Technosquare's Monster Naturally Aspirated Toyota 4AG part 2
The 83mm x 83 mm stroker motor gives 1800cc. The 200 extra cc's makes a huge difference, fortifying the 4AG's notoriously narrow power band. The engine cranks out over 250 crank hp, more than the most highly developed Formula Atlantic engines of yore and 30 more hp than the previous 1600 cc version of this motor. On a Dynopack chassis dyno the 4AG ripped out 216 whp, a hell of a lot for a naturally aspirated 4AG. You have to remember that a RWD set up typically robs 20% from the crank power levels vs the 15% more efficient FWD transaxles typically steal.
Wiseco forged pistons are used. To maintain the best possible stroke to rod length ratio, the pin is moved as high in the rod as possible. It even intrudes into the oil ring groove slightly. Strut type construction is used for light weight. The skirts are moly coated for less friction and better wear of the skirts and cylinder walls. The dome volume is set to give a 13:1 compression ratio. Technosquare deburrs and cleans the dome of all sharp edges which could create a hot spot causing detonation. The piston feature double pin oilers and use a tapered wall lightweight 20mm tool steel piston pin for minimal weight. This is the post 1988 4AG pin diameter, up 2mm from the older, weaker, flexy 18mm pins. The pin is retained with Cosworth style round wire retainers.
This close up shot shows the radical pin placement, intruding into the oil ring groove. This sort of design usually has higher oil consumption but allows the longest possible connecting rod for an improved rod ratio and lower piston speed. The piston has anti detonation grooves between the top of the piston and the top ring. These grooves buffer the detonation shock waves and protect the top ring. The also reduce the contact area high on the piston for less friction. The grooves also reduce expansion at the top of the piston to reduce the likelihood of scuffing under extreme heat. A pressure equalization groove between the compression rings improve ring seal at high rpm by acting like a reservoir for leaking combustion gasses, improving the seal of the second ring.
The rings are fairly conventional. The upper compression ring is a narrow 1mm chrome face design. The second compression ring is a 1.2 mm iron ring. Both rings are low tension for reduced friction and less likelihood of flutter at high rpm. The oil ring is a conventional 2 rail design.
The stock 4AG valvetrain uses shim on bucket valve adjustment. This system has trouble at over 8000 rpm and 8.5mm of lift. The cam lobes tend to flip the valve shim off of the top of the cam follower. When this happens the cam lobe beats on the retainer and causes the keepers to fall out and the valve to drop, destroying the engine. To prevent this from happening the valve train is converted to TRD Formula Atlantic stuff. Double valve springs are used with titanium retainers that are designed to accept a small adjusting shim. The cam follower goes over the shim preventing it from getting flicked out by the cam lobe. You can see one of the small adjusting shims next to the cam followers. This is called a shim under bucket system and is used in motorcycle and other high RPM engines. TRD big valves are used, they are 2mm bigger than stock. TRD Formula Atlantic cams are also used, 320 degrees intake duration and 304 degrees exhaust duration. Both cams have 11mm of valve lift. The cams are set on a 102 degree lobe center.
The cylinder head is rough ported by Hasselgren and finished by Technosquare. A late post 1988 4AG head is used as it has a smaller but better flowing intake port. The intake and exhaust ports have smoothed short side radius, moved back port splitters and raised flattened roofs. The combustion chamber is unshrouded by the valves and the deck surface is fine milled and lapped on a surface plate for better sealing.
The combustion chamber valve unshrouding is shown here, you can see how the edge of the combustion chamber is moved back from the valve and valve seat. This really helps flow as the valve starts to open and close. A 3 angle valve job is used with the top 30 degree cut blended into the combustion chamber and the 70 degree throat cut blended into the seat and port wall. The 45 seat is kept to around 1 mm in width on the intake side and 1.2 mm on the exhaust side.
The intake port starts with the valve bowl. The valve seats are opened up for the bigger valves and opened though to the valve bowl. The valves are supported by bronze TRD valve guides. The entrance to the bowl is opened up and material is mostly removed at the port roof. The short side radius is rounded.
You can see the extent of roof flattening in the intake port here. This gives the best flow with the least volume for good port flow velocity. This gives the best throttle response and widest power band. The port splitter is moved back to give more flow area without increasing volume much. The EGR bosses in the roof of the intake ports are plugged.
The exhaust ports have the same sort of treatment as the intake ports. The valve seating surface width is slightly wider at 1.2 mm to improve valve cooling.
Like the intake the exhaust has more material removed at the roof and has the port splitter moved back. You can clearly see how the bowl area is opened up leading into the port.
The sides of the valve bucket hole have to be milled out to clear the high lift TRD cam lobes.
You can see how the cut outs work to clear the cam lobes here.
The assembled head with the TRD big valves.