Why under-drive a pulley system?

By Martha Maglone:

If under-driving a pulley system robs less power from the engine, why doesn’t Concept One do it?

“We don’t under drive the pulley system,” says Concept One’s Kevin Redd, “because we are more concerned with driveability than saving a couple horsepower. When you under-drive the pulley system you slow it down and as a result you can lose performance at low rpm and at idle. Reducing the pulley speeds can also have a detrimental effect on alternator charging as well as power steering performance and cooling efficiency. For these reasons we don’t under-drive our pulley systems.”