In our last SR installment of the KA vs. SR battle series (Issue #90, June 2010) we sent a brand new SR cylinder head core to TOMEI POWERED (TP) in Japan. There, TOMEI POWERED’s cylinder head specialist Mr. Tahara put the head through the TOMEI POWERED Genesis Phase One and Two cylinder head program. Phase One involved replacement of the valve seats, upgrading to Phosphorus Bronze valve guides, +1mm oversized Supertech valves, Type A valve springs and titanium retainers. Phase Two added solid lifters (to replace the hydraulic units), shims, 270-degree PROCAMS and rocker arm stoppers. Solid lifters are not adjustable, requiring Tahara-san to adjust shims instead to achieve proper valve clearance. However, converting to solid lifters reduces the chance of a rocker arm being thrown from its position between the valves and the cam lobes, allowing for a higher redline. We added TP rocker arm stoppers as a further measure of insurance. For our SR, we asked Mr. Tahara to reshape and cc the combustion chambers in anticipation of a 90mm bore, which is 4mm greater than the factory SR20 bore size.
Go With The Flow
Once the SR head returned to America, it paid a visit to Tom Fujita at Port Flow Design to be flow tested on the Super Flow SF-600 flow bench that measured our KA24 cylinder head (see Issue #91, July 2010). Employing the same testing method as had been performed on the KA24 head, Tom measured the flow of a stock SR20 head as a baseline to compare against the TOMEI POWERED Genesis cylinder head. On the intake side, the best improvements were noted at only 0.05-inch lift, where a 26-percent improvement in flow was measured. At the maximum lift tested (0.45-inch), the gain was 8-percent. On the exhaust side, the best gain was noted at 0.40-inch lift where a 15-percent improvement was realized. At max lift, gains were on the order of 14.8-percent. These results were interesting, as TOMEI POWERED did not modify the ports dramatically. The transition from the throat to the bowl was smoothed after seating the +1mm valves, but the ports remained largely untouched. When we inquired with Allen Lorenzo of TOMEI POWERED, he commented, “We smoothed the ports and transitions between the throat and the bowl for the larger valves, but we prefer to leave as much cylinder head material as possible for the consumer, should further customization of the head become necessary.” When we compare the percentages against the KA, it would appear the SR head realized greater improvements. However, when comparing the Exhaust-to-Intake Ratios (E/I ratio - see chart), a different story is told. The SR head falls below the desired 80-percent range from 0.25 inch through maximum lift. This lower E/I ratio indicates room for improvement but also means the turbocharger may spool up more quickly, eliminating the KA24’s 97cc displacement advantage.
After flow testing, the SR20 head went to JE Pistons to be digitized. JE Pistons’ engineer Dane Kalinowski made a computer model of the SR combustion chamber using JE’s Zeiss Prismo 3D digital scanner. The scanner features an articulating probe that measures every aspect of the combustion chamber. The data can then be extrapolated into a three-dimensional model of the chamber. This process ensures precise compression ratios and a properly designed piston crown. Once the combustion chambers were digitally mapped, the data was used to create a CAD model of the custom piston. The CAD model was then uploaded to a CNC machine where forged aluminum billets were machined into custom 89.5mm pistons sporting a 9.5:1 compression ratio. These custom slugs feature the same optional thermal barrier crown and anti-friction skirt coatings as our KA24’s pistons.
The SR20 Goes To Idaho – Sleeves Not Spuds
While JE Pistons handled our custom piston order, the SR20 shortblock took a trip to Dan Benson Racing in Laclede, Idaho. Dan Benson has been sleeving and machining engines since 1975 and he’s no stranger to building high-power import engines. Dan resleeved the SR20 block with Benson’s Signature Sleeves and then bored the cylinders for the new JE Pistons. Since we also planned to stroke the engine, Dan line honed the main journals and clearanced the block and girdle to accommodate the 91mm TOMEI POWERED stroker crankshaft. He then blueprinted and assembled the bottom end with ACL Race Series main bearings and ARP main studs. There are different schools of thought when it comes to blueprinting an engine, and Dan presented his perspective on bearing clearances. “I like an engine that is tight. When I machine blocks and blueprint rotating assemblies, I aim for the tightest and most precise tolerances. The end result is a high-performing, free-spinning engine.” Blueprinting and machine work go hand-in-hand as much of the work depends on the application, whether it’s for drag, street or endurance and whether it is naturally aspirated or forced induction.” ACL main bearings feature three metal layers and a hardened-steel backing. ACL states that this proprietary alloy ensures the greatest relative load carrying capacity and high-RPM performance demanded of a high-performance engine. ACL bearings feature a “one-size fits all” tolerance, with an optional 0.001-inch “looser” bearing option. When Dan torqued down the ARP main studs, he used ARP’s Ultra-Torque fastener assembly lube. Unlike motor oil and other thread lubes, Ultra-Torque enables the engine builder to duplicate the torque applied to each stud with exacting precision for equal preloading. The even distribution of clamping force takes best advantage of the clamping force the ARP studs provide. The custom JE Pistons make the connection to the stroker crankshaft by way of TOMEI POWERED connecting rods and rod bearings.