Basic performance increase (BPU)

Also known as: Basic Performance Upgrade

The BPUpgrade ensures that the engine generates more power by increasing the boost pressure produced by the turbos.
As standard, the loaders produce 0.76 bar (11 psi). This pressure can be increased. It is generally known that the ceramic turbines of the J-Spec models can cope with 1.2 bar (17.4 psi). The export models (UK / Euro / US) have steel turbines, which can generally withstand higher pressures. There are BPU Supras driving the 1.4 bar (20.3 psi).
1.2 Bar pressure is generally considered as an upper limit in terms of significant extra power and a certain steadiness of the engine.

Boost pressures above 1.2 bar make sense only if the heat dissipation in the engine compartment has been improved, since the additional heat inevitably warms up the intake air, which again reduces the effect. In addition, the series loaders above 1.2 bar no longer work efficiently.

For both J-Spec and export models, the BPU tuning boosts performance to around 400 - 450 hp depending on boost pressure. Not only the final performance of the engine is higher, but also the response of the loader is significantly improved, ie the power is available spontaneously.

First step in the BPU: the exhaust system

Technical background:
The speed of the exhaust gas turbines in the superchargers is dependent on the flow rate of the exhaust gases. Apart from the wastegate ( a valve which, when the maximum charge pressure is reached, directs exhaust gases past the turbo in order to maintain the boost pressure at the level reached ), the permeability of the exhaust system is primarily responsible for the boost pressure that the turbos build up.
The series exhaust system has a relatively low permeability, which is particularly due to the catalysts. If you replace the series Kat's with a racing cat, best in combination with a complete system with a larger pipe diameter, a noticeable increase in output is noticeable (about 30 hp) and the car drives "liberated".

Attention ! This modification has different effects with J-Spec and Export-Spec! The J-Spec models require a flow restrictor ("Restrictor Ring"). The following explains why.

J-Spec:

In the J-Spec model, if the cat was removed, the serial twins produce over 1.6 bar pressure without further modifications. This goes far beyond the safe level. At such pressures, the rotor can come off its axis and destroy the whole loader.
This huge increase in pressure comes about because the small wastegate in the J-Spec models is not designed for such exhaust flow.
To prevent this you have to reduce the exhaust gas permeability again a little. The purpose of the flow restrictor in the form of a perforated metal plate, which is simply mounted either between the two Kat's or behind the second Kat. The hole in this metal plate must logically have a slightly smaller diameter than the rest of the system, usually 2 inches. A reasonably sized restrictor should throttle the boost pressure to about 1.2 bar or less.

UK / Euro / US (export) SPEC:

The increase in exhaust gas throughput poses no problems for the export model, since a larger wastegate is standard equipment. Therefore, the boost pressure usually settles after conversion at about 0.85 bar (12.3 psi).
If you want to increase the pressure even more, you need a "Boost Controller" to influence when the wastegate opens.
A flow restrictor is not needed in the export model.

Annotation:
In both models, it is possible to increase the boost pressure by means of a boost controller without modifications to the exhaust system. Due to the still existing dynamic pressure by the Kat's, this is strongly discouraged. The pressure causes a significant increase in the exhaust gas temperature in the manifold, which can lead to serious damage in the engine.

Required parts:

Obviously, a cat substitute (called "downpipe") + flow restrictor (J-spec) or a cat replacement + boost controller (export model) is all it takes to boost boost. However, in order to be able to drive the car permanently and with increased performance, you should do the following in both model variants:

1) Exhaust system from Cat also replace
The standard exhaust system (the pipe run from where, as standard, the two Kat's sit) brakes the exhaust flow through their small diameter and the many bends pretty much. In order to allow a nice, as uninhibited and thereby faster exhaust flow, not only the Kat's should be removed, but the whole system. There is a wide range of exhaust systems specifically designed for the Supra MKIV that are designed to do just that.
So you need:

  • Downpipe with welded racing cat (replaces the Y-piping, which includes the two Kat's as standard)
  • Cat-Back (= Kat to the end) exhaust system

2) Prevent the "fuel cut"

The standard engine control (ECU) will stop the gasoline flow as soon as it evaluates a boost pressure of 1.0 bar or more for a period of 3 seconds. This is a simple security mechanism.
There is a ready to buy part, which is looped into the signal line of the pressure sensor for the ECU. This part is called "Fuel Cut Defender". It allows the pressure sensor signals to flow freely to the ECU up to a certain value (approximately 0.9 bar), and above this limit, the unit continues to trickle the ECU into this limit. Thus, permanently higher pressures are possible.
Instructions: Installation of a BCC

3) Replace fuel pump

In the J-spec model, the standard fuel pump is not sufficient for the BPU performance. In the export models, it is sufficient, but since most Supras are already over 13 years old, it should also be replaced.
Very popular thanks to price / performance the Walbro pump. But even better is the Denso pump, which is also standard in the export model installed. It is slightly more expensive, but also has a higher specification and works quieter.

4) Replace spark plugs

Higher boost pressure makes higher demands on the spark plugs.
In the US, the following candles are recommended for BPU Supras:

  • Iridium IK22 (longer, sometimes over 15000km, price is over 50, - € for 6 pieces)
  • NGK BCPR7ES (have to be replaced every 5000km, cost just under 12, - €)

Recommended but not essential parts:

1) Boost pressure display

An electronic or mechanical boost display should be installed in the cockpit. The easiest way is where the watch is standard, because this has 52mm diameter and that is also the usual display diameter.

2) mixture display (Air / Fuel Ratio)

This gauge has a broadband sensor that measures the level of oxygen and gasoline in the exhaust stream. It is important to monitor this value for the following reason:
Japanese turbo engines generally run very fat, ie more gasoline is injected than is burned. This is done because the unburned gasoline cools the combustion chamber.
The higher the boost pressure, the more gasoline must be injected. If the fuel pump and / or nozzles do not come to a stop at high engine speeds and high boost pressure, the combustion chamber will become too hot and the pistons may melt! Although an ad does not allow a correction of the mixture, a timely off-gas can save the engine.

3) Intercooler

A larger intercooler is not mandatory with BPU. But the series LLKs are not in good condition for many supras due to their age. One should, before upgrading to BPU, check its LLK for condition and replace it if necessary. Corroded or even damaged by rockfalls slats can significantly affect the cooling performance.