Distributor Advance Testing on the Sun 404 Tester
Advance curve testing is carried out on a Model 404 Sun Distributor Tester. Functional tests include electrical resistance, breaker point tension, cam lobe accuracy, breaker alignment, point dwell and variation, centrifugal advance calibration, and vacuum breaker and advance (when applicable). Results are compared against targets and/or factory specs. A typical advance curve and datasheet can be found here.
When available, factory distributor advance curves are used to compare test results. Below are examples for several distributors and types. Advance measurements are plotted at the distributor (deg.) and at the crankshaft (2X, deg.). No initial advance is added to data, though a 5° starting advance is normal for Porsche 1600 motors.
BR18 Distributor Example Data
The BR18 distributor has a cast iron body with the main shaft through the bore at its base (unlike the later 022 that used bushings). Maximum mechanical advance is 30° +/-3° at the crankshaft starting at about 3000 RPM, not including a 2-5° initial advance. The total advance should be about 32-33° and no greater than 35°, a bit lower than original factory specs with today's gasoline.
Modified Advance BR18 Example Data
Distributors can be modified to limit the upper advance, allowing for an increase in static timing without increasing the total advance. In addition, since the ~35° of total advance of 383/BR18/022/031 distributors can be too much for higher compression ratio motors, especially with higher octane fuels, limiting upper advance may be desired. Re-curving Bosch distributors involves a combination of spring choice and reducing the travel allowed for the advance limit pin in the lower drive plate (see parts 29 and 15 in the BR18 exploded diagram. This is done by adding a shim stop into the drive plate limiting hole. An example of this modification in a BR18 is shown below, where measured data is shown together with results when 7° of additional static timing is added (resulting in 12° total static timing). Modification can be done to limit advance to a lesser degree as well.
022 Distributor Example Data
The 022 has a cast iron body with the main shaft running through bushings at the base. Maximum mechanical advance is 30° +/-3° at the crankshaft starting at about 3000 RPM, not including a 3-5° initial advance. As with the BR18, the total advance should be about 32-33° and no greater than 35°.
Modified Advance 022 Example Data
An example of reduced advance 022 distributor is shown below, where measured data is shown together with results when 6° additional static timing is used (resulting in 11° total static timing). Modification can be done to limit advance to a lesser degree as well.
031/061 Distributor Example Data
The 031 and 061 aluminum body distributors exhibit advance characteristics in mid- and high-rev ranges equivalent to the older BR18 and 022 distributors. Advance at low-revs start a bit later (above 1100 RPM at the crank), allowing for a more stable idle when above 800-900RPM. At 900-1000 RPM with BR18/022 distributors, small changes are on the slope of the curve while for 031/061 distributors advance hasn't yet risen. This was designed into the distributors as idle settings increased with 90HP 365C and 912 motors.
050 Distributor Example Data
The 050 distributor has been used as the closest Bosch replacement for original distributors. The total advance is about 13° at the distributor and 26° at the crankshaft, not including a 5° initial advance. This is just below the factory spec of 27° but may be better suited for today's lower octane gas.
009 Distributor Example Data
The Bosch 009 is a mechanical advance distributor intended for use in a VW engine. Although it will allow for some degree of performance in a 356/912 engine, it has only about 16° of straight line mechanical advance between about 1000 and 3000 RPM. This can have limited use for high compression track engines if static is set high, around 12 to 15° - even more dramatic than the modified BR18/022s above. Otherwise, the Bosch 009 can serve a useful purpose as a spare distributor for a 356/912. In a pinch, being prepared with a 009 distributor that has been checked out and adjusted will present no harm beyond sluggish performance at lowered advance settings. Many Porsche 1600s have logged many miles with a 009 distributor in place, though driving performance was likely compromised. There are variations in performance so it is best to confirm specifications for any individual distributor.
We will test your distributor on our Sun 404 test machine at no cost (or obligation). Please contact us for details.