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Excellent article in the June 05 "Motor Trend" re Chevy Small Block. One question. What is the definition of a "small-block"? A '65 'Vette could be equipped with a "big-block"; 396CID/427 hp; the new LS7 will be termed a small block @ 427.6 CID/500 hp??? Wouldn't the larger displacement infer a "big-block" designation?

Is it the fact that the original (1955) length of crank was shortened to 21.75" or that the short stroke (2.93") allowed a short engine deck height?

Thanks,
 

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The confusion exsists because
1. many believe Cu In Displacement is the determining factor for the SB vs BB designation, which it is NOT.
2. The overlap of bored and stroked SB and destroked small bore BB contributes to the confusion.
3. Further contributing to the confusion is Chevy's revival nomenclature such as LS6, LS7 which indicated 60's & 70's BB's and current production HP Gen III SB's

BB Vs. SB designation is determined by bore center line seperation.

And thence the ability to establish larger bores (> 4.2" dia.) or NOT.

Current Gen II - Gen III SB have 4.2" bore centerlines and have a bore range of 3.76" - 4.165" (with siamesed bores).
Deck height range = 9.025" - 9.525". Thus allowing a displacement range of 265 cu in - 462 cu in

Gen II - Gen VII BB have at least 4.5" - 5.0" bore centerlines (including Olds DRCE, KBlack & Donovan blocks) and have a bore range of 4.118" - 4.9". Deck heights range = 9.2" - 10.7" thus allowing displacements of 366 cu in - 750 cu in.

Note that the soon to be introduced LS7 at 427 cu in, is a small block where as the '70's LS6/LS7 454 was a BB.

Also the 8.1L Viper V-10 is considered a small block due to bore spacing and piston size. It just has more cylinders. Conversly some older inline 6 cylinder engines were considered BB due to their massive pistons and bore spacing but were under 300 cu in.

SB can develop nearly the same HP but at higher RPM's. Consider the newer 2.5L F1 engines developing 850 HP at 20,000 RPM.

The advantage of a BB is the massive torque & HP developed at lower RPM's. Thus most BB started out as truck engines. The BB has a greater HP potential as it can move more air more efficiently as evidenced by current top fuel engines producing 3,000 HP
 

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I don't know if there are any internal or external measurement criteria that differentiate a "small block" from a "big block". However, the "big block" is dimensionally a much larger engine, including a taller, longer, and wider block, larger heads, crank, pretty much everything. The first big block was based on the 409, but the Chevy "big block" that most refer to was a different engine started as a 396 and 402 in 1965, then became the 427 in 1966, then became the 454 in 1970, then became the 502 in 1991 (I think). All of these big blocks share the same basic block and head design, and with the exception of minor generation changes, are all born from the same block, and look almost the same externally. All of these different big block displacements were made by simply boring and stroking the original 396. For example the 454 is simply a stroked 427 (which is why 427 and 454 heads are interchangeable), and the 502 is simply a bored out 454, and shares the same stroke. All of this displacement increase was relatively easy for the big block because of the large and thick cylinder walls, and tall block dimensions, allowing plenty of room for bore and stroke increases, which is much more difficult with a small block because there is less material to work with (cylinders are much closer together and the block height is shorter). The big block is still sold in Chevy and GMC trucks as an optional engine today, as the "Vortec 8.1L", which is the 502.

If you look under the heads of a LS7 427 and compare it to a 1967 big block 427, you'll see the LS7's cylinder spacing is razor thin, and the length of the piston stroke goes right to the bottom edge of the block, whereas the big block 427 has plenty of room between the cylinder walls and lengthwise.
 
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