1945 P-51 D Shutdown procedure

[Jacques]

Does anyone know the rationale behind moving the throttle full forward after the engine winds down under 700 RPM (and not before) as outlined in the 1945 manual?

Interesting to note that a 1950's manual warns that the throttle should not be advanced during shutdown!

I was just curious as to what benefit was being, or was percieved as being derived by accomplishing the action?

[stephan.cote.1]

I do not know.. but I would gess: To allow unrestricted flow of air in the engine to ensure all fuel left in the carb and cylinders is burned out properly... remember the merlin has a pretty massive carburator and even after the mixture is cutoff, their is probably a sustential quantity of fuel that might have access to the combustion chambers?

Could this also help the engine to smoothly spin to halt since the carburator is a sealed/pressurized type?

Maybe Dudley could step in here with corrections to my hypotheses?

Regard,
Steph

[DHenriques_]

I do not know.. but I would gess: To allow unrestricted flow of air in the engine to ensure all fuel left in the carb and cylinders is burned out properly... remember the merlin has a pretty massive carburator and even after the mixture is cutoff, their is probably a sustential quantity of fuel that might have access to the combustion chambers?

Could this also help the engine to smoothly spin to halt since the carburator is a sealed/pressurized type?

Maybe Dudley could step in here with corrections to my hypotheses?

Regard,
Steph

There were several pressure carb modifications done to the Merlin during the production runs. The initial carb was a two position carb then they modified that to the single RUN position carb. I can't speak to the early versions as our Mustang had the single RUN position. On ours the throttle also controlled the spark advance henceforth the 1 inch start position.
We never ran the throttle up on shutdown as that could really screw up the advance for the next start.
Normal shutdown procedure was to run in place for a few minutes at around 1500RPM to stabilize the internal pressures and temperatures then simply bringing the throttle back below 700RPM and the mixture into ICO.
Dudley Henriques

[robert41]

I got this from a Merlin engine manual dated 1944.
This manual is specifically for the Merlin 65 thru 85 engines with cut off, run and full rich throttle settings.
"The opening of the throttle after operating the cut off ensures a clean cut off without afterfiring."

[Jacques]

Thanks Gents! An interesting detail is that the 1945 manual actually does describe the two different mixture controls (dual and single position) and the spark advance set by the throttle lever in the single position version. Yet the recommended shut-down is to push the throttle full-forward once RPMs are below 700!
Perhaps this particular manual is a bridge between old and evolving technology and that the shitdown described was left in by mistake. I don't think I'll test whether A2A modeled "really screwed up" spark advance!