Power valve size?
Thread Starter
Race Director
Joined: Mar 2001
Posts: 11,322
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From: Living in the Hartland
Power valve size?
Going to pull the carb and do some cleaning and adjusting. Got a couple questions on powervalves and sizes. According to Lars' adjusting paper in the tech section, the powervalve opens when the vac pressure hits the valves rating ie at 6.5 (which is states is stock) the valves opens.
1. Does this mean above 6.5 it is open or below 6.5 it is open. Seems by his description the vac pressure sucks the valve closed so anything above 6.5 it is closed. Also, it it progressive or open/closed?
2. If I am running 16 to 17 lbs of pressure at idle (yes pretty small cam) then by his suggestion I should have a 14 to 15 lbs valve. That seems like a big jump from stock 6.5 to 14 to 15. Is this right or what size would you suggest?
I'll throw in a jet questions: What should be the difference between main and sec jet size? I am running 67's in the main and not sure about the sec. What size jets for the sec assuming I am right on with the 67's?
Thanks
Chris
1. Does this mean above 6.5 it is open or below 6.5 it is open. Seems by his description the vac pressure sucks the valve closed so anything above 6.5 it is closed. Also, it it progressive or open/closed?
2. If I am running 16 to 17 lbs of pressure at idle (yes pretty small cam) then by his suggestion I should have a 14 to 15 lbs valve. That seems like a big jump from stock 6.5 to 14 to 15. Is this right or what size would you suggest?
I'll throw in a jet questions: What should be the difference between main and sec jet size? I am running 67's in the main and not sure about the sec. What size jets for the sec assuming I am right on with the 67's?
Thanks
Chris
Racer
Joined: Sep 2001
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From: Prineville Oregon
Re: Power valve size? (Fevre)
The power valve opens under a heavy load such as full throttle. At full throttle your engine vacuum will drop below 6.5, this opens the power valve and feeds more fuel to the engine.
We need the carb part number to tell you what your jetting should be...it is found stamped on the front air horn and will be a number starting with an "R". Your carb probably has a secondary plate rather than jets to be changed out. You can buy plates or enlarge the holes if you have access to "numbered" drills.
Your going to find that "little" 650 way to small of CFM when you switch to your trick flow heads...you'll need at least a 750 Holley and 1 3/4" inch header to extract the power above 5,500 RPM. Deen
We need the carb part number to tell you what your jetting should be...it is found stamped on the front air horn and will be a number starting with an "R". Your carb probably has a secondary plate rather than jets to be changed out. You can buy plates or enlarge the holes if you have access to "numbered" drills.
Your going to find that "little" 650 way to small of CFM when you switch to your trick flow heads...you'll need at least a 750 Holley and 1 3/4" inch header to extract the power above 5,500 RPM. Deen
Thread Starter
Race Director
Joined: Mar 2001
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From: Living in the Hartland
Re: Power valve size? (DeenHylton)
Thanks Deen
My carb is a 4150 with vac sec. I know it has jets since I put bigger ones in when it was on my 383, just don't recall what size they were. I am going to check the valve size when I pull it this week end. What size powervalve do you think i need with an idle vac pull of about 16lbs?
I think I am going to run both carbs (have a 750 dp too) on my 383 and do some dyno test to see where I stand. I don't need all out horse power unless I am going to race it. I am looking more for low end torque and mid range horse. But if I like the dp better, then it will stay on the engine. I am going to get headers for the 383 and probably a new exhaust system.
My carb is a 4150 with vac sec. I know it has jets since I put bigger ones in when it was on my 383, just don't recall what size they were. I am going to check the valve size when I pull it this week end. What size powervalve do you think i need with an idle vac pull of about 16lbs?
I think I am going to run both carbs (have a 750 dp too) on my 383 and do some dyno test to see where I stand. I don't need all out horse power unless I am going to race it. I am looking more for low end torque and mid range horse. But if I like the dp better, then it will stay on the engine. I am going to get headers for the 383 and probably a new exhaust system.
Safety Car
Joined: Mar 1999
Posts: 3,715
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From: VASS, NC, USA
Re: Power valve size? (Fevre)
I did what TOM454 recommended
Hook up a vacum gaue to a ported source of vacum. Watch vacum gauge at idle, and at a crusing speed. Get a PV thats rated, 1.0-2.0 points LOWER
Worked for me
Hook up a vacum gaue to a ported source of vacum. Watch vacum gauge at idle, and at a crusing speed. Get a PV thats rated, 1.0-2.0 points LOWER
Worked for me
Thread Starter
Race Director
Joined: Mar 2001
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From: Living in the Hartland
Re: Power valve size? (MELVIN)
Melvin
I guess you mean use the lower of the two vac readings to determine the baseline? Also refresh me on ported, is that the one which is always open or opens like for the vac adv dist?
Thanks
I guess you mean use the lower of the two vac readings to determine the baseline? Also refresh me on ported, is that the one which is always open or opens like for the vac adv dist?
Thanks
Le Mans Master
Joined: Apr 2000
Posts: 5,202
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From: cincinnati ohio
St. Jude Donor '09
Re: Power valve size? (Fevre)
Beg, borrow or steal a good vacuum gauge and hook it to direct manifold, put it inside the car with you and go for a ride. Observe the readings at various steady cruising speeds. Select a power valve that opens about 2 in. below the lowest steady cruise reading :cheers:
Thread Starter
Race Director
Joined: Mar 2001
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From: Living in the Hartland
Re: Power valve size? (yellow 72)
Or I could just take it out of my tool box. ;)
But I will do the cruise vac test before I take the carb off so I know where I stand.
Thanks
But I will do the cruise vac test before I take the carb off so I know where I stand.
Thanks
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Racer
Joined: Sep 2001
Posts: 354
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From: Prineville Oregon
Re: Power valve size? (Fevre)
Chris, your "4150" is not the part number, that is only a "style" of Holley carb. As I said before your part number begins with an "R". My 750 is a 4160 series carb that I have modified to become a 4150 series carb...my part number is an R3310-4. From the part number I can tell you what the stock jet sizes are , correct power valve, renew kit, color of secondary vacuum spring,etc.
If you have jets on the secondary side your probably have an R2818-1, if that's the case your secondary jetting would be 76's
If you have jets on the secondary side your probably have an R2818-1, if that's the case your secondary jetting would be 76's
Thread Starter
Race Director
Joined: Mar 2001
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From: Living in the Hartland
Re: Power valve size? (DeenHylton)
Deen
I know that 4150 is not the part number but does intake jets in the secondaries instead of a metering block like in the 4160 series. I am trying to tailor this carb to the specific characteristics of my engine and I was trying to use Lars' paper on Holley tuning in the tech section. I just thought it would be a huge leap from a 6.5 rated power valve (which was what was in my carb) to a 14 to 15 which Lars' formula called for. In my case I am 16 to 17 lbs at idle so according to formula I get a valve that is 2 lbs less that what I am pulling hence the 14 to 15. Does that seem right to you? I have to admit I don't think my vacuum will drop to 6.5 so I don't think the power valve would ever open. The biggest one I could find in stock was 9.5 but one of their techs said that 8.5 was what i needed. I thought, well at least it would be closer to what Lars calls for. Of course instead of just putting the power valve and testing it, I adjusted a few other things so now i have a slight hesitation. Would really like a concrete answer on power valve size calculation and if 14 to 15 rating seems correct. Any one running one that high?
I know that 4150 is not the part number but does intake jets in the secondaries instead of a metering block like in the 4160 series. I am trying to tailor this carb to the specific characteristics of my engine and I was trying to use Lars' paper on Holley tuning in the tech section. I just thought it would be a huge leap from a 6.5 rated power valve (which was what was in my carb) to a 14 to 15 which Lars' formula called for. In my case I am 16 to 17 lbs at idle so according to formula I get a valve that is 2 lbs less that what I am pulling hence the 14 to 15. Does that seem right to you? I have to admit I don't think my vacuum will drop to 6.5 so I don't think the power valve would ever open. The biggest one I could find in stock was 9.5 but one of their techs said that 8.5 was what i needed. I thought, well at least it would be closer to what Lars calls for. Of course instead of just putting the power valve and testing it, I adjusted a few other things so now i have a slight hesitation. Would really like a concrete answer on power valve size calculation and if 14 to 15 rating seems correct. Any one running one that high?
Racer
Joined: Sep 2001
Posts: 354
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From: Prineville Oregon
Re: Power valve size? (Fevre)
The book I have shows a 10.5 power valve as the highest you can purchase unless you go to a two-stage power valve. The two stage valves are generally used on tow vehicles...you can get a 12.5 first stage with a 5.5 second stage. The first stage opens under part throttle with a heavy load and then goes to the second stage under full throttle as when your passing or towing up a steep hill.
Race Director
Joined: Nov 2000
Posts: 11,872
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From: Waterloo ontario Canada
Re: Power valve size? (BB wowbagger)
The formula for a correct power valvis:Vac at idle x 2+2 /2..Thats what i`ve learned..
_______
Am I reading this Wrong? If a motor has a idle vacuum of say 14 then 2 x 14 equals 28 plus 2 is 30 divided by 2 is 15. So the correct power valve for a motor idling at 14 would be 15. I don't think so.
_______
Am I reading this Wrong? If a motor has a idle vacuum of say 14 then 2 x 14 equals 28 plus 2 is 30 divided by 2 is 15. So the correct power valve for a motor idling at 14 would be 15. I don't think so.
Le Mans Master
Joined: Aug 2000
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From: Semper ubi, sub ubi
Cruise-In VI Veteran
Cruise-In VII Veteran
Re: Power valve size? (Fevre)
I believe the formula is idle + 2 / 2, If your idle vacuum is 15 then it would be, (15+2=17) /2 = 8.5 for the power valve. If your idle vacuum was 13, it would be (13+2=15)/2 = 7.5 for the power valve.
At least this is what the guy at the speed shop told me, and he could have been wrong.
At least this is what the guy at the speed shop told me, and he could have been wrong.
Thread Starter
Race Director
Joined: Mar 2001
Posts: 11,322
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From: Living in the Hartland
Re: Power valve size? (Fevre)
Thanks you all for the input.
I am going to get a new vac gauge and some vac hose this weekend to test my actual driving vac under normal and hard acceleration which should tell me a lot. From what I read and understand about the power valve and how it operates, it should be a couple pounds below my idle vac. Thing is I seem to be running rich at idle/off idle but when I jet down a size I get a big hesitation on acceleration so the jet size seems in the ball park but could go down with the correct power valve size. Odd that the largest single stage is 10.5, might have to try the two stage. Guess I could make it easy on my self and get a bigger cam to drop my vac down to where I need a smaller power valve.
:)
I am going to get a new vac gauge and some vac hose this weekend to test my actual driving vac under normal and hard acceleration which should tell me a lot. From what I read and understand about the power valve and how it operates, it should be a couple pounds below my idle vac. Thing is I seem to be running rich at idle/off idle but when I jet down a size I get a big hesitation on acceleration so the jet size seems in the ball park but could go down with the correct power valve size. Odd that the largest single stage is 10.5, might have to try the two stage. Guess I could make it easy on my self and get a bigger cam to drop my vac down to where I need a smaller power valve.
:)
Race Director
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From: Waterloo ontario Canada
Re: Power valve size? (Fevre)
The main jet shouldn't have anything to do with off idle hesitation. The main jet only comes into play when you are cruising. Off idle stumble is effected by the transferr ports, accelerator pump and squirters. I jet my carbs fairly lean and use the power valves to richen the mixture for passing, hill climbing etc. I have a oxygen sensor and meter so under normal cruising loads using the jets it is around 15 to 1 but a little acceleration and the power valve richens it to 13 to 1. MY gas mileage is around 12 mpg around town with no loading problems at idle.
