Merlin supercharger and 150 octane fuel.

[Black-Witch]

1. What changes were made to Merlin engines to get the extra boost, on a Merlin 66 for example, when using 150 octane.

 

2. Why, with 150 octane, do you need to change the supercharger gear manually?

 

Cheers

[357th_KW]

These engines used a manifold pressure regulator to avoid overboosting below the critical altitude of the engine.  By simply adjusting the aneroid switch on the regulator, it will allow more boost to be run (assuming the supercharger can produce it).  The supercharger system uses a similar aneroid switch to control the switch between low blower and high blower.  For some reason the USAAF adjusted the switches on the P-51 to allow them to continue to switch automatically, and the RAF didn't do this on Spitfires.  Possibly to make it easier to swap between fuel types?

 

http://www.wwiiaircraftperformance.org/mustang/V-1650-3_7_power_curve.jpg

 

You can see on a boost curve like that how the higher boost levels (72" shown on this chart) just fall right on the same curve as before, at lower altitudes.  The downside to the increase in boost is that the superchargers on these engines weren't sized for these boost levels, and so critical altitudes at those higher boost levels are quite low as a result. 

 

http://www.wwiiaircraftperformance.org/mustang/p-51h-booklet-pg10.jpg

 

As another example, here's a series of boost charts for the V-1650-9 from the P-51H which was designed around running 90" with water injection - the engine can hold boost much higher then on the earlier -3 and -7 engines (merlin 63 and 66 respectively) because the supercharger has been sized to run those boost levels.

[Black-Witch]

6 hours ago, 357th_KW said:

These engines used a manifold pressure regulator to avoid overboosting below the critical altitude of the engine.  By simply adjusting the aneroid switch on the

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Thank you sir, very well explained. I appreciate it.

 

Cheers

 

[Vortice]

On 9/16/2023 at 4:22 PM, Black-Witch said:

Why, with 150 octane, do you need to change the supercharger gear manually?

Because the RAF wanted you to have the opportunity to detonate your engine at the moment of your choosing, rather than let the decision be taken by a mindless mechanical system acting on your behalf.