Plug Gap
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Track Day
Joined: Jun 2001
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Plug Gap
What is the largest possible plug gap you can run on the 84-89 911 Motronic ignition cars? The shop manual states 0.7mm. Would a larger gap improve combustion? I'm not concerned with plug wear. How about 0.9mm?
RL Technical Advisor
Joined: May 2001
Posts: 11,870
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From: Portland Oregon
Originally posted by jpa:
<STRONG>What is the largest possible plug gap you can run on the 84-89 911 Motronic ignition cars? The shop manual states 0.7mm. Would a larger gap improve combustion? I'm not concerned with plug wear. How about 0.9mm?</STRONG>
<STRONG>What is the largest possible plug gap you can run on the 84-89 911 Motronic ignition cars? The shop manual states 0.7mm. Would a larger gap improve combustion? I'm not concerned with plug wear. How about 0.9mm?</STRONG>
If it were me, I would stick close to the factory spec of .7mm. You should be OK with .8mm, too.
There is certainly some power to be gained by opening the plug gaps up but only when the ignition system has enough current capacity and coils are constructed to handle it. The OEM Porsche ignitions are very good but not in the same league as MSD, in this regard.
Opening the gaps to where you will make some measureable HP gains at 1.0mm to 1.1mm, will place a large heat load on the coils that may result in premature failure, not to mention the inability to saturate and fire reliably at 5000+ RPM.
FWIW, I have snipped away the side electrodes on Bosch plugs using the MSD 6AL and their coil with very reliable starting & running up to 7000 RPM.
I was truly amazed and it was good fun but hardly recommended for the MSD 6AL.
