1978/06/14 Caterpillar

All Engines

Because of the type of information in this article, it is not written in Caterpillar Fundamental English.

There has been considerable confusion in the field concerning the correct definitions, usage, and applications of the following nomenclature.

1. Inlet Manifold Pressure

2. Bring The Engine To Normal Operating Temperature Before Measuring Inlet Manifold Pressure

3. Engine High Idle RPM

4. Altitude Consideration In Horsepower Rating

The following explanations should help clear up this confusion and lack of understanding.

Inlet Manifold Pressure

Inlet manifold pressure (frequently referred to as "boost") is the air pressure being delivered to an engine. This pressure is most commonly measured in inches of mercury gauge, but sometimes you will find it specified in absolute pressure or PSI. It has been common practice to compare measured pressures on engines with published figures in the Rack Setting Information Book to try to determine if an engine is producing its rated horsepower. These two quantities, manifold pressure and horsepower, are not always related, therefore, inlet manifold pressure by itself can not be used as a definite indicator of the horsepower being produced. Other factors have to be considered.

To use inlet manifold pressure as an indicator of the horsepower being produced, the guidelines listed must be followed. If there are deviations from these guidelines, the inlet manifold pressure measured is not a true indicator of the horsepower being produced by the engine and should not be used as such.

1. The engine is in good working condition.

2. Proper adjustments have been made to the governor, fuel system, timing, etc.

3. Correction factors for barometric pressure, ambient air temperature and fuel API have been taken into consideration.

4. The engine has been brought up to proper temperature.

5. Special attention has been given to any qualifications explained in the Rack Setting Information Book.

6. There are no leaks in the inlet or exhaust systems.

7. Air inlet and exhaust restrictions are within the published limits.

8. Adherence to information in Special Instruction Form FEO36044-06, Engine Performance Evaluation and Adjustments with 4S6553 Instrument Group.

If the guidelines are followed, the measured inlet manifold pressure should fall within the specified range published in the Rack Setting Information Book. A comparison can now be made between measured inlet manifold pressure and horsepower to get an indication of whether the engine is performing properly or not.

It should be pointed out though, that inlet manifold pressure can vary from engine to engine. Therefore to assume that a measured pressure at the low end (or high end) of the range is an indication that the horsepower is at the low end (or high end) of the allowable tolerance is not necessarily correct.

Bring The Engine To Normal Operating Temperature Before Measuring Inlet Manifold Pressure

An engine being checked for inlet manifold pressure should be at normal operating temperature before this measurement is taken. Also, if the inlet manifold pressure is to be checked at the torque converter stall speed, the torque converter must be at normal operating temperature before this check is made. This is necessary to stabilize all systems for obtaining valid measurements to compare with published specifications.

If the engine to be checked is on a dynamometer, it can easily be warmed to operating temperatures by loading it with the dynamometer. (Refer to Special Instruction Form GEGO1024-02, Dynamometer Testing Caterpillar Diesel Engines). If the engine is in a chassis or in its normal operating environment, Special Instruction Form SEHS7050, using the 5P2160 Engine Horsepower Meter Arrangement, lists techniques for loading engines in their several different types of applications.

High Idle RPM

The high idle column in the Rack Setting Information Book is described as "engine high idle" or "high idle-engine only" and "high idle-in vehicle." The terms "engine high idle" and "high idle-engine only" indicate the high idle for the engine when it is out of the vehicle or its working environment and the parasitic loads such as fan, hydraulic pumps, power take offs, free wheeling torque converters, etc. have been removed. The term "high idle-in vehicle" is the high idle that should be expected or set to when the engine is in a vehicle. It will usually be a lower value than the "engine high idle" or "high idle-engine only". This is due to the parasitic loads imposed on the engine when it is installed or in its working environment. You can usually expect a 15 to 20 RPM reduction in high idle between "high idle-engine only" and "high idle-in vehicle". This is only an approximate value and can be significantly affected by the amount of parasitic load the engine is subjected to.

Altitude Considerations In Horsepower Ratings

The Rack Setting Information Book lists the maximum altitude at which an engine can develop its maximum horsepower when set at the given chart values. When the engine is set for a particular altitude, but is to be operated at a higher altitude, the rack setting, high idle and in some instances, timing, turbochargers and pistons may need to be changed. This must be done or engine and turbocharger damage could result.

Similar cautions should also be taken into consideration when an engine is set for a special high altitute rating and then is brought down to a much lower altitude to operate. If the engine timing, turbochargers or pistons have been changed to operate at the higher altitude, a check should be made to determine if the changes made are still compatible with the lower altitude or if they need to be changed again.