Racecar engines (and apparently Chevy Vegas) sometimes use unlined aluminum blocks. For the race cars, they only use the engine block for about 250 miles. For the Chevy Vega, it was a maintenance nightmare. Apparently having no liner improves cooling and allows for tighter tolerances.
What I thought I knew about these engines (I thought the cylinders were anodized) was incorrect. So I won't spout any more "information".