It's a function of geometry. The larger foot on a mushroom lifter allows a more radical angle on the cam. Very helpful on engines with small diameter lifters. The Y-Block lifter, although technically a mushroom, really doesn't have much of an advantage over the typical lifter, except they're a lot lighter. That's why Ted got upset when they used the wording of the rule, rather than the intent, to disqualify him last year. The person/persons who wrote the rule never saw a Y-Block lifter, which is why they might change their minds this year. (After Ted beats them up again)

Frank/Rebop

Bristol, In ( by Elkhart) 

Chuck in NH

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Unfortunately there’s still a lifter restriction as far as the Y is concerned.  Here’s the excerpt from the 2009 rules.

CAMSHAFT/LIFTERS/TIMING BELTS-CHAINS

Camshaft must maintain OEM journal diameters as originally manufactured in

production for a given engine type.  Roller cam bearings are prohibited. Custom designed and custom-ground camshafts are acceptable.  Flat-tappet, solid roller and hydraulic roller designs are legal.  Mushroom, ceramic, or composite lifters, for example “Schubeck” lifters are prohibited.  Lifters must be unmodified out-of-the-box parts, hydraulic or solid, and cannot be altered in any way from their manufactured as-new state.

I have mailed in the required application for a Y entry as there’s always the chance for a rules exception and I’ll push for that as was done in previous years.  This year looks to be more promising in getting that exception passed as compared to the last couple of years so it’s just wait and see for right now.

Lorena, Texas (South of Waco)

Yes, it's true. For 2009, there is no lifter restriction, even rollers are allowed.