HEY DDMCSE checked it out and found this, so im assuming the car cant run lean then just cuts the fuel.
what about the stock head gasket i fitted and arps will the head take it. cheers.
oh one last thing mate i have gurgling behind dash. wouldnt of thought head gasket. temperture fine water levels fine no smoke no water in the oil etc. plus only just finished head gasket replacement and gurgling was there straight away. suppose it could be air in the system
The boost pressure of the Toyota Supra can be raised to a certain limit. If the boost goes beyond this limit, the computer will cut of the fuel delivery for a certain period of time (about two or three seconds) because it assumes 'something is wrong'.
This boost cut or FCO (Fuel Cut-Off) is a nice thing to have, nobody wants their Supra to blow up of course. But there is one bad thing about it; it's way too early. The fuel gets cut off at about .70 kg/cm2 (that's about 10 PSI).
This really depends on the weather etc. In my situation this was (I live in Holland where it rains a lot), on other situations the boost cut can be a little bit higher, but after all, it's way too early.
Before I started to search for a solution to get rid of the annoying FCO, I first researched how the FCO gets activated and why exactly.
The computer of the car, monitors the amount of air sucked in via the Air Flow Meter (AFM) of which you see here a picture:
As you might be able to imagine, all air filtered by the intake gets through the AFM before it hits the wheels of the turbo. This AFM measures the amount of incoming air. Only a certain amount if air can be measured, e.g. the AFM has a physical limit. The exact limit I don't know.
The more air is sucked in by the turbo, the more fuel needs to be injected into the engine. This is achieved by giving the fuel injectors a 'longer' pulse. How long the pulse is, is normally counted in percentage 'duty cycle'. A duty cycle of 0% means no fuel at all and 100% means completely open. 100% is in practice impossible; the highest duty cycle in fuel injectors is something about 80%.
Now what is the physical limit causing the boost cut? This is a combination of the limits for both the AFM and the fuel injectors. At the boost cut level, the AFM cannot measure any much more air, and the fuel injectors are at their highest duty cycle. If people can confirm this (and the levels) please email me!
So the solution to this FCO problem, is somehow telling the computer the amount of air going thru the AFM is less than the actual intake air mass. This can be achieved by tampering with the Air Flow signal. A device taking care of this tampering is the HKS Fuel Cut Defenser (FCD).
The HKS fuel cut defenser
The fuel cut defenser from HKS tampers in such a way with the air flow signal, that the computer is told there is about 80% of the air sucked in, instead of the actual 100%. So the FCO is just a device to get rid of the fuel cut temporarily.
Although this can solve most of your problems, an FCD can be a highly dangerous device. As the computer is told there is about 80% of the original air flown in, the injectors are controlled to flow this amount of fuel as well, so in fact the engine will run more LEAN than originally planned. And this is NOT GOOD.
Lean conditions on engines can be really harmful. Pistons and valves can get too hot and your engine can be gone within minutes! So be warned, know what you do when you put in a FCD. When HKS designed this unit, they assumed your car was running way too rich, which is a fact. But running cars too rich is done to prevent lean conditions. So don't screw the air flow signal too much when you're not absolutely sure, ok?
Conclusion: The HKS FCD is a nice device for just a little tuning whatsoever, but when you start 'serious' stuff, keep your hands off the FCD.
So when we're not really 'allowed' to tamper with the airflow signal, how do we get rid of the boost cut then? Answer: Just REPLACE the airflow meter.
Replacing the AFM is a very common 'upgrade stage' in tuning cars. HKS has their own unit for replacing the airflow meter called the 'Vein Pressure Converter' VPC. This unit is quite expensive and totally not needed, as I found out after doing the Lexus Riemer upgrade.
The Lexus Riemer upgrade, is called as the two words stand for the most important ingredients: Lexus for the Lexus Air flow meter and Riemer for Reg Riemer, a Supra enthusiast from the states.
With this upgrade we replace the air flow meter of the Toyota Supra with one from a Lexus LS-400. This air flow meter is completely interchangeable with the Supra one; the electronics on the airflow meter even has the same serial number!
The only difference the Lexus AFM has with the one from the Supra is the size. The Lexus AFM is approximately 25% bigger, so the amount of air measured is about 25% less than the 'real' airflow.
Now when the computer gets this tampered signal, it will control the fuel injectors as if 25% less air was taken in; the duty cycle will drop with approximately 25% as well. Your car will run way too lean. To compensate this lean condition, the Lexus Riemer modification will tell you to replace your stock fuel injectors with 550cc fuel injectors.