Senior Tech Team
Join Date: Jan 2005
Location: Granite Bay, Ca.
Re: BBC pulley sizes
Are we talking about SWP or LWP here?
The LWP uses a creative set of pulleys on the BBC with two different diameters on both crank and water pump. When I measured the crank I came up with 6.75" and 7.75" diameters. On the water pump it came out to 5.5" and 6.25" diameters. Both of these result in a 1.25 overdrive between the crank and the water pump based on pulley circumference (which is what the belts see).
I don't think I would recommend increasing the drive ratio. If you rev to 6000 RPM, your fan/WP is going to be turning 7500 RPM. This is fine if they are designed for it but if not...
It sounds more like you need a better fan. The factory used the thermal fan clutch in there for a reason. They could use a big high pitch fan and gear it down through the fan clutch viscous coupling. This would make it easier to turn and also make it turn slower. Then when things got warm they could lock up the clutch and take advantage of the overdrive ratio built into the pulleys for extra air flow at slower/idle speeds.
1968 El Camino, 402 with TKO