Weber conversion manifold

[RootesRacer]

The hierarchy of carbs went from twin 36mm throttles (36WIP/A), to twin 32mm (Solex 32PAIA) then to twin 38mm variable venturi carbs (CD150s).
Its my opinion that the 36WIP were the best OEM carbs offered on the alpine, they seem to make more power on 1725s than the CD150s did.
ALL these carb setups have the exact same issue and this is that the adjacent cylinder has its induction stroke only 180 degrees out of phase with its neighbor and not symmetrically (2 to 1 and 3 to 4 are 180 degrees apart but 1 to 2 and 4 to 3 are 540 degrees apart). All alpine intakes are log plenums
and really rely on a large volume of "port uncommitted" air-fuel in order to balance out the mixtures. From this standpoint, the solex 2bbl is no worse than the twin zenith carbs were, what makes the solex 2bbl suck is that its a crappy carb with small throttles and venturis.

 

[Tom H]

Did the original Solex carburetor with PCV and brake booster ports on the intake manifold cause lean running on cylinders 1 and 4? If not, what is different in the design of the Weber that creates this issue? Sorry for my ignorance, but what is meant by "log" manifold? I am replacing my old Solex with a Weber now, but had planned on using my current intake so any help would be appreciated.

Just a note regarding the effect of Booster and PCV connections to any manifold. While the PCV connection may have a significant effect, the booster connection will have virtually zero effect. Under normal driving conditions there is zero flow of air into or out of the booster once the vacuum is established in the first couple seconds that the engine is running. Only after the brake is applied and released, typically with the throttle closed, of course, is there a momentary passage of air to re-establish full vacuum in the booster chamber.

Tom

 

[hierogk]

The AI manifold is the only manifold designed (To My Knowledge) to have each cylinder runner have its own unique entrance into the plenum under the carb. It's the most likely option to provide near equal F/A charges to each cylinder, for that reason. As for the OE Solex manifold and end cylinder A/F mixtures, I doubt anyone has tried to do any real world testing on the subject. On the other hand, there are results found during tear downs that could be interpreted as having A/F mixture being a probable or contributing cause.

By design, the OE manifold has the PCV air entering the plenum under the carb. That design allows the air that flows into the induction tract, from the block, to mix into the A/F mixture before distribution to all cylinders. This PCV air can be compensated for with the carb's tuning. If the PCV flows into just a single cylinder runner, then the PCV air displaces part of the A/F mixture and fails to replenish the missing fuel for that cylinder's charge.

The OE manifold is certainly usable. There are too many Rootes' engines using that, and similar design, manifolds to think it's not a viable option. Most of what has been discussed is "best design" as opposed to "usable design". Every "engineered" part or system probably has some type of compromise included due to design choices.

The sV's return to twin carbs can probably be viewed as Chrysler's desire to market the Alpine as a "performance" car in the US. IIRC, the later Rapiers continued to use the Solex carb with the 1725. The Rapier 1725 wasn't rated to the same output as the Alpine V 1725, though.

Hope this helps. I apologize if this seems too pedantic, but I tried to answer your questions without any idea about your knowledge of the subject.

Thank you very much husky drvr for that thorough explanation. I think I'll stick with my manifold and continue with the PCV as designed. (The early Zeniths look the part much better though.) This car is not likely to see high mileage again until the next owner gets his/her hands on it so I doubt the Rootes manifold will cause any significant problems in the foreseeable future. I am interested to see if there is any noticeable power and drivability difference. Thanks again. This forum is great.