Boost pressure is the amount of air pressure (measured in PSI or psi - pounds per square inch) that a turbocharger or supercharger forces into your engine above atmospheric pressure. Atmospheric pressure at sea level is 14.7 PSI, which is the baseline amount of air your naturally aspirated engine receives naturally. Boost pressure adds to this baseline. For example, 10 PSI of boost means your engine is receiving (14.7 + 10 = 24.7 PSI) of total air pressure. More air means more oxygen available for fuel combustion, enabling larger explosions in the cylinders and generating more horsepower and torque.

Horsepower gains from a turbocharger depend primarily on boost pressure and secondarily on engine displacement, fuel quality, and tuning. As a general rule, each PSI of boost adds approximately 0.07 horsepower per baseline horsepower (for the simplified method). More precisely, the pressure ratio formula shows that doubling pressure (14.7 PSI boost) doubles power output. At 10 PSI boost on a 350 HP baseline, expect 420-490 HP (20-40% gain). At 15 PSI, expect 560-700 HP (60-100% gain). At 20 PSI, expect 700-900+ HP (100-150% gain). Real-world results vary 10-15% depending on engine condition, tuning quality, and supporting modifications.

The primary boost horsepower formula is the pressure ratio method: HP_boosted = HP_na × [(PSI + 14.7) / 14.7]. This accounts for the proportional increase in air pressure and available oxygen. A simplified version approximates: HP_gain ≈ 0.07 × PSI boost (for average engines). Another method: HP_boosted = HP_na × (1 + boost_percentage). For superchargers, the average real-world formula is: HP_boosted ≈ HP_na × 1.46 (46% average gain accounting for mechanical losses). Choose your formula based on calculation method in the calculator.

Safe boost depends primarily on engine construction and compression ratio. Stock production engines with factory compression ratios (9.5:1 to 10:1): Maximum safe boost is 5-7 PSI on 87 octane, 7-9 PSI on 93 octane. Mildly built engines (lower compression 8.5:1-9:1): Safe to 10-15 PSI on premium fuel. Heavily built race engines (low compression 7:1-8:1): Safe to 15-25+ PSI with proper tuning and race fuel. Rule of thumb: Lower compression ratio allows higher boost. Always consult your tuner before exceeding factory specifications. Listen for knock/ping (detonation warning sign). Excessive boost without supporting modifications causes catastrophic engine damage.

Superchargers: Mechanically driven directly off engine (instant response, no turbo lag), sap 10-15% engine power to run, add 30-50% power typically, simpler installation, higher cost for power gained. Best for: Drag racing, street performance where instant torque matters. Turbos: Exhaust-driven (more efficient), slight spool-up lag (50-200 ms), add 40-100%+ power depending on size, lower cost, can reach higher boost safely. Best for: Highway cruising, spirited driving, efficiency-minded owners. Both work great; choice depends on priorities: turbo lag acceptable for efficiency gain, or pay more for supercharger responsiveness.

This calculator provides reasonable estimates typically within ±10-15% of real-world results for properly tuned vehicles. The simplified method is less accurate (+/- 20%). The pressure ratio method is more accurate, especially for higher boost levels. Real-world results depend heavily on fuel quality, ECU tuning quality, intercooling efficiency, air temperature, altitude, engine condition, and supporting modifications. Use dyno testing for precise measurements of your specific vehicle. These calculations should inform decisions, not be treated as guaranteed outcomes. Consider this a planning tool rather than a definitive prediction.

Atmospheric pressure is the weight of air pressing down on Earth's surface due to gravity. At sea level, this pressure is 14.7 PSI (pounds per square inch) or 101.325 kPa (kilopascals). This is the baseline air pressure that naturally aspirated engines receive for free. As you increase altitude, atmospheric pressure decreases (Denver at 5,280 feet = ~12.2 PSI). This matters for boost calculations because your engine's absolute pressure is: atmospheric pressure + boost pressure. A vehicle with 10 PSI boost at sea level receives 24.7 PSI total. The same vehicle at high altitude receives only 22.2 PSI total (12.2 + 10), reducing power output despite same boost reading. Boost is always measured as pressure above atmospheric baseline.

Moderate boost (6-9 PSI) is generally safe on stock unmodified engines when tuned properly. Higher boost (10-15+ PSI) requires engine modifications: lower compression pistons, stronger connecting rods, reinforced block, quality fuel injectors, and proper ECU tuning. Excessive boost without supporting modifications causes knock/detonation (uncontrolled fuel combustion), which destroys pistons, rod bearings, and can catastrophically fail an engine. Temperature control is critical—an intercooler prevents inlet temps from climbing excessively. Quality tuning prevents knock by adjusting ignition timing and fuel mixture. Without these precautions, boost severely damages engines. Consult a qualified tuner before boosting any vehicle. Stock engines were engineered for naturally aspirated operation and boost fundamentally changes the loads they experience.

Intercoolers are heat exchangers that cool compressed air after the turbo/supercharger but before the engine. Compressed air heats up (compression generates heat), and hot air is less dense than cool air. By cooling the air, you increase its density, packing more oxygen into each cylinder—more oxygen enables more fuel to be burned, producing more power. Intercooling also reduces surface temperatures, lowering knock risk and allowing safe higher boost without detonation. Quality intercooling improves performance 15-25%, depending on design and ambient temperature. Larger intercoolers are more efficient but add weight and complexity. In hot climates or summer conditions, intercooler efficiency drops significantly, reducing boost effectiveness. Winter and cold air intake kits improve intercooler efficiency. Neglecting intercooling when running moderate-to-high boost is a common mistake causing suboptimal results and detonation risk.

Torque typically increases MORE than horsepower percentage-wise when adding boost. If you gain 40% horsepower, you'll often see 50-60% torque increase. This is because torque is calculated as Power × 5252 / RPM, and boost increases power across a broader RPM range (wider power band). A supercharger adds average 31% more torque according to real-world studies. Turbochargers often provide even greater torque improvements because they push air at low RPMs (when naturally aspirated engines are weakest). This is why boosted vehicles feel so responsive off the line. Torque increase is actually the more noticeable improvement for daily driving compared to the top-end horsepower gain. This torque is important for towing, acceleration, and responsiveness that drivers experience most frequently.

Higher octane fuel resists detonation (engine knock) better than lower octane. Detonation is uncontrolled fuel combustion that damages engines. When adding boost, compression and combustion temperatures increase, raising detonation risk. Lower octane fuel detonates at lower pressures/temperatures. Therefore, you can safely run higher boost with premium fuel. On 87 octane (regular): Limit boost to 5-7 PSI for safety. On 91 octane (mid-grade): Safe to 8-12 PSI with proper tuning. On 93 octane (premium): Safe to 12-18 PSI depending on engine. On 98+ octane (race fuel): Can safely exceed 20+ PSI on built engines. Using too low octane fuel for your boost level risks knocking, which destroys your engine instantly. Always use fuel octane appropriate for your boost level. Premium fuel costs more but is insurance against catastrophic damage when boosting.