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Optimum Spark Plug Gap

R RonK
I have a 1835 engine with a breakerless electronic ignition system made by Universal Corp. It uses a photo optic sensor to detect the cam lobes (which are painted flat black leaving shiny flats)and has a finned aluminum amplifier box that mounts on the firewall. I have a Bosch blue coil and Bosch W8AC plugs . This is not a CDI system. The vendor says that this system does not significantly alter the voltage, only the dwell time. I had really black sooty plugs. During a period of trouble shooting, I increased the plug gap from stock .026 to .046, thinking that a larger gap would provide a greater spark profile. Changing the gap did not seem to make any difference in how the engine ran or the plugs looked. I finally found issues with my carbs and have since got them running fine. The plugs have gone from black to off white in about 15 miles of driving. The question is should I go back to the .028 gap? Can someone give me a lesson on plug gap selection?
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R RonK
I have a 1835 engine with a breakerless electronic ignition system made by Universal Corp. It uses a photo optic sensor to detect the cam lobes (which are painted flat black leaving shiny flats)and has a finned aluminum amplifier box that mounts on the firewall. I have a Bosch blue coil and Bosch W8AC plugs . This is not a CDI system. The vendor says that this system does not significantly alter the voltage, only the dwell time. I had really black sooty plugs. During a period of trouble shooting, I increased the plug gap from stock .026 to .046, thinking that a larger gap would provide a greater spark profile. Changing the gap did not seem to make any difference in how the engine ran or the plugs looked. I finally found issues with my carbs and have since got them running fine. The plugs have gone from black to off white in about 15 miles of driving. The question is should I go back to the .028 gap? Can someone give me a lesson on plug gap selection?
Gordon NicholsGordon NicholsMassachusetts 1993 CMC Supporting Member
I don't know about a lesson, but if they are light tan to gray in color you're running right and the mixture is correct (finally!).

From the official VW Service Manual by Bentley:

Stock gap for carburetor engines is .024"

Stock gap for EFI engines is .028"

***********************************

Both of those are for a stock coil. If your "Blue" coil is the beefier one at 40,000 volts, you could stay at the .046" you have now. If it exhibits high rpm misfire, then reduce your gap to something closer to .028" (I like .032")

The real question is, if it's running well set up as you have it and it's not misfiring anywhere in the rpm range and the plugs look good, why change??

Go have a Guinness!!!!

gn
D DaveTart
Your unit sounds like a Crane. I got one in the TR6.

This is a generic topic not just for VW types. Do some googling . I did a month or so ago.
My impression was unless you got a Mallory distributor, .45 will lead to arcing there.

1) Only one of the ign path components (plugs OR wires OR distrb rotor) should be resistor type.

2) .35-.38 seems to be the max , without CDI

3) Dry Climates may handle a bigger plug gap . Wet ones more prone to condensation.

Readup, your conclusions may be diffrenet.
FM Former Member
All this is very cool and intersting .. However for the Factory 1600 cc engines the bosch manual gives a tuning option with a gap of 28 on a carbed engine, said to improve idle and a gap 26 was said to impove hwy speed performance.

I have always split the differance at 27 on the plugs,, and 17 on the points (if used)..

I am curious as to how that would apply to compufire.. and what you might recemend in that case..
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