When Chris Henderson of Henderson...
When Chris Henderson of Henderson Machine looked to build a top-level street engine, he looked no further than the Chevy 400 small-block. With a big factory bore and ample stroke, the 400 is an excellent foundation for a hot Chevy combination.
When it comes to building power on a budget, the surefire formula for success is to start with the biggest factory engine you can find and use that framework to its maximum potential. In the world of Chevrolet small-blocks, that framework is the factory 400 block. The capacity for power is already built into the 400, with a bore size that dwarfs the other factory small-blocks at 4.125 inches, standard, and a factory stroke of 3.75 inches. When looking to make Chevy power on a budget, the appeal here is that you really do not have to make radical alterations to the factory combination in search of cubes; you can spend your time and money optimizing those cubes for serious horsepower.
That was just the plan Chris Henderson had in building a small-block Chevrolet for the AMSOIL Engine Masters Challenge. The idea wasn't to reinvent the small-block. Rather, Henderson makes the most of the best of what's already there. Henderson's philosophy is to pay attention to the details, and letting all of those small improvements add incrementally to a good basic package.
If you are going to build...
If you are going to build serious horsepower, you'd better pack some beef into the block. This '76 vintage 400 (the last year of 400 factory production) received a partial fill in the water jackets, Eagle billet four-bolt mains, and ARP main studs.
This engine was based upon a production 400 block, but Chris was looking to improve on the factory's original layout. First thing on the hit list were the short OEM rods. One of the drawbacks of the original 400 Chevy was the short 5.565-inch connecting rod, which increased cylinder wall loading and led to accelerated bore wear when the engines were delivered as OEM. While a common upgrade when building the 400 is to use 5.7-inch connecting rods, Henderson is a believer in the long-rod advantage, leading him to employ a 6.00-inch rod combination. Henderson explains his thinking: "I selected a 6-inch rod so that there will be a longer piston dwell around TDC. This will require a small enough port to keep an accelerated air velocity for good cylinder filling, but with a large enough port to make enough higher-rpm power."
While it seems like this situation would create a difficult balancing act, the port size was largely determined by the intake manifold of choice, Edelbrock's Performer RPM Air-Gap. Henderson tells us, "My rod-to-stroke combo has a 1.6:1 ratio, which responds better to larger port volumes, with larger cfm carbs relative to the displacement. My ports are as big as I can practically make them with the intake."
The Build
Starting with the production block, Henderson began with modifications to ensure the block's integrity at the expected high power level. The block was prepped with a partial fill using Moroso block filler in the water jacket, a move that increases the cylinder rigidity and also helps with piston ring seal. Eagle billet four-bolt main caps clamped with ARP studs add even more
security to the lower structure. Prepped like this, the factory block can be very reliable at high power levels.
The crankcase was relieved...
The crankcase was relieved for reduced windage and improved drain back. The grinding and polishing here is tedious and time consuming, but it is just one of the small details that add up to less parasitic loss-and free horsepower.
You will not find the factory crankshaft residing within the reinforced block structure, but rather a Scat Aero-wing crankshaft in the factory stroke. The crank is made with 350-sized main journals rather than the larger 400 mains, with COMP bearing spacers taking up the slack. This move reduces bearing speed and drag, and Henderson is always looking for ways to reduce parasitic drag in the engine assembly. Henderson favors this crankshaft for windage reduction, a theme he carried through to his block prep by relieving and polishing the inside of the crankcase for increased clearance to the crank as well as improved drain back.
To further reduce the possibility of unnecessary windage-related power losses, the lifter valley was fitted with standpipes, and the oil drain directed to the front and back of the engine. Henderson says, "Up front with the Jesel beltdrive, there isn't anything for the oil to get caught up in; it's all smooth up there." Completing the rotating assembly, Henderson filled the bores with off-the-shelf Mahle domed pistons with a compression height of 1.125 inches, giving a compression ratio of 11.4:1.
Taking the windage reduction...
Taking the windage reduction further while substantially increasing the strength compared to the cast-iron stock crank is the Scat Aero-wing crankshaft. The profiling on the counterweights allows the crank to cut though any oil mist or froth with minimal drag.
A large part of Henderson's...
A large part of Henderson's build strategy centered upon the conversion to long 6.00-inch rods. Henderson says that the piston dwell at TDC helps optimize the combination. The Eagle H-beam rods feature a standard .927-inch Chevy pin and are configured for 2.100-inch crank journals. All of the bearings used in the engine featured antifriction coatings, while the bolts are ARP.
Note the short 1.125-inch...
Note the short 1.125-inch compression height of the piston when the longer connecting rod is used. These Mahle pistons feature a short dome to deliver a compression ratio of 11.4:1. The ring package is comprised of Total Seal's gapless 1.5mm top, Napier-faced 1.5mm second ring, and a low-tension 3mm oil ring.
Up at the lifter valley, the...
Up at the lifter valley, the astute observer will note that the return of oil from the top end of the engine received plenty of attention. The standpipes prevent oil from splashing on the rotating assembly, and the lifter valley area has been ground smooth with pathways for oil return to the front and back of the block.
The cylinder heads and the...
The cylinder heads and the associated valvetrain components are a key to performance and reliability. The heads are CNC-ported Pro Comp castings that were hand-finished. The valves are hollow-stem Ferrea pieces in 2.125/1.625-inch sizes. Henderson extols the virtue of the tulip exhaust valve form. Note the fire slot in the quench area of the head. Henderson claims it helps reduce detonation and increases turbulence.
Looking down the intake ports,...
Looking down the intake ports, the handwork done to the passages is evident. The final intake port volume ended up at 217 cc, but Henderson tells us the 407 could have handled even bigger ports.
A stiff valvetrain with minimal...
A stiff valvetrain with minimal flex helps reduce the possibility of valvetrain deflection. Henderson selected COMP Cams 3/8-inch hardened pushrods to work in conjunction with COMP Hi-Tech 1.7:1-ratio stainless steel rocker arms. The springs are from K-Motion, while the lifters are COMP's standard hydraulic rollers.
In keeping with the idea of...
In keeping with the idea of creating a stiff valvetrain while looking to achieve this goal at a moderate cost, the studs were upgraded to 7/16-inch ARP units, with a COMP stud girdle. This arrangement is very appropriate for a hydraulic roller valvetrain at a fraction of the cost of a high-end shaft-mounted rocker system.
A Jesel beltdrive system turns...
A Jesel beltdrive system turns a COMP dual-pattern custom cam, ground with 246/258 degrees duration at .050. The cam features a wide lobe separation angle of 113.5, looking to make torque over a broad rpm range. With the beltdrive, it was a simple matter to vary the installed centerline adjustment until the best power curve was discovered.