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Author Topic:   Power valve question...
MrXerox
Gearhead

Posts: 322
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 06-07-2001 01:12 AM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
After calculating the correct power valve selection with the idle vaccum method, should the same size be used in the secondary side of the carb as well?

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69maverick
Gearhead

Posts: 890
From: Thomaston,CT.
Registered: Jan 2001

posted 06-07-2001 08:22 AM     Click Here to See the Profile for 69maverick   Click Here to Email 69maverick     Edit/Delete Message   Reply w/Quote
I have always wondered about these things to!
can someone explain the whole thing?

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capri man
Gearhead

Posts: 3258
From: doerun, ga.
Registered: Nov 2000

posted 06-07-2001 10:50 AM     Click Here to See the Profile for capri man   Click Here to Email capri man     Edit/Delete Message   Reply w/Quote
ALEX, WE NEED YOU!!!!!!!!!!!!!!!!!!!!!!!
MIKE R-----CAPRI MAN

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Mike_R
Gearhead

Posts: 150
From: Indianapolis, IN 46237
Registered: Feb 2001

posted 06-07-2001 12:06 PM     Click Here to See the Profile for Mike_R   Click Here to Email Mike_R     Edit/Delete Message   Reply w/Quote
I won't pretend to be Alex, but I'll throw 2 cents worth in. I would think since the power valve opens at it's designated vacuum rating that you would want front and rear to be the same. There is another issue to consider here also. If you have a car that launches really hard you probably don't even want a power valve in the rear of the carb. The front is okay with a power valve becuase it will stay covered with fuel under acceleration but the rear can become uncovered and cause a temporary lean condition. This is the same reason people use jet extensions in the rear of the carb. The power valve is actually higher up in the bowl than the jets so it is even more prone to being uncovered. If you plug the rear power valve you then have to jet the rear up by about 6-10 jet sizes. It's just something to consider. I really don't know what the cut-off point is as far as 60 ft times, but maybe somebody else does.

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Ryan Wilke
Gearhead

Posts: 1450
From: Stanton, Michigan 49707
Registered: Oct 2000

posted 06-07-2001 01:21 PM     Click Here to See the Profile for Ryan Wilke   Click Here to Email Ryan Wilke     Edit/Delete Message   Reply w/Quote
Here's another thought on the subject (maybe only 1-cent worth )

Since we're in the Racing Forum, I'll assume that you're application is a race car. My understanding is that you want the fuel delivery requirements of the primary circuit to be shared by using a PV and jets. This allows you to use smaller primary jets and thus, to idle in the pits and in the staging lanes without an over-rich condition that loads up your sparkplugs and burns your eyes. Once you're in the burnbox & jump the throttle this drops the manifold vacuum, and then PV would kick in to supply additional fuel with the jets. In the rear or seondary circuit, I agree you are better off to remove the PV (if you even have one there, DP'rs don't) and installing a plug or dummy PV. Yes, if you install a plug you have to install much larger 2ndary jets (maybe 6-8 or larger), but you don't need to worry about blowing that PV too, in the case of a violent backfire.

If you don't want to go with the plug in the rear, then I'd do as Holley does and use stepped PVs, to prevent a potential bog or stumble as a result of too much fuel coming on all at once. One example of this is in Holley's #3310, they ship them with a 10.5" PV in the primary and a 8.5" PV in the 2ndary circuit. Of course, barring personal preferences, actual testing is the best teacher.

Good Luck!! Ryan

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capri man
Gearhead

Posts: 3258
From: doerun, ga.
Registered: Nov 2000

posted 06-07-2001 04:38 PM     Click Here to See the Profile for capri man   Click Here to Email capri man     Edit/Delete Message   Reply w/Quote
hey mike r. that was a very informative and educated answer on the power valves. i hope someone gets us mixed up and thinks it was my answer!!!!!! hehe
thanks-----MIKE R------capri man

[This message has been edited by capri man (edited 06-07-2001).]

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MrXerox
Gearhead

Posts: 322
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 06-07-2001 08:05 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
How about for a primarily street driven car? It is by no means a daily driver, just a toy to go spank chebbies with Anyway it is a 351C with closed chambered heads, an x-cellerator manifold, 850DP, 244 @ .050 .623/.623 solid roller C-6 with 3.89s in the back...I would assume that plugging the rear PV would not cause any streetability problems as the secondaries would not be used unless I really got on it anyway...? I bought the carb used and rebuilt it and it already had the rear jet extensions and plug in the back.

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Mike_R
Gearhead

Posts: 150
From: Indianapolis, IN 46237
Registered: Feb 2001

posted 06-08-2001 08:52 AM     Click Here to See the Profile for Mike_R   Click Here to Email Mike_R     Edit/Delete Message   Reply w/Quote
Hey MrXerox, I have set up a couple of 850's and 1050's this way for the street and they seem to do fine. You may just have to experiment and see what works for you.

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MrXerox
Gearhead

Posts: 322
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 06-08-2001 08:35 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Do you guys think I should stick with this 850 or try and find a 750? I just got the motor done today and was talking with a guy that said he thought a 750 would run much better on my motor...I remember kid vishus saying that he never could get an 850 to work worth a damn on his 351C do to a weak signal...

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capri man
Gearhead

Posts: 3258
From: doerun, ga.
Registered: Nov 2000

posted 06-08-2001 09:13 PM     Click Here to See the Profile for capri man   Click Here to Email capri man     Edit/Delete Message   Reply w/Quote
i think you might have a bit to much carb for your engine. the 750 will probably be closer to what you need.
good luck---mike r---capri man

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