Performance Tuning: Part 1

Overview

This section is provided to help the tuner optimize the performance potential of their 911 when equipped with Weber, Solex, Zenith or PMO triple throat carburetors. Although the body of the following text will specifically address components as used in the Weber carburetors, all of the processes are applicable to these four types routinely used on Porsche 911 engines.

Weber carburetors are instruments used to realize your engine's performance potential; where "performance potential" is your decision: to maximize fuel economy, to maximize engine torque for ease of driving in traffic or to maximize peak horsepower for high speed sporting purposes. While it is possible to achieve improvement in two of these criteria, it is not possible to simultaneously satisfy all three. Engine performance is maximized through carburetor tuning by first determining your goals and then by conducting performance testing. By evaluating the results of your testing in light of your established goals and by utilizing the guidelines provided in this section you may then make educated changes in the jetting and other configurable components to achieve the best compromise for your driving style.

Jetting a carburetor is the task of selecting and adjusting the various components to supply vaporized air/fuel mixture to match an engine's performance demands. Performance testing is required to determine the best selection and configuration of these components. The effort to understand the interactions of the different fuel delivery systems in the carburetor, conduct performance tests, evaluate the results and repeat the test with changed components is time consuming but the task is well worth the effort! If the jetting is rich then surplus fuel will wash the oil from your cylinder walls, dilute your engine oil and create carbon deposits on your valves. On the other hand, a lean fuel mixture will cause the engine to run excessively hot causing detonation and eventually result in burned valves or holed pistons. Either rich or lean fuel mixtures will affect the engine's potential for performance and will negatively affect reliability, performance and service life. However, carburetors do not have the flexibility in tuning like EFI systems provide; compromises may be necessary to insure you maintain adequate mixture strength at peak torque (as an example) and as a result, accept a non-optimum and non-damaging mixture during other operational ranges.

The primary feature of Weber carburetors is their adaptability. Different engines have different demands for air flow to fully realize their performance potential: small engines need less air flow (CFM or cubic feet per minute of intake air flow) while larger engines need more CFM and high performance engines require more CFM than a lower performance engine of the same displacement. Webers are designed to use a common throttle body and by selection of different main venturi (choke) they may be used to provide for these different engine demands. However, optimization of the throttle body size is required to maximize the performance potential and efficiency of operation of any particular engine. (See Throttle body and main venturi sizing for more information regarding this topic.) The ability to tailor the carburetor to best fit the demands of the engine assures optimum performance whether it is for street or performance applications. However, this tune-ability comes with the obligation of the user to perform the adjustments correctly or suffer the results. You 911 engine, when equipped with two triple-throat Weber carburetors essentially has six single-throat carburetors but unless they all work together they will work against each other. Along with multiple tuning features comes the opportunity to get them all wrong.

Carburetors are easily the most commonly blamed item for engine performance faults, this is most likely due to the ease in which they may be accessed/adjusted and how prominent they are once the engine lid is opened. Any random adjustment may easily lead to a series of missteps that will ultimately necessitate a careful tuning effort to recover acceptable performance. Careful performance tuning will tailor the fuel delivery system to match the requirements of your engine and will minimize the need to mask poor functioning with overly rich settings resulting in greater performance under all operating conditions that ultimately improves engine longevity and driving pleasure.

AFR meters are almost equivalent to a chassis/engine dynometer for today's enthusiasts and tend to encourage devotion to the quest for "perfect" mixtures for all possible driving situations. Be wary of the quest for perfection; the AFR readings are best used as a tool to provide guidance for jetting changes but the best path forward is to provide what the engine needs.

In our time of easy access to instant, digital data and acquisition systems (AFR readings and laptop computers) and with our modern fuel/ignition management systems that may be programed to provide perfect spark timingand mixture for any throttle position/engine RPM/load situation/atmospheric condition and phase of the moon, we may be lead to believe our carburetor should be able to replicate that level of perfection with enough manipulation of jets/venturis and data reductions. Unfortunately it is a mission with little hope of achieving; better to achieve what is reasonable and let the driver use his own microprocessor (located between the ears) to adjust throttle position based upon their feedback system (feedback sensor located in the driver's seat).

Complicating the jetting process is the reality that air and fuel are different in nature (gas vs. liquid) and in viscosity; this means the mixing and metering is not a linear process. Carburetors are simple devices that incorporate different strategies to accommodate the non-linearity of the mixing/metering demands. Specifically you have a low speed mixing/metering system, a high-speed system and an accelerator system. The low speed and high speed systems overlap in their effective service range while the accelerator pump system helps smooth the transitions during rapid throttle opening. The low speed system is called the idle and progression circuit and the high-speed system is called the main circuit.

The idle and progression circuit typically affects engine operation from idle through 3500 RPM with diminishing effect through 4500 RPM and the main circuit affects operation from as early as 2500 RPM through top engine operational speed or redline. Since these engine RPM regions overlap it follows that a jetting change in one area of operation will affect operation in another RPM range during the overlap region of operation. It is therefore important to know the basic operation of the carburetor; please see the web page regarding this topic for more information and to reinforce your understanding: Carburetor Operation.

Prior to beginning your testing/optimization program it is important to first establish your goals and then make fundamental selections for the main venturi and main circuit jetting based upon the general guidelines (provided below) or from reliable sources with jetting for an engine of similar specifications and application. The available fuel for your engine, the altitude where you will be using your car and the type of driving you expect to use your engine for are also important considerations when defining your goals of performance. The following testing procedures will help direct you to the best selection of components for your application and conditions but many times the final jetting selections are based upon a trial and error process where careful notes are taken for each jetting configuration. Methodical testing will pay great dividends in the optimization process. Also, making singular changes in jetting provides clearer resolution of what change affects what performance result. When making initial jetting changes it is good practice to make a significant change so that the response to the change is clear to discern. Additionally a larger jet change will provide a scalar to judge a clearer direction to work toward with the next test iteration.

One assumption to be made clear before delving into the complexities of carburetor performance tuning is that the engine and all of its components are in good operational condition and are matched as a package that is balanced in design and application. For example: a 3.0 with race camshafts, large headers and a street muffler will suffer un-resolvable tuning issues due to the restrictive exhaust.

Tuning your Webers for performance is not difficult but the processes are exacting and interrelated; follow these procedures methodically and you will achieve a success beyond that of improved performance; you will know they are right, why they are right and you will be rewarded in your personal accomplishment.

Operation of components

The following discussions provide information regarding how individual components of the three main carburetor circuits work. The operation of these three basic circuits are discussed on the web page Carburetor Operation and review of that information is recommended prior to continuing with the following.

Idle jet

Fuel is supplied to the idle jet from a fuel gallery that taps into the bottom of the emulsion tube well. Fuel travels up this gallery to the top of the throttle body where it enters the tip of the jet and then exits the body of the idle jet through two holes. It is due to the vacuum in the intake tract, below the throttle plates that generates the force to draw fuel from the emulsion tube well and into the idle jet. Fuel that escapes from the body of the idle jet is mixed with air from the idle air correction jet located directly above the holes in the idle jet body. The mixing of air with the fuel emulsifies it which helps the fuel atomize into fine mist when it passes through the idle mixture adjusting screw hole and the progression circuit holes and is subsequently subjected to the low pressure in the intake tract.

The idle jet is installed into a jet holder (a tight, slip-fit) that screws into the side of the throttle body near the top. The tip has a beveled end that mates with a beveled seat creating an intimate seal. Later versions of Weber throttle bodies incorporated an o-ring seal to help assure no un-metered air would interfere with the emulsification of the fuel in the idle jet/air correction jet area.

The idle jet affects mixture strength in a uniform manner throughout the effectiveness of the idle/progression circuit operational range.

Idle air correction jet

The idle air correction jet provides air for the fuel delivered to the idle/progression circuit which mixes with the fuel passing through the idle jet. The mixing of air with fuel is referred to as emulsification and the result is fuel that is readily vaporized upon its entry into the low pressure region of the intake tract, below the throttle plates.

The idle air correction jet has a fixed size orifice and is installed by pressing the jet into a recessed hole in the top of the main throttle body, directly above the idle jet. This jet is not an adjustable item but a modification of the jet is possible which allows for changing the size of the metered hole thereby providing additional tuning options for adjusting the idle/progression circuit operation.

The idle air correction jet affects the mixture and timing of the fuel delivery for the idle/progression circuit in these ways:

Larger orifice diameters shorten the duration and leans the fuel mixture delivered through the idle jet
Smaller orifice diameters prolong the duration and enrichens the fuel mixture delivered through the idle jet
This jet has effects that are more pronounced in the upper regions of progression and transition circuit operation

One more function of the idle air correction jet: It breaks the siphon action that would otherwise drain the float bowl and empty its contents into the cylinder bore through the idle/progression holes. This siphon would operate if the air correction jet was blocked from debris or by a gasket without an appropriate hole to allow air to enter the jet.

Idle and Progression circuit

The idleand progression circuit provides fuel to the engine during operation when the throttle valve is nearly closed. This region of engine operation occurs during mild acceleration from low engine speeds and during cruising operation at moderate highway speeds. The primary region of fuel contribution is from idle speed up through 3500 RPM with continued fuel being delivered for engine speeds as high as 4500 RPM. During these conditions the engine power demand is low and the partially opened throttle valves restrict airflow into the engine which generates a vacuum in the intake tract. This vacuum acts upon the holes in the wall of the throttle body that are slightly above the level of the throttle shaft. These holes of the progression circuit and the hole for the idle mixture screw provide all the fuel delivered to the engine during low power demands. All fuel for the idle and progression circuit is supplied from the idle jet.

The idle mixture screw is located below the holes of the progression circuit and the rate of fuel flowing through this hole is adjusted by the tapered needle on the tip of the mixture screw. The fuel flow through the idle mixture hole is a function of the vacuum in the throttle bore which decreases as the throttle valve is opened.

The progression circuit is more complicated in its operation than that of the idle mixture screw. Classic carburetor design has the edge of the throttle valve blocking the lowest of the holes of the progression circuit when the engine is at idle. The remaining holes of the progression circuit are located above the lowest hole and are therefore exposed to air pressure in the intake tract above the throttle valve. Since the air pressure in the intake tract above the throttle valve is nearly that of atmospheric air pressure these holes act like the idle air correction jet and add air to the fuel being delivered through the idle mixture screw hole. This additional air has the same emulsifying effect as the air from the idle air correction jet and leans the mixture.

As the throttle valve is opened, the lower progression holes are exposed to low air pressure in the intake tract and fewer progression holes above the throttle plate are exposed to atmospheric air pressure. The result is a richer fuel mixture being passed through the progression holes in accordance with increasing air flow and engine power demand. Concurrently with the changing fuel delivery of the progression circuit is the continuous fuel delivery from the idle mixture screw. Of course, as the throttle valve is progressively opened the vacuum in the intake tract below the throttle valves decreases which degrades the ability to deliver fuel as a function of vacuum. This is where the main circuit begins to take effect and the region of operation where the idle and progression circuit bridges with the main circuit is referred to as transition.

Due to the subtle characteristics of the interaction of progression circuit hole sizes and spatial alignments with the throttle valve it is very risky to try to modify them and expect to achieve a positive outcome in fuel delivery profile. The only methods available to the tuner are making adjustments in idle jets and idle air correction jets and then by making adjustments in the main circuit to help remedy transition performance issues.

The progression circuit operation is similar to that of the emulsion tube as utilized for the main circuit except it operates in an opposite fashion. Where the emulsion tube provides an ever leaner fuel delivery profile with increasing engine speed the progression circuit provides an ever richer fuel profile with increasing engine speed until the vacuum in the intake tract diminishes.

Main jet

All fuel delivered to the engine via the idle and progression circuit and via the main circuit first passes through the main jet. Since the fuel demand of the idle and progression circuit ends with increasing demand from the main circuit there is no conflict with the ability to supply fuel to both circuits simultaneously.

The main jet is screwed into the tip of the hollow bolt referred to as the main jet carrier. The main jet carrier is screwed into a bung at the very bottom of the fuel bowl which places the main jet near the bottom of the fuel reservoir. The bung has a hole that allows fuel to enter the main jet through holes drilled in the body of the jet carrier. The main jet is located so that fuel exiting the jet is fed into the bottom of the emulsion tube well. There is no beveled seat creating a seal between the main jet and the emulsion tube well, the only seal that keeps unmetered fuel from flowing into the emulsion tube well is that of the fit of the threads on the main jet carrier and those threads internal to the bung in the throttle body.

The main jet affects mixture strength in a uniform manner throughout the effectiveness of the main circuit operational range.

Main air correction jet

The main air correction jet provides air for emulsification of the fuel delivered by the main circuit. The resulting emulsified fuel is readily vaporized upon its entry into the low pressure region at the waist of the main venturi.

The main air correction jet is selectable based upon the diameter of the orifice thereby becoming an effective tuning aid for the main circuit. It is located in the top of the main throttle body and one is used for each emulsion tube which when screwed in place keeps the emulsion tube seated in the emulsion tube well.

The main air correction jet affects the mixture and timing of the fuel delivery for the main circuit in these ways:

Larger orifice diameters shorten the duration and leans the fuel mixture delivered through the main jet
Smaller orifice diameters prolong the duration and enriches the fuel mixture delivered through the main jet
This jet has effects that are more pronounced in the upper regions of main circuit operation

Emulsion tube

The emulsion tube is easily the most subtle tuning component of the Weber carburetor and thereby the most difficult to understand and to select; their purpose is to adjust the fuel delivery curve during partial throttleand gentle acceleration operation during main circuitoperation. The emulsion tube concept is rather simple and due to the many possible variations of tube design provides tuning opportunities that showcase Weber carburetors excellence. It is installed into a vertical well and is held in place by the main air correction jet which is screwed into the top of the main throttle body. The well is filled with fuel that passes through the main jet and when the engine is running at slow speeds the fuel level in the emulsion well is equal to the level of fuel in the main float bowl.

Emulsion tubes are typically hollow brass tubes with holes of various diameters and spacial locations along their length while the diameters of the tubes are varied both internally and more significantly, externally. Air from the hole in the main air correction jet enters the hollow center of the tube and is passed through the small holes along the length. The air passing through the holes is jetted into the fuel that is in the annulus between the outside diameter of the emulsion tube and the inside diameter of the well. As the velocity of this jetted air can become sonic, the mixing action is quite active and creates an emulsion of fuel with air which is drawn into the main throttle bore where low air pressure and high air speed vaporize the emulsified mixture.

Fuel flow through the main jet is the direct result of the pressure difference between atmospheric pressure within the fuel bowl and the low air pressure generated within the throttle bore. As airflow increases at the waist of the main venturi there is a corresponding decrease in air pressure. The bottom of the auxiliary venturi (secondary venturi) corresponds with the waist of the main venturi and the co-location actually "boosts" the pressure drop within the auxiliary venturi making it more effective as a signal to start flow of fuel through the main circuit. It is the low pressure at the waist of the auxiliary venturi that creates the differential that causes fuel to flow from the main float bowl, through the main jet and emulsion tube well and finally out the discharge nozzle within the auxiliary venturi.

The emulsion tube is referred to by Weber as a "brake" which is meant to describe its purpose as a device that slows the flow of fuel from the main jet to the discharge tube in the auxiliary venturi. The reason for the need to slow or "apply the brakes" to fuel delivery is that as the pressure drop in the main venturi increases with increasing air flow to the engine with increasing RPM (air pressure decreases compared to atmospheric pressure) and resulting fuel flow would be too great without a tuning device to moderate the flow to match the engine's demand.

As fuel demand increases so does the pressure differential between the fuel in the emulsion tube well and that of the fuel in the float bowl which causes the fuel in the well to rise relative to that in the main float bowl. Obviously smaller main jet sizes slow the rate of fuel transfer so fuel supply would be restricted (leaner) than with larger main jets.

Also, as fuel demand increases and the fuel rises in the emulsion tube well the main air correction jet delivers more air into the interior of the emulsion tube. This is due to the air correction jet being supplied with atmospheric air pressure while the fuel is being "lifted" in the well due to lower air pressure generated in the auxiliary venturi. The air in the interior of the emulsion tube displaces the fuel and begins to pass from the interior and mix with or emulsify the fuel in the annulus. As the fuel continues to rise around the emulsion tube more air is introduced into the tube and the lower the air reaches inside the tube. As the air level reaches the lower levels, air is bled into the fuel surrounding the emulsion tube via the holes lower in the tube body. In this fashion it follows that holes lower in the emulsion tube have a leaning effect for higher fuel demands (upper RPM operation). Obviously, larger main air correction jets tend to equalize the differential air pressures between the main fuel well and the auxiliary venturi at a faster rate than smaller jets and thereby delay and lean the fuel mixture.

It is important to reiterate that without the introduction of air into the fuel delivered through the main circuit the mixture would run progressively richer with increasing RPM. Therefore, the emulsion tube and the main air correction jet are essential to help moderate this fuel flow to be in accordance with engine demand.

Once the fuel flow has reached the point where no fuel is held within the emulsion tube the mixture to the engine is solely based upon the main jet size and the air correction jet size, the emulsion tube no longer functions as a tailoring device for fuel delivery for partial throttle operation, it acts only to emulsify the fuel with all orifices injecting air into the fuel flow.

Another characteristic of emulsion tubes is their stepped diameters and the resulting effects they have on fuel delivery. The emulsion tube well is machined to a close-toleranced inside diameter. Emulsion tubes with varying outside diameters fit within the well and the space for fuel in the annulus between well inside diameter and emulsion tube outside diameter is a function of the clearance between them. As the pressure differential increases between that in the fuel well and that at the discharge port in the auxiliary venturi the rate of fuel flow through the annulus increases. Where there is a large annulus (small emulsion tube diameter) the fuel is easily made up to supply the demand without dropping and exposing more air holes but when the annulus is small there is a decreased amount of fuel available to fill the demand and the fuel level in the annulus will drop quickly which is a leaning effect due to the rapid exposure of more holes to emulsify the fuel.

An emulsion tube with no steps along its length or with a smaller outside diameter would have a larger supply of fuel in the annulus which would enable fuel delivery during rapid accelerations (racing demands) without causing a delay of fuel replenishment. Since the fuel has already passed through the main jet and is residing in the well it is available for instant demand acceleration without needing to be passed through the main jet for resupply.

General comments regarding emulsion tube operation:

Emulsion tubes provide control of fuel delivery for the main circuit in two fashions:
- They have a STRONG contribution upon when the main circuit is activated (Remember that fuel delivery at Transition is the sum of fuel from the Progression Circuit plus that during initiation of fuel flow from the Main Circuit)
- They greatly affect the engine's partial throttle and gentle acceleration mixture strength throughout the main circuit's range of operation up to nearly redline.
The main circuit does not begin to become effective until fuel has been drawn high enough in the emulsion tube well to pass through the transfer port and into the auxiliary venturi
During mid-RPM operation some of the upper emulsion tube holes (normally submerged in fuel at idle speed) will be exposed and discharging air into the fuel in the annulus thereby emulsifying it
During mid-RPM operation a mild acceleration demand will draw fuel from the annulus above the stepped outside diameter of theemulsion tube. The higher up the stepped diameter extends, the less fuel will be available for mild accelerations. The mild acceleration allows a rush of air through the main venturi in the attempt to satisfy the low pressure (intake vacuum) below the throttle plate.
During mid-RPM operation, a wide-open-throttle demand has the following realities:
- The airflow through the main venturi changes as for the mild acceleration situation (above) but this mode of operation passes quickly; fuel delivery through the main circuit is a function of airflow through the main venturi which responds directly to engine speed.
- The accelerator circuit will need to provide the supplemental fuel during the sudden throttle opening; the delivery volume and duration must bridge the fuel demand until the main jet and main air correction jet have taken over fuel supply requirements.

General comments regarding emulsion tube tuning:

If there is a rich fuel delivery issue at a given RPM (steady state, mid-RPM operation) then a selection of a new emulsion tube or the modification of the current tube may rectify the issue by a trial and error process. Try adding a hole or two along the length of the emulsion tube and re-run the engine noting what RPM the added holes generated a lean response. It is then easy to solder the holes closed and relocate them either higher or lower along the length of the tube to move their effect into the correct RPM region. Holes higher up will create an earlier leaning effect while holes lower in the tube will affect mixture at higher RPM.
Emulsion tubes with small outside diameters and with holes in the lower portion of the tube will have a maximized reserve of fuel and serves an engine requiring strong acceleration (large throttle openings) from low engine speeds up through high RPM. If there is too much reservoir for a given engine then the mixture will be rich. A small outside diameter of the tube provides fuel for acceleration without requiring replenishment fuel from the fuel bowl, a delayed response since the fuel must pass through the main jet to maintain fuel level within the well.
Emulsion tubes with small outside diameters typically are paired with large main jets and are selected for use on competition engines.
Emulsion tubes with large outside diameters are less sensitive to fuel level changes within the float bowl.
Emulsion tubes with large outside diameters and air holes up high will be leaner at high RPMdue to the restrictive fuel flow in the annulus and the preference to flowing air is satisfied by the air holes up high in the tube.
Emulsion tubes with large outside diameters will be lean-out the fuel mixture more quickly than those with small diameters, they require larger main jets to help make up for its lean characteristics compared to that of a smaller diameter tube. The reason they are "leaner" than smaller diameter tubes is that during increasing fuel flow, the fuel level in the annulus will drop more quickly than for a smaller tube and will therefore expose more holes for air flow through the tube into the delivered fuel.
At wide-open-throttle the fuel inside the emulsion tube itself is depleted and the main jet is directly supplying all fuel for the demands of the engine, this fuel is emulsified by all the holes in the emulsion tube injecting air as it flows through the annulus.
Holes up high on the tube cause a leaner mixture at all engine speeds and the more holes there are the further up the RPM range the weakening will reach
A change of emulsion tubes will usually be accompanied with main and air correction jet changes
The float level in the main fuel bowl is extremely critical for proper emulsion tube operation and timing of the main circuit. Since the fuel level in the emulsion well can vary dramatically compared to that in the fuel bowl proper fuel pressure is required to properly supply fuel demand in the well. Especially critical is if the upper holes in the emulsion tube are covered by fuel. This will result in a very rich mixture until fuel level in the well drops during acceleration.
Holes located above the idling fuel level in the float bowl allow air passing through the main air correction jet to suppress the vacuum signal generated within the auxiliary venturi. This acts to delay initiation of fuel flow through the main circuit and to lean-out the low RPM range of the main circuit operation. Typically a strong signal occurs when small main venturis are selected and an early main circuit activation would result in a rich mixture.
Holes located at the idling fuel level in the float bowl help activate the main circuit quickly due to air passing through the main air correction jet blows across the surface of the fuel in the annulus thus carrying fuel with the airstream.
Holes located below the idling fuel level in the float bowl have the effect of lowering the fuel level in the annulus around the emulsion tube, the level becoming equal to that of the lowest holes that are blowing into the fuel and creating an emulsion. Since the liquid fuel is lower that the level of fuel in the float bowl there is the natural tendency for fuel to flow from the higher level in the float bowl to equalize with the lower level in the annulus. (This is equivalent to raising the fuel level in the float bowl.) These emulsion tubes will activate transition onto the main circuit the quickest.
An emulsion tube with more holes or larger holes will flow more air (once the main circuit becomes active) than a tube with smaller or fewer holes.
Holes along the length of the emulsion tube affect mixture during mid-RPM operation, holes at the top of the emulsion tube affect low RPM mixtures, middle holes affect mid-RPM mixtures and lower holes affect upper RPM operation.
Holes toward the bottom of the tube have a more pronounced leaning effect at upper RPM operation than those holes higher up.

Notes:

Remember that the float level, as it maintains the fuel level in the bowl, also sets the idle fuel level in the emulsion tube well. Because the emulsion tube well is so much smaller by comparison to the bowl, its fuel level will drop dramatically quicker than the main bowl. Proper fuel pressure is required to make sure the bowl can try to maintain a constant fuel level, to properly supply fuel to the emulsion tube well. Fuel bowl level is critical; especially for the upper holes in the E-tube; covering the upper small holes will result in an extremely rich mixture until the fuel level in the well is reduced during continued acceleration.
Once the correct emulsion tube has been selected for partial throttle operation then the entire fuel curve may be enriched or leaned by changing the size of the main jet.
The main air correction jet has the most influence on mixture strength during high RPM, wide-open throttle operation.