Webers and 102s. And maybe 106s - Alfa Romeo Bulletin Board & Forums
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post #1 of 93 (permalink) Old 08-25-2019, 04:53 PM Thread Starter
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Webers and 102s. And maybe 106s

A recurring topic. Many conflicting theories, some not-necessarily intuitive evidence. Very little empirical data.

I suggest here that we strive to present things that have some degree of supporting data. Opinions, no matter how fervently held or frequently expressed, do not advance the cause. In fact, they tend to make following a thread fairly tedious.

Observations.

Alfa has used a lot of Webers on their engines, with fairly good success. Acceptable mileage for their day. No comment on emissions.

All such installations had fairly short, individual runners, one per cylinder, with fairly smooth bores and little to no bends between the carb and the smooth curve down to the intake valve.

To my knowledge, no Weber installations supplied heating water that bathed and heated the intake runners.

There are seen displaced inlets for the float bowl vents on some installations. These tubes, when included, are of different lengths, diameters, and open into different places from one installation to the next.

Webers...

Webers use a system of circuits to provide for effective fuel feed and mixture control. There is an idle circuit that also feeds fuel through early throttle positions. Then, there is the main circuit that overlaps the progression circuit, and continues up through full throttle. There is also an acceleration circuit that can add fuel at any time, depending upon jetting and operation.

The intent of these systems is NOT to simply provide fuel. It is to provide the correct fuel amount for the existing airflow. A general maxim is that best power is provided if the fuel to air is maintained at 14.7 to 1. However, in most performance oriented installations, a ratio of 12.5 to 1 is felt to give good power while avoiding flat spots, or damaging over-lean conditions.

REGARDLESS OF ANY OTHER ELEMENTS, IF AN ENGINE IS FED FUEL AND AIR IN A CONSISTENT RATIO OF 12.5 TO 1, IT CAN BE SAID TO BE IN GOOD TUNE, AND EXPECTED TO OPERATE SMOOTHLY IN BOTH CRUISE AND DURING ACCELAERATION.

Yes, something nearer to 14.7 is closer to optimum, but without the sophistication of fuel injection, problems can occur with sudden throttle openings, fuel formulation variations, and even component wear in higher mileage engines.

A loss of power (actually a negative discontinuity of torque) will be experienced if the mixture goes significantly higher or lower than our target. This can happen in a Weber due to poorly matched jetting.

The two most common torque losses are from idle to transition zone, and between the transition zone and the main circuit. These are due to poor mixture control, and can be either lean or rich.

In practical experience, I have found that most idle to trans losses are due to excessive leanness. The correction will be found in idle jet selection.

The bigger problem is during the trans-to-main transition. As two separate circuits overlap here, almost always the problem is due to being too rich. Too lean can occur, but is less common.

Lastly.... intake manifold and plenum design.

A history of the Rolls Royce Merlin engine pointed out that, when WW2 was on the horizon, and better performing aircraft engines were required, the first person Rolls Royce hired to improve their V12 aero engine was an aerodynamicist. His first area of exploration was the intake tract. It was assumed fuel could always be accurately regulated, but air tubes had a way of creating drag and standing waves that would play havoc with their needs. A great deal of time was spent getting rid of bends, managing constant-area port shapes, and avoiding anything that would disturb the desired laminar flow into turbulent flow.

At this point I’ll pause for you all to plot your challenges, but also hopefully ponder the importance of laminar and turbulent flow, and constant area ports.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...

Last edited by DPeterson3; 08-25-2019 at 08:53 PM.
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post #2 of 93 (permalink) Old 08-25-2019, 05:11 PM Thread Starter
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BTW.... my comments will generally apply to the earlier “non emission” carbs.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #3 of 93 (permalink) Old 08-25-2019, 09:03 PM Thread Starter
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Emulsion tubes are the most complex, and critical component. They exist primarily to control the changeover from the idle circuit to the main circuit. In a perfect world, the idle circuit flow would trail off at a matching, but opposite rate to the main circuit. This perfect system would maintain our targeted mixture throughout the RPM range, delivering the maximum, and consistent torque.

Note there is a different behavior between a gradual throttle advance and a slammed-open, full throttle. The latter largely eliminates the idle circuit, kills the air flow speed, and depends upon the accelerator pump system to make up the fuel flow loss from the idle circuit. This discussion will focus primarily on more gradual, natural throttle openings.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #4 of 93 (permalink) Old 08-25-2019, 09:10 PM Thread Starter
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While I cannot know what happens in a welded-up, Siamesed two-port manifold, I believe it is safe to say it will not exhibit the smooth-wall, mostly laminar flow of individual runners. Perhaps worse, we know that any protrusion along the tubes will trigger a standing wave reversion. Mapping that behavior would be worthy of a master’s thesis, and might not apply to every manifold in the same manner, as each would exhibit different locations, shapes, and depths of protrusions.

So, we have, a variable-variable.

Next.., what to do about a rich or lean transition?

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...

Last edited by DPeterson3; 08-25-2019 at 09:14 PM.
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post #5 of 93 (permalink) Old 08-26-2019, 06:20 AM Thread Starter
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With an appropriate manifold, it is generally possible to remove the E-stacks and tune the idle circuit. The goal being good synchronization, good idle, and smooth acceleration up to the point the progression holes are past, and the declining fuel flow causes a fall-off of power. It is relatively easy to swap idle jets until you get acceptable part throttle behavior.

After the reinstallation of the E-stacks, acceleration should be smooth and continuous to redline. If it is smooth down low, and smooth after a certain point, but not in between, we have a transition mixture problem.

Brief commentary... we do not know at this point what is causing the inappropriate mixture. If the engine operates OK low and OK high, but not in the transition, it is likely NOT due to the rough choice of main or air jets. It is likely to be the E-tube, float bowl level, or manifold reversion. More on this later.

The first thing to do is determine if you have an over-rich, or over-lean transition. It makes no sense to assume you have one or the other, and make corrections based upon those assumptions. Generally, an over-rich transition will sound boggy and wet, but will not backfire. A lean transition will often cough or backfire while seeming to lose all power for a brief moment. I strongly recommend having a baseline dyno session so your early efforts are at least in the correct direction.

The main circuit comes on line as a result of the airflow through the auxiliary Venturi creating enough vacuum to “lift” the fuel in the E-well until it reaches the transfer port from the well to the Aux Venturi. The E-tube acts in concert with the aux Venturi to regulate the fuel flow to match the air flow. The main jet sets the basic flow capacity, and the E-tube+air corrector modify the flow for mid and upper RPM.

IMPORTANT CONCEPT

The float bowl vent does two things. It allows in the air that is used to emulsify the fuel in both the main and idle circuits. Thus, it should be filtered, lest eventually dirt enter the float bowl. Secondly, it is intended to ensure that the air pressure entering the carb inlet is the same as what is acting on top of the fuel in the float bowl. So long as the pressures are the same, the vacuum generated by the aux Venturi will predictably and reliably “tip in” the main circuit when it is tuned to do so.

Brief interruption.... Must save and continue later.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...

Last edited by DPeterson3; 08-26-2019 at 07:03 AM.
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post #6 of 93 (permalink) Old 08-26-2019, 08:11 AM Thread Starter
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Editorial about Weber.... the company.

I worked for about 15 years for BAP/Geon, who was at that time the sole importer of Webers into the US. This did not make me an expert tuner, and I do not claim to be one. However, it exposed me to many insights into Webers, as well as the Weber company methods and strategies.

Bottom line, they were never much engaged in the after market. They would work diligently with their OE customers to perfect installations, but aftermarket customers were left to use what was available, and figure out how to make it work. This sort of explains the variety and irrationality of their emulsion tubes. Our assumption was that the numbering system indicated some sort of job order rather than indicate anything about the tube. If someone has better info, have at it.

In any case, the DCOE was introduced around 1960 +/-. There were only a few early customers, Alfa possibly being the single largest. As their market expanded, more E-tubes were designed IN CONCERT WITH CAR MANUFACTURERS USING DYNOS AND OTHER INSTRUMENTATION. Fuel economy was a commercial concern, and so tiny changes that would never be felt by the driver were nonetheless added. The cumulative value of a 1 MPG improvement when multiplied by a fleet of tens of thousands of cars was significant. I doubt many of us with one-off modified cars are prepared to expend the testing time in pursuit of a tiny economy improvement.

But.... back to E-tubes and transitions....

If your car has a rich transition, you’ll want to delay the main circuit tip-in and/or use an E-tube with a more gradual slope. As a very general statement, you won’t be far wrong using an F16 and varying the static float level to set the timing. As the purpose of this section is concepts rather than actual tuning, I’m not going to attempt a survey of E-tubes. Not my area of strength, anyway.

The point here is that novice tuners will assume there is one, fixed float level. There is not. This what controls the tip-in timing, in concert with the Aux Venturi.

Pressure regulation of the Float Bowl Vent.

People are looking at Alfa installations from the 50’s and 60’s, spotting FBVs that are displaced from beside the carb intake, mixing this into their problems with tuning Webers attached to a woefully inappropriate manifold, and leaping to the certainty that the displaced vents were Alfa’s magical solution to tuning their engines. All of the offered arguments are devoid of math that might describe the scale of the concepts.

Scale matters.

1. Optimally, the FBV is Intended to be close to the carb inlet. This is one of Weber’s better design elements, as it creates a stable reference against which the float level and aux Venturi can achieve a reliable tip-in.

2. There is not a significant air flow across the FBV. There is a small flow going into the vent. There are also small currents in the vicinity, but next to none flowing neatly across the FBV in a manner that would create a significant vacuum. There is generally turbulent air in the plenum, except in the case when trumpets are installed to smooth the air into the carb bore.

3. All of the Alfa plenums I have seen have a smallish feed pipe from the air cleaner opening up into a much larger plenum. This accomplishes several things. It slows the air flow, and in so doing, increases the pressure. Slower air will generate less drag, as drag increases as the square of the airspeed. Slowing the air also decreases turbulence (which itself creates drag). Slowing the air also reduces any tendency to generate an induced vacuum by flowing over an open port. Likewise, any vacuum created by air flowing over an open port will diminish as the square of the size of the hole.

Ever wonder why the FBV is such a large hole, when it flows only a small amount of emulsifying air? The amount of vacuum generated by recently slowed, and turbulent airflow, over the large FBV hole would be minuscule. Would it be enough to impact the column of fuel in the E-wells? I seriously doubt it, but I’ll work on some math to get a better sense of it.

So, why displace the FBV?

Pressure fluctuations from power pulses and reflected intake reversions can disrupt the otherwise predictable flow characteristics of fluids. Looking at Weber plenums over the years we see less and less displacement, and more of a simple shield that minimizes the power and reversion pulses from disturbing the FB stability.

In my two operational 102s, one currently has short trumpets inside the plenum, while the other opens directly into the plenum, immediately next to the FBVs. The trumpets may slightly shield the FBV from power and reversion pulses, but at this point the car without the trumpets exhibits the smoothest, and most continuous transition power. Neither show any sign of a transition “sag”.

What is the single most significant difference between two 102s giving great power and driveability, and those struggling with transition issues?

The item that Rolls Royce felt was the top priority to increase power.

I propose that chasing FBV induced vacuum solutions is a false assumption. First of all, the problem is during the relatively low RPM transition zone, when pressure creating ram tubes would generate very little effect. Worse, their pressure creation would increase into the upper revs where simple jetting should be sufficient. Meanwhile, do you know whether your transition power loss is due to weakness or richness? Adding pressure to the FB MAY slightly depress the fuel in the well, which might slightly delay the tip in, but if your problem is a too-lean transition, you’ll aggravate the problem.

Must run for now.

Sorry, not as simple as trying to duplicate a 60s Alfa solution for a manifold not intended for the job.
horsewidower likes this.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...

Last edited by DPeterson3; 08-26-2019 at 08:18 AM.
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post #7 of 93 (permalink) Old 08-26-2019, 09:12 AM
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MATH...you're using math...

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post #8 of 93 (permalink) Old 08-26-2019, 11:38 AM Thread Starter
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BTW, back in the 70s when I helped a few guys try to use the Solex manifold on 102s, none of which were de-siamesed, the solution to getting them to run was to go shockingly rich. We could get past the 1,500-2,000 RPM “GASP!” only by pouring gas through them. Not an acceptable solution. My thought at the time was that one barrel’s reflection through the shared port simply stopped the incoming fuel charge, causing the lean slump.

Of course, I won’t claim an unmodified Siamese manifold tells us much about a de-siamesed unit, just like I don’t think a modified Solex manifold will behave much like a bespoke Weber design.

Lastly (in this post anyway)....

The ram tubes I see in the early Weber manifolds are all opening in a necked-down area upstream of the much larger plenum body. This necked down area will have a higher velocity, and therefore lower pressure than inside the larger plenum. Even assuming the displaced FBV tubes generate a positive pressure, can we say with any certainty that this increase is above, below, or the same as the higher static pressure of the plenum? Plus, the vent tubes are fairly small, as well as long. Well this dampen or otherwise modify the pressure felt at the float bowl?

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #9 of 93 (permalink) Old 08-27-2019, 05:40 AM
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Quote:
Originally Posted by DPeterson3 View Post
They (Weber) would work diligently with their OE customers to perfect installations, but aftermarket customers were left to use what was available, and figure out how to make it work. This sort of explains the variety and irrationality of their emulsion tubes. Our assumption was that the numbering system indicated some sort of job order rather than indicate anything about the tube. If someone has better info, have at it.
Thanks for the above help sorting out Webers. In "Weber Carburettors (sic)(book) 2 Tuning & Maintenance" John Passini states, "If the rules for the first choice of an auxiliary venturi are considered undesirably vague, then the rules for the first choice of an emulsion tube can be said to be non-existent. On pages 12 and 13 of the little green book, he offers 'Appropriate Technical Specification of Emulsion Tubes' -tables provided by Weber with the comment "one could feel reasonably happy (with the suggested tubes), though it does not mean that the engine would share this feeling".

Mark
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post #10 of 93 (permalink) Old 08-27-2019, 06:18 AM Thread Starter
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Mark

Thanks!

When I came off the counter at BAP and began my meteoric rise to fame and glory, the only books/catalogs I took with me were the Weber manuals. About two weeks ago I went down to the hangar where they’d been stored for about a decade, and found everything was a glued together mess. Dang. I had the Passini stuff, but didn’t recall your quotes. I agree.

The point of what I’ve written so far is not to guide tuning, or to proclaim one, true god. Based upon what I’ve read about 102s and Webers over the years, it seems people are trying to reinvent science and misapply the laws of physics. Kind of like “intelligent design” nonsense.

The latter might get donations when the hat is passed, but magical thinking won’t solve Weber tuning problems.

Gotta take boy to school. Constructive comments later.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...

Last edited by DPeterson3; 08-27-2019 at 06:50 AM.
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post #11 of 93 (permalink) Old 08-27-2019, 07:37 AM Thread Starter
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Undigested data points...

I’ve been studying ram-air effects.

The general observations on cars that have employed ram air to enhance power output is that they observe about a 0.02 psi increase at 60 MPH. My 1964 Mooney has an operable ram air system that can be opened when up in clean air, as it circumvents the air filter. At about 150 Kts (about 170 MPH) it adds roughly 1” of pressure, some of which is due to deleting the restriction of the air filter. That’s 1 inch of mercury. I don’t have a inch-mercury to psi conversion at hand at this moment. I’ll get back to that.

The air in the plenum of a 102 is flowing at what speed? I dunno. Research required.

One thing I am confident of is that the air in the plenum is not flowing in a smooth, coherent manner across the FBV. As other posters have pointed out, I should be expected to understand how a pitot tube works, and I do. I am specifically trained in the physics of the Bernoulli principal as part of earning a flight instructor certificate. We also must be technically conversant with laminar and turbulent flow concepts. Having said that...

There is a reason that pitot tubes are mounted waaaay out on the wingtip of an aircraft. Sometimes way out in front on a long pole. It’s because any obstructions in the vicinity can set up unpredictable waves and interferences that would disrupt the stable measurement of the ram air pressure.

It would be enlightening to have a sort of Doppler 3D picture of the air flow in a 4-cylinder Weber plenum. To my knowledge, we don’t have those. So, we have to use our imaginations, but we should also use what physics teaches us about flow of gasses.

Gasses don’t like to make right angle turns. When asked to do that, they become turbulent. Turbulent air creates more drag and less lift than laminar flow. This is why I believe everyone’s drawings of air flowing across a tube are simplistic to the point that this idea simply does not apply.

So, why does Alfa sometimes have displaced FBV tubes?
Note that the later versions of the GTA plenums do not use this concept.
Why does the Solex plenum have simple entries into the plenum?

We can get lost in conjecture, but I suggest that won’t help very much. But, just for fun, a brief thought experiment.

Let’s assume that the Alfa engineers had good dynometers, and the ability to instrument any and all points where they might want a data point. A reasonable assumption, I think.

We know, from Passini, that E-tubes are more black magic than predictable science.

I can imagine Alfa doing a fair amount of tuning to the point an E-tube was working reasonably well. However, they’d like to get just a touch more power or economy.

A long tubular plenum will have a natural resonance, as well as overtones. It might be possible to identify the point near the inlet where these resonances would either add or subtract from the pressure due to wave reinforcement or cancellation. Just like with Weber inlet horns, this effect is understood, but subtle. Perhaps Alfa found a way to very slightly tweak the FB well height between small, partial, and full throttle operations, to improve either or both fuel consumption or power?

My point being, simply assuming displaced FB vents were there to depress the well height at increasing RPMs, and so a hobbyist could solve their flat-spot problems by a random placement of similar-looking tubes, is folly. Alfa did pretty thorough engine development.

Can anyone describe a late-model Euro Weber plenum? I used to have one for an 86 FNM, but traded it away. It might hint at what Alfa was doing toward the end of Weber E-tube development.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #12 of 93 (permalink) Old 08-27-2019, 01:23 PM
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Quote:
Originally Posted by DPeterson3 View Post
The air in the plenum of a 102 is flowing at what speed? I dunno. Research required.
Let's try to calculate the air speed at the entry of the plenum : The section is perfectly circular. It is at this place that Alfa put their FVB tubes on the "2000 Sportiva".
We call this speed "v"

Hypothesis :
Engine : 2 liters
Engine speed : 3000 rpm (why not ?)
Plenum diameter : 80 mm (Plenum radius = 40 mm)

Calculation :
Plenum section = S = PIx40x40 = 5 000 mm2 = 0,005 m2
3000 rpm = 50 revolutions per second (rps)
Engine is a 4 strokes, that mean it sucks its 2 liters in 2 revolutions.
Inflow is 2 liters x 50 rps x 1/2 = I = 50 l/s = 0,05 m3/s

I = Sxv
v=I/S=0,05/0,005=10 m/s

v=10 m/s

I hope I didn't made any mistake ...

Here, I have considered that the air flow was the same in all the section, that is not the case, but this quick calculation gives a good idea of the speed scale at the entry the plenum.
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post #13 of 93 (permalink) Old 08-27-2019, 02:23 PM Thread Starter
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Serge,

Thank you for adding to the conversation.

I’ll have to check your math later, but at this moment I’m trying to grasp the mental picture of something the size of a can of oil departing the tailpipe at 10M/sec. That doesn’t seem right.

Math check later.

However, to extend this, we’ll need to measure the expansion into the larger plenum, which will allow us to apply Boyle’s law to determine the pressure increase and speed decrease.

Also...

The full consideration needs to consider not just the average volumetric efficiency, but the VE at various throttle positions. This is because the VE at a slow speed is likely higher than at a high speed, leading to a changing vacuum ratio between the float bowl, and the remote vents.

It may well be that this delta is what Alfa used to modify the E-well height to obtain a mixture curve that the E-tubes of the 50’s and 60’s were not sophisticated enough to achieve. We certainly haven’t seen such radically displaced FBVs in nearly 50 years, but we have seen continued development in E-tubes, even to KF’s abandonment of emulsification in the last few years.

Don P
Carson City, NV

Past Alfas...
59 102 Touring (first Alfa $500 running)
65 Sprint GT (2nd Alfa, $500 daily driver)
102 Sprint (never did anything with it, but wish I had)
74 Berlina (first new car - now certainly rusted into oblivion)
61 Giulietta Spider G-Prod Race Car (where is it now?)
84 Spider Veloce (rarely drove it, so sold it)
86 Quadrifoglio (Dull car - no more 115s for me)
1971 Montreal "The Full Monty". Fair winds and following seas

Current Alfas
59 102 Touring Roadster - restoration complete, enough Alfa for any rational man. Or irrational, for that matter
Oops. Add to the "present" list, 10204 01488, 2000 Touring Roadster project

And past...
Two 1946 Stampe SV4C (c/n 294 "Rocinante" - wife's favorite airplane. RIP), and c/n 235 "La Bon Temps Femme" (gone to a new home, but never forgotten)
Zlin 50LA (+9 -6 gees, titanium spar, 1200 lbs, 260HP rumored to now be in Brazil)
1946 Luscombe 8A
Starduster Too (recently spotted at the Nevada City, CA airport - over 20 years and an entire continent separating it from our stewardship in Binghamton, NY)
1955 Cessna 170B (wife taught me to fly tailwheel in this)

And present...
64 Mooney M20E ("Rambo". My faithful steed for over 40 years) Over 55 years old, and just returned from a trip to Argentina in him
Newest in the fleet
1967 Piper Super Cub on Wipline amphibious floats (a true "all terrain vehicle")
2010 Triumph Thunderbird


You can snap roll an Alfa only one time...
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post #14 of 93 (permalink) Old 08-27-2019, 05:00 PM
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Quote:
Originally Posted by PS70 View Post
Engine is a 4 strokes, that mean it sucks its 2 liters in 2 revolutions.
I think this should be 2 liters in 4 revolutions, not 2 (i.e. it takes 4 revolutions until any of the strokes repeat, and each of the 4 cylinders is filled once over 4 strokes).

-Ruedi
[SIZE="1"]'63 2600 Touring Spider (AR 191437, the car that started the 2000/2600 International Register, reassembly in progress)
ex-'65 2600 SZ (AR 856043, the car in my avatar, sold as resto project to Austria)
Maintainer of a private 2600 SZ register (not the one in the Netherlands).[/SIZE]
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post #15 of 93 (permalink) Old 08-27-2019, 11:47 PM
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Quote:
Originally Posted by tubut View Post
I think this should be 2 liters in 4 revolutions, not 2 (i.e. it takes 4 revolutions until any of the strokes repeat, and each of the 4 cylinders is filled once over 4 strokes).
Not sure. Let's see in detail :
Each cylinder is 0,5 liter

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With 2 turns of crankshaft, the engine sucks 2 liters.

To simplify (because it is not easy with 4 cylinders, we mix brushes quickly) imagine that the engine is a 2 liters but with a single cylinder.

Don, when I practiced paragliding, I often used an anemometer to measure the wind speed on the take-off place.

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If I can find it (not very confident) I'll made some measurement at the plenum entry.
BTW, 10 m/s at 3000 rpm sound good to me. 10m/s = 36 km/h = 22 m/h
That represent around 3 m/s at the idle speed (11 km/h = 7 m/h)
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