Boost referencing
09-06-2002 | 04:15 PM
#3
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Joined: Oct 2000
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From: Cleveland, OK
A friend of mine has a 427 with a 8-71 Hampton and two Holley carbs that are supposed to be for blower motors in a '69 Camaro. He is blowing the power valves on initial start-up. What could his problem be?
09-06-2002 | 09:40 PM
#6
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Blower carbs?
A look at how, and what carbs need for supercharged engines. 1st, we go back to the basic function of Volumetric Efficiency, which is what you need to accurately calculate to ensure proper carburetion for whatever you are building.This topic has came up in a couple other threads, and now it's time to touch on it a bit more so it makes a little more sense... Boost referencing is plumbing a rubber vacuum hose from the carburetor power valve vacuum source, to the intake manifold. The reason is to allow the power valve to reference or "see" engine intake manifold vacuum, below the supercharger. This allows the power valve to operate correctly, providing additional fuel to the main circuit during conditions of low manifold vacuum.
Volumetric Efficiency
Before you can determine the correct carburetor size for your engine, you must know its volumetric efficiency. Volumetric efficiency is an indicator of how well an engine can breathe. The better an engine's "breathing ability" the higher its volumetric efficiency. It is expressed as the ratio of the actual mass (weight) of air taken into the engine compared to the mass which the engine displacement would theoretically take in if there were no losses. The ratio is expressed as a percentage. It is quite low at idle and low speeds and varies with engine speed. Volumetric efficiency should be computed at the expected operating RPM or your engine application.
Use the following examples as a guide to estimate the volumetric efficiency of your engine.
(A) An ordinary low-performance engine has a volumetric efficiency of about 80% at maximum torque.
(B) A high performance engine has a volumetric efficiency of about 85% at maximum torque.
(C) An all-out racing engine has a volumetric efficiency of about 95% at maximum torque.
A highly tuned intake and exhaust system with efficient cylinder head porting and a camshaft ground to take full advantage of the engine's other equipment can provide such complete cylinder filling that a volumetric efficiency of 100%, or slightly higher, is obtained at the speed for which a system is tuned.
Supercharged engines generally require carburetors with 40% to 50% more C.F.M. than normally aspirated engines.
How to determine needed CFM
Engine Size (CID) X Maximum RPM / 3456 =CFM @100% Volumetric Efficiency
EXAMPLE: 350 CID X 6000 RPM = 2,100,000 / 3456 = 608 CFM
Here are some basic, and simple modifications for Holley carbs on Supercharged engines. These are examples, and you must procede using information for your application.... no one mod is good, or right for everyone.... Running without power valves can be done by increasing the jet size 5 - 7 sizes and will work, but with less fuel control. Many other things are involved like restrictors and passage sizes, and should be done on a flow bench.
Idle Feed Restrictions
In many supercharged engines an off idle lean stumble is due to insufficient transition fuel. First check that that the throttle blades are not to far open at idle. If more than 0.040"-0.050" of the transition slot uncovered, try opening the secondary throttle plates a bit to let more air in. This will allow you to close the primaries a bit to gain transition slot area. If that does not work, some extreme cases require drilling a 1/16" hole in the main throttle plates, but this is usually only required on long duration race cams. Once you determine that the throttle plates are in the correct idle position and that accelerator pump tuning has little effect on the stumble, look at modifying the idle feed restrictions. You can increase the size of the idle feed restrictions, increase them only 0.001" at a time. Stock Holley's will have idle feeds in the 0.028"-0.032" range and when dealing with holes that small, 0.001" is a big jump.
Power Valves
When running high boost 15+ psi, the blower can make enough vacuum under the carb to pull the power valve shut and lean out the engine. Some people plug the power valve and then jet up to compensate, but this is a mod that I do not recommend on street engines. Plugging the power valve will force you to jet up and kill part throttle cruising mileage. The better solution is to modify the base plate of the carb to give the power valve a signal form a vacuum line that is connected to manifold, not carb vacuum. Then when under boost, the is no way the power valve can close. The modification is simple. Simply plug the power valve vacuum supply in the throttle plate with lead shot and drill an adjacent hole from the font of the plate connecting it to the power valve vacuum passage. You can the press in a steel vacuum fitting and run it to manifold vacuum. This way you get a functional power valve without the risk of high speed lean out. As far as power valve selection, the basic rule applies, at least 1.5-2 inches below cruising vacuum. Just make sure you take the vacuum reading at the point of the power valve vacuum signal.
Primary Main Jetting
Supercharged carbs need a little more fuel that a normally aspirated engine. A good starting point is about 4 jets sizes richer. You can then jet down checking the plugs and dial it in, but always start richer to be safe. Running too lean can quickly damage any high performance engine and the situation is aggravated further with boost.
Secondary Main Jetting
The secondaries should go about 8 sizes richer to give a safe starting point. If you have a secondary metering plate instead of a metering block, drilling the main restrictions 0.010" larger is usually a safe bet, but it's best to go up in 0.003" increments so avoid going too rich. It's not as easy making the holes smaller as it is to drill them larger.
Power Valve Channel Restrictions
If you find that you need more fuel, it's best not to jet up any more than 4 or 5 sizes on the primaries and 8 on the secondaries. Instead, increase the size of the power valve channel restrictions in 0.005" increments going a maximum of 0.015". Going up too much on main jets will do nothing more than cause a rich condition in no boost conditions. Increasing the power valve fuel will only richen up the heavy load and high throttle setting, leaving your cruising untouched. If your car cruises fine and runs lean under heavy load, the power valve channel restrictions need to be larger.
Simple heh?.....
Volumetric Efficiency
Before you can determine the correct carburetor size for your engine, you must know its volumetric efficiency. Volumetric efficiency is an indicator of how well an engine can breathe. The better an engine's "breathing ability" the higher its volumetric efficiency. It is expressed as the ratio of the actual mass (weight) of air taken into the engine compared to the mass which the engine displacement would theoretically take in if there were no losses. The ratio is expressed as a percentage. It is quite low at idle and low speeds and varies with engine speed. Volumetric efficiency should be computed at the expected operating RPM or your engine application.
Use the following examples as a guide to estimate the volumetric efficiency of your engine.
(A) An ordinary low-performance engine has a volumetric efficiency of about 80% at maximum torque.
(B) A high performance engine has a volumetric efficiency of about 85% at maximum torque.
(C) An all-out racing engine has a volumetric efficiency of about 95% at maximum torque.
A highly tuned intake and exhaust system with efficient cylinder head porting and a camshaft ground to take full advantage of the engine's other equipment can provide such complete cylinder filling that a volumetric efficiency of 100%, or slightly higher, is obtained at the speed for which a system is tuned.
Supercharged engines generally require carburetors with 40% to 50% more C.F.M. than normally aspirated engines.
How to determine needed CFM
Engine Size (CID) X Maximum RPM / 3456 =CFM @100% Volumetric Efficiency
EXAMPLE: 350 CID X 6000 RPM = 2,100,000 / 3456 = 608 CFM
Here are some basic, and simple modifications for Holley carbs on Supercharged engines. These are examples, and you must procede using information for your application.... no one mod is good, or right for everyone.... Running without power valves can be done by increasing the jet size 5 - 7 sizes and will work, but with less fuel control. Many other things are involved like restrictors and passage sizes, and should be done on a flow bench.
Idle Feed Restrictions
In many supercharged engines an off idle lean stumble is due to insufficient transition fuel. First check that that the throttle blades are not to far open at idle. If more than 0.040"-0.050" of the transition slot uncovered, try opening the secondary throttle plates a bit to let more air in. This will allow you to close the primaries a bit to gain transition slot area. If that does not work, some extreme cases require drilling a 1/16" hole in the main throttle plates, but this is usually only required on long duration race cams. Once you determine that the throttle plates are in the correct idle position and that accelerator pump tuning has little effect on the stumble, look at modifying the idle feed restrictions. You can increase the size of the idle feed restrictions, increase them only 0.001" at a time. Stock Holley's will have idle feeds in the 0.028"-0.032" range and when dealing with holes that small, 0.001" is a big jump.
Power Valves
When running high boost 15+ psi, the blower can make enough vacuum under the carb to pull the power valve shut and lean out the engine. Some people plug the power valve and then jet up to compensate, but this is a mod that I do not recommend on street engines. Plugging the power valve will force you to jet up and kill part throttle cruising mileage. The better solution is to modify the base plate of the carb to give the power valve a signal form a vacuum line that is connected to manifold, not carb vacuum. Then when under boost, the is no way the power valve can close. The modification is simple. Simply plug the power valve vacuum supply in the throttle plate with lead shot and drill an adjacent hole from the font of the plate connecting it to the power valve vacuum passage. You can the press in a steel vacuum fitting and run it to manifold vacuum. This way you get a functional power valve without the risk of high speed lean out. As far as power valve selection, the basic rule applies, at least 1.5-2 inches below cruising vacuum. Just make sure you take the vacuum reading at the point of the power valve vacuum signal.
Primary Main Jetting
Supercharged carbs need a little more fuel that a normally aspirated engine. A good starting point is about 4 jets sizes richer. You can then jet down checking the plugs and dial it in, but always start richer to be safe. Running too lean can quickly damage any high performance engine and the situation is aggravated further with boost.
Secondary Main Jetting
The secondaries should go about 8 sizes richer to give a safe starting point. If you have a secondary metering plate instead of a metering block, drilling the main restrictions 0.010" larger is usually a safe bet, but it's best to go up in 0.003" increments so avoid going too rich. It's not as easy making the holes smaller as it is to drill them larger.
Power Valve Channel Restrictions
If you find that you need more fuel, it's best not to jet up any more than 4 or 5 sizes on the primaries and 8 on the secondaries. Instead, increase the size of the power valve channel restrictions in 0.005" increments going a maximum of 0.015". Going up too much on main jets will do nothing more than cause a rich condition in no boost conditions. Increasing the power valve fuel will only richen up the heavy load and high throttle setting, leaving your cruising untouched. If your car cruises fine and runs lean under heavy load, the power valve channel restrictions need to be larger.
Simple heh?.....
09-07-2002 | 12:38 AM
#7
Registered
Joined: Jun 2002
Posts: 529
Likes: 1
Great info FindMe. Simple Huh? Actually if you have worked on or understand carbs this is a good an explanation as I have seen. One thing I would add is to smake sure you make SMALL changes and ONE at a time. You are in the realm of carb engineering here. The boys that do this stuff all the time pretty well know what works or can get you real close with good information but like always it costs $$$. 
If you like doing this type of stuff and have patience it can be done. Rich is safe and lean is mean. $$$ I personally like to boost referance my blower carbs. Just my .02
If you like doing this type of stuff and have patience it can be done. Rich is safe and lean is mean. $$$ I personally like to boost referance my blower carbs. Just my .02
