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I recently purchased a 1966 Plymouth Belvedere II 4 door sedan powered by a 225 c.i. slant 6 engine. I was wondering what octane level of gasoline must I use in this car? Should I use 94 unleaded gas and add a lead additive? Or just use 87 octane unleaded and not add a lead additive? Thanks to any and all responses.
 

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I used to have a 71 Dart Swinger with a slant 6 that ran just fine on regular unleaded octane up until the day it died at 273,000 plus miles. Oil pump went out and locked the engine up on the interstate.
 
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87 octane with no additives will be fine. There is usually enough carbon and lead deposits on the valves to protect them.

Great car, I had the wagon with slant-6.
 

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Definitely save the money by using 87 octane. There are enough additives added to the fuel to prevent valve damage and pinging isn't an issue.
 

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Only thing to be careful about is timing. Set to EXACTLY 2.5 degrees BTDC. Conversion to electronic ignition is highly recommended. Set valves to .010 intake, .020 exhaust.
 
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These engines can tolerate a lot of advance without pinging. I always ran my at at least 5 BTDC with no pinging, and also with light advance weight springs.
 

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When this car was new, octane ratings were 93 for regular and 97 for premium. They were recalculated some time in the 1970s. Your car should run on 87 with no problems.

Scrounge
 

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Congratulations! That's a bulletproof engine, if you do some simple regular maintenance. DO watch the oil pump. They tire, eventually.
 
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