Are Lead Additives Needed?

At regular intervals the question “should I use a lead additive in my gasoline” comes up on a number of antique car discussion forums. The answer is, of course, “it depends”. For owners of cars with the Plymouth L-6 (flathead) engine, I believe the answer is no.

First we should examine why lead was added to gasoline. I suggest that you take a few minutes and at least scan the Gasoline FAQ located at the Internet FAQ Consortium website.

If you don't read the whole FAQ, at least take a look at the following sections:

• 4.4 History of Gasoline
• 4.6 Why lead was added to gasoline
• 6 Octane
• 6.13 Can Higher octane fuels give me more power?
• 7.2 The effects of compression ratio on octane requirement
• 8.8 Can I use unleaded on older vehicles?
• 8.9 Valve seat recession

Keep in mind the following:

• Only the last of the Plymouth L-6 engines had a “high” compression ratio of 8:1. In 1933 the compression ratio was 5.5:1, a compression ratio that calls for gasoline with less than 81 octane. I defy you to find gas with that low an octane rating at any gas station in the US. Fortunately, using a higher octane fuel than needed does no harm.
• You are not likely to be using you antique car for towing heavy trailers at high speeds. Most valve seat recession is under high speed, high load conditions.
• All Plymouth L-6 engines came from the factory with hardened exhaust valve seat inserts.

So basically any 1930s engine can run on modern unleaded fuel. The Plymouth engine of that era is even more suited to modern fuels as it is fitted with appropriate exhaust value seats.