Mk III-T Engine

Basic Engine Assembly

The engine assembly, ready for trial fitting.

The powerplant is based on a Honda VF700FII “Interceptor” motorcycle engine, 699cc V-4, with dual, gear-drive overhead cams and 4 valves per cylinder. It was chosen for its narrowness, very short and stiff crankshaft and robust bottom end. It would require re-sleeving to 588cc to meet SCCA requirements for forced induction engines in D Sports Racing. This would be later changed to 620cc, necessitating re-sleeving the engine and re-ordering pistons before the engine ever ran.

Piston and Con Rod

Custom pistons were designed by Arias from combustion chamber molds. Mechanical compression ratio is 8.75:1, to retain some engine flexibility off-boost. The piston skirts are treated with Heanium low-friction coating, and the crowns are ceramic coated to retain heat in the combustion chamber and reduce risk of detonation damage under high boost conditions.

Connecting rods are by Carrillo, using high-strength ARP rod bolts.

Crankshaft and engine cases

The crankshaft was prepared to RC30 specs by Falicon; straightened, balanced, micropolished for its intended high-stress use. Note the short length of the crankshaft and narrow spacing between main bearings, giving a very strong bottom end to withstand the rpm and power output to be expected. Cylinder heads were lightly ported and given a racing valve job, with wider seats for reliability by V-Tech. Stock Interceptor cams were retained, since they provided nearly ideal timing for turbo use, having relatively short overlap and late exhaust cam timing to retain boost pressure for optimum power.

Exhaust headers and Turbocharger

Headers are fabricated from thick wall mild steel exhaust tubing, gas welded and joined at a collector at the turbo flange. The turbo is an IHI RHB52, exhaust turbine from a Subaru EA-81 and large custom compressor. The system was prepared by Turbo and Induction Systems. System targets were: operation up to 30# boost, 14,000 rpm. Computer models predict 190-195hp at peak boost.

Fuel Injection Throttle Body and Inlet Plenum

The throttle body is from a Subaru EA-82, appropriately sealed for the purpose. The inlet plenum and injector ports were fabricated from mild steel sheet and tubing. Injectors are Lucas, race prepped by RC. The Lucas injectors were chosen for their disc valve, which has much lower mass than the typical pintle valve, therefore better metering at low boost and low RPM, where the duty cycle of the large injectors would be so short as to be difficult to meter.

Showing fabrication of the plenum chamber and inlet horns

The intake trumpets were swaged, and installed ½ diameter above the floor of the plenum to avoid flow stagnation between adjacent trumpets. Injector bosses were cut from bar stock, bored, stepped, and angle milled, then TIG welded to the trumpet bodies.

Engine Being Test Installed

Preliminary installation of the complete engine, showing placement of the turbo, including installation of oil return line to engine case. Some features of the engine mounting system can be seen, showing the triangulated tubing structures at the front which feed loads into the main cage. There are also lateral mounts, seen just above the clutch housing of the engine, as well as rigid bolt attachments at the rear bulkhead of the chassis. Thus the engine serves as a critical stressed member, providing both beam strength and torsional strength to the rear of the chassis, conducting suspension loads into the central cage structure of the car.

Engine and Final Drive/Rear Brake

Left side of engine in chasses, showing shift linkage, final drive spool carrier and rear brake. Drive chain tensioner is milled from billet and TIG-welded, using a threaded abutment bolt for adjustment and thru-bolt to lock adjustment in place. Chain is D.I.D. #530 o-ring chain, with fabricated aluminum chain guard.