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, Sat, 14 Sep 2002 15:08:44
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The T5 does not have the motorized throttle. So when you increase the boost with an external controller (MBC), the ECU can only do two things.
One is to map the fuel at the higher pressures as long as the ECU has not being set up to cut all fuel at a set pressure. When operating within this pressure range of overboost, air/fuel ratios are perfectly maintained. No problems with fuel pressure or injectors at all.
The other, you guessed it, it cuts the fuel when the pressure reached at a certain point. For the 5spd T5s, that pressure is reported to be 1.05 bar.
A modified ECU will move that fuel cut pressure and create more boost as well.
The MAP: sensor signal can be modified/hacked. This makes the ECU see a smaller pressure level than reality. That will throw off the AF mixture, but the ECU quickly adapts via the O2 and recalibrates. The ECU just thinks that it is running at a rather high altitude, and does not seem to care that it is fueling more than sea level. Its rather adapative, but really has little idea about whats going on. You see it also has to adapt to variations in the set points of the (non adjustable) fuel pressure regulators. So this hack will also increase the boost. This can be used alone or to shift the fuel cut point of the ECU with a MBC. Without a ECU mod, one can only go so far where the ECU will throw a CEL as the injector duty cycle is getting to large in seeking O2 adaption. That problem will go away if you increase the fuel pressure regulation. But a minor change will not have such problems and that minor MAP hack with a MBC will allow for good boost without creating a O2 adaption CEL. A minor MAP hack might be useful to avoid MBC pressure overshoot fuel cuts. Thats great in theory, but many folks will want to increase the boost to dance with the fuel cuts at a higher level of boost. But that is not a technical problem, but a human factors problem.
A fuel cut is rather harmless, it just is an event where the ECU will not fire the fuel injectors for a moment, until the pressure drops, which is does rather quickly. In the lower gears, this can feel rather alarming to the driver, who is motivated to not do this very often. The ECUs will log the events, but don't throw CELs when this happens.
I hope that is useful. We are not going to get technical dislosures of the internal workings of these ECUs, but they are not that hard to understand at a behavioural level.
And the stardard firm caution. It is considered good practice when increasing the boost pressure on an engine, to reduce intake and exhaust restrictions. Restrictive intakes and exhaust systems not only reduce the power, but they increase temperatures which can lead to knocking prone conditions. Yes, you can purchase an Abbott modified ECU and install it into a stock system. If there are any problems and you have the BPC/APC functions intact, the ECU will cut the boost. So there is a fail safe. With the MBC you don't have that. But the ECU controls knocking very well with timing and mixure control when the BPC is overridden with a MBC. So we know that to be true in a properly fueled well maintained engine that is in decent mechanical condtion. You can probably slap a MBC onto a an otherwise stock setup without any problems. But don't come whining to the BB if you have any knocking or other adverse problems if you don't heed this recomendations. Most folks here follow these guidelines, which are not just mine, and problems just don't exist.
If you installed an Abbott ECU on an otherwise stock system, you would be really limiting the potential of the engine. So you need to make these changes also because they increase power and reponsiveness.
And I need to point out that the BPC valves are electrically moldulated basic bleeder valves. So it is no surprise when you understand why bleeder valves create boost lag, that the factory BPC setup creates lots of boost lag too. This is one good reason to run a MBC, other than more boost. The MBC greatly decreased boost lag.
And to put a balance on things, a MBC does lead to fuel cuts until adjusted to the point where the pressure overshoots don't cause this anymore. But in cold weather the boost pressure comes on so much faster that fuel cuts can come back. Much of this goes away with a throttle foot that knows more than bashing the petal down. But such problems remain. And the basic MBC install that taps pressure from before the IC leads to boost droop at high RPMs (CFMs). Some of that is solved with a relocate and a Viggen IC, but the MBC will always have some downsides. But many people are very happy with the MBCs and will gripe about such issues, but few want to go back to stock. And a fair number have moved on to modified ECUs. And others go for modified ECUs and MBCs and go after 22 PSI of boost.
So I hope that this is informative and you can see the thinking and issues involved. (But there will be some who still persist that it is optinion, not information.)
posted by 65.68.10...
, Sat, 14 Sep 2002 15:08:44, Sat, 14 Sep 2002 21:02:01
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