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The 4AG engines were introduced in 1984 as the replacement to the 2TG. Although a lot of knowledgeable Toyota enthusiasts welcomed the 16 valve head, many kept back their excitement because the engine looked fragile. Internal pieces did not follow the usual Toyota robust design. In less time than the engines needed rebuilding, modified units were performing and surviving in competition. Toyota has rewarded it's Twin Cam diehards- with a new engine!
The secret in the 4AG's compact and lightweight design was better metallurgy. This is how Toyota was able to guarantee 4AG durability equal to its performance potential. The iron block was preferred over all-aluminum units, with a minimal weight disadvantage. The 4AG can be modified without the disastrous results of all-aluminum blocks. For it's physical dimensions Toyota was wise to keep the iron block.
As with any Toyota engine design- the 4AG evolved. The first generation and second generation can be differentiated by their large or small ports, and third generation with 5 valves! The later versions had piston oil coolers, and higher compressions. The 4AG was very popular and during it's introduction and out ran any 4 cylinders 1600 with 16 valves.
Toyota Motorsports U.S.A. initiated a development program for these engines, and have been rewarded with a successful Formula Atlantic series. These heavily modified engines maintained all the 4AG dimensions, with modifications to the original block and cylinder heads. As a full racing engine the 4AG has proven itself.
The 4AG originally powered 3 distinct drive arrangements- RWD (Corolla GTS), Mid-Ship (MR2), and FWD (FX16). There is 2 possible 4WD conversion available, but must only be used with stock HP output. The later model Toyotas all went FWD, by which time the 4AG was in its 2nd generation (1990). The 4AG disappeared from the U.S. market by mid-91. By the 3rd generation, in Japan, the GTS 4A-GE and GTZ 4A-GZE Supercharged engines were replaced by the 4AG 5 valve (1992). The 5-valve has also a 2nd generation (1996). The 4AG series evolved from 115HP to 170HP.
The 4AG, because of its lightweight and compact design, is a very popular choice for diverse applications. Desert racing buggies (qualifying for the Toyota Contingency program), Lotus 7 replicas, and the older Toyota Corollas and Starlet all support the "modern" 4AG engines. Engine conversion kits are available. There is tremendous respect for these power units, specially for RWD applications.
In Japan there is resurgent interest based on a comic hero, whose 4AG Corolla has taken and beaten all comers. This fantasy based on reality. Japanese performance parts manufacturers, after discontinuing sports part production, have again scheduled 4AG parts for production. Just like the engine it replaced, the 2TG, the 4AG is the Toyota engine for a whole new generation.
The U.S. 4AG-ZE in the MR2 developed 140HP with the supercharger and with strictly bolt-on modifications produced 188HP. There was available a Twin Charger kit from HKS that developed 210HP and after further modifications produced in excess of 350HP! Also available was the HKS 5AG kit (not a Toyota designation), which included a stroked crank and over-size pistons for 1720cc.
We have rallied the 4AG Corolla AE86 in many international FIA events. The cars are very competitive in Group N. One of the first Corollas we prepared was for SCCA GT 2, which ended up in South America. We have exported prepared 4AG engines to 5 countries! MR2s in SCCA IT (Improved Touring) are disadvantaged in the class, but with a little tuning (late model blocks) become really competitive. We have set-up both cars for SCCA Solo ! and II.
The fastest set-up is the Twin Charged MR2, and is completely streetable. Unfortunately the kit is no longer available. Turbo engines can be set-up for ultimately more HP, but ac lot less tractable. The turbo engines, with forged or supercharger pistons, easily outrun the 4AGZ, with the same psi output. The 4AGZE engined MR2 and RWD/FWD Corolla conversions with the HKS (10 psi) big pulley puts out at least 175 to 200. There are bigger pulley kits, example: Cusco (14 psi), that will definitely need additional EFI programming. Mild cams and injectors can be combined for phenomenal cc. to HP output ratios.
THE 4A-GE 20 VALVE
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).
The 20-Valve engines never came officially into the US (and many other foreign markets where the old 4AGEs were available- and of course it imparts a very exotic presence. The individual throttle bodies look like they came off the fabulous 503 race engines. Think about it, no Toyota factory RACE Engine had 5 valves per cylinder!! Yamaha- who works with Toyota on cylinder head design (and manufacturing), have been a very avid pioneer and supporter of the 5-valve design. The short-lived Yamaha Formula 1 engines had 5 valves per cylinder. In terms of maximum valve to surface area- the 5 valve design cannot be beat- theoretically and geometrically.
Aside from the obvious 5-valve and variable timing cylinder head design- the difference from the 4-valve is the dramatically reduced weight of the internal components. Pistons, connecting rods, and to a lesser degree, crankshafts have all been updated with lighter weight design. The good news is that it remained an iron block- the engines were still on solid foundation for heavy and serious modifications.
Between the Silver and Black top- the later Black models came with still lighter rotating components. Laying out the engine components side by side?there are miniscule differences, Toyota was tuning the engines to a higher degree by rotational mass reduction, a lesson advanced from the earlier lightweight block design (of the pre-multi-valve engines). The difference in the connecting rods between 16 and 20 valve engines is dramatically surprising. However for purposes of radical modifications a Silver top will probably be a better engine to work with (slightly stouter rod).
In Japan these engines were used in Formula Toyota, a series much like Formula Atlantic where there is only one SPEC engine available to all competitors. The magic of 20-valve is apparent on the ability to attain and sustain a high RPM threshold. The addition of variable cam timing allows the engine to surpass both low-end torque and high RPM horsepower figures of the older 4AGEs. This is the closest to race spec engine Toyota has ever produced.
In terms of Toyota technology, the twin versions of the 20-valve engine- out specified any previous offering in naturally aspirated form. This is the last true iron block with the ultimate head design and intake set-up. The newer 1ZZFE and 2ZZGE come with a more advance VVTi head set-up- but with 1 less valve (per cylinder) and aluminum blocks. The 4AG 20-Valve wins!! What will Toyota use for the next Formula Atlantic engine??
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.
Some Recommended Set-ups:
For Stock configuration- Overhauling the 4AG is straightforward. Machine Shop experience in rebuilding the head is a must. Bores should be kept at the maximum 83mm. If available newer rods with the bigger wrist pin must be used- with the later model high compression pistons. Clean up the ports and EGR passages at the same time. Verify crank pin sizes since there are differences.
For Modified configuration- Use the 2nd generation blocks. The smaller port heads (non-TVIS) heads have better air velocity, specially important when using carburetors. Depending on the EFI system used, choose the mildest cam available- you will have better results. Adjustable cam pulleys will ensure accurate timing and cylinder pressure tuning. Forged high compression pistons are reasonably priced and are a good upgrade. Modifying to a late model head will require fabrication of a new intake layout for RWD. The throttle body can also be enlarged to maximize air intake.
The 4AGZE conversion for normally aspirated models will need the correct power steering pump, air conditioning compressor, and alternator. This pieces are unique to this application- and the proper bracket is also a must. The top mounted intercooler will fit under the normally aspirated MR2 hood and will need a hood scoop on FWD Corollas (but a front mount intercooler is still the best set-up. The bigger (14 psi) crank pulley is a must.
There are as many tuning theories as there are tuning shops. The most important step in preparing an engine is to finalize the exact set-up before buying any part. Displacement increase that does not affect the bore / stroke ratio, to the detriment of RPM potential will give the engine the edge. The stock 16 valve head has undergone a lot of development time in terms of cam configuration- unlike a displacement increase (done properly), a wrong camshaft choice will result in a completely unsatisfactory set-up.
For Racing configuration- The preparation of any engine in sanctioned Motorsport events are tightly regulated. Know the rules before starting. FIA Group A or N parts homologated for use in these engines are available. Any serious preparation will start by converting the valve adjustment to the racing type (TRD) under-the bucket shims. This will allow the use of any camshaft combination. Formula Atlantic spec con-rods may be used with the correct piston (19mm). For supercharged or turbo applications stay with the 20mm pins. Lightened flywheels are available, to ensure the fastest throttle response. Cylinder port and chamber configuration approaching Formula Atlantic specs can be used- but never the full Atlantic spec if the engine is not going into a formula chassis or dune buggy. (The 5-valve engine, used in Japan's Formula Toyota, is also supported with tuning parts.)
