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-   -   Ignition for a high-revving B20 (https://forums.turbobricks.com/showthread.php?t=355776)

fatcatbestcat 06-30-2020 10:37 PM

Ignition for a high-revving B20
As the title says; doing a partial rebuild on the B20 in my 144S, and I'm throwing some components in it that I suspect might mean I can rev past 6,000 rpm. I'm not sure the stock, 40-year old distributor can handle that much.

I have an MSD 6A box lying around that I could probably install with the stocker, would that be a better solution (for a cheapskate like myself) than buying a new 123 distributor?

EDIT: Just realized I posted this in 'maintenance'. My bad.

vwbusman66 06-30-2020 11:03 PM

Lol, my points seem to have no issue clearing 6k.

MSD 6A and a Pertronix in the stock dizzy should be alright.
JohnMc runs a vintage Mallory distributor with a Crane optical pickup and MSD 6AL (I believe).

JohnMc 06-30-2020 11:07 PM

That is correct. Mallory dual point distributor body with a Crane optical sensor and box. I used to use an old style 60's hot rod Mallory coil as well. It always pulled nice and hard up past 7500 rpm with that setup.

I added the MSD 6A box and Blaster 2 coil because I had them laying around after removing them from my 240 wagon's motor (went COP on the 16V). That adds a noticeable bit of pep at lower RPM's.

fatcatbestcat 07-01-2020 12:54 PM


Originally Posted by vwbusman66 (Post 6093398)
MSD 6A and a Pertronix in the stock dizzy should be alright.

Alrighty then, I'll order up a pertronix unit. Assuming I can get my hands on fuel line and an L junction for my carbs, I'll probably be attempting to start my engine this week. I probably won't need to rev it super high for a first-time startup.

culberro 07-01-2020 01:40 PM

I recently helped a friend install the basic 123 distributor, it was 2 wires and used the stock coil. It was nice being able to select different timing curves (10 or so) that are based on different Bosch distributor curves.
The instructions are not the best, but once we figured out WTF they were talking about.... it was pretty easy.

Pertronics is pretty easy, but you’re stuck with 1 curve... unless they’ve updated them.

JohnMc 07-01-2020 01:45 PM

AFAIK the pertronix doesn't do any curving at all, it's just got the sensor attached to the plate in the distributor, and all the pertinent vacuum/centrifugal advance/retard comes from the distributor itself.

Pertronix just uses one kind of sensor (Hall?), Crane uses an optical sensor. Dunno which one is more reliable, they're probably both soe much better than points-n-condenser that it's not worth splitting hairs over.

I can say that I bought my Crane system used on eBay. It's actually so old it says 'Allison' on it. It's been on the PV's motor for 20 years and 65K miles (more or less) - with never an issue.

The MSD6a and Blaster 2 coil did burn a hole in my rotor once, that part might be wretched overkill, too much zap. But with the 6A in play (it's quite easy to bypass) it starts noticeably better when cold, has a slight but noticeable bit more pep and throttle response ant low/medium RPMs.

142 guy 07-01-2020 02:04 PM

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.

740atl 07-01-2020 02:07 PM

Be different. Install a wasted spark or coil near plug setup. Ditch the dizzy and install a 36-1 crank trigger setup.

vwbusman66 07-01-2020 04:20 PM


Originally Posted by 740atl (Post 6093567)
Be different. Install a wasted spark or coil near plug setup. Ditch the dizzy and install a 36-1 crank trigger setup.

At that point, you need an EFI computer, so you'd be better off using coil on plug and individual throttle bodies.

JohnMc 07-01-2020 04:30 PM

And a rare 16V dual cam B20 cylinder head.

283SD 07-01-2020 08:13 PM

I have a 1961 MK1 MG midget, With a nasty [100HP/Liter]1275cc A series motor. I run points, I shift @ 7500k,red line is 8500K. Never had a miss fire, Used it for a season in the FP race car that I was crew chief for, [We were waiting for our race motor to be built] so that we could qualify for the SCCA Run Off's. If every thing is in good condition points work fine

JohnMc 07-01-2020 08:39 PM

Probably less of an issue with a 4 cylinder vs. more cylinders (with a single coil).

The original stock system on my PV was a bit weak, but that could have the ancient coil.

fatcatbestcat 07-01-2020 10:09 PM

Thanks for the responses everyone.
On a side note (probably a dumb question, but I'll ask anyway);
I know the distributor is geared to the camshaft, but is is a bad idea to take out and replace the cam with the distributor in place?

JohnMc 07-01-2020 10:14 PM

It's not geared to the cam, it's slotted onto the oil pump drive, which is geared to the cam. And the cam can't come out with that gear in place. Also, pretty much the head has to come to access the lifters.

fatcatbestcat 07-01-2020 10:18 PM

I figured as much. Already have the head off, I'm switching it for a B20F head. I suppose I have to take the distributor apart anyway, especially if I'm going to be putting that pertronix thing in.

142 guy 07-01-2020 10:23 PM


Originally Posted by fatcatbestcat (Post 6093684)
Thanks for the responses everyone.
On a side note (probably a dumb question, but I'll ask anyway);
I know the distributor is geared to the camshaft, but is is a bad idea to take out and replace the cam with the distributor in place?

Because of the way that the gear on the distributor / oil pump drive engages the camshaft, I am going to take a flyer that it is an impossibility. Perhaps by turning the cam clockwise as you remove it and rotating the distributor shaft at the same time you might be able to get the cam out (unless the cam lobes hit the drive gear as you pull it out. Even if the cam lobes didn't hit the drive gear I am pretty confident that you would never get the cam back in - at least phased correctly.

vwbusman66 07-01-2020 10:28 PM

You cannot remove the camshaft without removing the distributor and oil pump drive.
Furthermore, it takes all of 2 minutes to pull both. Not a major time suck, not worth ruining a block or something else over.

mitch1971 07-02-2020 02:42 AM

I have a megjolt ignition on my Amazon and a 123 on my 142. The 123 is simplicity. Do megajolt ECUs come up used in the US? If so that might be a cheap way to get programmable ignition. It’s more faf than the 123 as you have to build up the system; wiring, coil pack, trigger wheel and sensor etc.but works well when done. The 123 is a great swap if you have a worn dizzy and is a modern like for like upgrade in reality.

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