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Author
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Topic: Power Valve?
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-10-2004 10:16 PM
   
Would someone please enlighten me as to what exactly is a powervalve and what is it's function in a Holley carb.I had read something previously on sizing. What are the guidelines? Thanks, Charlie
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steve'66
Gearhead Posts: 9104
From: Sonoma,CA,USA
Registered: Mar 2000
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posted 07-10-2004 10:28 PM
Charlie,A power valve is a valve that opens when engine manifold vacumm drops to add more fuel to the engine. It's purpose is for normal driving, not WOT drag raing. Say you are cruising along at a steady speed and approach a hill. You can keep your foot in the same position as the vacumm drops and the power valve opens adding more fuel to the engine. The more load an engine sees the lower the manifold vacumm drops, so the power valve is usually sized a few inches of vacumm lower than the engine's idle vacumm. Otherwise it would be open at idle really screwing things up. Hope that helps, if your car is a street/strip ride eliminate the power valve(s) and jet up 6 numbers to make up for it.  SteveW
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-11-2004 11:21 AM
Thanks Steve. That defines it well.
Speaking for a midly cammed, strictly street motor: How come it doesn't work at WOT? How does it know? 
Also, if stock PV's are like a 6.5 and average vacuum on a street motor is 11-14" of vacuum (I'm guessing on that one) why would you drop to a 4.5? Wouldn't that lean things out? Charlie
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Dave_C
Gearhead Posts: 994
From: Gadsden, Al
Registered: Aug 99
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posted 07-11-2004 12:56 PM
Charlie, I'm not Steve, but I'll give it a shot. The main reason for a PV is to increase driveability. On a street car if you jetted the carb for best wide open throttle power you would be way to rich at part throttle. Doesn't matter on a race car since you are either at WOT or idle (and on the carb's idle circuit) with very little time spent at part throttle. Most stock Holley carbs have a 6.5 power valve. I have always tuned for them by taking whatever vacuum the engine makes at idle and then dividing that by half. IE: a 13" vac engine would be perfect with a 6.5 pv. A 9" vac engine would take a 4.5 If it's uneven I round down. A 8" vac would use a 3.5 pv. Watch a vac guage at idle and then snap the throttle. The vac drops instantly and will open the pv. The needle also bounces around at idle if there is an healthy cam. That bounce us why I go to 1/2 the rating. If you put a 6.5pv in an engine with only 7-8" of vacuum the pv will partially open due to the vac bouncing around. Makes for an uncontrollable idle mixture that is far to rich. Hope that helps some. David Cole
"who has given up on carbs on his race car and gone to the promised land of mechanical fuel injection" [This message has been edited by Dave_C (edited 07-11-2004).]
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steve'66
Gearhead Posts: 9104
From: Sonoma,CA,USA
Registered: Mar 2000
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posted 07-11-2004 07:09 PM
Perfect Dave! SteveW
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-11-2004 08:53 PM
Thanks guys. I feel smarter already.
Seriously I appreciate the education. I am still looking for clues to stop the valve pinging. I get it at half and wide open throttle in the 1500-2500RPM range. I am going to take a vacuum reading this week, and a compression check next. Charlie
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capri man
Gearhead Posts: 6652
From: doerun, ga.
Registered: Nov 2000
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posted 07-11-2004 09:18 PM
charlie, what is your timing set on?------------------
mike r
racing is real
everything else is just a game.
81 capri-302-7.33 @92mph 1/8
1.54 60 ft. http://www.angelfire.com/ny5/billswebsite/pg06.html
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-12-2004 06:04 PM
capri man, it has been set everywhere. I had it all the way back to zero, yes (0), and it would fire right up, but still ping between 15-25 mph, which I calculate to be approx 1500-2500RPM. I currently have it at 12. If I go easy it's okay, but half throttle or more and it fusses until I get into the higher RPM range. I was going to check vacuum tonite, and I pulled the plug from the vacuum tube under the primaries bowl and it picked up about 200 RPM and was running smoothly. I still haven't gotten vacuum measured yet, but I verified it wasn't lean (at least at idle) with squirting propane from a torch down the carb. What do you think? Charlie
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capri man
Gearhead Posts: 6652
From: doerun, ga.
Registered: Nov 2000
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posted 07-12-2004 07:06 PM
what is your compression ratio and what kind of gas are you useing??------------------
mike r
racing is real
everything else is just a game.
81 capri-302-7.33 @92mph 1/8
1.54 60 ft. http://www.angelfire.com/ny5/billswebsite/pg06.html
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-12-2004 09:58 PM
Compression ratio started at 9.5 to 1. The machine shop just skimmed it to true the surfaces, so even a couple thousandths should keep me below 10 to 1 with fly-cut, flat-top pistons. As for fuel, just 93 hi-test. I was hoping it would behave on regular, but no such luck. The only other thing that is haunting me in the back of my mind is that maybe the cam was degreed wrong, or not at all. It may be off alittle. I asked Alex and he said one tooth off was 22 degrees, BUT that would equate to a cam gear with 16 teeth. The crank would have at least double that, which would put it 11 degrees out. I haven't gone into negative initial advance, but if the cam is opening and closing the valves out of sinc with the crank, I was told by another mechanic that I could have high static compression. This would be where the cylinder fills and the valve closes at the bottom of the stroke, due to short overlap, the piston would compress the whole upward stroke, thus compressing the mixture like a 12 or 13 to 1. This would explain the valve ping. That is why I am preparing to do a compression check. If it is around 120#, it's okay. If it is like 170#, I will know there are problems. I would ask the machine shop guy, but even though he's a nice guy, I wouldn't trust him to tell me the truth. He probably can't remember anyway. I will never let him touch this or another engine of mine. Charlie
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-13-2004 10:03 PM
Dave C- Vacuum is 12" at idle in park, and 8-10" in drive with the brake on. What do you think? Charlie
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Fastymz
Moderator Posts: 19583
From: Reno Nv USA MEM#1240
Registered: Apr 2001
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posted 07-13-2004 10:07 PM
I'd try a 4.5 or a 5.5 PV. BTW I've been asking every one I know how to help you. ------------------
SCOOP oddly obsessed with big scoops on little Mustangs 65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc. My Pics The older I get
The better I was [This message has been edited by Fastymz (edited 07-13-2004).]
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-14-2004 06:06 PM
quote:
Originally posted by Fastymz:
BTW I've been asking every one I know how to help you.
Ron, that is very kind of you. I appreciate all the suggestions you and everyone else have offered here. It has become my summer project to whittle away at this problem, clue by clue, until the pinger becomes a screamer. Not only that but I am learning a good bit as well. Thank you for your concern.
quote:
Originally posted by Fastymz:
BTW I've been asking every one I know how to help you.
Scoop, I'm beyond all help. I got a terminal case of Mustangitis and Go-Fast flu. Charlie 
[This message has been edited by MidLifCrisis (edited 07-14-2004).]
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Fastymz
Moderator Posts: 19583
From: Reno Nv USA MEM#1240
Registered: Apr 2001
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posted 07-14-2004 07:31 PM
quote:
Originally posted by MidLifCrisis:
quote:
--------------------------------------------------------------------------------
Originally posted by Fastymz:
BTW I've been asking every one I know how to help you.
--------------------------------------------------------------------------------
Ron, that is very kind of you. I appreciate all the suggestions you and everyone else have offered here. It has become my summer project to whittle away at this problem, clue by clue, until the pinger becomes a screamer. Not only that but I am learning a good bit as well. Thank you for your concern.
quote:
--------------------------------------------------------------------------------
Originally posted by Fastymz:
BTW I've been asking every one I know how to help you.
--------------------------------------------------------------------------------
Scoop, I'm beyond all help. I got a terminal case of Mustangitis and Go-Fast flu. Charlie
[This message has been edited by MidLifCrisis (edited 07-14-2004).]
Charlie you do know that SCOOP is Ron and Ron is Scoop just one person here. Sorry to let you down so hard, alittle like finding out that Clark Kent is really superman. :P
------------------
SCOOP oddly obsessed with big scoops on little Mustangs 65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc. My Pics The older I get
The better I was
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MidLifCrisis
Gearhead Posts: 508
From: Frederick County, MD
Registered: Oct 2001
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posted 07-14-2004 08:09 PM
Thank you sincerely for the expalnation, but I know Ron, Scoop, and Fastymz are one. Kinda like Twoface in Batman, I wished to address the two personas separately.
As I see it, Ron is the detail guy, getting specs from Alex to share with everyone; working on getting the body cherry before paint. AND Scoop is the other guy that blows Ricers in the weeds  (while Ron apologizes  ) and wants flames  (while Ron puts them off for the future  ) I love 'em both. They remind me of me and me (MidLifCrisis and G-freak Charlie).The web is the ultimate form for freedom of expression. Thanks, Charlie [This message has been edited by MidLifCrisis (edited 07-14-2004).] [This message has been edited by MidLifCrisis (edited 07-14-2004).]
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capri man
Gearhead Posts: 6652
From: doerun, ga.
Registered: Nov 2000
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posted 07-14-2004 08:21 PM
HUH????? ------------------
mike r
racing is real
everything else is just a game.
81 capri-302-7.33 @92mph 1/8
1.54 60 ft. http://www.angelfire.com/ny5/billswebsite/pg06.html
IP: Logged |
Fastymz
Moderator Posts: 19583
From: Reno Nv USA MEM#1240
Registered: Apr 2001
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posted 07-14-2004 09:04 PM
quote:
Originally posted by MidLifCrisis:
Thank you sincerely for the expalnation, but I know Ron, Scoop, and Fastymz are one. Kinda like Twoface in Batman, I wished to address the two personas separately.
As I see it, Ron is the detail guy, getting specs from Alex to share with everyone; working on getting the body cherry before paint. AND Scoop is the other guy that blows Ricers in the weeds (while Ron apologizes ) and wants flames (while Ron puts them off for the future ) I love 'em both. They remind me of me and me (MidLifCrisis and G-freak Charlie).The web is the ultimate form for freedom of expression. Thanks, Charlie [This message has been edited by MidLifCrisis (edited 07-14-2004).] [This message has been edited by MidLifCrisis (edited 07-14-2004).]
Now I'm real worried ------------------
SCOOP oddly obsessed with big scoops on little Mustangs 65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc. My Pics The older I get
The better I was
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Big D
Gearhead Posts: 5249
From: WELLS, NEVADA USA
Registered: Nov 2000
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posted 07-15-2004 04:31 AM
Hey,, I like that name SCOOP.. Don
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Fastymz
Moderator Posts: 19583
From: Reno Nv USA MEM#1240
Registered: Apr 2001
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posted 07-15-2004 04:37 PM
quote:
Originally posted by Big D:
Hey,, I like that name SCOOP.. Don
------------------
SCOOP oddly obsessed with big scoops on little Mustangs 65 coupe,351w,C4,Big Boss 429 hood scoop,8" 3.40 TracLoc. My Pics The older I get
The better I was
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