What did lead do in gasoline? How did it work?
Also, I noticed that lead is in the same family as carbon, so it ought to be chemically similar. To “lead” gas, were some of the existing carbons replaced by lead, or was the structure of the molecule changed?
I beleive that it was used as a lubricant for the piston rings because the grades of oil they were using at the time could not stand up to the use. Better lubricants and ring materials made lead in the gas unnecessary. At least that’s how I kind of recall it.
Should always google first. It was mainly for anti-Knock. Here: leaded gas knock yourself out. 
While we’re waiting for someone who knows what they’re talking about to come along, you might want to take a gander at the Wikipedia article on tetra-ethyl lead, the additive in question. In particular: no, the gasoline molecules in leaded gasoline are the same as those in unleaded.
The lead was tetraethyl lead, a liquid that was mixed into the gasoline, not molecularly combined with.
The main purpose was to prevent preignition (knock) in the engines. A secondary benefit was what Si Amigo said, it provide some lubrication/protection to some components, mainly valves & valve seats.
I think that today’s valves or valve seats are specially hardened to handle this. I never personally heard, though, of it being a problem for folk who religiously used Amoco unleaded gasoline back then, so I can’t personally say this is complete gospel.
you are right that you can replace some carbon atoms directly with lead in many carbon molecules. However the stability of the resulting molecule is much less than that of the carbon analogue.
Engines are designed to be *lower compression * now, to compensate somewhat for lost anti-knock properties of fuels that had lead.
Engines prone to knocking - turbos and supercharged engines - usually require higher octane, along with an engine computer and sensors particularly programmed and reactive to engine damaging knock.
Advances in computer controlled timing, ignition, valve train and compression have helped to overcome the loss of lead in fuels. The last engines to be designed around non-leaded fuels were smaller/older designs, such as outboard engines and small motorcycle engines that seemed to rely on the lube benefit of leaded fuels.
The company I work for manufactured TEL. The wikipedia information that MikeS posted a link to is pretty accurate. I’d draw attention, though, to the fact that it was as much issues with the catalytic converter that obsoleted TEL as environmental and health concerns; the main impetus for unleaded gas came from the automakers (who still dominate the industry’s decisions).
Switching to unleaded gas required some changes to deal with seal compatability, but TEL has little to no lubricity function.
There are still a few countries that make use of leaded gas – although Venezuela, the largest remaining user, recently abandoned its use. Meanwhile, other gasoline performance additives were developed to provide an octane boost to gas for knock-prevention. When gas companies are marketing their special technologies (e.g., gas with “Techron”), they’re usually adding more of these GPAs. Premium grades of gas also use more GPAs to help meet their octane rating.
This is not true. Back in the days of muscle cars and leaded gas (the 60’s and 70’s) , CR of 8 or 9:1 was typical. Most cars today are at least 9.5:1 and high performance cars (which are spec’d to run premium gas) are sometimes over 12:1. I own an air (and oil) cooled BMW motorcycle that runs ~11:1
Fuel injection is the primary factor allowing this, though improved combustion chamber shape,combustion dynamics, optimized cooling (esp with Al heads and blocks) also play a part.
A carburated engine doesn’t have great mixture control, and the cylinders farther from the carb tend to run leaner. The lean cylinders knock initially, and the richer cylinders start knocking after carbon builds up raising the effective compression ratio (and creating carbon “glow plugs”). Low compression and high octain fuel provided a broad range of mixture where knock could be aoided.
Finally most of todays engines incorporate knock sensors. This allows a high CR for efficient cruise operation. The ignition advance is reduced under high load, which hurts effiency, but hopefully the engine doesn’t see sustained operation at this condition, so the high CR gives a net boost in effiency.
To see how far technolgy has come in this area:
Recip airplane engines are mostly 1930’s technology. The Lycoming O-360 is specified for 100 octane low lead (which is actually still more lead than they used to use in car gas). This engine operates at 8:1 compression ratio, and is considered a high compression engine in aviation community.
Just a clarification on this part of your post. Reading it literally, you are correct. Most engines of that day were low performance and did have low CRs.
However, I kinda “interpret” this in light of the actual muscle cars themselves, and just wanted to point out that from the 1960’s through around 1972 or so, they, at least boasted CRs in the 10.x to over 11:1 range.
Just for comparison, there is a tech sheet showing the MoPar engine specs from 1966 to 1974 located here.
Note that for the performance engines, the ratios were in the 10.0:1 to the 10.5:1 area.
Here is a similar one for Pontiac GTO engines 1964-on
When the era of the “smog motor” came along, the ratios did indeed drop through the floor, as did performance.