I am currently watching modern marvels "engines" episode and there is an entire segment devoted to this very topic. Their conclusion was that there was no cear answer

__________________
TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth