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Regency Generators Knowledge Base

How to... Measure Ambient Clearance, For Open or Canopied Sets

Why Measure Ambient Clearance?

Calculating ambient clearance of an engine is useful in a number of scenarios, including testing new canopy designs; confirming ambient clearance of engines and ensuring airflow is sufficient when placed in a difficult location.

Requirements for measuring ambient clearance

You require the following items:

  • Tools for your engine to remove the thermostat.
  • A way of jamming the thermostat in the fully open position, or a spare thermostat for the engine concerned.
  • The correct mix of coolant in the radiator and cooling system.
  • A 'K' type probe for measuring the ambient and return water temperature.
  • Suitable personal protective equipment for testing generators.
  • The maximum top tank temperature of the engine you are testing. This can vary by a lot between engines, so ensure you check the specification sheet. If you have used a non-standard radiator that isn't supplied by the engine manufacturer, you need to check the same figure for this radiator. We call this figure "MaxTopTank"


Remove the engines thermostat. Fix the thermostat into a full open position to achieve maximum coolant flow. Note, for some engines this can lead to destruction of the thermostat and therefore having a suitable genuine replacement in stock is advised.

Replace the thermostat.

Measure the ambient air temperature, using the K type probe. Record it. We will call this "AmbientTemp"

Replace pipework removed during thermostat removal, fixing the K type probe into the water pipe after the thermostat.

Run the engine on full load (or the load at which you want to test the clearance at) until you reach a steady state. This of course may take some time.

Now record the value that the engine has settled at. We call this "MeasuredMaxTemp"

Calculation Of Ambient Clearance

To calculate the maximum ambient clearance for this engine, use the following calculation.

AmbientMax = MaxTopTank - MeasuredMaxTemp + AmbientTemp.

Therefore lets take a Perkins 404A-22G. Its MaxTopTank is 113 degrees. Assuming the current ambient is 40 degrees C and we measure a top tank temputature of 90 degrees, the ambient clearance can be calculated as:

AmbientMax = 113 - 90 + 40

AmbientMax = 63 Degrees C.

Additional Notes

It is generally assumed that for worldwide clearance, 48 degrees is the target (it used to be 46). For EU clearance it is 41 degrees clearance.

Note that 55 Degree radiators refer to the 'air on' temperature, not the ambient. Therefore these things are not the same. You should not therefore aim to show 55 degree clearance on a 55 degree radiator, you won't get there.

Depending if the set is in a canopy, you will see a reduction in the ambient clearance compared to an open set, due to the additional air flow restriction on the canopy.

After you have performed this test, if you are using a controller that is possible to adjust / configure the curve of the sender, you can adjust the unit to match the K type probes measurements.