Version 1 - July 12, 1999
The purpose of this document is to lay out the facts and assumptions that are behind my upgrade plan, that will -- safely, I believe -- bring my MR2 Turbo to 12psi maximum boost and an expected 240hp (flywheel). As of the time of writing, I have not proceeded with any engine modifications beyond performance intake and exhaust. When I have completed this first round of planned upgrades, which I hope to do in 1999, I will report on its success.
Power (Acceleration, actually) is only one part of the Holy Trinity of Performance (the other two parts being Handling and Braking). Certainly all three merit study, but this discussion will be limited to Power.
Disclaimer
What follows is a summary of the knowledge that I have accumulated while researching the Toyota MR2 Turbo, and automotive performance in general. Some of my sources are more authoritative than others. I am sufficiently satisfied as to the completeness and veracity of my understanding of the subject that I will modify my own MR2 Turbo based on what I know, with reasonable confidence that I can avoid unexpected engine damage or safety issues. I present the following only in case it might guide the researches of other enthusiasts. I make no claim that any of the following is true or correct; I am not suggesting that anyone make the changes here discussed to an automobile or similar system; I take no responsibility for the consequences of any actions taken by anyone based on my statements. I do not condone nor suggest illegal activity with respect to The Highway Traffic Act or any other laws.
I am not affiliated with any automobile manufacturer, seller or repair shop, nor any parts manufacturer, supplier, seller or installer, nor any automotive publication. I am in no way associated with the automotive industry except in that I own an automobile, and I declare that I am not an expert in any way. The opinions that I express in relation to automobiles and their performance do not necessarily reflect those of my employer, which is similarly not affiliated with the automotive industry.
My researches have revealed that the 3S-GTE engine in the MR2 Turbo can run at 12psi boost pressure, safely and reliably, with almost no other modifications. As the stock boost is about 7psi, and assuming that airflow is the only limiting factor in performance at these near-factory levels, this boost increase represents a torque increase of ((12+15)/(7+15)=) >22% over 200, or about 44 lb.ft.s, with a numerically similar increase in horsepower.
A car in stock form has some extra capacity built into the fuel, ignition and control systems, as a safety margin against unexpectedly high airflow. The corollary of this fact is that engine performance can be increased simply by increasing airflow into and out of the engine, at least until the capability one of the other systems is exceeded. Therefore, the first few performance mods focus on increasing airflow through the engine. One must keep in mind that safety margins are being reduced. Either know the limits of your other engine systems, or install the appropriate gauges, sensors and warning circuits, and watch them carefully. As further mods continue to increase airflow, it becomes necessary to upgrade these other systems to keep pace.
It should be mentioned here that, as performance increases, the limitations of parts and systems other than fuel, ignition and control will be reached: notably the pistons, rods and crank, the block, the head bolts and head gasket, cooling system, clutch and drivetrain, etc. These consequences are beyond the scope of this discussion.
The MR2 Turbo is a mid-engined car, with air intake vents on both sides of the car beside the engine. The driver's side air vent feeds directly into the engine air intake. Air flows into the air filter box; through the mass airflow sensor (MAS, aka AFM (air flow meter)) which tells the ECU how much air is being drawn in; through the "cold side" of the turbo which pressurizes it; out to the intercooler (IC) which is positioned in the passenger's side air vent and which cools the intake air; through the throttle body, intake plenum and runners, past the valves and into the cylinders. Following combustion, the exhaust gases flow past the exhaust valves; through the exhaust manifold; through the "hot side" of the turbo, doing the work of spinning the impeller in the process; through the catalytic converters, muffler and finally the exhaust pipes.
The turbo boost pressure is specified to be between 7.2 and 11.8 psi [BGB], stock. In line with the vacuum lines that control the turbo is a VSV (Vacuum Solenoid Valve) which the ECU uses to control turbo boost. Under normal operating conditions the VSV is open, allowing full stock boost, which we understand to be 7-8 psi. The ECU can close this VSV to limit boost if conditions exist under which full boost could be dangerous. These conditions include low engine temperature and cold intake air, among others.
As the Toyota boost spec shows, the engine is equiped to handle airflow up to almost 12psi boost. If the ECU detects boost in excess of 12psi, it will trigger Fuel Cut. This is apparently a very traumatic experience for both car and driver. Once Fuel Cut has been triggered, the engine will not run above 3000 RPM until the Fuel Cut is reset. Resetting the Fuel Cut is apparently as simple as switching off and restarting the engine, although I have also been told that a circuit breaker must be reset manually.
Therefore we conclude, and it has been widely demonstrated so, that it is safe to increase maximum turbo boost from the stock 7-8 psi to close to 12psi. The stock fuel, ignition and control systems know what to to with up to +12psi gauge of air.
Concerning performance exhausts: many people regard an exhaust upgrade as an advisable first or second stage. Shops and parts sellers say this because they make a lot of money on exhaust parts. Enthusiasts buy it because a big shiny exhaust system is stylish and impressive, and certainly sounds high-performance. The fact is that the stock MR2 Turbo exhaust is not very restrictive, so there is not much power to be gained if this is your only mod. Too large an exhaust can actually cause a loss of power. Considering the cost of a good performance system, I say leave the exhaust until it has become a significant restriction -- unless, like I did, you have the opportunity to buy a moderately bigger, used system at a very reasonable price.
Detonation Protection
Even though the fuel system is not outstripped at 12psi, the base probability of detonation has been increased. For more detail on this subject see Cosmo's Treatise On Detonation. The bottom line is that, to avoid accelerated engine wear, loss of compression, damaged spark plugs, knock sensor, head gasket, pistons, and more severe engine damage, steps should be taken to protect against detonation.
At 12psi boost, it should be adequate to ensure that all of the stock systems that help protect against detonation are working at their peak efficiency. Heat is a major concern. Flush the cooling system and refill it with premium coolant. On the MR2 this is a tricky operation which can easily be done wrong, resulting in dangerous air bubbles. Install cooler spark plugs. While you're at it, switch to copper plugs, and be prepared to change them regularly (see ignition system upgrades). Cool intake air is important to detonation avoidance; clean the intake piping between the turbo and the throttle body, including the intercooler, of oil. A clean intercooler will cool much more effectively. To keep it clean, take steps to prevent the flow of oil from the PCV line into the intake. The intercooler efficiency can be further increased by installing a SPAL fan on the intercooler.
Fuel System
As for the fuel system itself, only take extra care to guard against a lean burn condition. Change the fuel filter, and change it regularly. If the car has seen high mileage or a lot of hard use, consider having the injectors cleaned and balanced. And of course, use quality, high-octane gasoline; high-octane pump gas should be sufficient.
Ignition System
Regarding the ignition system: as turbo boost increases, cylinder pressures increase. As cylinder pressures increase, the resistance between the spark plug terminals increases, making successful spark delivery more difficult. The spark generated at the ignition coil, unable to jump the plug gap, may "leak out" at the distributor, plug wires, or through the spark plug insulator. This problem will be experienced as hesitation and missing under boost. If this problem is left untreated, damage to the plugs, plug wires and/or distributor cap can result. In addition, the unburnt fuel exiting a cylinder following a failed ignition event can cause the catalytic converters to overheat and even to melt.
Some owners have experienced these problems at 12psi boost, but not all. Whether an ignition problem occurs may be dependant upon the condition of the ignition system. One should be prepared to deal with ignition problems at the 12psi stage.
Inspect plugs, plug wires and the distributor for signs of spark leakage, which will be black dots, burn marks, or broken plug ceramic. Check the contact gap in the distributor, and check the plug gaps. Gaps wider than the factory spec should be suspected. Replace worn or burnt parts as necessary. Consider replacing the stock platinum plugs with copper plugs. From now on, life for your plugs is only going to get harder. Will you trust platinum plugs to stay clean, whole, and to maintain their gap for the advertised 100k km? Better to install copper plugs which you will change more often. That way you will be inspecting your plugs regularly (plugs are your window into the combustion process), and you can be sure of their gap. Use resistor plugs to protect against glitches in the control system caused by EM interference from the ignition system. Consider also reducing the plug gap to less than stock. A narrower gap gives a greater certainty of spark delivery, but also reduces spark energy. Consider replacing plug wires and distributor cap with premium high-performance units.
If all of these changes do not solve hesitation and back-firing problems under boost, but turning down the boost again does eliminate them, then consider upgrading your igniter at this point. The cause seems to be none other than too weak a spark for 12psi.
Control System (ECU)
The stock ECU and sensors can operate normally up to 12psi. If the boost climbs to 12psi or beyond, Fuel Cut may be triggered. Some boost controller types are more susceptible than others to changes in temperature and barometric pressure. It is prudent to leave some margin of safety between the boost setting and 12psi, at least until you learn the way your boost controller reacts to weather changes. Monitor your boost gauge closely, day to day.
There are many ways to increase the capacity of the fuel system. As an increase of only ((15+15)/(12+15)-1=) ~12% is required to keep the mixture the same, there are inexpensive options available.
I have not researched as extensively the changes required to run safely at 15psi, but I know that I will, before I do so. I suggest to anyone else considering it that they do the same.
In stock form, the turbo regulates itself by the boost pressure at its outlet (safe). If you have changed this arrangement so that the turbo wastegate monitors the boost pressure at the throttle body instead of at the turbo outlet, then a leak in the intervening airway can cause the turbo's early demise via over-rev'ing.
Tech: Cosmo's MR2 Turbo Engine Upgrades Part 2
thesandmage@netscape.net
www.angelfire.com/sc/cosmo
