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I know that by next year "everyone is required" to have a driver that is conforming .830 COR rating...

My question is, how much distance is the difference between a conforming and non conforming driver?

Lets say a .830 vs. .850 vs. .890

Thanks!

The following is from an August, 2002 Golf Digest article. While it does not quantify distance for varying face thickness, it does give you a generic answer to your question.

How COR affects you

Basically, increasing COR means more springlike effect in a small point on the clubface. Ball speed still increases with off-center hits, too, says Tom Wishon, a member of Golf Digest's Technical Advisory Panel. "Clubheads designed with variable face thickness," he says, "are able to show strong springlike capabilities even when hit one-half inch off-center." Frank Thomas, Golf Digest's Chief Technical Advisor, says "average golfers benefit more from a bigger, more stable clubhead than the increase in COR." Still, some believe the new limit means longer drives by 10 yards. Dick Rugge, USGA senior technical director, says it will be less: "This has been blown way out of proportion by the industry and by us."

Says Geoff Goodman, senior vice president at Callaway Golf: "High-COR drivers can help all players hit it farther. But that doesn't mean it's guaranteed."

What you might see in your golf shop

COR has gone right over the heads of most consumers, says Joe McOwen, vice president at Washington Golf Centers. "We're not promoting drivers now as being at the limit," the retailer says. "It's not something that's a selling point." But if .860 becomes the rule, there could be a rush by manufacturers to fill that void, and presumably a fire sale on some under-.830 drivers. By some estimates, that could mean as many as 500,000 drivers already in the pipeline that in some cases could have their in-store prices chopped by a third.

However, don't assume "old rule" drivers are obsolete. The USGA's nonconforming list already includes what Rugge calls "a fair number of drivers" just barely over the .830 limit. "Your ruler would be enough to measure the distance difference, and a small portion of your ruler at that," he says. Some manufacturers say the proposal only adds to consumer confusion. Says Ping chairman John Solheim: "[The USGA and R&A] are rules-makers. They need to stay out of marketing."

What the manufacturers might decide to do

At least half a dozen manufacturers plan to market high-COR drivers, while many others say they have prototypes in waiting. Most companies that market clubs internationally already sell drivers that do not conform to current USGA rules, so presumably they would be ready with a new driver at a moment's notice. Mizuno, for instance, has nine drivers on the USGA's nonconforming list (none marketed in the U.S.). There are 26 nonconforming drivers on the list that are made by other companies and sold in the U.S. Regardless of any rule change, be leery of "off-brand" companies claiming to have a new high-COR driver. Says Callaway's Goodman: "It's not easy to make a high-performance, high-COR driver. Without doubt, there will be stories of high-COR drivers that have durability problems."

Jesse Ortiz, chief club designer for Orlimar, believes face cave-ins will increase the closer a driver gets to the new limit. "Realistically, something approaching .860 is not going to be the most durable club on the market."

You also have to look at what COR really is. Coefficient of Restitution is the amount of velocity maintained after an impact. COR = (velocity after impact)/(velocity before impact).

This is how the test for COR is performed: you drop an object at a known velocity off the surface and then measure how fast it bounces back off the surface, and then you calculate what the COR is. So, take a conforming driver (all the OEMs manufacture right at the 0.830 limit today) and a golf ball hits the driver face at 10 m/s. If it hit right on the sweet spot, and perfectly square, it will bounce back off at 8.3 m/s. Now, obviously you swing the club and hit a stationary ball, but the idea is the same, the COR limits how fast the ball can bounce off the driver face. If the COR was 0.860 in the exampke above, the ball would bounce off at 8.6 m/s, or a mere 4% or so increase.

A 4% increase in distance would be to absolute maximum you could get from a .860 driver, meaning about 10 yards on a 250 yard drive. But, it probably wouldn't even be that much, because at the higher velocities, the drag force is greater, meaning I'd bet that you'd only get 2-3 yards more. Probably not even enough to be completely significant on most amateur's swings -- meaning that most amateurs wouldn't be able to distinguish between a swing fault/improvement and a "hot" club.