I am pretty sure the HD TBI engines have the 4 bolt mains and lower compression to help to stave off detonation that is more apt to occur when the truck is loaded or trailkering and being operated at WOT for extended period of time.

The HD engine has the lower compression, the LD has the higher compression.

HD is rated at 190 hp 300 lb/ft torque 8.75:1 comp ratio

http://www.jegs.com/webapp/wcs/store...4892_-1_761929

flat tappet hydraulic (.382''/.402'' lift, 165.9°/174.8° at .050'' duration, and 111.9°
Cylinder heads: 76cc chamber, 1.94''/1.50'' valves, swirl port intake, center bolt valve cover, and 7-bolt exhaust flange. These are not performance heads. They are high-swirl intended for good low end torque and fuel economy. The intake runner is restrictive because it has a ramp cast into the bottom of the bowl.


LD is rated at 200 hp 300 lb/ft torque 9.4:1 compression ratio

http://www.jegs.com/webapp/wcs/store...53961_-1_10763

382''/.402'' Lift, 165.9°/174.8° @ .050'' Dur., 111.9° Lobe Sep. Angle
Cylinder Heads: 65.3cc Chamber, 1.94''/1.50'' Valves

According to this, the LD engine uses 65.3 cc chambers while the HD engine uses 76cc chambers.


peace
Hog