Over the years I have heard more stories about how so and so shot somebody's car and cracked the engine block... Understood that a properly placed .22 could stop an engine (fuel line / coil / etc.) but for actually punching through or cracking a cast iron block, what is possible and what is out and out horse hockey?

.499 L-W  or the 50 beowulf

when in doubt: 40mm HEDP

.450 Nitro Express, .460 Weatherby Magnum, .50BMG SLAP are all suitable rounds for cracking engine blocks.

Too man factors to account for

Depends on the block, and where on that block you shoot it.
Depends of the type of bullet used as well.

It's safe to say a AP round from a 50 BMG will penetrate at Detroit Diesel Series 60 at 25 yards.

.22 short

Take off the valvecover on your car's engine block.  You'd be surprised how thin the metal is.  There are channels throughout the block for the oil and coolant.  I would not be surprised if a .38 revolver will punch through and cause a leak.  Coolant and oil are under pressure in a running engine, all you need is a tiny leak somewhere and it'll break down fast.

.17 HMR

How about aluminum engine blocks on the cars today?

But in Blackhawk Down it looked like they nailed the block with a 5.56 or .308....

Quite a few cars today are going to aluminum blocks & heads, so I would doubt you would need much of a caliber to mess one up. I would be safe to say that 7.62x51 ball could ruin most engines by either penetrating thin areas of iron blocks or aluminum blocks, as well as the other systems like radiator, fuel, oil lines, electrical, computers and such. It may not stop one dead, but the car will crap out soon. Going with AP rounds in any caliber would help.

On a side note, I can say first hand, that 180gr 8mm Mauser,  M2 ball .30-06, and  M80 7.62 NATO will NOT go all the way through a Toyota torque converter at 50yds even after many shots. It was ruined and all of the fluid drained out but the hard metal inside was just too strong for regular FMJ rounds. The 8mm mauser round went right through a cast iron control arm though.

Back in '03 at the Norcal arfcom shoot I was shooting my 1911 at a cast iron intake manifold from about 50 yards. Busted it all to hell.

Definitely depends on the BIQ (Block in Question)

My dad has an ancient Chevy truck on the range with a straight 6 that we shoot at occasionally.

5.56 FMJ will punch through the metal to get to the motor, but not really do any damage.  All of the 7.62 rounds (AK, .308, 30-06, 7.62 Russian) will punch through the metal sidewalls and blow a hole in the cylinder walls.  All it took was a single shot to blow the entire distributor system out of the block.

The side wall metal on this truck is extra thick too.  They made them tough in the old days, you know.

I would think that anything in 7.62 would seriously fuck a thin skinned vehicle up.  That's why my bug out rifle is an AK in 7.62.  We are such a vehicle baseed society that I really want my survival rifle to be able to deal with cars and trucks if the need presents itself.

Plus, to take out a vehicle you don't need to screw up the block.  The cooling systems (esp the radiator) and the electrical systems are seriously vulnerable.  From the front, you really can't miss the radiator . . . and the vehicle WILL stop soon with no coolant.

Disconnector.