Japanese Aircraft Performance

Nakajima was so ambitious to beat Mitsubishi that IJA had relied on it, and Japan had relied on it.
Mitsubishi survived the war as it was but Nakajima.

Hello Tomo Pauk, Shortround6,

I don't think you are quite understanding how REALLY POOR the Japanese fuel quality was.
In the book Genda's Blade, there were descriptions of combining Pine oil extracts to extend the fuel, but the problem was that the resulting "stuff" had an octane rating probably in the mid 80's. How do you run a high performance engine on 85 octane fuel? (You don't use even moderately high manifold pressures.)

Japanese aviation fuel was only 91-92 octane at its best. Some captured stuff tested higher but that was the standard. I think the Army used 91 octane and Navy used 92 octane but I may have it reversed.

Japanese did not use Water Methanol as ADI in quite the same as other countries.
It wasn't just for War Emergency Power.
Here are a few numbers for the Nakajima Ha-45 (Homare) as installed in the Ki-84:

2000 HP @ 3000 RPM Altitude: Sea Level +500 mm Hg Boost T-O & Emergency
1790 HP @ 3000 RPM Altitude: Sea Level +350 mm Hg Boost Rated Model 21
1890 HP @ 3000 RPM Altitude: 1700 m +350 mm Hg Boost Rated Model 21
1430 HP @ 2780 RPM Altitude: Sea Level +200 mm Hg Boost
1550 HP @ 2780 RPM Altitude: 2500 m +200 mm Hg Boost

I believe the last two numbers are maximum continuous.
Note however that for this engine, any boost pressure above +180 mm Hg uses Water Methanol injection, so without ADI, this engine would only be making about 1400 HP or somewhere around there. Basically at anything above cruise power, these engines were using Water Methanol. This was pretty typical of Japanese engines.

The manifold pressures listed here also are not particularly high either.
For a conversion to more common units
+500 mm = 49.606 inches
+350 mm = 43.701
+200 mm = 37.795
+180 mm = 37.008

The point is that because of the poor quality fuel, the Japanese were using ADI to achieve boost pressure that most folks were able to achieve without ADI.

- Ivan.

There are some descriptions of fuel coming from the Dutch East Indies as being rich in aromatics which, if true, would help account for the higher than 91-92 octane rating when captured examples were tested. How much higher is certainly open to question as is when. The Japanese may not have been able to break the raw material down and recombine it. In fact since the PN scale was not developed until 1941 (??, it was after the BoB) the Japanese may have had no accurate way to measure fuel over 100 octane which makes it hard to blend service fuel.

From Wiki "Development of the Homare started in 1940, and certification was completed in 1941." this has a bearing on what the Japanese had for fuel during development and where they expected fuel to go in capability in the future. What the Japanese actually had for fuel in 1944/45 maybe a totally different story.
The Japanese certainly using water injection for high power cruise or climb ratings much more than anyone else.
The Homare weighed about 835kg (1837lbs?) which is rather light for a 2000hp engine, Given American fuel you might be able to run it at the same power levels without water injection as the Japanese did using water injection (cylinder head temps might need careful monitoring) but trying get much more power (more boost used) might be difficult due to the strength of the engine and not due to detonation.

Japanese had the 100 octane fuel imported from the U.S. though not much quantity.
It was mainly used for testing new engines like Homare.