SVDO can't be tuned by people who are used to old piggybacks because they can't figure out closed-loop lambda-based ECU operation. Old MMC ECUs are lame. Misinformed people say they're "good" just because they're easy to be fooled by piggybacks. SVDOs are more advanced than MMC ECUs any day, that's why they're harder to fool and are "poorer" for tuning purposes.

MMC ECUs use mass airflow sensor to read the incoming air charge. Then, it determines the injector duty cycle based on the internal map. Easy as that. If you plug in higher flow injectors, your engine will run richer all the time. Plug in lower flow injectors, your engine runs leaner all the time. The mapping of MAF readout <-> injector duty cycle is static.

Traditional "tuning" on such ECUs involve only the interception of MAF output. An altered MAF signal is provided to the ECU to fool it into using a different injector duty cycle (by fooling the ECU into thinking there's more or less air than there actually is).

For SVDO ECU, it uses the lambda sensor for closed-loop operation at idle up to around 3500 rpm. Furthermore, it uses a MAP sensor that uses manifold vacuum to determine airflow.

If you intercept the vacuum signal to fool the ECU into thinking there's less air, the lambda readout will still see the actual air-fuel ratio. Within the closed loop mode, the injector duty cycle target is based on air-fuel ratio. If you wanna tune the SVDO ECU, you'll have to intercept the lambda sensor value from idle to 3500 rpm.

Above that, the SVDO ECU goes into open-loop mode, and functions similarly to the older ECUs.. by using a static injector duty cycle map for the MAP sensor inputs. It won't take into account the air-fuel ratio.

Hence, overall, you'll have to intercept the lambda sensor signal from idle to 3500rpm, and alter the MAP sensor signal above 3500rpm.

The SVDO MAP sensor in theory can detect up till 0.8 bar of positive pressure in the manifold. However, the output will be a full +5V back to the ECU. The SVDO ECU will see values like that as a fault condition. One solution is to clamp the output voltage with a zener diode so that the MAP sensor output never exceeds a certain point, e.g. 4 or 4.5 volts. That way, the ECU won't detect a fault condition. The disadvantage is that the MAP sensor response will be non-linear at higher manifold pressures, since we're cutting off the maximum output abruptly. Another solution is to use a voltage regulator at the MAP sensor input to reduce the input voltage from 5 volts to something like 4.5 volts. In effect, we're scaling down the entire range of output for the MAP sensor proportionately, so the sensor response will still be linear.
SVDO ECU has the added advantage of having a built-in knock sensor that ties back to the ignition advance mechanism (electronic ignition advance, not like in the older ECUs). Thus, engine knocking due to lean mixture can be mitigated by the SVDO ECU and it compensates by enriching the fuel mixture. However, I've heard that the knock sensor input isn't present on the 4G15, it's only on the Satria GTI's 4G93.

The closed-loop function can actually help somewhat. In theory, you can practically tune a turbocharged SVDO ECU without a piggyback and by using an AFR meter to check your air-fuel ratio. At above 3500rpm, the ECU goes into open-loop static map. During that time, adjust the fuel pressure regulator until the AFR shows a slightly rich value (e.g. 12:1 or 12.5:1 AFR). Some tuners will set the AFR as rich as 10:1 with piggybacks, which is quite unnecessary unless you're running big boost. With a low boost, a slightly rich AFR will do.

Below 3500rpm, regardless of the fuel rail pressure, the ECU will try and adjust the injector duty cycle to achieve a target AFR. In the case of SVDO ECU, its target isn't really stoichiometry.. it's a bit richer, at around 13:1 to 13.5:1 because a slightly rich mixture will burn cooler than a stoichiometric mixture. If the engine can handle the thermal load (which should not be a problem at low boost), you can just let the closed-loop mode sort out the injector duty cycle automatically.

SVDO tuning on a shoestring budget will thus probably only involve: MAP sensor clamp/regulator; adjustable fuel pressure regulator; uprated injectors (240cc should be fine for mild boost); and an AFR meter for ensuring your engine doesn't lean out.

For the BOV, venting off the excess air at such a low boost might hurt the engine response. And don't vent it into the intercooler piping, as that part of the air plumbing also has compressed air (roughly the same pressure as the vented air, so you'll just keep the intake side choked with no actual blow-off to relieve the pressure). Vent it back to the turbine's "cold" side (air filter side), i.e. send the vented air back into the air inlet of the turbine. This is the standard blow-off path used by stock turbo setups on light pressure turbo cars.

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Added on November 17, 2008, 6:50 am

1 way is of course to use an external wastegate with a soft spring.. which I think is not so common. You'll vent off the excess exhaust gases into the atmosphere, so your car would be a pretty noisy machine with the wastegate dumping exhaust at 0.3 bar.. that would be pretty often!

The most cost-effective solution is to cut the stock actuator arm, and alter it. If you have a spring pressure gauge to accurately measure the amount of deflection required to produce the amount of wastegate opening (it's related to the actuator's diaphragm spring rate), then you can derive how much of the actuator arm you need to add to open the wastegate sooner. Longer arm = shorter travel to open wastegate = lesser pressure to open the wastegate = lower boost. Shorter arm = longer travel to open wastegate = more pressure to open the wastegate = higher boost.

Some people DIY to make the actuator arm length adjustable, by threading the cut actuator arms and then inserting nuts and an additional hollow sleeve.