- stock fuel system, pump gas: 15 psi, or an O2 sensor reading of 0.85V minimum, or an EGT reading of 1650 degF maximum.
- upgraded fuel pump, stock turbo, pump gas: same as above.
- upgraded fuel pump and injectors, stock turbo: same as above. You will probably hit fuel cut first. Here's why.
- turbo upgrades: varies, but typically lower than 15 psi. You'll need a fuel management system before you can do much of anything more than that. Here's why.
Owing to individual variations between cars, the 0.85V O2 reading mentioned above should be treated with extreme care. Most people prefer 0.90V or better.
By changing the intake pressure in your car, you are changing the mass of air entering the engine. In order to maintain a proper air/fuel mixture, more fuel must be delivered to the engine to compensate. Therefore, the 'safe' amount of boost on any DSM is primarily determined by the fuel delivery system in the car. Also, local factors will change the actual mass of air entering the engine, which again changes the required fuel.
Under all circumstances, remember this simple rule: intake air pressure does not equal intake air flow. Flow is what matters.
The amount of boost pressure you can run without risking damage to your engine depends on the following factors:
There may be other factors that affect maximum intake pressure. Those interested in a more detailed discussion can read Morgan D'Antonio's post about what limits boost levels on DSMs.
In the above list, the fuel pump and fuel injectors determine the amount of fuel that can be delivered to the engine in a given time. Aftermarket pumps and larger injectors increase fuel delivery capacity.
Turbo and intercooler efficiency determines the temperature of the air going into the engine at any given pressure. Lower temperatures reduce combustion temperatures, but also increase the actual mass of air entering the cylinders. Colder ambient temperatures also increase air mass. Higher altitude cars, though, have less dense air to push than sea level cars, meaning less air mass enters the engine.
MAS modifications introduce an additional element of error into the mass air calculations done by the ECU. Because the ECU, under some conditions, does not check to see that the amount of fuel provided is adequate to safely operate the engine, the operator must monitor the engine operation. Most 'free' mods do not change the MAS operation enough to cause problems, but there is always a small possibility of engine damage, as well as idle and misfire problems.
Gasoline octane is important in that it also reduces the occurance of knock. Running too much boost on bad gasoline with leave your engine pinging like crazy, forcing the ECU to cut boost and reduce timing, thereby losing power.
The rule, accoding to leading DSM performance authorities, is to run at least 0.85V from the oxygen sensor at all times. This corresponds to exhaust gas temperatures of about 1650 degrees F maximum. Failure to observe these limits will often result in melted engine parts.
As a general guideline for those without air/fuel or EGT gauges, sea-level cars may safely run 15 psi (1.0 bar), with the stock fuel pump and turbocharger. The addition of other mods does not generally change this figure, provided the intake pressure remains at 15 psi maximum. This is the level that most owners run in their cars, at least until further upgrades are possible.
For those not operating near sea level, the 'safe' boost you can run seems to increase by about 1 psi (0.07 bar) for each 1000' over sea level. For example, a car at 3000' over sea level can safely run as much as 18 psi of boost. This rule is less reliable than the above 15psi rule, though, since high-altitude owners are understandably reluctant to repeat the boost-related engine damage experienced by some unlucky sea-level owners.
Those owners with upgraded fuel pumps may also run higher boost levels than 15 psi. If you want to do this, presumeably you have installed air/fuel (A/F) and/or exhaust gas temperature (EGT) gauges in your car to monitor the engine operation. Moving to higher boost levels without at least one of these instruments can be hazardous to your cars health. Provided you meet the above-mentioned 0.85/1650 rules, there is no limit to the boost you can run until you hit fuel cut.
It should be noted, however, that the stock DSM turbochargers are pretty much incapable of making more power somewhere around the 15-17 psi level. Above this limit, the turbocharger heats the intake air so much during compression that any power gained from additional air density is lost. Many people have reported this effect, which leads to less power at higher boost pressures.
Also, higher boost pressures exact more stress on the engine components, regardless of the air mass. After all, 18 psi is still higher than 15 psi. Owners that run pressures higher than 15 psi may experience failures on other components that cannot take the stress. One good example is the intake manifold gasket, which was made of rubber in 1990-1992 cars, and sometimes cannot handle the increased pressure. (Fortunately, the replacement 1993-1994 gasket is metal.)
Those interested in the factors responsible for limiting boost should read Morgan D'Antonio's take on what really limits your boost levelson a DSM.
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