Power Valve question

[bandit]

Rebuilding the carb this weekend while the wife is out of town and I can bring this stinky thing inside to work on it. Got a rebuild kit and it came with a power valve but the size already in the carb is a 7.5 and the kit came with a 6.5. I do not have time to order up a different size unless I want to hold this off until summer. I know the size is determined by manifold vacuum, but my question is, what will happen if I just put in the 6.5. (I did not take a vacuum measurement before removing the carb) Meaning what should I expect as far as performance difference with a smaller power valve installed if it is supposed to take a 7.5. This is on a 750 cfm Holley. Going to use it for street driving. Previous owner raced it.

 

[lewtot184]

6.5 should be fine

 

[bandit]

Thanks

 

[HEMI-ITIS]

Go with the new ones and verify the vacume on the restart.

 

[Photon440]

It's not really a smaller power valve, but it will open slightly later. The 6.5 and 7.5 is the manifold vacuum that will trigger when the valve opens. Under WOT they'll flow the same.

 

[RustyRatRod]

Just make it simple. Plug the power valve port off and jump up several sizes in jets. Power valves suck. Just one more thing to go wrong.

 

[bandit]

I've heard of that but wouldn't that create a rich problem at low rpm.

 

[Meep-Meep]

The 7.5 Hg power valve will open sooner than the 6.5 Hg one and that may be OK or not OK depending how your engine is set up. If you are pretty stock then the 7.5 may be just fine. I would not block it on a street car for the reason you said.

 

[RustyRatRod]

I've blocked the pv on several hot street car and never had a problem fouling plugs. Power valves are a pain in the ***.

 

[rev.ronnie]

Dialing in the correct power valve is critical to how well they work. If you have a valve with a vacuum value that is too low, say a 2.5 on a relatively hi-vac engine, it will open too late when you roll on the throttle. This will create the bog or lag before it starts pulling fuel and accelerating the car. Conversely, a power valve that is too high for the engine's vacuum will open too soon and you will get a soggy bisquet that reeks of gas at idle. Many guys block the power valve on the secondaries for easier tuning. I run them on both front and rear most of the time.

Every engine is different. Some engines with smaller carbs may overwhelm the booster with fuel without running a power valve, some bigger carbs atomize it better if it all goes that route. It's all understanding each function of the carb's circuitry, the individual air bleed circuits, the power valve system and making them all work together.

As a rule of thumb, if you are fuel fouling spark plugs, you are probably murdering your piston rings and contaminating your oil with fuel.

Carburetors are really just "semi-controlled" leaks at best. They sure are fun to play with though.

 

[lewtot184]

power valves are rated in ".5" increments for a reason. the reason being is that they don't exactly hit their rated number. example; a rated 6.5 power valve may open at 7" or 6", a plus or minus of .5. the only true way to find the exact power valve that is needed is by road testing and that isn't exact science. generally use a power valve that has at least a couple inches rating lower than the idle vacuum (in gear for an automatic). removing the power valve in a street car is like giving the carb a lobodomy. basically the carb will run on the idle circut or main metering all the time, no cruise mode. running no power valve belongs on the track and at times i question the use there. removing the power valve and just jetting up a few steps is rather crude. the area of the power valve restriction orifices needs to be figured out and added to the jet area. take the total area of both and then find a jet with the same area. also be prepared to loose several mpg's. we hot rodders always try to re-engineer our parts, but the truth is most of the time we shouldn't.

 

[RustyRatRod]

I've never had a lick of trouble eliminating power valves. I always jumped up between 6-8 jet sizes and never looked back. Never had problems with fouled plugs or running too rich. Keep in mind I have actually DONE it, not just read about it on the internet. This is straight off the Holley site:

"The purpose of a power valve is for it to add the needed extra fuel when you are under wide open throttle. It is not intended to be an enrichment circuit during normal throttle cruising.

If you are lean or rich during normal driving conditions, you need to tune the jets, not the power valve.

The reason you check the vacuum at idle and take that reading and divide it in half, is so you do not have a power valve that will open at idle as this will cause an excessively rich mixture at idle. . Under WOT the vacuum of the engine will drop to 0 or near 0 and the power valve will then open adding the extra fuel needed during WOT."

Plugging the power valve is fine as long as you jet accordingly. It has zero effect on cruise conditions, unless it is opening prematurely. I've run them with and without and I happen to prefer without. Just because somebody does something different than the "norm" does not make it incorrect.

 

[72RoadrunnerGTX]

Wow, a whole lot of carburetor/fuel injection engineers have wasted a ton time over the years designing load variable fuel enrichment systems for use on internal combustion engines. It appears now, it is all unnecessary.

Your Holley quote says nothing about recommending the removal of power valves, on the contrary it states that fuel mixtures requirements are different for cruise and WOT. On a street application, power valve timing aside, if plug the valve and jet correctly for cruise, it will be too lean for WOT, jetted correctly for WOT it will be unnecessarily rich for cruise,"6-8 jet sizes"?, simple as that. For a straight out race application where there is little to no “cruise” phase, its idle or WOT, the power enrichment system serves little purpose. On a street application, it’s there for a purpose.

 

[lewtot184]

i've done it too. it's not on my recommended list for drivers. my experience with this stuff dates back to the late '60's and a chassis dyno shop i used to frequent. the things i done then i wouldn't do now. some of the things we did back in the day was thru ignorance. we didn't have the information or technical expertise to make good decisions.

 

[RustyRatRod]

Wow, a whole lot of carburetor/fuel injection engineers have wasted a ton time over the years designing load variable fuel enrichment systems for use on internal combustion engines. It appears now, it is all unnecessary.

I never said that, either. I simply said I found something that has worked for ME for years. Simple as that. No need to be a smartass about it. I don't understand when someone makes something work against the grain so to speak, why there have to be people who make it personal. It's all good though. Carry on.

 

[HEMI-ITIS]

I have a vacume gauge tapped into the carb spacer.This tells me what the vacume drops to at WOT and at idle.I was told I had to use boost referenced carbs on my blower which I did not.When I tried the plug it method I developed an off idle/cruze bog when I opened the secondaries.I reinstalled the 10.5 PV"s and the bog dissappeared.It idled with 13 inches of vacume,cruze was 25 and WOT was 4.So I had a better set uo using the power valves.Your results may differ.

 

[moparmarks]

Have several friends that live by the Leaner is Meaner concept and don't run a P/V. Most problems that I've seen is they don't fatten up the jets enough causing a lean condition and burning a E/valve. Holley recommends running a P/V but yet they make the delete plug. I, myself have had better luck running the P/V but have friends with good luck without the P/V.

 

[HEMI-ITIS]

These circuits all work together,including the air bleeds to really dial in a carb.After many years with a pair of 750 DP's,I stepped up to a pair of SV1 carbs by Pro System.A new design by Patrick himself.I have dialed them in best I can untill I get a data logger,then I can really turn the boost uo over 10 lbs.I have not raced with more than 5 lbs YET!

 

[65_Satellite]

Dialing in the correct power valve is critical to how well they work. If you have a valve with a vacuum value that is too low, say a 2.5 on a relatively hi-vac engine, it will open too late when you roll on the throttle. This will create the bog or lag before it starts pulling fuel and accelerating the car. Conversely, a power valve that is too high for the engine's vacuum will open too soon and you will get a soggy bisquet that reeks of gas at idle. Many guys block the power valve on the secondaries for easier tuning. I run them on both front and rear most of the time.

Every engine is different. Some engines with smaller carbs may overwhelm the booster with fuel without running a power valve, some bigger carbs atomize it better if it all goes that route. It's all understanding each function of the carb's circuitry, the individual air bleed circuits, the power valve system and making them all work together.

As a rule of thumb, if you are fuel fouling spark plugs, you are probably murdering your piston rings and contaminating your oil with fuel.

Carburetors are really just "semi-controlled" leaks at best. They sure are fun to play with though.

I know this is a very old post but wanted to ask about the second power valve on the secondaries. Ive never had a carb with both of them, I've only had them in the primary and there wasn't even a spot for one in the secondary. My new double pumper 750 has both front and rear. If my car is pulling 9hg of vacuum at idle (big cam and a 4spd with idle at 950) I would normally run a 4.5 power valve in the primary but what would I do for the secondary? Would that be the same or would I go slightly smaller like a 3.5 or 2.5?

 

[RemCharger]

If they were the same, keep them the same.