TeamFC3S -- ECU & Fuel System Hints and Tips

ECU & Fuel System Hints and Tips

Boost Cut. What is it?

When Mazda designed the car, they wanted their production Turbo rotary to be considered reliable and not tarnish the rotary reliability reputation. They imployed many tricks to achieve this and one of them was to maintain a maximum level of boost that you couldn't exceed. I guess they worried that if Joe Shmo V8 was under the hood pulling vacuum lines and happened to rip out the wastegate line, the engine wouldn't try to launch the apex seals into orbit. So, they programmed the computer to cut fuel off to the rear rotor at approximately 8.6psi of boost. This is measured by the Pressure Sensor found behind the stock air box on the strut tower. They ran a vacuum line from the intake manifold to this sensor to control boost. Just to make extra sure everything worked the way they wanted, they used that box to control the secondary injectors which come into action over 3800rpms. So, just pulling the vacuum line won't help. There are many ways to defeat this cut off. One of the least expensive, most popular ways is the Racing Beat Defender. This just lies to the ECU by introducing more resistance and lowering the signal seen by the ECU low enough so that it doesn't sense enough boost to cut fuel. This can cause the fuel mixture to lean out but, I assume Racing Beat felt this was better than having the rear rotor melt down from no fuel.

Detonation is a voilent explosion that can damage the engine severely. It also sounds and feels very much like Pre-ignition. The things that can cause detonation are
Too much air, not enough fuel (running lean) Running extremely rich Excessively high intake temperatures (too much boost, small turbo, small IC) Incorrect timing or malfunctioning Knock Sensor Poor Ignition (discussed in 'Ignition Upgrade' section) Too low of an octane fuel
You can usually hear or feel detonation when it's happening. The sound is similar to popcorn popping or tapping on metal. You can usually feel it either through the pedals or shifter as suddenly jerks and vibrations. Detonation will cause very rapid engine wear and sudden apex seal breakage or other seal failures. The only instant cure for detonation is to immediately get off the throttle. So what if you lose the race, at least you can go on to race another day. Check to make sure your timing is correct, you have enough fuel (use a EGT or A/F ratio meter), make sure you aren't over-spinning the turbo, etc. Detonation will not happen for no reason so make sure you check everything. Pre-ignition is when the air/fuel mixture gets ignited before it's proper time in the combustion camber. It is most often caused by hotspots in the engine such as carbon buildup; HKS and GReddy both have kits that allow up to 350HP with the stock fuel pump. The key on their setup is additional injectors. The stock injectors are roughly 550cc and you have 4 of them, staged in pairs. The primary injectors are used for idle and under 3800rpms. The secondary injectors come on when needed above 3800rpm and supply extra fuel. With the stock ECU, injectors, and fuel pump, you are limited to about 230HP or 12-14psi of boost with the stock turbo and 92 octane fuel. Even with an adjustable fuel pressure regulator, the stock pump can only get so much fuel out. The options are to either install additional injectors and a controller and/or upgrade the fuel pump. So people have tried to simply replace the secondary injectors(530cc) with injectors out of a GSL-SE (~680cc) which allows more fuel with a stock fuel pump for the high end but mid-range will suffer because the ECU doesn't know to adjust the pulse width for the increased injector size and you get rich mixtures. Primarily, if you plan on upgrading the turbo, you need to look for ways to fuel it because you are increasing air flow. There are two schools or thought, or methods for upgrading the injectors. The first is what most bolt on companies like HKS and GReddy recommend which is to mount the injectors (usually two 550cc for 350HP applications) onto the upgraded intercooler piping (the intercooler usually needs to be upgraded before you worry about fuel). This has the advantage of being very easy to install yourself. The disadvantage is that you are placing injectors before the throttle body and thus spraying fuel onto a dry throttle body which will cause carbon build up and poor atomization of the fuel. The second method, which I think is much more effective, is to plumb the injectors directly into the upper intake manifold. This gives you a more direct method of injection, better atomization, low carbon buildup, and more fine-tuning of the A/F mixture. When using two additional injectors, the most popular runners to plumb the injectors into are the primaries (outside) but there is no rule a tuner must abide by. Just whatever you find that works best.
: First, think about how much peak HP you'll want and maximum amount of turbo boost needed. Then, we calculate the maximum CFM of air flow we will be flowing. This is done by; (CID * MAX RPM * 1.10)/1728; Example; (80 * 8000 * 1.10) / 1728 = 407.41 CFM; Then we create a Pressure Ratio (PR); (Additional Boost + Pressure Loss) / 14.7; Example; (6psi + 1.5psi) / 14.7psi = 0.51psi; The Additional Boost is the amount of boost you wish to run minus the maximum amount of boost your fuel system can handle. The stock system maximum ratio would be the maximum boost you can achieve with a 12.7:1 - 13.1:1 Fuel ratio with your new or upgraded turbo. You don't need to worry about additional injectors with the stock turbo because you will overspin it before you need additional injectors. It's always best to underestimate your stock system than to overestimate and end up with too little fuel for your target HP. The Pressure Loss is the loss in boost from the compressor side of the turbo to the intake manifold (through the intercooler). The stock intercooler has a pressure loss of appromiately 1psi per 6psi of boost produced at the turbo. So, my above example was if I wanted to run a total of 16psi with a stock fuel system, stock computer, and an upgraded intercooler that flows slightly better than stock.
Next, we figure out how much extra air will need to be fueled that our stock system can't handle.; Base CFM * PR * Compressor Efficiency (CE) * Intercooler Air Density (AD); Example; 407.41 * 0.51 * .72 * 1.30 = 194.48 Total CFM needing fuel; So, now we know how much air is being flowed that needs fuel, we can now find out what size injectors are needed to fuel the air and help convert it into power. The above formula used two values we found in the past two formulas, the Base CFM and Pressure Ratio. The two new values are something that depends on your application. The Compressor Eff. can be found by looking at the compressor map of your turbo. At given boost and CFM, you get a certain efficiency. If you are unclear about this, refer to the Turbo Hints page for info about reading a compressor map. The Intercooler air density can also be found from your intercooler map. Much like a turbo map, the intercoolers are most effiencent at certain CFM's and how fast air is blowing over the core. Since more dense air needs more fuel, use the 1.30 value if you do not know the exact Air Density of your intercooler. This value will insure you get enough fuel no matter what.
Now we figure out the Base Injector size needed.; (Total CFM * 3.75 * MAX RPM) / (# of additional injectors * 6000rpm); Example; (194.48 * 3.75 * 8000) / (2 * 6000) = 486.20cc; This formula now makes a few assumptions. The first being that you will want to run a A/F ratio of 11.5:1 @ 36psi of fuel pressure. If you want to plan on a leaner mixture, you can use the value 3.1 for a ratio of 13:1 @ 36psi. This might be helpful if you plan on running higher fuel pressure.
Now we have the injector size we need if we were running to run it at 100% duty cycle. Since this isn't a good idea, we are now going to increase the size of the injector to allow the same fueling capabilities at only a 80% duty cycle.; Base Injector size * 1.2; Example; 486.20cc * 1.2 = 583.44 cc; So, we now have figured out that we are going to need at least a 583cc injector or higher to supply enough fuel. Since most injectors aren't sold in custom sizes, we have to figure out the closest size that matches what we want. In this case, I would suggest going with 550cc injectors and installing a Fuel Pressure Regulator (FPR) to insure enough fuel pressure. Smaller injectors has an advantage over larger injectors in that you can control the fuel flow better. Think of it like this, you have building that's 20ft tall and one that 30ft tall and you have to make a ladder to reach the top but to are only given 20 steps per building to make each ladder. Obviously, the 20ft building and ladder combination is going to allow closer steps, just like a smaller injector allows more fine tuning.

How exactly does a Fuel Cut Defender work?

The Racing Beat Fuel Cut defencer is a basic resistor that plugs into the pressure sensor behind the stock air box. It's function is to alter the signal sent to the ECU and cause the fuel cut to be moved to a higher level. Some people have tried adjusting the values of the resistor to gain higher boost without fuel cut but this is also going to cause the engine to run more and more lean under boost applications because the ECU thinks you are running lower boost all the time.
Currently, a few members of TeamFC3S are developing a less expensive, more effective fuel cut defencer. Once this device has been properly tested, detailed instructions on its construction will be made available exclusively on the Turbo II Performance page so Stay Tuned!

Last Updated: April. 18, 1999, by: Mike McLeish