How to modify your Holley Power Valve Circuit

[Tom Kirkham]

Originally Posted by 69boss429

Are you doing this modification to decrease or increase fuel volume while still retaining the original PV vac setting ie. 6.5 etc.
Or,is this for track use such as where performance suffers coming out of corners and a boost from more fuel is required.
Or,are you just seeking better overall street driving...

We do this modification to get the air fuel ratio where the engine needs it. Most of the time (but not always) Holley has the fuel curve set very safe (rich.) We are just trying to get the air fuel ratio back so the engine runs right. This mod does not change when the power valve opens; only how much fuel is being delivered to the engine when the power valve opens.

Originally Posted by 69boss429

...I realize many Cobra engine combos out there have cams that provide poor at best vaccum and wonder what PV's are being used,how low of a setting?...

I have seen the PV selected in two different ways. The first way is to have the PV come in before the main jets such as at cruise to give a slight enrichment when climbing hill or passing a car on the interstate. The second way is to have the PV come in after the main jets are activated. This is typically done in a more aggressive situation. What we try and do is to get the air fuel ratio fairly lean during cruise and richen up to about 12.5:1 under full throttle. Better combustion chamber designs will require a leaner mixture for max power.

Originally Posted by 69boss429

...The reason for the questions is the 2 -715 Holleys on my Boss 429 run pretty well and the engine produces 10 in of vac at idle with 6.5 PV's in both carbs. However, when I accelerate at full throttle I look like I'm leaving a vapor trail out of the exhaust. No hesitation, just acceleration, but adding insult to injury as I bested a ZO6 and washed him with the vapor trail at the same time.He was peeved, I might say I was embarassed ,or not.

You are running way rich and giving up horsepower, fuel economy, and throttle response.

Originally Posted by 69boss429

I also realize that the sewer size ports combined with dumping all that fuel at WOT may not be a cureable situation but am exploring all options.
Anyone else with 2X4's experiance this?
Carbs are from Carl's,and overall I'm very happy with the performance.

I feel like a broken record but you need to get a wide band O2 sensor, and see what your engine is doing. We had a dual 4 427 (482 CID)come in and it was terrible. It would hardly run until 1/2 throttle at which point it would take off like a rocket. The car was undrivable. When it left it was almost as good as factory EFI (once it was warmed up). You will be much happier once the engine is tuned up.

[69boss429]

Originally Posted by Tom Kirkham

We do this modification to get the air fuel ratio where the engine needs it. Most of the time (but not always) Holley has the fuel curve set very safe (rich.) We are just trying to get the air fuel ratio back so the engine runs right. This mod does not change when the power valve opens; only how much fuel is being delivered to the engine when the power valve opens.




I have seen the PV selected in two different ways. The first way is to have the PV come in before the main jets such as at cruise to give a slight enrichment when climbing hill or passing a car on the interstate. The second way is to have the PV come in after the main jets are activated. This is typically done in a more aggressive situation. What we try and do is to get the air fuel ratio fairly lean during cruise and richen up to about 12.5:1 under full throttle. Better combustion chamber designs will require a leaner mixture for max power.



You are running way rich and giving up horsepower, fuel economy, and throttle response.


I feel like a broken record but you need to get a wide band O2 sensor, and see what your engine is doing. We had a dual 4 427 (482 CID)come in and it was terrible. It would hardly run until 1/2 throttle at which point it would take off like a rocket. The car was undrivable. When it left it was almost as good as factory EFI (once it was warmed up). You will be much happier once the engine is tuned up.

"Safe is good,"
"At cruise I'm in reality running a 2 barrel and the power band is very steady as I accelerate with no stumbling or hesitation. "
"Yes,I'm sure it's running rich when I floor it and everything opens up,but the secondaries are vaccum and I'm thinking maybe the power valves are dumping to much fuel all at once at initial WOT,after the initial blast out the pipes everything is clear as a bell.The plugs and tailpipes both have the same coffee with cream coloring and driveability is actually fantastic. "
"I'm sure the 482 FE is cammed differently.I can drive off idle with no issues,and if and when I've achieved 1/2 throttle I'd be exceeding the speed limit in any gear including first.And if I was any happier when driving it I'd likely get arrested
Thanks for your thoughts,I may try closing down those holes a bit"