Posted by Aaron Bunch on Feb 9 2017
Starting with a 100% bone stock MR2 (do those still exist anymore?).

We are also going to assume you have kept up with maintenance. So no 150,000 mile timing belts, no knocking, no overheating, no puking oil. No smoking. A few drips of oil here and there are acceptable (sometimes I think MR2 means "leaks oil" in Japanese).

Step 1: Full Tune Up

The stock ignition system is really good on a 3SGTE. You will find lots of old articles about cap/rotor wear etc. Those were wrong. We routinely make over 700whp on basically stock ignition systems, and field tons of "help me" calls about malfunctioning aftermarket coil on plug systems. Coil on plug systems are available, but require stand alone EFI and additional hardware. That adds up to money that could be put to good use elsewhere.

ATS sells a full tune up kit here (for 91-92) or here (93+). The included NGK plugs are pre-gapped a little bit too big. They will work for stock applications just fine, but we suggest dropping the gap down to .028 inches. While you are changing the spark plugs you should take the extra time to do a compression test. This is a good basic indicator of piston/ring condition. You are looking for 140-185 psi of compression. And it's very important that they are all within 5% of each other. Compression testers are available at your local autoparts store for under $30.

Step 2: Let It Breathe

Your car will be louder now. If you are not comfortable with a louder than stock car then sell it and go buy a hybrid. Removing breathing restrictions is fundamental to making power. There are some quiet exhausts occasionally available on the market, but everything will be at least slightly louder than stock.

  1. Intake: You've got a few choices here. You can go with an Apexi bolt in open element air filter like here. They flow great and still filter well. Or you can make your own with parts off ebay. Be sure you have the skills and patience to make a bracket, otherwise your filter will beat itself to death on the fuel filler pipe, quickly develop a hole, and no longer filter. Filters from HKS, Blitz foam, and Greddy Airnex should be avoided.

    What a cheap freebie? Take the stock air box base, cut or drill more openings, specifically on the front and outside sides. Remove the stock air bladder from the firewall and reinstall with a new genuine TOYOTA air filter. Be careful to leave enough base material that it can still be bolted down. This flows great, and filters as well as OEM.

  2. Exhaust: The $199 Ebay dual muffler dual tip exhaust flows awesome. We have seen 800whp from these before. Seriously. The bad news is that they are LOUD, especially when combined with downpipes and larger aftermarket turbos. The Berk exhaust does not flow as well, but is much quieter. TCS occasionally offers two exhausts: a quiet SP style exhaust that looks like a 3" version of stock. This is my favorite. It's difficult to make it fit perfectly, but it is quiet and flows reasonably well. The other TCS is the 3.5" dual muffler, dual tip, it's beautiful and BOLD (read: loud).

    Downpipes: I'll get to those later. An exhaust can be changed out in about 2 hours at home. A downpipe takes 2-3 times that amount of time. I will also note that adding performance exhaust parts like a downpipe or a 2.75" b-pipe in front of a stock muffler is dumb. There is almost no gain in power because you still have a big restriction at the end. ATS Throttle Body Inlet–hey, it's my page I can push my own products if I want–The 1991-1992 cars NEED this. It's a 4-5 horsepower gain. Want proof that it helps? Well the 1993-1995 cars have a redesigned throttle body inlet that mirrors ours.

Step 3: Cooling

You have three choices (for now) based on budget.  There is almost ZERO natural air flow through the stock intercooler.

  1. So a quick FREE modification is to unplug the temperature sensor on the engine lid. This will force the factory intercooler fan to be on anytime the engine is running (or key is in the on position).

  2. Upgrade the fan. An 8” puller fan will fit on the engine bay side of the intercooler. These are $99. The install takes about an hour and hopefully you know someone with small hands willing to help you install the push lock mounting tabs. If you have the motor out, or you feel like punishing yourself you can fit a 9” pusher fan on the front side of the intercooler. It sounds like just 1” bigger, but it flows about 30% more air!

  3. The ATS Side Mount Intercooler kit. These are on sale for $499. This upgrades you to a higher flowing, more efficient core, plus you get cool piping upgrades, a 9” fan with shroud as well as a mounting shroud to make sure we are getting the maximum flow possible through this core.

If you are shooting for more than 350whp in the long term will will outgrow all these options, but you will also outgrow the stock ECU, fuel system, stock pistons, etc, etc...there is already another blog about that.

Step 4: More boost. Finally we get to the good stuff. The stock MR2 runs about 9-11 psi in stock form. If you have been following this path you may have a car running as much as 13 psi even without a boost controller. First, buy a gauge. It doesn't have to be fancy, but avoid brands like GlowShift and Prosport (I've seen more broken ones than working ones). I like VDO, Autometer, and any of the big Japanese brands. Some electronic boost controllers have built in gauges, so that can offset some of the cost.

You will also have to disable fuel cut if you have a 91-92 car and want to run more than 12 psi. The 93+ cars have a fuel cut set right about 16 psi. For stock turbo people on pump gas, this is usually plenty. You can buy a fancy FCD (fuel cut defenser) or do a $5 zener (pronounced Zee ner) diode mod if you are handy with a soldering iron. Or you can just disconnect the vacuum hose that goes to the stock map sensor. That sensor does only two things: it runs the stock (useless) boost gauge and it activates fuel cut. Leave it electrically plugged in, remove and cap/plug the vacuum hose and voila: no fuel cut. Now to actually raise boost you will need a boost controller. Basically the stock turbo has a 7 psi wastegate actuator. When boost hits 7 psi the actuator opens up and routes exhaust around the turbine wheel to control the boost. The factory setup has a turbo VSV that bleeds off approximately 3 psi if all conditions are met (warm enough outside, warm enough coolant temp, no recent knock detections). This is typically disabled when you install an aftermarket boost controller. Again here are several choices in order of price:

  1. Manual boost controller. The ATS MBC is a ball spring type for fast spool and accurate boost control. Not bad for $49. However expect boost to fall off 1-3 psi by redline with most turbos. This is because the exhaust back pressure increases inside the turbine housing and pushes on the wastegate flapper door. The ball-spring boost controller cannot compensate for this additional pressure on the wastegate flapper.

  2. Electronic Boost controllers. These race in cost from about $329 to $600+. Greddy, HKS, Apexi, and Blitz all make or made electronic boost controllers. The old Greddy Profec B Spec 2 is probably the easiest to use ever, and the Apexi AVC-R has the most features, but is the hardest to setup. The HKS EVC4 has a somewhat difficult initial setup, but after that it is VERY EASY to change. I think it's the only one that you can actually just tell it what boost you want to run and it does the best it can to hold that boost all the way to redline regardless of gear.

  3. Now before you go drop $600 on a new electronic boost controller, realize that is more than half the cost of a good stand alone ECU that will have boost control built it, or available as an inexpensive option. However stand alone ecu's require tuning and unless you do it yourself, that isn't free. Typically budget about $2000 for a full stand alone ecu with tuning and options (knock sensor, boost control, wideband)

So what is a safe boost level? Well that depends. On US 93 octane pump gas we typically quote 17 psi as the maximum safe boost level. The fact that 93+ cars had a 16psi fuel cut from the factory seems to agree with this number. However, as these cars get older we have seen some with excessive carbon build up that couldn't handle more than 13 psi before we heard detonation. I highly suggest renting some local dyno time so you can see how the car behaves under controlled conditions.

Step 5: Gauges: I already mentioned in step 4 that a boost gauge is necessary. The other gauge I recommend is a wideband O2 sensor. This will give you some insight into what the ECU is thinking as well as early detection of problems. There are lots of brands, Innovate is one of the biggest, and one of the worst. They eat sensors for breakfast. We have had excellent luck with PLX, Zeitronix, AEM, and recently a very affordable one from APSX. All of these have outputs that can be integrated into stand alone ecu's for later tuning use. Zeitronix and PLX offer logging software that can be VERY useful, especially if you are trying to street tune. So expect results of 14.7 at idle and cruise, and 10:1 at full boost. 11:1 is good (and safe) for full boost under most conditions, but you will need something to tune with to get there.

We've reached a fork in our path. You can proceed to step 6a or 6b, or choose to do both depending on your budget.

Step 6a: Tuning. The way we have laid out (and limited) these modifications you can stop at any point and still have a great running car. This step is really about optimizing it all and getting the most out of the investment you have made. Again, choices are laid out in order of cost:

  1. nothing. It all runs pretty good as-is.

  2. Piggybacks. Way back in the day we thought these could be the answer. Drop in some 20% larger injectors, and use an AFC to dial back the air flow meter signal 25% to correct the fuel map. The only problem is that the Air Flow Meter also effects the ignition timing map. So everytime we reduced the signal to lean out the air fuel ratio, we advanced the timing a little. This wasn't good. There is another complication that we didn't know about until we started doing rom tunes. There are TWO fuel and timing maps inside the ECU. When they ecu detects excessive knock it switches to a richer fuel map and more retarded ignition map. Even when tuning on a dyno it's almost impossible to know when this happens. Many tuners went along unaware and optimized the car to run great in it's limp mode...sounds fine until one cool day the car comes out of limp mode and suddenly there is too little fuel, and too much timing, and snap there goes a piston ringland. So two simple rules: 1. avoid them, and if you just can't then 2. don't use more than 10% correction (unless you have a way to compensate for ignition changes).

  3. Rom Tunes ($800)– We've done literally hundreds of these. There is a better description on the rom tune product page, but the short version is this allows us to change the actual fuel and timing maps inside the stock ecu. So instead of manipulating a sensor we are changing the way the ecu thinks. They come pre-tuned for your car based on cars we have already tuned at ATS.

  4. Stand alone ecu ($1000-$2500)– This used to be a huge undertaking. We had to build a new engine harness, people weren't sure if they could keep AC, or cruise control, no one knew if power steering of ABS would even still work. That is all fixed now. Link and ECUMaster both offer plug-n-play ecu kits for the MR2 (we developed the pnp for the ECUMaster so it is exclusive to ATS Racing), and all the original accessories and options still work. These come with a base map and you should be able to start up and run with an hour.

    These start at a very reasonable prices (about $1000) but will require tuning. If you can do this yourself then great, but if not then expect to spend $400-$1000 at a local tuner. The other part of the price is the options. Things like boost control, built in wide-band O2, flex-fuel, knock sensor, oil pressure sensor, fuel pressure sensor, bluetooth or wifi output can be added to the emu. The best part about stand alone ecu's is that they can grow with your car without needing to be replaced. I run my 872whp MR2 on the same $1000 EMU that we put on near stock MR2's. There are other brands available. My experience with AEM, Megasquirt, and Hydra Nemesis are to avoid them. They may have a lower initial cost, or be available used because someone else already figured out how much they suck.

Step 6b: turbocharger upgrades

1. CT27 (or CT21 if you have a Gen3). This is a stock turbo that is upgraded on BOTH sides for additional flow. We replace the original compressor wheel with a larger billet wheel, and machine the compressor housing to match. We don't publish the exact size because there are many competitors out there who are willing to just copy someone's work instead of doing actual research themselves. We also do a CNC machine operation to the exhaust housing that optimizes the merge of the twin entry volutes. Don't try this one at home though, because if you overshoot the cut by .5mm (that .02 inches) you will ruin the exhaust housing. Externally these look like stock turbos so they are very popular in states with serious emission laws.

If you are doing the work to install a new turbocharger you should also install a 3” downpipe at the same time. The ATS downpipe is ceramic coated stainless steel and the wastegate opening/flow has been verified with each one. Berk makes a downpipe too, but the wastegate openings of the turbocharger must be enlarged because of an inadequate design or you will have boost creep (uncontrollable boost beyond target).

If you intend to run more than 15 psi we recommend the billet wastegate actuator and a 15 psi spring. I mentioned before that back pressure becomes significant at higher boost levels and can actually push the stock 7psi wastegate flapper open on it's own regardless of what signal you are sending from your boost controller. Raising the base spring pressure of the actuator will help prevent this.

The CT27 typically makes 250-300whp on pump gas, 350whp on E85/race gas and the current balls out fully modified everything record is 389whp

  1. GT28RS Disco Potato – basically a ball bearing version of the CT27. Similar performance with slightly faster spool. Great for autocross or track days.

  2. GT3071R – literally one of my favorite turbochargers. I have run these at 250whp on a bone stock MR2 and I have run them at 500whp on a fully modified MR2. This turbo has a HUGE range of power, really fast spool, and can grow with your needs, Plus there are options to upgrade it like the new line of GTX billet wheels. I haven't done one yet, but a 550whp GT3071R is a possibility.