Plugs for higher compression?

[Mac-Porsche]

I know this is one of those topics that is debated endlessly... But gotta ask.

Rebuilt '81 911sc engine. Upgraded to 10:1, 97mm P&C set. Car runs great, but feels a little sluggish when cold (first ~5 minutes on road). I drive without lambda (02 unplugged), set the afr to 13.0 when at warm idle - intention is to get a little more power.

I installed NGK BP7ES spark plugs after finishing the rebuild, because that's what the standard recommendation is for later 911sc's. However, I read that increased compression may benefit from different plugs?

[Paulporsche]

My engine is similar to yours, it was an 80 with 98 mm P&C, 10.3:1 and no O2. I still have CIS. I have always used Bosch W7DC or NGKBP6ES, on the recommendation of the original installer. I have done this for about 29 years/90,000 miles. I prefer the NGKs but that's really from a butt dyno standpoint. I don't have any particular data to back that up.

[Paulporsche]

Bump for further input

[Trackrash]

NGK BP7ES is a good starting point. If they are not fouling and the plugs are not looking cooked, you are most likely OK with them. IMO, for street driving those are the right plugs for your motor.

If you tell us more about your motor and how it is used, you may get more opinions.

[911pcars]

While that NGK might be in the ballpark, I’d hate for you to be guessing about something that could potentially destroy your engine. Some variables that affect heat range requirements (detonation threshold) include effective compression ratio, altitude and fuel octane, single/twin spark ign., ign. timing, maybe other factors I’ve missed. In addition, the more monitoring devices you have, the better the picture (EGT, AFR, CHT, Knock sensor, etc.). The monitoring requirements could correlate with the replacement value of your custom engine.

Minimally, start with plugs used in race cars (in the cold range), then inspect the plugs for fuel/ oil fouling and signs of possible electrode wear/erosion/damage. One heat range may not be ideal for all operating conditions, street vs track.

Sherwood

[mikedsilva]

Originally Posted by Mac-Porsche

I know this is one of those topics that is debated endlessly... But gotta ask.

Rebuilt '81 911sc engine. Upgraded to 10:1, 97mm P&C set. Car runs great, but feels a little sluggish when cold (first ~5 minutes on road). I drive without lambda (02 unplugged), set the afr to 13.0 when at warm idle - intention is to get a little more power.

I installed NGK BP7ES spark plugs after finishing the rebuild, because that's what the standard recommendation is for later 911sc's. However, I read that increased compression may benefit from different plugs?

Any updates to this Mac?

[faapgar]

Best plug for this setup is WDPO-3.The one and only true Platinum.Pricey but accurate.In the end cheaper than an engine repair.I am not a fan of plug tips extending into the combustion chamber and getting close to the top of the piston.Fred