Take a look at a Boss block for the modular crowd. With 5.0 liters of displacement while still 4.6-liter deck height, FRPP's new cast-iron Boss 5.0 block (PN M-6010-BOSS50) may be just the ticket for mega-power street and drag 'Stangs. Suggested retail is $2,050.
Horse Sense: A non-Siamese block has water passages in between the cylinder bores. The more rigid Siamese block has nothing but solid metal between the bores.
Lately, the midnight oil has been burning fiercely at Ford Racing Performance Parts, especially in the realm of crate motors and, more to the point, engine blocks. It seems just a short while ago ("It's Good to Be Boss," Jan. '07, p. 104), we were scoping out FRPP's ultra-stout Boss 302 pushrod block, and now we're taking our first look at its modular counterpart, the Boss 5.0.
Displacing-as you likely guessed-an even 5.0 liters, this is much more than a bored 4.6-liter block. In fact, the Boss 5.0 is a new casting that incorporates many strength, cooling, and durability improvements compared to its factory modular counterpart. On a historical side note, it's also among the final series of engine blocks to be poured in FoMoCo's famous Cleve-land casting plant, scheduled to close its doors sometime around the end of 2007.
Despite its eight bigger holes-3.700 inches in diameter versus a 4.6's 3.552-inch bore dimension-the Boss packs about 10 pounds more muscle than the OEM iron casting, most of which can be found in the main web/bulkhead areas. Stiffening this area surrounding and supporting the crankshaft is a huge ingredient in the recipe required to contain the ludicrous amounts of power we're producing these days. That explains the rationale for casting the Boss 5.0 in iron instead of lighter aluminum.
Getting to some cooling detail, note that on the Boss block (below), a cooling passage routes around the front-most head bolt on the passenger side. Contrast this to the passage configuration on the stock block shown here.
Road racers may shun this block in favor of aluminum to get the lighter nose weight-and that's OK because they're normally not running more than 500 hp. As power levels increase beyond that, aluminum blocks have a tendency to develop more flexibility or "walk" in the bottom end, which, among other things, adversely affects piston ring seal. When the rings aren't sealing, power is leaking away, as is durability. "At as low as 500 hp, you'll make more power with an iron-block than an aluminum block because the rings will seal and last longer," says FRPP's Jesse Kershaw. One obvious application we see for the Boss 5.0 is on '03-'04 Terminators, many of which now routinely pound out 700 or more high-boost, rear-wheel ponies. As those bottom ends wear-and they will-the Boss is the ideal replacement, one that also brings along the obvious benefit of additional displacement at a stock 4.6-liter deck height.
Still on the topic of crankcase, the Boss 5.0 main caps are machined so they can easily be drilled for an additional pair of bolts, but it seems the power threshold necessary to require such additional clamping measures is high. How much power can this block handle as issued with its two-bolt caps and cross-bolts? "I think we're talking 1,000-1,100 or more horsepower," Jesse says. "The Sutton drag car is currently making more than 1,000 hp, and they've been running this block all season long. They actually set the EFI Renegade record for e.t. and mph in the NMRA without any issue, and that's with the two-bolt mains." So yes, this is a stout chunk of iron. "For a street car, I think this is the last block you'll ever have to buy."
A factory 4.6's cooling circuitry gives a 60/40 cooling advantage to the driver-side bank, which can cause head gasket or cylinder head issues on the less-cooled passenger side under extreme conditions. With the Boss 5.0, coolant is equally divided side to side. | 
Notice that the main web thickness remains constant on the Boss block (right), whereas it has a fairly uneven topography on the stock counterpart. Minimum web thickness on a stock block is about 7 mm; this measurement never goes below 17 mm on the Boss 5.0. | 
Also note the "windows" (arrow) through the web near the cylinder bases on the Boss, which relieve crankcase pressure and oil foaming by allowing air to move to adjacent cylinders as a piston moves down through its bore. We shoved a piece of paper down the bore to make the window more obvious-another high-tech 5.0&SF visual aid. |