this is a great read. getting to 275rwhp with our stock setup and lil bit ah money!!
3S-GTE Power Primer
Toyota designed the 3S-GTE for racing and they made it very capable. The second generation of the engine was very conservatively configured for the MR2 Turbo to provide high reliability and meet emission requirements. The stock US version of the 91 MR2 produces 200 HP at the flywheel. With a 20% drivetrain loss that works out to around 160 horsepower at the rear wheels (160rwhp). Since the 3S-GTE is substantially toned down from the factory, it requires little effort to get 50% to 70% more power out of the engine.
This primer provides a roadmap to those wishing to enhance the performance of their 3S-GTE. It explains what I currently believe to be the best way to gain quick, reliable power. My analysis and direct experience doing many dyno runs and configurations concludes that many cars have been modified in ways that are expensive, ineffective and unreliable. I have compiled a straightforward set of modifications that, if properly applied, will get you to 250-275rwhp. Properly applied means that when I say go to the dyno and tune you need to go to the dyno and tune. Sure it costs money and you don't get a part that you can point to and show off (you do at least get a chart that you can scan in and brag about) but it will yield good, reliable power benefits and tell you if something is keeping you from making the kind of power you should be. I will also discuss how to upgrade beyond 275rwhp, which requires a lot more money and effort.
While I mention specific products in certain cases, I do not believe in spending much time agonizing over which particular intake or intercooler or exhaust to buy as long as it meets the requirements specified. There is little extra power to be made by trying to find the "best" of any of these. In many cases, what is "best" at a particular stage becomes just adequate or even marginal or wrong at another, so my suggestion is to choose between products that meet the spec based on price, availability or personal preference and move on to the next stage.
The Basics of Power
Power in an internal combustion engine comes from mixing the right amount of fuel with the right amount of oxygen and inert gases in the cylinders and lighting it off at just the right moment. When the air-fuel mixture burns, it heats the gaseous byproducts of combustion and inert gases present in the cylinder. This heat raises the pressure with which the gases push against the piston and produces the mechanical energy that powers the wheels.
How can we produce more power? The hotter the gases get, the harder they push against the piston and the more power is produced. There are, however, very serious limits to how hot you can allow the gases to get before they damage the engine. Tuning the air-fuel mixture to keep combustion temperatures at the optimal range to produce the most power without destroying the engine is critical. It is also important to expel exhaust gases, because any exhaust that stays from one cycle to the next effectively lowers the temperature of the combustion chamber and reduces the power produced.
The more gases are compressed inside the cylinder before they are ignited, the more power will be produced but there is a serious limit here as well because compression heats the air fuel mixture quickly and cause detonation, which is the uncontrolled explosion of the air-fuel mixture. Detonation literally goes off like a bomb inside the cylinder and can blow spark plugs apart, punch holes in piston rings, shatter ring lands (the area on the side of the piston between the top and the first compression ring) and damage rods and bearings. Detonation is an engine killer and needs to be kept in check at all times.
The more air-fuel mixture that moves through the engine in a given time, the more power is produced. There are four ways to get more air-fuel mixture to move through the engine and every one of them has been used singly or in concert to increase power. The first way is to increase the volume that is displaced between the point where the piston is all the way down (Bottom Dead Center or BDC) and the point that it is all the way up (Top Dead Center, or TDC). The sum of the volumes of every cylinder is the displacement of the engine. The stock 3S-GTE displaces 1998cc (cubic centimeters) or 121.9 cubic inches. Divide by 1728 to convert cubic inches to cubic feet. There are three ways to increase displacement. You can increase the bore of each cylinder, which is the diameter of the cylinder. You can increase the stroke of the piston, which is the distance that the piston travels from TDC to BDC. Finally, you can add more cylinders which, as you might expect, is not easy to do without moving to a completely different engine.
The second way to move more fuel-air mixture through the engine is increasing the speed at which the engine rotates. The 3S-GTE is a four stroke engine. This means that its valves open to let air into a cylinder once per ever two rotations of the crankshaft. Given that, the amount of air in cubic feet per minute (CFM) that should move through the 3S-GTE is:
CFM = (121.9 / 1728) * (RPM / 2)
For various reasons, an engine never moves exactly this much air through itself. A fancy term called Volumetric Efficiency (VE) is introduced to modify the equation to reflect how much air is actually moving through the engine:
CFM = (121.9 / 1728) * RPM / 2 * VE
One very important thing to keep in mind is that VE is not constant, but changes with RPM and engine load. Most of the time, it is approximated with a value like 0.9 or 0.95 which is the same as saying that the engine is moving 90% to 95% of the potential amount of air that it is theoretically capable of moving. This brings us to the third way to move more air-fuel mixture through the engine: improve its VE.
The fourth way to increase the the amount of air-fuel mixture that moves through the engine is to increase the density of the air-fuel mixture. There are two ways to do this. The first is to lower the temperature of the air-fuel mixture. The lower the temperature of a gas, the denser it is. The second way to raise the density of the air-fuel mixture is to increase its pressure. Normal atmospheric pressure at sea level is approximately 14.7 pounds of force per square inch (psi). Superchargers and turbochargers are the most common devices used to increase the density of the air-fuel mixture. These devices can raise the pressure of the air-fuel mixture above ambient atmospheric pressure. The difference between the pressure they produce and nominal atmospheric pressure is called boost pressure.
Applying the Basics to the 3S-GTE
The first and most important thing to do before embarking on any power modifications to the 3S-GTE is to get the basic engine in shape to handle them. Perform any basic maintenance that may have been neglected by either you or the previous owner. An oil change, new oil filter, spark plugs, distributor cap, rotor, spark plug wires and a flush of the cooling system should be the first priority of the day before proceeding. Also, keep in mind that a modified engine is less tolerant of poor maintenance and the price that you pay for increased performance is prompt and diligent maintenance.
Strength, a moderate compression ratio and an intercooler allow the stock 3S-GTE to easily handle higher that stock levels of boost pressure. Maximum stock boost pressure is 10-11psi. With the addition of three simple, inexpensive devices, this can be safely increased to 15psi for a gain of nearly 10rwhp per additional pound of boost. After doing this, you can expect to be producing approximately 200rwhp. First, you need an accurate boost gauge to be sure that you are not boosting above 15psi. The stock boost gauge is slow to respond, peaks below 15psi and has no numbers to tell you how high you are boosting. Second, you need to raise the point at which the protective fuel cut feature kicks in from around 12psi to 16 or 17psi. This can be done easily and inexpensively as shown here. Finally, you need a boost controller to raise the actual factory set boost pressure to 15psi. Manual boost controllers are inexpensive and do the job while electronic boost controllers are more expensive but allow easy adjustment from the cabin. If you get a manual boost controller, go with a ball and spring type instead of a bleeder type. Ball and spring controllers keep the wastegate shut until you approach the desired boost limit while bleeder types let the wastegate partly open at lower boost and result in a slower transition to full boost. Several boost controllers are available with CARB exemption stickers, so you can take advantage of this performance enhancement in all 50 states.
To help keep the cylinders cooler under high boost and insure that the stock ignition keeps the spark going strong, you should replace the stock plugs with a set of NGK 6097 copper spark plugs. These are one step colder than the stock plugs to help keep detonation in check. You should gap them at 0.028", which is a little less than stock to make it a little bit easier on the ignition system to keep the spark going strong and reliably even under higher boost pressure. These will need to be replaced every 5K miles or so. Note that you do not need to make any other changes to the stock ignition system until you reach a higher power point. It is a myth that you can make more power with aftermarket ignition components on the 3S-GTE. All that the ignition can do is lose power if it is weak or not properly functioning. The stock ignition and components, except for the spark plugs is adequate to 17-18psi in good working condition.
Because the engine needs air to produce power, a small but increasingly important amount of power will be gained by replacing the restrictive stock air intake with an aftermarket high flow air intake. At 15psi boost, you can expect to gain another 2-3rwhp from a relatively inexpensive air intake. The air intake is not as critical on a turbocharged engine as on a normally aspirated engine because the compressor can compensate for a restrictive intake better than a normally aspirated engine can. After upgrading the air intake, you should be making nearly 205rwhp. Many aftermarket air intakes are CARB exempt.
Much more important to the stock 3S-GTE than a less restrictive air intake is a less restrictive exhaust. The most restrictive portion of the stock exhaust is the b-pipe between the downpipe and muffler. This pipe contains a small catalytic converter so it is not legal to upgrade it in some states even though the primary catalytic converter in the downpipe cleans the exhaust enough to meet emission requirements. There are many aftermarket exhausts that replace the stock muffler and b-pipe. Choose one constructed with 2.5" or 2.75" mandrel-bent pipes. Any of these will yield an extra 10rwhp. If you are fortunate to live in a location without emission restrictions or to afford an additional exhaust system to put on just for off-road use, replacing the stock downpipe with a 2.5" downpipe will gain you another 5rwhp. At this point, you should be able to net around 220rwhp at the dyno.
To set a good foundation of reliability for the next stages, it is prudent to make the next modification to your 3S-GTE one that prevents detonation. A good aftermarket side-mounted intercooler will add another 5rwhp to your setup. The larger intercooler will make power in two ways. First, it will cool the air charge more than the stock unit. Second, there is less of a pressure drop across the larger intercooler core than there is across the stock unit, so the turbo has less work to do to reach the desired boost pressure. This means that cooler air will reach the manifold and produce more power at the wheels. Now that you have more cooling on your side, add another pound of boost to take you up to 16psi and get you another 5rwhp. If you have made all the modifications mentioned so far, you should be making around 230rwhp.
The intake manifold and TVIS were designed to provide good low end torque and high top end flow using a dual runner per port setup with a set of butterfly valves kept closed at low RPMs to increase air velocity into the port and open at higher RPMs to increase flow capacity. The proper TVIS opening point lowers as VE improves (better exhaust, turbo, valves or cams). The ECU keeps a fixed opening point around 4200RPMs which is too high for modified engines. Consequently, there will be a torque dip from around 3800 RPMs to 4200 RPMs. The ECU also uses the TVIS to reduce engine VE if it has detected knock in the recent past. This can cause variations in performance for no apparent reason. The TVIS be kept functional as long as you continue to use the stock intake manifold but you should control the TVIS with an MSD RPM switch. You will need to purchase an MSD 8950 along with an appropriate RPM module, which is going to be included in wither the MSD 8743 (for even 100 RPMs) or MSD 87431 (for odd 100 RPMs) opening points. Alternatively, you can make your own fully adjustable RPM module. Most modified 3S-GTEs will need to open the TVIS between 3600 and 3900 RPMs. The only way to know for sure is to do a dyno pull to 4500RPMs with the TVIS open and another one with the TVIS closed keeping everything else the same. Then, the point at which the two torque curves cross is your best opening point.
The best place to install the MSD 8950 switch is in the engine bay near the igniter/coil. Cut the red and blue loops on the switch since you are going to run on a 4-cylinder engine. Ground the black wire to any bolt that goes into the chassis. With the ignition key off, pull the 2 pin connector from the coil and splice the red wire of the switch to the black wire with a red stripe. Splice the white wire of the switch to the black wire with a white stripe. I recommend using good solder or crimp connections to do this. Plug the connector back into the coil. Now go under the intake manifold and locate the TVIS valve. This valve has one vacuum hose going to the TVIS actuator, another vacuum hose going to a blue/purple canister mounted under the intake manifold and a 2-pin electrical connector. Disconnect the connector and cut the green wire with a black stripe about two inches from the connector. Cover the end of this wire from the engine harness with electrical tape. Connect the other end of this wire going to the connector to the gray wire from the MSD 8950. Use a pair of spade connectors so that you can easily set the car back to stock or control the TVIS by hand to help you find the switch point at the dyno. Reconnect the connector
For the interim, install a 3800 or 3700 RPM module in the switch. Now you just need to find the best switch point at the dyno as explained above. To keep the TVIS open, you can disconnect the gray wire from the TVIS control valve and leave it hanging in air. To close the TVIS, ground the wire going to the TVIS valve. If you notice no difference, your TVIS system is not working properly and you must follow the directions in the BGB on how to diagnose it.
The next modification is a tricky one that requires the use of a wideband O2 (oxygen) sensor to do properly. Most performance shops that rent dyno time will hook one of these up to your car while you are using the dyno (and if they don't, go elsewhere). The stock ECU (Electronic Control Unit--the computer that controls the engine) is programmed to dump a lot of extra fuel after the boost pressure reaches 12psi to keep the engine safe from detonation. This extra fuel, however, reduces power by lowering combustion temperatures that we can reclaim by "leaning" the air-fuel mixture so that there is a little less fuel and a little more air than the ECU is programmed to deliver. The "best" air-fuel ratio (AFR) to run the 3S-GTE at on pump gas is around 11.5:1. The least expensive way to lean the AFR safely is to get an adjustable fuel pressure regulator, an inexpensive fuel pressure gauge and a fuel computer such as the S-AFC. Some performance shops will tell you that you only need an S-AFC, but the safest combination is to use all three. Install both the fuel pressure regulator and the S-AFC. Set the S-AFC to zero adjustment across the board and adjust the base fuel pressure to 35psi (base fuel pressure is the pressure when the fuel pump is running but the engine is not, insert a paper clip in your diagnostic connector between +B and FP to produce this condition). Take the car to a dyno and hook a wideband O2 meter to it. Readjust the base fuel pressure to 30psi and add about 10% fuel at every point in the RPM range. Do a pull on the dyno and print out the AFR chart. It should start at around 14.5:1 and quickly drop to near 10.0:1 or even lower. Use the S-AFC to take out a little fuel in the ranges above 3500 RPMs where the AFR is under 11.5:1 but don't go under the stock (0%) level. When you (or the tuner) is finished, you should be at or close to 11.5:1 AFR under high boost and you should have picked up another 10-15rwhp to get you in the 240-245rwhp range. This is around 300 horsepower at the flywheel, which is 50% more power than you get out of the stock 3S-GTE. Not bad for a $1000-$2000 dollar total investment so far.
This is about the most that you will be able to get out of the 3S-GTE with the stock CT-26 turbo. You now have a big decision to make: do you stay where you are at and call it good enough or do you want just a little bit more or do you want to go for truly serious (and expensive) power?
If you want just a little bit more power, you need to upgrade the CT-26, purchase a CT-20b which will bolt right on to the stock exhaust manifold or, if you are a little more adventurous and live in a state with less restrictive emission laws, invest in a TD05, TD06, GT25R, GT28R or T3/T4 turbo kit. If you upgrade the CT-26, get a CT-27 upgrade from ATS Racing. This is your best choice because you are now at the point where the biggest thing preventing your engine from producing more power is the restriction that the turbine wheel and turbine housing are placing on the exhaust. If you have been paying close attention to your boost gauge, you probably noticed that the CT-26 is unable to keep producing 16psi of boost beyond 5.5K RPMs. The CT-27 upgrade machines the turbine housing to allow more exhaust to escape the engine. This will allow you to continue producing 16psi of boost until 6.5K RPMs and add another 15-35rwhp to the car. This will take you up into the 255-275rwhp range. As you approach this range you will have to re-do the AFR tuning and you will reach the point where you will actually need to raise the fuel pressure back up to the 38-40psi stock levels to keep a safe AFR. At these levels the S-AFC will be zeroed out again and can even be removed.
If you go with a CT-20b, you can expect to pick up another 15-35rwhp after you re-do your AFR tuning. Depending on the state of your fuel system, you may or may not be able to make over 275rwhp with the stock genII fuel system. If you can, then for less than $3000 dollars (if you have done your own labor), you should be at or near 275rwhp. You are now sitting at or close to the limits of the fuel system and you run a serious chance of damaging your engine if you exceed those limits. You now are or have already surpassed the limits of the stock clutch and need to be thinking about replacing it with an aftermarket clutch that can hold more torque. ACT, Clutch Masters and SPEC all make good aftermarket street clutch kits for the 3S-GTE. While replacing the clutch, you can also install a lighter flywheel. The flywheel will not produce more power, but it does lower the rotational mass that the engine has to spin up and thus transfers power to the wheels faster. It will improve drivability once you master the easier-to-bog nature of the lighter flywheel.
A Few Myths
Let me say a few things about what not to spend your money on unless you want to do it just to show off.
An aftermarket Blow-off valve is not needed and will gain you nothing except maybe a loud sound and rough running or stalling after it goes off. The stock bypass valve is strong, fast and quiet.
Fancy wires and ignition boxes don't do much for you. Also, these devices never produce power (except through ignition timing tuning, which is not essential to getting to 275rwhp).
Getting larger fuel injectors or a larger fuel pump won't do you any good until you have the proper electronics to control them properly. An S-AFC is not the proper electronics. Since the stock fuel system can be pushed to 275rwhp, you definitely don't need or want bigger injectors or pumps before then.
You do not need to get rid of the AFM, the TVIS, the EGR or upgrade your stock cams, or port your head, or get a larger throttle body, or an aftermarket intake manifold, or get forged pistons or anything like that until you go over 275rwhp (and way beyond that for some of these things). Don't waste your money on these.
You don't need to mess with your base ignition timing or your cam timing to hit 275rwhp, so leave that stuff alone and mess with the things I mention because those are the ones that will make you safe and reliable power.
A 2.5" downpipe and exhaust system is perfectly adequate to support 275rwhp. There is no need to invest in a 3" exhaust system unless yo are seeking to make 300rwhp or more.
The notion that a CT-26 just puts out hot air above 15psi is nothing more than hot air. All turbos produce hot air and the higher the boost the hotter it is. Proper intercooling allows higher boost to be safely used regardless of which turbo is being used to produce it.
Contrary to popular belief, bigger is not better in the power game. Balance is essential and "just big enough and no larger" is usually optimal. For example, if your intake runners or valve ports are larger than they need to be, you will lose power instead of gain it. If your compressor wheel is larger than you need, you spool slower but get no additional power to make up for it. So, if there is something that you feel you need to compensate for, don't go doing it with engine parts, otherwise you are going to be feeling pretty bummed the next time that you go to the dyno.
Big Power, Big Money
If you are still reading, then you must be looking for serious power and willing to spend some serious money to get it. Beware that street manners and very high power are very often at odds. A very high power 3S-GTE is not necessarily a pleasure to drive every day in traffic. Be mature when you set you performance goals and realize that you cannot have it all. In particular, very high power and high reliability are not cousins in the world of 4-cylinder engines. If you cannot afford to be without your 3S-GTE, then leave it relatively unmodified.
Somewhere around 275rwhp, you will reach the limits of the stock fuel system and you must upgrade the fuel system to safely move on. If you do not, you run the risk of letting the air-fuel mixture get too lean and burn your pistons, particularly on cold mornings when the air is denser.
At this point I strongly recommend investing in an aftermarket engine management system (EMS). An EMS is a completely programmable fuel and ignition management computer that can be programmed to deliver the precise amounts of fuel and timing for all situations. There are several reasons to do this. First, the stock ECU is programmed with stock fuel injector sizes and stock VE values and the next few stages are going to change those values dramatically. Second, some of the stock engine sensors have reached and surpassed their measurement ranges and, in the case of the AFM, are becoming a serious power restriction. Third, black boxes like the S-AFC that can be used to some extent to trick the ECU into changing its behavior will quickly multiply and create an inscrutable and untenable tuning infrastructure. By replacing the heart of control system, you greatly increase what can be done and greatly simplify how it is done. There are several good EMS units to choose from. Expect to pay $1500-$3000 if you do the installation yourself or go with a plug and play system or if you go with a non plug-and-play system $3000-$5000 and have somebody else do the installation.
Having chosen an appropriate EMS, you can now break free of the stock fuel system and install a fuel system capable of taking you into the 400-500rwhp range. Read the center-feed fuel system section for details on what this entails. The fuel system upgrade itself will not produce any more power, but it is essential to providing the fuel that you will need to safely produce over 275rwhp.
Along with an EMS and an upgraded fuel system, you need a turbo capable of flowing enough air to take you to your power goal. When choosing a turbo, you need to pick one that will operate well with the 3S-GTE from as low an RPM as possible to 7K-8K RPMs. You should aim for a turbo that is in the middle of its efficiency island with the 3S-GTE at a VE of 0.90-0.95 at around 18-20psi boost pressure. See the turbo sizing section to learn more about how to do this. You will also need to consider the boost control aspect of the turbo. While the boost controller that you chose earlier will still be adequate, the wastegate that comes with the turbo should be properly sized to the turbo and exhaust system to prevent boost creep, which is a dangerous situation where the wastegate cannot divert enough exhaust around the turbine to keep boost levels under control. Turbos that can be set up to produce reliable power in the 300-400rwhp range include the TD06 and the T3/T4 46-trim. For 400-500rwhp, the T3/T4 50-trim, T67 and GT30R are solid, proven performers. Above 500rwhp, the GT35R is a capable turbo.
Longer duration cams and are also needed to move past 325rwhp. The stock cams have a duration of 236 degrees and a maximum lift (distance that the valve top moves from it's closed position) of 8.2mm. Aftermarket cams are available in various durations and lifts. Longer duration cams trade off some low end torque for higher end power but once the cams become the performance bottleneck, there is little to do but go with at least a modestly longer duration set of aftermarket cams. Since you will be taking the cams out, this will be a great opportunity to replace the stock lifters and shims with lighter shimless buckets to allow the valve springs to easily handle 8K RPMs. Setting a higher redline will also give you that extra fraction of a second to shift out of first and second gear. Once installed, cams must be properly degreed and the EMS should be retuned for the improved VE. For the 300-500rwhp range, 264 degree cams provide a good combination of flow without giving too lumpy an idle. For +500rwhp, 272 degree cams are recommended. There are theories that using staggered cams, meaning different duration cams for intake and exhaust, have benefit. If you choose this route be sure that your longer duration cam is mounted on the intake side of the valve train.
Before you install the big turbo and crank up the boost, you should take more anti-detonation measures. If you plan to run high boost on premium pump gas, install a water injection system and use the EMS to control it so that it comes on at around 10psi of boost. Install an 0.7-0.9mm orifice between the intercooler and the throttle body. Run it with either distilled water or, in areas where freezing is a concern, a 50-50 water/methanol mixture. If not interested in water injection, switch to higher octane fuel when running over 18psi of boost. In addition to that, get a 160 degree thermostat to keep the coolant at a lower temperature and install one or two engine lid fans to draw more air though the engine bay.
If your EMS does not come with a knock sensor capability or an upgraded ignition system, you will need to purchase these and install them as well. The J&S Safeguard unit is well recognized in the industry to provide knock detection. Several manufactures (MSD, Crane, AEM and Electromotive) make ignition systems that can deliver the extra power required to reliably bridge the spark plug gap at boost pressures above 18-20psi. Direct fire systems, while not absolutely necessary to produce prodigious amounts of power, do eliminate the need to replace the distributor cap and rotor frequency.
Install the bigger turbo and take the car to the dyno to tune it. With the new anti-detonation measures, carefully crank up the boost to the 18-22psi range and back off on the ignition timing if you detect any knock. You are better off running a little less timing and a little more boost than vice-versa. Also, monitor your AFR with a wideband O2 meter to insure that your upgraded fuel system is doing its job. If you have several compressor wheels and turbine housings available to go with your turbo, monitor intake and exhaust manifold pressures to select the best combination for your setup. If the exhaust manifold pressure gets much higher than the intake manifold pressure, get a turbine housing with a larger A/R. If the intake and exhaust manifold pressures drop as RPMs increase, get a larger compressor wheel. In every case, use only the smallest compressor wheel needed to stay in the desired boost range to redline.
Now it is time to improve flow a little bit more. There are many ways to do this, most of them quite expensive, but a lot can be done with just a few changes. Eliminate the double 90 degree intake pipe elbows between the intake and the turbo with softer 45 degree bends or straight pipes if you can move enough components to do so. If you have a 91 or 92 engine, get a throttle intake pipe from ATS Racing. This pipe does not narrow as the stock pipe does. Get a larger throttle body or have the stock throttle body bored out to permit more flow. Most importantly, though, you will need to address the stock intake manifold, particularly at 350rwhp and above. The stock intake manifold has very poor distribution characteristics which cause more air to flow into the #2 and #3 cylinders than to the #1 and #4 cylinders. If using the stock intake manifold with an EMS at power levels above 250rwhp, apply some fuel trim to the inner cylinders to keep them from going dangerously leaner. A properly designed intake manifold with a larger surge tank than stock is critical to making safe power above 350rwhp.
When producing 400rwhp or more, the larger aftermarket air-to-air intercooler that made such a nice impact at lower power levels is starting to reach its limits. It now heat soaks very quickly, since the turbo can keep putting out large blasts of heat farther across the RPM range and the increased air volumes means a larger pressure drop across the intercooler. It is now time to look for a bigger better intercooler and your best choice is an air-to-water unit with a heat exchanger mounted on the front. By placing the intercooler between the turbo and the throttle body right over the engine, the long turbo pipes are no longer required and this improves throttle response because the turbo has substantially less volume to pressurize before the boost hits the intake valves.
The exhaust system will also need to be reconsidered as you approach levels above 400rwhp. At this level, 3" pipes or larger will be required. Also around 400rwhp, it starts to become difficult to produce more power without some head work. At higher power levels, there is power to be gained by pulling the head to make throat adjustments and port match the head, intake manifold and the TVIS elimination plate. Going to 1mm oversize valves is probably the best step with which to start your serious head work. The larger valves by themselves will bring the critical throat to valve ratio of the 3S-GTE head to near optimal requiring only a small amount of throat adjustment work to produce a very good head that delivers high air velocity into the combustion chamber across most of the RPM range.
Also while the cylinder head is out, you should put in stronger valve springs to keep the valves from floating at higher RPMs under higher boost. A metal head gasket and ARP bolts and fasteners will also be needed to prevent the head from lifting under immense pressures and the head gasket from blowing. With these changes, you should be able to raise the boost pressure to 22-30psi with appropriate tuning and race fuel.
One sure way to increase drivability is to stroke the motor. Stroking means increasing the distance that the piston travels from TDC to BDC. There are pistons, rods and crankshafts available to turn a 3S-GTE into a 2.2 liter engine. Alternatively, you can start with a 5S-FE block, which is already a 2.2 liter motor and put in better pistons and rods (the 5S crankshaft is a monster and can be used as is). The increased displacement means that more exhaust flow is available sooner to the turbine to spool up the compressor and generate boost. Ultimately, this may not result in more power because high RPM torque, which translates into horsepower, is determined by how much air-fuel mixture and exhaust can enter and exit the engine, which is limited by the cylinder head. Thus, it is the cylinder head and not the displacement of the engine that determines peak power potential.
Thermal coating of the piston tops, cylinder head and valves as well as the exhaust manifold and turbine housing should be considered as they will improve peak torque, fuel efficiency and turbo spool while reducing the amount of heat that the cooling system has to dissipate. Coatings can also reduce the chances of hot spots which can lead to detonation. Given that there are really no downsides to proper thermal coatings, it should be an easy decision to use them if you are building a 3S-GTE for high power applications.
Nitrous can also be used at different power points to increase power. Nitrous adds power by injecting a gas into the cylinders that is 33% oxygen by composition instead of the 21% oxygen present in the atmosphere. Since oxygen is essential to combustion, the availability of more oxygen means that more fuel can be combusted in the same space, thus producing higher cylinder pressures translating into more power. With the introduction of additional oxygen, more fuel must also be provided to maintain a proper AFR. If enough fuel is not added with the nitrous, the walls of the cylinder and the top of the piston will provide the material needed to feed the combustion process at the expense of the engine. Because of the much greater cylinder pressures produced when using nitrous, it is essential to also retard timing while it is in use to protect the head gasket, prevent the lifting of the cylinder head and to keep rods from bending and bearings from being deformed. At lower power levels under 350rwhp, nitrous can be injected into the throttle body along with additional fuel using a fogger-type setup. At higher power levels, it is very crucial to ensure that the nitrous and fuel are precisely distributed into each cylinder and directly injecting into the intake manifold runners in a ported setup with proper EMS control that retards timing and increases fueling during nitrous injection is essential to engine longevity.
A few more myths
A gen3 3S-GTE is no better a platform with which to make big power than a gen2. Although the gen3 produces more power in its stock configuration, all of that power comes from simple improvements that can be applied to a gen2 engine. Once you reach about 300rwhp, the playing field is essentially level.
Larger displacement does not equate to more peak power as long as the cylinder head is the limiting factor. Larger displacement will result in more low end torque and faster spool, but ultimately torque will fall faster on a higher displacement engine and peak power will be about the same.
The notion of keeping the stock head gasket to provide a weakness that will protect the pistons and rings from detonation is wrong-headed. A blown head gasket can cause just as much if not more damage than a blown ringland. The best and only real way to deal with detonation is proper intercooling, fuel and tuning.
Where are the limits?
How much power can a 3S-GTE be configured to reliably produce? The jury is still out on that one. It appears that 700rwhp is attainable and more than that is theoretically possible. The time and expense required to get there is tremendous, but as MR2s in North America are nearing the 9 second 1/4 mile mark, the amount of effort and experimentation going into producing more power with this little Toyota engine and transferring it to the ground is quite respectable. At this point it would be hard to make any predictions about limits, especially given that new techniques and materials are being introduced every so often that allow temperatures and pressures to be increased while improving the overall management and control of the engine.