early 911 air horn question?

[MountainRoads]

This is a follow-up to my earlier post about MFI cold-start enrichment. Per suggestion of several posters, I checked to see if the air horns on my '71 E were plastic or metal before reactivating the cold-start system on my reinstalled MFI. Unfortunately, they were indeed plastic, so I swapped in some spare metal ones that I swiped from some Webers I had. Upon instalation, I noticed that the diameters on the metal ones were larger than the plastics, leaving a reduction "step" where the air horns meet the injector stack castings.

1) Does anyone know if this makes a difference? Arguably, the larger air horns offer less restriction, but I'm wondering if the step adds some unwanted turbulence to the air flow that would adversely affect performance. Or, is the difference only theoretical and not a practical consideration?

2) Do the metal horns come in various sizes depending on model (E/S/T) and and year or displacement?

Lastly: Also, what color is the wire that should be connected to the enrichment solenoid? Wiring diagrams I've seen haven't helped much and no obviously loose wires are back there.

Thanks in advance,

- MR

[304065]

Yes, you want a smooth transition all the way from the mouth of the trumpet to the intake valve if possible. What is the diameter of the plastic horns?

There is no "enrichment" solenoid on a 71 MFI, there is only the overrun cutoff solenoid. It's a gray wire with a red stripe. Do you mean the cold-start solenoid? It's a gray/white wire, goes to #12 in the diagram (where it is called "electromagnetic control valve.)

[edmayo]

I don't remember that the Weber air horns are an interchange for the MFI ones. I think you'll need to get the correct MFI air horns.

[MountainRoads]

Yes, you want a smooth transition all the way from the mouth of the trumpet to the intake valve if possible. What is the diameter of the plastic horns?

Thanks. I was afraid that might be more than a purely theoretical problem. I'll need to check the horn diameters. The car is on the other side of the mountains, so it might take a couple of weeks. I'd estimate the metal ones to be about 2 mm larger, leaving a pronounced 1 mm step all the way around the circumference where the air horn meets the injector stack.

Yes, I mean the "cold-start" solenoid, not the overrun solenoid on the MFI pump. Thanks much for the info, which helps a lot. I wasn't clear on which was the cold-start solenoid from the wiring diagram legends I saw.

- MR

[MountainRoads]

I don't remember that the Weber air horns are an interchange for the MFI ones. I think you'll need to get the correct MFI air horns.

I heard similarly, but the bolt patterns for the horns I have are identical, so I was hopeful until I noticed the internal diameter difference. I know I have the correct '71 E throttle bodies and am aware that same-year 'S' butterfly valves/ports are slightly larger, so either there truly is a difference between carb and mfi horns, or I don't have metal horns for the correct displacement and/or engine model.

Thanks for the input.

- MR

[MountainRoads]

What is the diameter of the plastic horns?

Sorry to take so long responding. The internal diameter of the plastic airhorn is 38 mm at the base. One of the local shop gurus told me that the airhorns were all the same for a given year. S's had larger butterfly valve diameters than the E's, but the air box trumpets were all the same. Perhaps they were even the same for all of the 2.0, 2.2, and 2.4's ??

- MR