Power Valve Question

[tittyman]

Just put on dual 450's on my BBC with tunnel...radical cam..starting the process of dialing the carbs in...I can't get a vac. reading because I have no vac port to take one with..these carbs come with an 8.5 power valve..Holley Tech line suggests that this application should use a 4.5 to 3.5 power valve..
Question 1. What is the function of the power valve..and how can you tell if you have the right size? Whats the difference between a 8.5 (original) and a 4.5 and 3.5. What changes should I look for..
Question 2. From some of your previous experiences with dual 450's on a tunnel..what size power valves are most of you running?

[tittyman]

So the power valve only comes into play during acceleration...there is no effect at idle even if you have to idle say at 1000-1200 rpm?? Another question..how can you tell if your secondary throttle plates are adjusted correctly at idle?
Just for the record..I had 660's on there. I'm going to hang on to those carbs..Once I get the 450's dialed in I will use them..I'm also going to hang on to the 660's...major difference in power..
Dragboats (Denis) it's Jess..just getting the 79 Sanger Picklefork dialed in..runs like a raped ape!

[tittyman]

Can you use the Vac. port above the mixture screws to help adjust the carbs with a vac. guage..Right now I have them capped off.. Also, if I understand you correctly a power valve with a 4.5 spring opens easier than an 8.5..and either way the power valve has no effect during idle..only at acceleration?

[tittyman]

This tunnel ram has no place to get a vac reading...and I'm really do not want to drill into the manifold..there are no ports below the plates and the only vac port on these new 450's is above the mixture screw..which I guess from what you are saying is useless when it comes to any kind of carb adjustments for idle mixture...
Anyone out there with a BBC, tunnel, dual 450's, mag, radical cam...what power valve are you using and any other helpful suggestions..thanks..

[Blown 472]

Check it out.
http://www.holley.com/TechService/Library.asp
And why two 450's? why not two 750's???

[tittyman]

Good question...after talking to a lot of guys on this board and after talking directly to Holley...Holley said without a doubt use 450'S in a dual tunnel ram situation...I will continue to try and dial these carbs in..if they don't work out I'll send them back to Summit..that's the carb they recommended as well..just need advise on dialing them in. If they don't work out..I'll put back the 660'S. Just wish I had a good vac source to get things done right..
What size power valve are most of you guys running with dual 450'S? I'm thinking 4.5..they come with 8.5..Did most of you have to change the jets as well? Did they run pretty good right out of the box?...Thanks....

[tittyman]

Just got off the phone with the local carb guy..in my town we got one to choose from..he said just remove the power valve, don't even try to size it correctly, use larger jets and plug the power valve off..he said there are no tunnel rams that pull enough vacuum to operate a power valve correctly..just want to get these 450's dialed in....
Do many of you guys just eliminate the power valve and re-jet? Thanks..

[Taylorman]

You need to know what your vaccum is to know what size pv you need. Go to harbor freight and get a cheap vaccum gauge. Your carbs should have a vaccum port. Read this.
http://www.holley.com/data/TechServi...wer_valves.pdf

[tittyman]

I read that article before..on these 450'S (list 9776) the only port on the carbs is located above the mixture screw..where you would hook up you vac to the dist. if you needed it...that port does not give true vac for adjusting carbs or dialing in power valves...You need a port located at the throttle base to get the true manifold vac...or directly on the manifold..at least thats my understanding..I think that vac. distributor port only draws at high RPM when the vac pressure is at its lowest point...I think you need to take a reading at idle when the vac is at it's highest point...someone please correct me if I'm wrong about the vac port that is used for the dist.and is not the place to get ture manifold vac pressure readings....it would make life easier..thanks..

[Blown 472]

Just got off the phone with the local carb guy..in my town we got one to choose from..he said just remove the power valve, don't even try to size it correctly, use larger jets and plug the power valve off..he said there are no tunnel rams that pull enough vacuum to operate a power valve correctly..just want to get these 450's dialed in....
Do many of you guys just eliminate the power valve and re-jet? Thanks..
I did, when you plug them make sure you dont double gasket the plug, and the back of the plug is on hitting the throttle body of the carb as it will hold the metering block off the body. If there is a rasied part on the plug where it is round, the part of the plug that faces the throttle body then file it down till it is flat. 7 or 8 jet sizes to compensate for the pv depending on what size passages you have. Did you check out that holley web site?