Jeff's Performance IMCA Modified Engine - Horsepower IMCA Style

Jeff's Performance sourced a used core for the block. Before beginning any costly machining processes, all cores should be mag'd for cracks. The cylinder bores should also be sonic-checked to make sure the thrust faces of the cylinder bores will be at least 0.125 inch thick after boring.

All blocks should also be line-honed. This is especially true if you are swapping out the cast main caps for high-performance steel units, which is what we're doing in this build.

Just a few years ago, CNC-machining was the domain of Nextel Cup teams, but now it's becoming affordable for Saturday night racing. After line-honing, Jeff's Performance installs the block on this CNC machine. That finishes every other critical machining process except the final hone. Hill says this method is affordable because the CNC machinery cuts several hours off of the time required to machine a block, versus the old manual methods, and the results are better.

Cylinder head specialist Jason Hartline hones the valveguides. Stock stem sizes are used. You should have approximately 0.0020-inch clearance on the intake and 0.0025 inch on the exhaust, with steel valves to prevent them from sticking in the guides.

Slightly longer than stock valve stems (0.150 inch longer to be exact) are installed to get the spring's installed height up to 1.900 inches and smooth out the rocker arm geometry.

Spring pressure is 130 pounds on the seat.

Stock Chevy engines use press-fit plugs to seal the oil galleries. These should always be removed for cleaning and then thrown away because they are prone to leaking. Tap the holes and use threaded pipe plugs to seal the galleries.

Brass freeze plugs are more expensive than standard steel plugs, but they don't rust and are less likely to leak.

Engine assembly specialist Fred Lazcano drives in the cam bearings. Hill used Clevite H bearings throughout for their increased reliability.

Jeff's Performance sells its own cranks and connecting rods made to their specifications. This crank is a 4340 forged steel unit that weighs only 44 pounds thanks to the pendulum-cut counterweights. The lightweight pistons required a bob weight of just 1,742 grams, which meant a few extra holes had to be drilled into the crank to get everything balanced.

This block is from a different build, but here you can see the four-bolt steel main caps Hill uses to strengthen the block. A few machining operations are required to get the block to accept the four-bolt caps, but if you are building an engine that will exceed 500 hp, it's time and money well spent.

Using motor oil to lubricate the ARP main studs, Lazcano torques the main caps.

The Crane solid flat-tappet cam features 0.374/0.381-inch lobe lift with duration at 0.050-inch lift rated at 258 and 262 degrees. Hill says this is one of the few times he's used a cam with more lift and duration on the exhaust valve, but the results more than spoke for themselves on the dyno.

Lazcano installs the 6.00-inch Jeff's Performance forged rods on the KB Pistons.

Hill chose Perfect Circle rings for this build. The top ring is a moly piece. The top two rings are both 11/416 while the oil ring is 31/416.