Quote:
Originally Posted by mcgarrett
alexiskai
Also, could you help me understand better how the Model B distributor works on the Model B engine but doesn't provide the proper advance characteristics on the Model A? I just want to get the nuances clear in my mind. Obviously, I'm not experts here, and you've spent a fair amount of time studying the differences to discover the associated problems. Thank you.  |
The B distributor provides 14-15° of centrifugal advance. It is limited to this amount by the design of the slots on the flyweights. Flyweights have been found with elongated slots that provide more advance, but it's not clear who made them.
A Model A engine with a stock cylinder head (4.2:1) uses a factory setting of 0° initial timing (spark advance at idle) and will typically need about 30° of total advance timing at cruising speed. The Model B engine uses a 4.6:1 stock head and is designed to run at slightly higher cruising rpms. The Model B timing cover, as mentioned above, assists a mechanic in setting the B engine's initial timing to 19°, which, combined with the 14° of centrifugal advance, yields total timing of approximately 33°. This can be adjusted a few degrees in either direction using a set-screw on the B distributor plate. (The B distributor does not allow manual real-time adjustment of spark advance.)
The problem when adapting a B distributor to the A engine is two-fold. First, you need to somehow reach the proper total timing for the engine. If you have a stock head, you need the distributor to deliver about 30° advance at cruising speed. Because the B distributor is mechanically constrained to 14° centrifugal advance, you need to set your initial timing, not at top dead center, but at roughly 15° before TDC. This is a difficult thing to do accurately without some kind of positioning device such as the B timing cover. Your starter then needs to be in sufficiently good condition to start the engine in cold conditions with the timing already at 15°. Some engines have trouble with this, and it could result in the loss of that characteristic low Model A idle sound.
Your second problem is that you need the centrifugal advance to begin advancing the timing within a certain rpm range and then get all the way to its maximum advance within a second rpm range. This is the timing curve. On a Model A, you would typically want the centrifugal advance to kick in somewhere below 900 rpm and to be all in by 1800-2200 rpm. In order to do this, the springs that control the rotation of the flyweights have to be of exactly the right length and strength so that the flyweights rotate out the right amount at the right rpm.
In my experience, the springs that people are getting in their B distributors do not deliver the correct timing curve. I don't know why. I don't know if the alloy used in the OEM springs was weaker or what. But across the board I've found that the springs are wrong. They're too stiff. Consequently you get no advance early in the torque curve and it does not reach maximum advance until you hit an rpm way above the Model A's operating range.
In my incomplete experimentation, I've found that modern stainless steel springs measuring 0.5mm gauge, 5-6mm diameter, and 15mm length are likely to give a timing curve that isn't ideal but is adequate. I'm still working on final settings and expect to have results in the next 1-2 years. (This is not my top priority project.)