power valves in racing carbs

Old 06-02-2008, 12:03 PM
  #11  
chevyart
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Default power valves

hey dcarr511 think i am a little confused by your last post. you say with a low number power valve (around 3.5) it will not be opened yet doing the burnout. My car is over (or around) 5000rpms doing the burnout. shouldnt the power valve be open long before the 5000 rpms. If it was not then i would probably be lean at that point. the staging process, which is around idle, i can see the power valve still being closed. Im really leaning towards using the power valve in front but i just want to be sure i am thinking this out properly and using the right power valve. moter is 355, ported iron heads, 12.3 to 1 compression, and a crane roller around .625 lift and the duration around 265-270 @.050, and a 5000 stall converter. i use a powerglide with 5.61 gears with 31 inch tires and come through the traps around 7700 rpms. im using proform center on a 750 holley with 74 jets in the front with a 3.5 power valve and 84 jets in the rear without a power valve. does this sound about right for my carb setup. i really appreciare you input on this as i am really concerned about getting the right combo. thanks again Chevy Art
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Old 06-02-2008, 08:52 PM
  #12  
nova77
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i had pro systems build me a carb for drag racing and he put a power value in it -- works great
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Old 06-03-2008, 08:20 AM
  #13  
mopar1968
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I run a dominator on a small block and i took them out just fatten up the jets 2 sizes.
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Old 06-03-2008, 03:31 PM
  #14  
jmarksdragster
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Originally Posted by mopar1968
I run a dominator on a small block and i took them out just fatten up the jets 2 sizes.
Unless it was already rich, this is a bad idea. Losing the PV generally requires 6 to 10 more than with with one on a Dominator.
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Old 06-03-2008, 05:48 PM
  #15  
mopar1968
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:? :? your saying to increase the jets 6 to 10 sizes up without power valves? i have never ran power valves.
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Old 06-03-2008, 06:44 PM
  #16  
jmarksdragster
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6 to 10 when removing a power valve. A 1050 typically runs 86 to 88 with power valves, 92 to 94 without. 3 to 4 more on an 1150, 6 to 8 more with a 1250. Modified Dominators may be a few off either way
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Old 06-04-2008, 12:46 AM
  #17  
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Ok, I have to disagree with this post at least part of it anyway.

" During the Burnout and Staging the Power Valve will likely remain shut and the Carb will only be receiving fuel from the Main Jets which are smaller ( than when run w/o a PV ) so it will be running leaner "


Durning the burnout at least mine anyway I'm almost always have the throttle at wide open or pretty close to it which would mean there is zero manifold vacuum and any power valve if installed would be open not closed since vacuum holds a PV closed not open.

Now I agree in staging it should be closed but this is where I get concerned with running a PV especially when "STAGGED", if you foot brake like I do while your torquing up the motor while STAGGED there is a chance that engine vacuum can drop enough to let the PV open or start to open which can cause a part throttle rich condition. Some people run a PV with a lower rating to try and combat this but this is primarily the reason I dont run one. Why take a chance on having a stumble or monkeying with my ET consistency?
Now if you run a box class you'll have it hammer down when stagged so it wont matter then.

JMO> Charles
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Old 06-04-2008, 02:45 AM
  #18  
chevyart
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Default power valves

hey cepx111 that is the post that really answers my original post and especially my second post questioning the power valve being closed for burnout. my second post describes my setup. if i go with a power valve in front and use the tranny brake always over 3000 rpms, which power valve number should I use thanks chevy art
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Old 06-04-2008, 09:07 PM
  #19  
cepx111
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Honestly since your almost over the threshold @ 3000rpm the 3.5 valve should be fine, but ultimatley you'll have to experiment to see exactly what the car wants so dont be afraid to make minute changes from weekend to weekend and see what works best for your particular set-up.

Note: I had a similar set up like yours on a 350 smallblock race only motor, Holley 750 dp with a proform center section, but I went with no power valves and had the carb jetted 82 square, worked really well for me.
Goodluck, Charles
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Old 06-04-2008, 09:42 PM
  #20  
chevyart
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Default power valves

hey cepx111 thanks again for your last post. guess what. I got my carb form my buddy who really knows his stuff and it was e3xactly like yours except he had the jets 80 front and84 rear, no power valves. I was the rocket scientist who changed it to 74 with 3.5 in front and 84 in rear. so if i go back to his original 80-84 it will be just about the exact same as yours. do you think I should go 83-83 square, as most guys recommend making the jets the same. pretty sure i will just go without the power valve, aand go with what you suggest as my jet size. thanks a million to you and all the other guys who took yhe time to help me with this important part of my engine tune. chevy art
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