View Single Post
Old 07-01-2020, 02:04 PM   #7
142 guy
Board Member
 
Join Date: May 2014
Location: Saskatchewan, Canada
Default

The bigger issue may be wear in both the distributor drive and the advance mechanism (if you have a B20 with the original distributor is more like 50 years old). On my 142 E I found that even with a Pertronix module which eliminated points bounce issues, when checking the timing with a strobe light the timing became very erratic just below 3000 RPM. The first step you might want to consider is if you can find one, take your distributor to a specialist and have them check its operation. If the shaft is not worn, new bushings and rebuild parts used to be available from GCP although getting the distributor apart can be a treat. I don't know whether parts are available to rebuild the advance mechanism.

Setting aside CD systems, any electronic module (Pertronix, Hot-spark, Crane ...) that replaces the mechanical points in a Kettering style ignition system potentially reduces ignition performance. When the points are closed the voltage across the points is zero and the full 14 volts of the electric system gets applied to the coil. The Pertonix, Crane and other systems use what is likely a Darlington pair open collector transistor to switch the coil. The open collector transistor is an imperfect switch and in the equivalent of the points closed position, the voltage across my Pertronix module was right around 2 volts. This reduces the voltage applied to the coil to 14 - 2 = 12 volts which reduces the maximum dwell current and increases the dwell time required to hit the same dwell current as with full 14 volts.

Despite what the hot ignition system vendors bark about peak voltages, what really matters in terms of ignition performance is spark energy and spark energy is determined by 1/2xLxI^^2 where L is the coil inductance and I is the max coil current. The coil designers can't bugger around too much with L since increasing L increases the required dwell time (not good at high RPM). The coil resistance becomes the primary determiner of the peak dwell current. True high energy coils have low primary resistance which creates its own problems (burned points). Pertronix and the other electronic switching system guys set limits for the minimum coil resistance that their systems can deal with (I think it is 1.5 ohms). The original Bosch coil resistance is around 2.5 ohms. Dropping to a 1.5 ohm coil (Pertronix and others sell them) will help some to make up for the voltage drop in the ignition modules. However, there is no free lunch. Every low resistance coil that I have seen has a higher coil inductance when compared to a normal resistance coil. The higher inductance potentially increases the spark energy, certainly at low RPM. However, the higher inductance increases the rise time of the coil current so at high RPM / shorter dwell time you may never achieve the peak coil current associated with low resistance coil. If you are prepared to troll through the old Bowling & Grippo on-line MegaManual they had a spark energy calculator. You inserted coil resistance and inductance, operating voltage and dwell times and it calculated spark energies. It allows you to asses whether your hot coil really makes a difference.

On the 140, the lowest effort / cost (less than $10) ignition improvement you can make is to address the deficiencies in the stock vehicle wiring. Because of the rather tortured path that the coil supply takes through the ignition switch, the voltage applied to the coil + terminal is quite a bit less than the alternator operating voltage. When I was still running the original ignition system, I measured the voltage on my ignition coil + terminal with the engine at idle. With the alternator running right around 13.8 volts the voltage on the coil + terminal was under 12 volts. I was losing around 2 volts through the ignition switch and car wiring. A simple improvement is to take a direct 12 or 14 Ga supply from the 12 v distribution block (fuse or no fuse is your choice) through a dedicated relay which supplies the coil + terminal. The relay coil is switched by the old coil + supply wire via the ignition switch. This arrangement applies full operating voltage to the ignition coil and reduces the amount of current flowing through your old tired ignition switch (you want to preserve that switch because 140 replacement switches now appear to be unavailable).

As a final observation, if you want a low up-front cost ignition improvement switch to the NGK iridium plugs. The fine wire plugs fire at a lower voltage and are much more resistant to fouling than conventional plugs. They are significantly more expensive than the conventional NGK plugs. However, they last much longer if your fuel system is well sorted. I run MS2 Extra sequential and I have been running the same iridium plugs for either 4 or 5 years without issue. If your carburation is not well sorted and you consistently run AFRs well below 14.7 then stick with conventional NGK plugs and change frequently because the on-going cost with replacing iridium plugs is too high.

Short summary
- if you don't want to fork over the $ for a new 123 distributor then make sure your existing distributor is not worn out
- wire up a relay to provide a direct supply to your ignition coil
- recognize that installing a Pertronix or similar module will reduce the peak available spark energy (which may or may not be a problem). If points bounce is a problem at high RPM then a Pertronix module will help with that particular problem.
- if you want to chase a hotter ignition coil then find the B&G coil energy calculator and the associated documentation and make an informed decision about whether a low resistance coil will actually do anything for you.
142 guy is offline   Reply With Quote