The 20-Valve 4AGE replaced the 4AGZE Supercharged engines. Just like the early and late 16 Valve 4Ges, they also underwent revisions. The Silver Top was introduced in the Corolla AE101 (1992), and the Black Top in the Corolla AE111 (1997). Both engines used Toyota�s version of Variable cam timing- VVT.
Bore and stroke remained as in the original 1st and 2nd Generation 4AGEs (early) big port (TVIS) and (late) small port (non-TVIS). Similarly compression was upped in the later models. The blocks are identical to the late model AE92 generation blocks (and naturally the SC blocks too). MODIFYING THE 4AG / 4AGZE Block: The 4AG block is a free revving and strong bottom end. A stock block will survive in excess of 350 HP if air-charged in the 8000 RPM range, or 250 HP in the 10000RPM range.
The 2nd generation blocks with oil squirters should be the minimum considered. The connecting rods are also beefier, similar to the 4AGZ. The 1st generation block had smaller wrist pins and rod journals, which should be fine for milder tunes. For serious competition the rods should be replaced. Stroking the crankshaft to a maximum 81mm will keep the RPM capability intact. Bores can be brought out to 83mm, but try to keep it under 82.5mm.
There is no need for the Formula Atlantic crankshaft. Reject blocks that will need 83mm to clean up, for sure there is head damage also. Stay away from the 7AF to 7AG conversion since the crank is not forged and the flywheel bolts are too small, with no provision available to upgrade. The rods are potential failures also when subjected to real 4AG RPM operation. It is an exercise in creativity but a total sidestep for any real performance-oriented application.
If you like the torque of the 7AF, use the complete engine. Cylinder Head: There are some issues about the Formula Atlantic head. The Formula Atlantic head is a modified 4AG head- port and polished with the valve adjusters changed to the under-the-bucket shim arrangement (similar to the 2TG and 18RG). The valves and springs are different too. This modification is required when running camshafts with high lift and/or long duration (above 300).
For street purposes there is no advantage converting to this adjustment method or preparing the head to Atlantic specifications. The port and chamber modifications will decrease performance- short of preparing the complete engine to Formula Atlantic specifications, which will render the package totally unstreetable. There is very minimal head preparation needed, aside from match porting and polishing the runners. Porting the heads on this 16 valve Twin Cam more often decreases the air velocity- resulting in poorer throttle response.
Despite others claim of a 20HP or more increase- on modified heads alone, this is simply impossible without camshaft or pistons changed. Toyota has casted and machined all their 4 valve heads to very close and accurate tolerances for street (and rally) use. Camshaft choices for stock EFI systems and pistons should be limited to less than 265 duration. Upgraded blocks with pistons of at least 10.5:1 compression may use up to 288. It is recommended to convert to under-the-bucket shims for durations over 300 (with corresponding higher compression pistons).
Installing high duration cams on the stock set-up will cause the shims to fall-off and instant result in instant engine seizure. As always stay away from reground cams with non-factory lobe centers, you will have problems getting the correct adjustment shims. On the 4AGZ intake manifold- the #1 runner, which is dog-legged, must be filled and recontoured. This is a simple procedure that must be done. Be sure to frequently check the supercharger oil also. Many units have died because of oil starvation. The 5 valve head is Toyota's bonus to the 4AG after the 4AGZE went away. With the additional valve and independent throttle bodies- the 4AG 20 valve is a unique head design. This head unfortunately has many proprietary pieces and the only way to do a conversion to a 5 valve is only with a complete engine. The distributor position may be a problem in RWD installations. (Note: There are 2 versions of the 5-valve engine, see related information under Japanese engines).
Supercharger / Turbo Upgrade- The preparation for the turbo set-up is to lower the compression. The 4AG / 4AGZ that will see boosts in excess of 10 psi should be set-up with forged pistons. Blocks with the oil squirters must be used as these come with the better rods. For boosts higher than 18 psi, the rods have to be replaced. Oil cooler is mandatory, and for all applications a bigger radiator is a good investment. Use the 4AGZ injectors and fuel pressure regulator for turbo conversions of the normally aspirated 4AG. 550cc injectors are available for serious boost / engine outputs. The fuel pump can be replaced with the Supra Turbo pump, a direct bolt-in. Fuel System- If at all possible avoid using the dual side-draft carbs. The cost for the carbs / manifold plus ignition conversion will come close to a new fuel injection system. The EFI system will be adequate for minor tuning. 4AG/Z EFI systems use either an AFM (air flow meter) or MAP (manifold air pressure).
The AFM model is actually preferred since it will allow the user some adjustability for modified cams; there is no compensation available with MAP sensors. There is no advantage to converting to a MAP sensor, the idea of loss HP due to the AFM restriction is not true. Ignition System: Even 4AGZs equipped with DFI (direct fire ignition), will see a performance increase with an ignition amplifier. This is one of very few Toyota ignitions that are marginal. Colder plugs must be used with any upgrade. For carburetor equipped 4AGs the distributor has to supplemented with a timing controller, MSD 6A, and a coil.