P-40 engine

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Stevenbrown

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May 26, 2016
P-40 engine change in North Africa. I'm not very good at identifying engines. Is that an Allison or Merlin?
 

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An Allison had about 7,000 parts. A Merlin had about 11,000 parts.

Why use 12 screws when you can use 36? Ask anyone who works on a Merlin!

That's why the V-1710 Allison had an average overhaul time of 191.5 man-hours and the Merlin V-1650 had an average overhaul time of 320.2 man-hours. From Table 115 of the Statistical Digest of WWII. There is simply more to deal with when you work on a Merlin. The high for the Allison for any month was 376 hours (first month that any overhauls were performed). The high for the Merlin was 592 (the second month overhauls were performed). The low for the Allison was 117 hours and the low for the Merlin was 240 hours. These both fell in 1943 when the sortie rate was at its highest. Overhauls man hours simply means the total number of man-hours expended to complete the overhaul. It tells us nothing about the engines themselves.

I want to emphasize that these were overhaul man-hour times, not time-between-overhauls, or TBO. The TBO for the Allison started out like the Merlin at something like 250 hours, but worked its way up. The Allison TBO was always higher than the Merlin, except right at the beginning when both started around 250 hours. Also, the TBO was not the useful life of the engine at all. The TBO was the point at which the USAAC decided that, if they overhauled it at that particular point, then 95% - 100% of the engine blocks could be reused. New engine production was slated for new aircraft use, and they hated to steal one for an engine block that was no longer usable during wartime at overhaul. As experience was gained, I believe the TBO for the Allison went up to 450+ hours and the TBO for the Merlin went up to 350+ hours. I think both eventually wound up some 50 hours higher than that, at the end of the piston use era.

Today, both can see 850+ hours in civilian use, but the TBO today is NOT when the block can be reused, it is when the engine starts exhibiting signs of wear-out. I'd say most of the blocks for either engine in use today CAN be reused, because they mostly ARE reused. I know personally of one Allison that went 1,100 hours between overhauls! And, it was running well when they overhauled it. I can think of only a couple of Allisons that could not be refreshed, and those were catastrophic damage from tractor pull racing. Most of the Merlins I know of were similar, but I know of some 5 - 6 blocks that were deemed not reusable. At least two of these were race engines.

I hasten to point out that I do NOT know the histories of these engines, and cannot say or speculate how they were operated or how many prior overhauls were performed. Generally, in my experience, both are reliable and perform as expected for long periods of time. Almost NONE of the Merlins are absolutely stock these days, particularly flying Merlins. Many have aftermarket rings, pistons, or rods. Some Merlins are running Allison rods (usually race engines running Allison G-series engine rods). Some run aftermarket induction systems, including fuel injection (usually tractor pull engines). The reason is simple ... they are running out of original Merlin parts. Many Allisons are essentially stock, with all Allison parts, but most of the flying Allisons are essentially 100-series engine internals regardless of the data plate. So, they run the late-model parts that are known to run better, including the 12-counterweight crankshafts, late valve rockers, late wrist pins, and late induction routing. There are VERY few flying early Allisons with the long nose case. I know of 1 currently flying; there may be another or even two. The long nose case is simply not up to the power that the engine block can make, so they have to fly it with some knowledge of the engine. No runs at 57" of MAP, they usually don't have the fuel for it, anyway.
 
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So the Allison was generally lighter, longer between overhaul, shorter to overhaul? Good to know.
 

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