If you change the overlap, you will change the lobe seperation which inturn affects cylinder filling by either forcing the mixture into the cylinder or using the exhaust gases to pull the incoming mixture. This inturn changes the idle characteristics, fuel economy or power, etc.

CompCams has some explained here.

And Crane say....

In a nutshell...

Decreasing lobe seperation:
1. narrows the powerband
2. moves the powerband lower
3. makes the idle choppier
4. makes more power
5. pumps up the midrange

Increasing lobe seperation
1. widens powerband
2. makes the idle smoother
3. takes away from the midrange
4. produces less maximum power

Racers usually use tight lobe seperations. The only exceptions are Pro-Stock, where 500+ ci are revved to the moon with cams so HUGE, tight LSA's produce too much overlap, and nitrous/blower/turbo applications where overlap sends all that extra fuel mixture out the tailpipe!

Tight lobe seperations increase overlap and can help increase velocity in overly huge intake ports (4bbl 351C). This will pump up the midrange, and make the engine feel less "lazy". Some 351C drag race cams have lobe seperations as low as 102-104 degrees. Automatics generally need a tighter lobe seperation for extra midrange.

Small engines with big ports LOVE tight lobe seperations, large engines with small ports work better with wider angles between the lobes.

A tighter lobe seperation also allows you to use a slightly larger cam in a lower rpm range. If you want maximum valve timing without revving the engine to the moon, a tighter lobe seperation can make it happen.

It's the dynamics of the tight (low) lobe seperation that produces the extra power.

Tightening the lobe seperation increases the cranking (effective) compression. The piston can only compress the air it traps in the cylinder. Long durations and wide lobe seperations push the intake closing event into the "compression" stroke. If air is escaping out the intake valve as the piston travels upward, it's not getting compressed, and cylinder pressure goes out the window! Narrowing the lobe seperation helps build more compression by making the intake valve close sooner, enabling the compression stroke to capture and compress more air. Advancing the camshaft will do this as well.

Some people mistakenly believe that compression is lost during the OVERLAP period... WRONG! Overlap occurs when the exhaust valve is closing and the intake is just starting to open... Nothing is being compressed yet! NARROWING the lobe seperation pulls the intake closing event out of the compression stroke. Advancing the cam will make the intake valve close sooner as well.

Here's another site with some lobe seperation info.

http://www.fordmuscle.com/archives/2001/04/camtech/

Just be sure to use an extremely free flowing exhaust with extra tight lobe seperation angles. Their scavenging effect from their extra overlap NEEDS some free flowing tubes, or the exhaust will just get backed up into the cylinders... Really bad exhaust systems will actually make the carburetor get all black with carbon. Open er' up, and let er' fly! I really like the dual 3" woofers that occupy the underside of my car.

Also ask yourself how high you are willing to rev the engine before deciding upon a camshaft.

Good Luck!

------------------
Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367