Post by Site Admin on Jul 12, 2004 19:58:51 GMT -4
POWER VALVES and ENGINE VACUUM
There is a lot of misunderstanding concerning power valves in Holley carbs. Many 4-barrels come with a particular power valve depending on the carb list# and application. Some carbs have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" and will remain closed above that amount. One of the misconceptions is that they can't be trusted to work because an engine backfire or "belch" can "blow out" the power valve. Many Holley performance carbs models and list#'s now come with built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install kit from Holley (PN - 125-500) that will also protect the power valves in case of an engine blowback thru the carb.
Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. In addition, when the power valve circuit is plugged, the part throttle fuel economy is worsened and may become overly rich. Plug fouling may become a problem at part throttle.
Stock engines can have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work correctly. Long duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lowered with these performance parts and the power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve and to do that you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "Drive" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.
Holley makes performance style "standard" flow or a "high" flow power valve which has a large opening. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". There are "two stage" power valves available that are more for "economy" minded users rather than "performance" enthusiasts.
There is a lot of misunderstanding concerning power valves in Holley carbs. Many 4-barrels come with a particular power valve depending on the carb list# and application. Some carbs have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" and will remain closed above that amount. One of the misconceptions is that they can't be trusted to work because an engine backfire or "belch" can "blow out" the power valve. Many Holley performance carbs models and list#'s now come with built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install kit from Holley (PN - 125-500) that will also protect the power valves in case of an engine blowback thru the carb.
Many tuners will automatically remove the power valves and use a "plug" thinking this is the "hot" ticket. However, if the power valve is removed and plugged, the main jet size must be increased 6-10 jet sizes to make up the required fuel amount lost by the removal of the power valve. In addition, when the power valve circuit is plugged, the part throttle fuel economy is worsened and may become overly rich. Plug fouling may become a problem at part throttle.
Stock engines can have high vacuum readings (10-18 inches at idle) and the Holley power valves with higher readings like 6.5 to 10.5 will work correctly. Long duration non-stock camshafts and other performance related parts can cause a problem, because engine manifold vacuum may be lowered with these performance parts and the power valve, if incorrect, will always be open, even at part throttle, leading to an overly rich air/fuel mixture. The solution is to choose the correct power valve and to do that you need a vacuum gauge. On a manual transmission vehicle, hook up the vacuum gauge and take the reading with the engine at idle. Then use a power valve that is rated 1-2 inches below that amount. For example, a motor that shows 7" of vacuum at idle should use a 6.5 or 5.5 rated power valve. If you have an automatic transmission, take the vacuum reading at idle in "Drive" (with the emergency brake on and the wheels blocked) and chose the power valve 1-2 inches below that figure. You can get a little more detailed information by driving the car with a vacuum gauge hooked up with a longer hose so you can read it while driving. Drive the car at medium loads and while cruising and note the various vacuum readings. Then chose the appropriate power valve rating.
Holley makes performance style "standard" flow or a "high" flow power valve which has a large opening. "Single stage" power valves are available in 1" increment sizes from 2.5" thru 10.5". There are "two stage" power valves available that are more for "economy" minded users rather than "performance" enthusiasts.