I read about this some time back. It was interesting stuff.
I think they use the small journal on those motors to reduce
oil shear.
Oil when under a lot of load and squeezed through a tight space like between a bearing and a journal gets torn apart at the molecular level (sheared) and looses it's viscosity and ability to effectively lubricate.
Plain bearing cranks run on a film of
oil under pressure to keep the journal metal away from the bearing metal. If the oil looses its viscosity the two surfaces can come into contact.
NASCAR engines spin at such high RPM that the main bearing surface speed is very high. Reducing the journal diameter reduces circumference and therefore the surface speed which reduces this shearing effect.
I think this is one of the reasons a 351C was a better choice for a high revving engine than a 351W. The 351C has 2.75" mains whereas the 351W has 3" mains. The 302W has much smaller mains @ 2.249"
To work it out determine the circumference of the bearing X the RPM
At 10,000 RPM the surface speed on the The 302W is 70,654 Inches per minute
The 351C is 86,394 inches per minute
The 351W is 94,248 Inches per minute (About 33% faster than the 302W)
Sorry I'm a nerd and the why? of things interests me.
Cheers