmed up and idling in gear. In either case, the power valve selected should be 1/2 the intake manifold vacuum reading taken.
EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5 power valve. If your reading divided by 2 lands on an even number you should select the next lowest power valve. EXAMPLE:
8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5.
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Re: Power valve selection?
[Re: ireland383]
#459206
09/04/09 08:26 PM
09/04/09 08:26 PM
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Joined: Jan 2003
Posts: 12,100
Benton, IL.
DaveRS23
Special needs idiot
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Special needs idiot
Joined: Jan 2003
Posts: 12,100
Benton, IL.
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I know Holley says 1/2 idle vac for an opening point, but my experience is that often causes a hesitation. There is no vac signal to the PV at idle so a higher (earlier) opening point will not affect the ide mixture. I usually run a 9.5 in almost everything I set up. In my Cuda right now, I have a 10.5 hi flow. My idle vac is 9" or so in gear. I can run a 9.5 and even an 8.5 if the weather is right. But by 6.5, I have a stumble at tip in. I am sure that Holley has forgotten more than I know about carbs, but I have dealt with this issue many, many times. If it helps, I get my PVs from O'Reilly's Auto Parts. They are the same price as Jegs, usually have it the next day and there is no shipping. 
Master, again and still
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Re: Power valve selection?
[Re: ireland383]
#459207
09/04/09 11:13 PM
09/04/09 11:13 PM
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Joined: Jan 2003
Posts: 5,443
Indiana
YO7_A66
master
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master
Joined: Jan 2003
Posts: 5,443
Indiana
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""I'm pulling 14" of vacuum at idle and 7 in drive."
I think that you have to address the 7"hg drop from Neutral to Drive before you choose your power valve. That is a big drop in vacuum. I would suggest that you try to tune the carb with the car warmed up with the emergency brake applied with the tranny in D. I think that you are lean in D. If this is true, then this would cause the hesitation off the line like you described.
1970 YO7 A66 [Canadian Export] F8 Challenger
340 (Currently in shop for stroker assy.)
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