When is it necessary to use a scattershield?

Here's a nice example of a clutch explosion

For drag racing, scattershields are normally required for anything running faster than 12.00 seconds in the 1/4. As a general guideline, I'd use one on anything over 350hp or 6000 rpm, although it's not a bad idea at lower performance levels as well. NASCAR certainly requires them for most of it's professional classes, and even NASA "recommend"s them for hobby classes. The NASCAR bigboys are running titanium bells, and there were several explosions in the past couple years. I remember one (Derrick Cope(?)) that even got some TV time showing the damage to the transmission.

Scattershields are generally approved to a "SFI 6.1" specification. The test generally involves cutting a flywheel apart while it's turning 7000 rpm - if the housing contains it, it passes. The SFI certification is for 5 years on 6.1, and 2 years on 6.2 & 6.3. The most common APPROVED scattershield material is steel, while the hi-$$$ (ie NASCAR) guys use titanium (~$3000). The are also aluminum scattershields - they are hydroformed from aluminum PLATE, NOT CAST, and typically run over $1000. There are NO Ford manufactured bellhousings which are SFI approved. SFI approval does NOT require membership in their organization either.

Nodular iron bellhousings were used on some trucks for their added torsional rigidity. They reduce flex in hi-torque applications resulting in longer clutch and transmission life. They were NOT designed for clutch containment.

Nodular or ductile iron is still a cast material. It's similar to regular cast iron, but has increased silicon content. It is about 30% stronger, but still fractures when impacted, unlike the 1/4" hydroformed steel in a typical scattershield, which deforms and deflects when impacted.

The link provided earlier give a basic overview of STEEL properties for various grades, but does not address nodular IRON. Iron and steel and NOT the same - kinda like glass and plexiglass - iron can be stronger (depending on what grade of materials we're comparing), but shatters on impact as it has limited elasticity. Here's a nodular iron link which gives a basic overview of nodular iron properties.