New Production Webers - Any Experience?

[M3pilot]

I'm starting a 2.8L short stroke build and haven't settled on induction. The new-production, Spanish-made Webers have been available for awhile now but there seems to be a rather surprising lack of feedback on them. I don't think I'm completely inept at Googling. Does anyone here have any experience?

FWIW, I was thinking about the 46s for the build (SC case, 66mm billet crank, 10-10.5:1, twin plug).

Thank you!
Stuart

[1QuickS]

I was sent a pair of 46s to review and re-jet for a 2.8 liter. I looked them over pretty well and I test ran them, they behaved well.

Comments regarding findings:

• Throttle lever arms are the short version as used on early race engines.
• Jetting as delivered was same jetting used on Carrera 6 engines, all of which was replaced for customer's 2.8 liter engine.
• Progression holes were drilled per original, race oriented 46mm Webers. Later 46s had a more street friendly progression pattern.
• Float bowls are vented via 4mm holes in the top covers, again as per original race oriented carbs. Production Webers used vent pipes in top covers.
• There were no emulsion well air screens supplied with the carbs but these would only be required for engines running without air cleaners.
• Castings and machined parts looked to be of good quality.

I have heard antidotal reports of tuning issues from reputable, commercial tuners. These may well be exceptions that are popularized and associated with early production carbs.

[uai]

I can't speak for IDA type carbs, but for DCOE the german importer tests each one and if necessary reworks or does return them.
On the other hand the progrssion holes are usually better on the modern 45 dcoe 152g.

[M3pilot]

Thank you both for your feedback.

It's interesting that they seem to have chosen "features" that might be a less-than-optimal for a street engine. Paul, you stated:

"Progression holes were drilled per original, race oriented 46mm Webers. Later 46s had a more street friendly progression pattern."

Care to elaborate what "street friendly" means in this case? My engine will be a street engine though limited to fair conditions, "sporty" driving, etc.

Thanks again, all!

[1QuickS]

A discussion of progression circuit design is akin in complexity to discussion of emulsion tube design and function. With that disclaimer I proceed prefaced by three photographs of progression drillings, first that of the early, race developed 46mm Weber, then by that of the later 46mm Weber as developed and sold in the later 1970s and finally the drillings used on the 40IDAs.







The progression circuit is controlled by three components:

• idle fuel jet (replaceable)
• idle air correction jet (fixed orifice but may be modified to be tunable)
• quantity, diameter and spatial arrangement of progression drillings in the throttle housing (not easily changed)

As the throttle valve is opened it sweeps past the progression holes in the throttle body. Low pressure in the intake tract below the throttle valve draws fuel from the idle fuel jet which is emulsified by air from the idle air correction jet AND by air introduced via the progression drillings ABOVE the throttle valve which is a high pressure region. The emulsification of the fuel leans the fuel mixture and promotes atomization of the fuel.

As the throttle valve continues to open, more progression holes are below the edge of the throttle valve and fewer are above it thereby changing the amount of air that is emulsified in the mixture, fuel is still emulsified from air introduced via the idle air correction jet.

So, the race 46mm Weber has a very small idle air correction jet (0.80mm) and two small progression holes followed by one large one. The four hole 46mm has holes of a more progression-like succession and the 40mm Weber also has a progression of hole diameters and uses a 1.10mm idle air correction jet. Therefore the race 46mm Webers have a large amount of air for emulsification at small throttle openings and a quick, rich surge at throttle openings which would typically be near transition which would be a cruising RPM on most cars.

I do not have better info such as a comparative driving test using 46mm race Webers vs. later 46mm Webers with three holes or 46mm PMOs (46mm PMOs use a four hole pattern VERY similar to the late, 4-hole 46mm Webers) so my comments are offered as cautionary as opposed to definitive.

Remember that the early, race-oriented 46mm Webers were developed for a 2.0 liter engine with 42mm main venturis (larger than what I would recommend for a 2.8 to be used on the street, probably 38mm venturis depending upon cams, etc.) and 906 cams and used in 1500 pound cars intended for class wins at LeMans with little need to work below 4000 RPM. Those venturis in a 2.0 liter engine would have a difficult time activating the main circuit until RPMs were fairly high but that was OK since not much power at lower RPMs was needed for "drivability". A larger displacement engine will draw in the main circuit more quickly than a 906 engine so there is less need to extend the low speed circuit so far up the RPM range. This earlier "drawing-in" of the main circuit in a larger engine with smaller venturis will conflict with the transition of the race oriented Webers that have a long and rich transition duration which is why I believe the race-developed, 46mm Webers may be less friendly for street applications than more contemporary options. I think the reproduction 46s were selected to accommodate the vintage racers in Europe.

I did mention that the idle air correction jets are fixed but easily modified to be tunable, this would allow for some modification of the strength of emulsification and duration of the range of the low speed circuit.

So, I would not black-ball the new 46s but I do caution that they may require some extra effort to tuning the low speed circuit, this in addition to replacing, in entirety the jetting components.

If you seek manifolds from PMO to install these "new" Webers in lieu of 46mm PMOs then be cautioned that Richard would be reluctant to accommodate you for obvious reasons.

[M3pilot]

Wow, Paul . . . what a fantastic reply! Conceptually, it seems to make perfect sense and motivates me to go back to your site and carefully study the wealth of information you have provided.

Thank you very much!

Stuart