C.O.R. (Coefficient of Restitution) is a measurement of how lively the face can be before it is deemed illegal by the USGA.

What was the COR of the old wooded heads? What is the COR of a one piece stainless steel cast iron head today?

.830 is the legal COR limit meaning that a golf ball shot out of an Air Cannon at the face of a clubhead at 100mph cannot rebound back any faster then 83mph or it is deemed illegal. Now they have a CT test (Characteristic Time) which is just another way of measureing face flexture.

Wooden woods with their array of different plastic to paper fiber to metal face inserts and normal 1 piece stainless cast irons both have a COR in the area of 0.770

In the history and evolution of COR as a performance factor in head design, designers didn't really become aware of COR all that much until around 2 yrs after the very first titanium driver heads came out in the early to mid 90s. Actually the first reason companies began to use titanium to make their driver heads was to enable them to make the head size larger than had been possible with 17-4 stainless steel, not because of the COR capability from titanium's strength to modulus relationship.

Titanium has a much lower density than steel (4.5 g/cc vs 7.6 g/cc) so this enabled designers to push the size of the driver head larger. Size was a big factor in driver design back in 1993-94 because Callaway had started this trend with their original stainless Big Bertha. When BB came out, it pushed the size of a driver from 160cc to 200cc and triggered Cally's marketing campaign of "bigger is better".

So the industry got into all this huff about making larger and larger drivers. And from that moving into Ti for the head material allowed them to do that.

It was only after a year or two that the industry started to take a look at the Ti drivers and realize, "hey, I guess this does hit the ball a little farther - wonder what is doing that?" And from that, then the industry found that titanium's strength to modulus relationship did have an effect on the COR, and from that, an increase in the smash factor for more distance.

Then from that point on, it became a matter of figuring out how to make the Ti heads so the faces could be thinner to keep pushing the COR up while still making sure the faces did not cave in, break or fail.

Also, In the 70s prior to the metal wood era, you would see companies use face inserts made from different hardness materials and saying the metal face insert hit the ball farther than the old paper fiber insert because it is so much harder.

Same thing happened in the early days of the stainless woods - they all were saying that the move to steel woods would bring more distance because steel was harder than wood.

Even today, with much better engineering knowledge in the equipment side of the business, you will still see such claims about harder being better, as in the case of maraging steel face woods you still see marketing departments try and push with totally false claims.

But in reality, what really happens when the face gives a little (pushes inward at impact with the ball) is the ball stays more round and doesn't flatten out as much at impact, thereby retaining more of its energy. Remember when you were a kid and "Superballs" were introduced? When you bounced them on pavement they bounced really high because they didn't flatten out at impact. Now drop a half inflated basketball on the pavement. It won't bounce back because it flattens out and loses all its energy. So a hard clubface that doesn't budge at impact will make the ball flatten more and the ball won't leave the clubface with as much speed therefore less distance.