Tuning secrets of the Pro's
Unlocking the secrets of reading spark plugs...
|
This is what the spark plug should look like if the fuel mixture and timing are dead on. Pulls the plugs and examine the ground straps (circle) after every run. The discoloring on the ground strap helps determine whether or not timing needs to be advanced or retarded. If the fuel and timing are where they need to be, the ground strap will display a rainbow-effect discoloration at the base of the strap. If the entire ground strap is discolored there’s too much timing; if there isn’t any at all, then timing is too far retarded.
|
After two or three passes, a cut-off wheel is taken to the plugs to reveal the porcelain core. Here the Yates team can see how rich or lean the fuel mixture is. The higher the ring (arrow) the fatter the mixture. But after about three passes, the plugs get harder to read and tend to look on the rich side regardless.
|
|
|
If the engine is excessively rich the plugs will look like the one on the left...
|
Since tolerances are extremely tight inside a Pro Stock motor, the plugs must be indexed with the spark gap down between the 10 and 2 o’clock positions. First the plug is marked on the backside of the ground strap.
|
|
Basic Carburetor and Fuel System Maintenance and Tuning Tips
Air Delivery
For maximum horsepower, the coolest, most dense air possible should be available at the carburetor inlet. Keeping restriction in the inlet path to a minimum - or better yet, pressurizing the air - is also desirable. The denser the air, the more you can get into the cylinders. This allows the engine to burn more fuel and make more power. We recommend that a hood scoop or outside air intake should be installed wherever rules allow. Under hood air is heated by the engine and headers and reduces the amount of power that can be produced.
There should be a minimum of three inches of clearance between the top of the venturis and a hood scoop. If an air cleaner is installed, the tallest possible element is preferred with four inches being the minimum suggested height.
When a hood scoop or external air intake is used, it is essential that the carburetor be sealed to it. Otherwise, air will flow across the top of the carb and out of the inlet tract rather than into the air horn. If air is forced past the carburetor it will siphon fuel, causing the engine to run lean. Windshield snorkels are especially notorious for siphoning unless the rear is sealed. Air pan kits for sealing the carburetor to the scoop are available or they can be fabricated from sheet metal and foam. In all cases, an air bell or radiused intake should be used whenever possible to increase air flow into the carburetor.
A Drag race car will not pick up ET after the scoop is sealed off if the scoop is too short or the fuel delivery system is inadequate. However, it’s not unusual for a car to improve ETs by 3/10 of a second and increase top speed by as much as seven miles an hour after installation of a sealed scoop. Depending upon track length, oval track cars will typically improve lap times by 1/10 to 1/2 seconds once an optimized air intake system is installed.
Fuel Delivery
Many racers experience fuel delivery problems without ever being aware that something is amiss in their fuel systems. State-of-the-art engines produce a lot more power than a race engine of ten years ago. The process of producing horsepower revolves around the conversion of fuel into energy. The more pounds of fuel an engine can burn efficiently per hour, the more horsepower it produces. Even though your car may not miss, pop, bang, skip or do anything else peculiar, it may not be getting all the fuel it needs to make maximum power.
In oval track applications, a BG Belt Drive Fuel Pump is preferred where use of a mechanical fuel pump is specified. This pump offers the highest fuel delivery volume of any mechanical pump yet maintains low fuel pressure at low engine speeds. This alleviates "loading up" of the spark plugs. The BG Six-valve and Super Speedway mechanical fuel pumps will also deliver ample fuel volume when used according to recommendations.
For drag race cars, a BG400-2 Electric Fuel Pump is the best way to guard against fuel starvation. If a drag car is "lazy" at mid-track or "lays down" then pulls well in higher gear, the engine may be experiencing intermittent fuel starvation. Typically, the carburetor bowls are full at the starting line so the car leaves hard but subsequently drains the bowls dry. In lower gears, the car accelerates rapidly with the engine picking up rpm very quickly. When the float bowl fuel level drops, the car "lays down" because of fuel starvation. In high gear, engine speed increases more slowly allowing the bowls to fill again.
To assure adequate fuel delivery, the fuel system should be capable of filling a one-gallon metal gas container at the carburetor fuel inlet in 25 seconds or less. Do not use a Super Jug or antifreeze jug - they are not accurate. Large displacement, high horsepower engines should have a fuel system that can pump a gallon in less than 20 seconds Remember that to get fuel into the engine, the jets have to be covered. Whenever a race car slows for no apparent reason, fuel delivery is the first thing to check. This holds true for both single and dual four-barrel installations. In fact, single four-barrel applications frequently have more severe fuel starvation problems than dual four-barrel installations. This is because a single four-barrel has only two fuel bowls and two inlet needle and seat assemblies. With dual four-barrels, there are four needle and seat assemblies and four fuel bowls. This doubles the fuel handling capability.
For the same reason, only filters specifically designed for racing, such as the BG5000 or BG Inline filter, should be installed. Use of a filter is strongly advised as long as it doesn’t restrict fuel flow. The fuel filter should be inline before the fuel pump. This filters the fuel, preventing any damaging material from entering the fuel pump or the rest of the system.
Fuel pressure should be set between 6 and 8 psi for a gasoline carburetor where an alcohol carburetor requires 4 to 5 psi at idle and 9 to 12 psi at wide open throttle. Fuel pressure is not a substitute for volume and if the fuel bowls aren’t full, pressure is meaningless. In fact, fuel pressure is simply an indication of the amount of restriction in the fuel system.
Most electrical fuel pump systems require the use of a fuel pressure regulator. One BG regulator is sufficient in many applications. The use of 2 regulators is recommended when using other types of regulators or in high horsepower engines to avoid excessive fuel restriction and provide adequate volume.
With mechanical fuel pumps, and some electrical pumps, a bypass is preferred rather than a regulator. A diaphragm bypass without an idle bleed is recommended when constant fuel pressure is needed from an electrical or mechanical pump. A belt driven fuel pump, using gasoline or alcohol, requires a diaphragm bypass with an idle bleed. Higher pressure mechanical fuel pumps delivering alcohol, such as our 15 psi BG Six-valve, require a throttle bypass to supply the variable fuel pressure required by the carburetor.
Idle Mixture Screws
The engine should be at optimum operating temperatures. Lightly seat the idle mixture screws, then back them out 1-1/2 turns to establish a starting point. With the engine running, slowly turn the mixture screws in or out as needed to establish the best idle quality. Do this twice. The first time is a coarse adjustment; the second one is a final fine adjustment. If idle quality can’t be adjusted properly, or if they can’t be backed out far enough to obtain proper adjustment, it may be necessary to have the idle circuits reworked (the cam may not be pulling enough idle vacuum). A common reason for lack of adjustability in the idle mixture screws is having the primary butterflies adjusted too far open.
The secondary throttles should be just slightly cracked open at idle. Even on carburetors with nonadjustable secondary idle mixture, secondary throttle position at idle may be adjusted. It may be necessary to open or close the throttle adjustment due to the cam design. It’s a trial-and-error proposition to find the setting that gives you the best idle. When setting idle speed, the butterfly opening should be adjusted equally both primary and secondary, but not so much as to expose the transfer slot more than .040". Secondary adjustment is set with a screw accessible from the underside of the carburetor.
Jetting
Whether you have a stock HOLLEY® carburetor or a flow-modified BG model, jetting should be fairly close if the carburetor is used in the application for which it was intended. For maximum performance, increase or decrease jet size two numbers (primary and secondary) as required. So long as performance improves, continue increasing or decreasing jet size. At some point, ET or lap times will start to fall off, which means the mixture has been moved past the optimum air/fuel ratio. At that point, move one jet size at a time in the opposite direction until optimum performance is achieved.
Jet for performance, not spark plug color. Most high-energy ignitions will leave very little residue on the plugs. With a drag car, plugs can remain bone white so attempting to "read" spark plugs is a waste of time. With an oval track car, the plugs will color, but the process takes longer with a high-energy ignition. If the car runs a little too hot, jetting up one or two sizes will alleviate the lean condition without hurting performance. (The ignition will burn off the extra fuel and cool the engine.) However, if jet size is increased but the engine appears to be running leaner, a fuel system problem is indicated. If a conventional ignition is used, jetting for best performance is still the way to go, but the plugs will take on a tan color after a short time.
Generally an engine will pop, miss or surge if it’s lean, although an excessively rich condition can cause the same problems. As a rule, cool, dense air requires larger jets, hot thin air requires smaller jets. Also, whenever a carburetor spacer is added or deleted, or a camshaft, cylinder head or intake manifold change is made, it will be necessary to retune the carburetor for maximum performance.
Jet Extensions
Another problem that will cause a stumbling is uncovering the secondary jets, and no amount of accelerator pump tuning will cure it. This occurs when a car leaves so hard that the fuel in the secondary bowl is being pushed up against the rear wall, uncovering the jets. BG Fuel Systems offers stainless steel jet extensions that correct this situation. Whenever jet extensions are installed in a carburetor equipped with a secondary power valve, the power valve must be removed and the hole plugged. Jet size must then be increase accordingly.
Accelerator Pump Circuitry
Even with jetting that is within the ball park, performance will be poor if the accelerator pump nozzle (squirter) diameter is incorrect. If a car is sluggish during initial acceleration and a puff of black smoke blows out of the headers when a drag car leaves the starting line, or when an oval track car comes off a corner, the accelerator pump nozzle diameter may be too large. Fuel spilling out of the vent tubes is another possible cause. The latter problem is easily cured by running a rubber hose form one vent tube to another, so long as a slot is cut at the top of the hose.
As with jetting, determining the best squirter diameter is accomplished by trial-and-error testing. Simply adjust size up or down until the best performance is achieved. And don’t forget about pump lever adjustment. For oval track use, the lever should be adjusted so that there is no play in the pump linkage when he throttle is closed. This will assure that there will be no lean stumble when the carburetor comes off idle. Tuning an accelerator pump for maximum performance off the corner frequently involves reducing, rather than increasing pump volume and discharge rate.
Drag racing calls for a slightly different approach. For the hardest starting line launch with a foot brake, the pump lever override spring should be adjusted so that fuel starts to discharge through the nozzle at an engine speed lower than launch RPM. If a car leaves the starting line at 5000 RPM, the pump shot should begin at 4700-4800; an 1800 RPM launch calls for the accelerator pump shot starting at 1500. The key is to have no slop in the accelerator pump system at starting line RPM, so that the pump shop isn’t "used up" below that RPM. Although adjusting the accelerator pump as described will create a lot of slop in the pump linkage at idle, (and may produce a stumble when driving in the pits) a car will leave harder. Cars equipped with a stick shift or trans brake, where starting line launch is accomplished with the carburetor wide open, requires adjusting like an oval track application.
Float Adjustment
When assembling a carburetor or reinstalling the fuel bowls, adjust the float approximately .450" from the top of the bowl (in line with the bowl screw bosses when the bowl is inverted). This dry setting is to get you in the ball park. Recheck the float level with the engine running and the sight plugs out. Fuel should just wet the outside of the bowl as the fuel seeps out. On the secondary side, where the sight plug is lower, fuel level should be slightly higher.
Power Valves
The power valve’s function is to supply extra fuel for wide-open throttle, high load conditions. When manifold vacuum falls below the vacuum level stamped on the power valve, the power valve opens and enriches the main circuit by about six to ten jet sizes. This occurs under high engine loads such as full throttle acceleration.
The power valve number should be at least 1.5 to 2 in/hg vacuum under engine idle vacuum. If an engine produces 8-9 in/hg vacuum, a 6.5 power valve would be a good first selection. Using a power valve with a rating lower than this will delay the enrichment and can cause hesitation. However, on an oval track car with restricted carburetor rules, using a lower rated power valve can sometimes help performance coming off corners.
Any drag race carburetor with a secondary power valve must be turned sideways to avoid fuel starvation. The power valve is higher than the jets and is the first to be uncovered as fuel is pushed to the rear of the float bowl. Since there is no way to put an extension on the power valve, the carburetor must be turned sideways to eliminate fuel starvation.
Throttle Linkage
Maximum performance cannot be achieved unless the carburetor is wide open when the throttle is pushed to the floor. Consequently, throttle opening should be checked regularly, immediately following any major changes in the engine compartment. Maximum air flow cannot be achieved if the carburetor does not reach wide open, or if it’s pulled past wide open. It may be necessary to install a stop to prevent the carburetor form being opened too far and to prevent damage to the linkage. Linkage should operated smoothly with no binding and should be free of obstructions which may cause the carburetor to stick open. An auxiliary return spring MUST always be installed to insure that the throttle positively closes.
Tuning for Changing Weather Conditions
Changes in air density due to changes in temperature, barometric pressure, and humidity will have a direct impact on engine performance. As the air density changes, adjustments to the fuel mixture are often necessary. These factors can change from afternoon to evening. Everyone knows the engine will make more power at night when the air cools down. The cooler, denser air can carry more oxygen per cubic foot and burn more fuel. The weather may not change enough from afternoon to evening to require mixture adjustments, but adjustments will certainly be necessary as the seasons are changing. Traveling to tracks in different climates or at different altitudes may also cause a need for fuel mixture adjustments. These factors can not be overlooked when tuning for best performance.
Spaces and Plenum Dividers
Spacers and plenum dividers provide an easy way to change the configuration of the intake tract and the relationship it has with the carburetor. Adding a plenum divider to an open plenum manifold will help keep the left to right fuel distribution balanced for oval track applications when the car is in the corners. This is especially helpful on alcohol engines. These dividers usually do not have a measured affect on torque or horsepower. Using spacers between the carburetor and intake manifold can produce dramatic results. The use of a four hole spacer can improve low end to mid-range by helping the carburetor draw and atomize fuel. An open center spacer increases the plenum area and can benefit the mid-range and upper rpm power. It is not uncommon to see combinations of spacer types or stacking of similar type spacers being used. It is also not uncommon to see results that are totally opposite from the general tendencies. The actual results from any spacer or combination of spacers can only be measured during a test and tune session on the specific engine combination being run. This information can be a very useful tool when tuning to find the best horsepower or to change the power characteristics to suit a specific track condition.
Carburetor and Fuel System Maintenance
For consistent performance, a carburetor must be kept clean. Spray the air bleeds with carburetor spray or WD-40 every week. Air bleeds become clogged from dirt and dyes in the fuel. Clogged or dirty air bleeds can cause a stumble or a high-speed miss in an otherwise perfect carburetor. If you use WD-40, you can also spin the engine over with the ignition off and the throttle open. A liberal spray of WD-40 down the venturi will leave a fine mist of protection on the valve seats and cylinders to prevent rust. Finally, your carburetor should be rebuilt at least once a year and more frequently if it’s operated in dirty conditions.
Applications running alcohol require additional maintenance beyond that of a gasoline fuel system. Alcohol, being extremely corrosive, should not be left in the fuel system or carburetor for an extended period of time. Proper care includes draining and flushing the entire fuel system, usually with gasoline. The most common method is to drain the system and add gasoline to the fuel cell allowing the pump to draw fuel through the lines and to the carburetor. Alcohol carburetors are much richer than gasoline, so when the engine begins to idle and die, the system is pretty well flushed. Always use a fuel additive that lubricates the fuel system for protection while racing.