High oil and coolant temperatures can also cause detonation. These, then, are two more things you must guard against. Always make power changes smoothly and evenly. This engine won't absorb a sudden blast of power without acting up. Move the controls slowly and smoothly to the desired settings instead of trying to pick up the proper setting with one swift movement. If you reduce the rpm first, you get a jump in manifold pressure which can cause detonation if the relation of rpm to manifold pressure is greatly altered. An engine running at constant power settings receives a constant amount of fuel and air from the blower. If you lower the rpm and the manifold pressure remains constant, the blower continues to supply the same fuel-air charge. The engine, running at a slower speed, cannot absorb this charge. As a result, pressures in the blower and cylinder heads build up. and cause detonation. Therefore, never reduce rpm before manifold pressure.

TO INCREASE POWER: 1 Increase the rpm 2 Advance the throttle, If you advance the throttle before you increase the rpm, the same thing happens as when you reduce rpm before manifold pressure in decreasing power. As the engine picks up speed, the manifold pressure drops. This is a normal and desired reaction. Note: Above 12,000 feet, increased rpm gives you increased manifold pressure. The P-40 F and L models are powered with Packard Rolls-Royce engines. For standardization purposes many of these have been replaced with Allisons. P-40 F and L models which have been refitted with Allisons are called P-40 R-I's and P- 40 R-2's respectively. ALL POWER SETTINGS GIVEN IN THIS MANUAL ARE FOR ALLISON ENGINES AND GRADE 100 FUEL. IN MANY CASES THE SETTING FOR OPERATING WITH GRADE 91 FUEL IS THE SAME AS THAT FOR GRADE 100. WHEN THERE IS A DIFFERENCE, THE SETTING FOR GRADE 91 IS STATED IN PARENTHESES AFTER THE SETTING FOR GRADE 100.