MAP vacuum accumulator size

[Walt Fricke]

The 2.8 I bought came with Electromotive TEC3r EFI. I've gotten around to installing the EFI. Here are pictures of the vacuum accumulator the previous owner of this motor fabricated.





I calculate this plastic tube to be about 4.9 cubic inches or 80 cc. Clewitt Engineering's website refers to a good discussion here on Pelican about installing Electromotive. Plus he has some recommendations, one of which is to install the MAP sensor as close to the accumulator as possible. I just left the hose that was on it, so I'm going to move it down onto the motor somewhere to shorten those hoses, and mount the sensor somewhere close by so I can shorten its hose.

Elsewhere I saw a suggestion that an accumulator should be used, and that it should be about 2 cubic inches in volume. That's about half the size of this one. On the other hand, in the Pelican discussion various people submitted photos of their accumulators. Neatly made "logs" with a bristling array of 1/8" hose barb brass fittings. I have some rectangular aluminum bar, drills, taps, etc, and could make up something like that. But those all looked to be rather smaller in volume than 2 cubic inches.

The TWM throttle bodies I have only take a signal from two intakes per side. Would this argue in favor of a smaller or a larger accumulator?

The function of this is to smooth out the vacuum pulses so the MAP sensor doesn't jump around as much, especially at lower RPM. Anyone got a theory on how to size such a thing?

My manifolds have a vacuum port I installed on one, using a spare boss down by the #5 intake on the outside. I used it to control a vacuum adjusted fuel pressure regulator for my carbs (a Cagle), and per instructions installed two small restrictors in the tubing which ran to the regulator. These were supposed to act as reservoirs and smooth out the signal. If that's all it takes for that purpose, maybe an accumulator can be pretty small?

Walt