The CHP 408 stroker started out life as a late-model production 351 Windsor block equipped with 4.03-inch bores. The block was further machined to accept the forged reciprocating assembly from Coast High Performance. The CHP stroker kit included a cast 4.00-inch stroker crank swinging a set of forged I-beam rods and 22cc, dish-top forged pistons. In addition to the dish, the pistons featured valve reliefs for use with inline valve locations. The combination of the 4.03 bore and 4.00-inch stroker produced a final displacement of just over 408 ci.
The 408 was equipped with an XFI stroker hydraulic roller cam that offered 0.579 lift, both intake and exhaust, and a 236/248 duration split. The XFI cam was tailor made for stroker applications and featured aggressive ramp rates to maximize the opening time of the cam relative to the duration figures. The Comp cam was combined with a double roller timing chain, 1.6 ratio Gold Series roller rockers, and custom length pushrods to work with the Pro Comp heads and hydraulic roller lifters.
The remainder of the buildup included as-cast 190 cc Pro Comp heads installed using Fel-Pro MLS head gaskets and ARP ½-inch head studs. Both sets of heads were run with an Edelbrock Super Victor intake and a Holley 750 cfm Street HP carburetor. Also employed were 1¾-inch Hooker headers, an MSD small-cap, a billet distributor, and a CSI electric water pump.
The new CHP stroker was treated to a 20-minute break-in procedure where the load and rpm were varied to properly seat the rings and bearings. We ran Lucas conventional 30W oil for the break in, and then switched over to 5W-30 synthetic for testing. All testing was performed on 91-octane pump gas. Equipped with the as-cast 190 cc heads, the 408 produced 463 hp at 5,600 rpm, and 480 lb-ft of torque at 4,300 rpm. Swapping on the CNC-ported heads resulted in a jump to 505 hp at 5,900 rpm, and 487 lb-ft of torque at 4,600 rpm. The porting resulted in a sizable gain in power, as much as 54 hp at 6,000 rpm, but there was a slight trade off in the lower rev ranges. This can be attributed to the drop in static compression caused by the CNC work performed on the combustion chamber. Ideally, we should have milled the CNC-ported head to equalize the chamber sizes for this test, but it is obvious that the CNC-ported heads from Pro Comp offer significantly more power than their as-cast counterparts. With street prices near $650 for an assembled set of as-cast Pro Comp heads (via the Internet), these look to be pretty popular with 5.0L enthusiasts on a budget.
After back-up pulls verified...
After back-up pulls verified the power output, off came the as-cast heads to make way for the CNC-ported version.
The CNC heads featured 215cc...
The CNC heads featured 215cc intake ports. The CNC porting improved the peak flow rate from 258 cfm to just over 300 cfm, both measured at 0.700 lift. We verified these flow numbers on the airflow bench at Westech. The exhaust flow was up as well, from 180 cfm on the as-cast heads to 205 cfm on the ported head.
Pro Comp Head Test-As-Cast...
Pro Comp Head Test-As-Cast vs CNC
It should be obvious from these power curves that porting really works. Equipped with the as-cast aluminum heads from Pro Comp, the 408 stroker from CHP produced 463 hp and 480 lb-ft of torque. After adding the CNC-ported heads, these numbers jumped to 505 hp and 489 lb-ft of torque.
After swapping the CNC-ported...
After swapping the CNC-ported heads, peak power numbers jumped from 463 hp and 480 lb-ft to 505 hp and 489 lb-ft of torque. The extra breathing offered by the CNC porting showed dramatic gains, with as much as 54 hp at 6,000 rpm. The slight loss in power below that point (4,300 rpm) can be attributed to the drop in static compression caused by the CNC work on the combustion chamber. Had we elected to mill the CNC-ported head to duplicate the compression ratio on the as-cast head, the power gains would have been even greater.