Mercruiser Combustion Air Requirements
Combustion Air Requirements – Marine Engine compartments with natural draft ventilation must have vent openings of sufficient size and location to accomplish the maximum marine engine performance. IMPORTANT: The size of ventilation openings must be increased if any auxiliary equipment is located in the engine compartment. The combustion air requirement (per engine) for the specified engines at Wide Open Throttle are given in the chart below:
COMBUSTION AIR REQUIREMENTS (PER ENGINE) AND VENTILATION INFORMATION
|
MODEL |
ENGINE AIR REQUIREMENTS AT W.O.T. |
MINIMUM COMBUSTION AIR VENT (AREA/ENGINE) |
ENGINE PHYSICAL VOLUME |
|
3.0L |
251 ft.3/Min.(0.118 m3/sec.) |
25 in2 (161 cm2) |
2.4 ft.3 (68.0 l) |
|
4.3L |
364 ft.3/Min.(0.172m3/sec.) |
36 in2 (232 cm2) |
4.6 ft.3(130.3 l) |
|
5.0L |
424 ft.3/Min(0.200 m3/sec.) |
42 in2 (271 cm2) |
5.0 ft.3(141.6 l) |
|
5.7L |
486 ft.3/Min.(0.229 m3/sec.) |
49 in2 (316 cm2) |
5.0 ft.3(141.6 l) |
|
350 Mag MPI |
506 ft.3/Min.(0.239 m3/sec.) |
51 in2 (329 cm2) |
5.8 ft.3(164 l) |
|
MX 6.2L MPI |
545 ft.3/Min.(0.257 m3/sec.) |
55 in2 (355 cm2) |
5.8 ft.3( 164 l) |
|
454 Mag MPI |
657 ft.3/Min.(0.310 m3/sec.) |
66 in2 (426 cm2) |
6.0 ft.3(170 l) |
|
502 Mag MPI |
726 ft.3/Min.(0.342m3/sec.) |
73 in2 (471 cm2) |
6.0 ft.3(170 l) |
IMPORTANT: The amount of vent area required, according to boating standards(NMMA, ABYC, etc.) and Coast Guard regulations, for complete (total) enginecompartment ventilation must include the engine combustion air vent area plus the engine compartment ventilation requirements.
The pressure differential between outside and inside the engine compartment must notexceed the following value.
|
Maximum Pressure Differential at Wide Open Throttle |
|
2 in. (51 mm) of water (measured with a manometer) |
Compartment Temperature – Specifications
Air temperatures inside engine compartments have been measured in excess of 200° F (82°C). The long term effect to fuel system components running at these excessive tempera-tures is not known at this time.
According to specification SAE J1223 for Marine Carburetors:
“The carburetor shall be capable of operation throughout an ambient (air temperature)range from +20° to +176° F (–7° to +80° C) without failure.”
Carburetors, throttle body injection (TBI) units and components for multi-port EFI systemsused by Mercury MerCruiser meet this specification.
Under the hottest outside air temperature condition at which the boat will be operated, theair temperature inside the engine compartment, measured at the flame arrestor, shall notexceed176° F (80° C). Also, the temperature of the fuel being supplied to the engine shallnot exceed 110° F (43° C) at any location between the fuel tank and the engine’s fuel pump.
Since many factors influence engine compartment temperature, temperature measure-ments should always be carried out.
Test as follows:
1. The boat being tested shall be a standard production boat fitted as it would be for deliv-ery to a dealer.
2. Temperature test meter used shall be of the type that can be read without opening theengine cover.
3. During the test, in Step 4, engine compartments are to remain closed. No outside air isto be forced into the engine compartment during the test and the bilge blower should notbe running.
4. Engine Running and Heat Soak Test:
a.Use 1 meter and 2 thermo couple. Place one thermo couple at the flame arrestorto measure the inlet air temperature. Place the second thermo couple at the fuelpump to measure the inlet fuel temperature. A third thermo couple is needed if thefuel supply line between the tank and the fuel pump is higher than the fuel pump.Place the third thermo couple at the highest point of the supply line to measure thetemperature of the fuel at that point.
b.Start engine to warm it up. After engine is at its normal operating temperature, runengine at 1500 rpm (in neutral gear) for 15 minutes. Record both temperature read-ings at 5 minute intervals.
c.After 15 minutes running at 1500 rpm, shut engine off and continue to record bothtemperature readings at 5 minute intervals for the next 45 minutes.
d.After the 45 minute heat soak test, start engine and idle (in neutral gear) for 20 min-utes. Continue to record both temperature readings at 5 minute intervals.
IMPORTANT: If the temperature at either location exceeds specifications, the enginecompartment will need additional ventilation until both temperatures remain belowthese specifications.