Delivering slow-speed torque is all about building cylinder pressure at low engine speeds. You must have proper-sized components and valve timing events to achieve the desired effect. You have already keyed in on the fact that you want to keep the intake runner small for the displacement of the engine. The LS6/LS2 cylinder head casting number 243 will be a great choice on your 4.0-inch stroked 6.2L LS3. The only issue is these cylinder heads have a small 64cc combustion chamber, which will kick up the compression on your 415-cid displacement. We will need to look into some dish-style pistons to take up some swept volume. As for the best inlet manifold for torque, you’ll want to hunt down a LQ4/LQ9 6.0L truck intake. This manifold will give you the longest inlet runners with a large plenum volume. These components with quite a short camshaft package should give you the tire twisting torque you’re looking for.
Check out Lunati’s complete rotating assembly, which features Wiseco 14cc dish pistons forged from 2618 aluminum and CNC profiled. The rods are Lunati H-beam made from 4340 steel and measure 6.125 inches, center to center. Rounding out the bottom end is a Lunati Pro Series non-twist forged crankshaft made of 4340 aircraft-quality steel, weighing roughly 45 pounds. This will give you a bulletproof foundation to build as much torque as you wish. With these dish pistons and your small 64cc combustion chambers, you’ll be around a 10.25:1 compression ratio. If you open up the combustion chambers to match your larger 6.2L bore of 4.065 inches, it’ll put you right into the 10:1 range.
As for camshaft selection, we need to go a little unconventional to build torque in the 1,800- to 3,000-rpm range. Again, your displacement is going to eat up most camshafts, making them react smaller than they spec out. Two selections should fit the bill. First, let’s talk about the COMP Cams XFI RPM hydraulic roller 259. This camshaft specs out at 206/212 duration at 0.050-inch tappet lift, 0.515/0.522-inch max lift, with the separation angle ground on 112 centers, and the intake centerline on 110 degrees. This is the smallest camshaft COMP offers, and it will close the intake valve 59 degrees ABDC. This will give you 121 degrees of crankshaft rotation to build slow-speed cylinder pressure. As for the second selection, we’ve stepped up the duration numbers a bit, but it’s still quite advanced for an EFI LS-type engine. This camshaft is the shortest in COMP’s lineup of LSr Cathedral Port camshaft profiles. The grind number is 265LRHR12. It specs out at 215/233 degrees duration, 0.604/0.610-inch max lift, the separation angle is also on 112 centers, but the intake centerline is advanced another 3 degrees to 107. This cam has quite a bit more overlap, but with its tight centers and advanced installation the intake closes at 60 ABDC, which is within 1 degree of the really short cam. This cam is what COMP offers for small-displacement, heavy vehicles needing to produce high slow-speed torque. This cam with the LQ4/LQ9 inlet manifold will run out of breath around 5,000-plus rpm, but it should pin you in the seat from 1,500 up. We’d try the larger of the two.
Please let us know how this experiment works out for you. Not too many individuals are looking to build high, specific torque engines these days. Most have switched to oil burners for their high doses of torque. We’re with you; we still like our petrol-burning engine. Good luck.