Also, another thing I've noticed is that Allison has very different fuel consumption to Merlin.
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Spitfire Mk.I Imp US
2500rpm +0psi (29.9) rich 49 58
1800rpm -4psi (21.8) weak 26 31
P-40
2500rpm 29.9 rich 74 (Imp/US?)
2500rpm 29.9 lean 59 (Imp/US?) (note: used power setting is above permitted for lean)
1800rpm 21.8 lean 23 (Imp/US?)
Pre-amble to question 2:
If we assume gallons on P-40 SHIFT+2 panel are US gallons, we notice that it has somewhat higher fuel consumption that Spitfire at higher power outputs (such as 2500rpm and MP at atmospheric pressure (29.9 inHg or +0 psi)). If it's Imperial gallons, then the difference only increases. Even if we assumed US gallons and run Allison in lean mixture (even though it's not permitted for such "high" MP) it still uses more fuel than Merlin running rich mixture. Of course the engines aren't identical. For example, displacements are slightly different (27L vs. 28L).
On the other hand, at low power settings, the situation is reversed: Allison uses much less fuel than Merlin (regarless whether P-40's ghp are US or Imp).
Some explanation for this increase of fuel consumption on high power output is no doubt the mixture curve and high power enrichment. Spitfire has next to none automatic enrichment for higher power if certain data is to be trusted, where as Allison probably has some.
If I make some rough synthetic calculations, 2500rpm time 29.9 inHg should pump 74750 "units" of air into the engine per second. At 1800rpm 21.8 inHg, it's 39240. Divide former by latter: 1.91. So if the mixture was constant, increasing power from the lower of mentioned power setting to the higher increases not just airflow but fuel consumption by 1.91.
If we however look at Spitfire's gph readings, and divide 49 by 27 we get only 1.88, so airflow increases more than fuel consumption, i.e mixture becomes weaker with added throttle (at least within that power range), so much actually, that even putting the mixture to rich setting, it's still weaker than in the lower power setting.
Doing the same math with P-40, dividing 74 by 23, we get 3.21 fold fuel consumption with same increase (1.91) of airflow through cylinders. Mixture is enrichened by a factor of 1.68 (3.21/1.91). Again, the higher power setting is with rich, and lower with lean. (Running the higher power setting with lean mixture would give 2.57 times the fuel consumption and 1.34 times the mixture enrichment.)
QUESTION 2: Do these calculations seem reasonable? Does Spitfire actually lean out with added throttle so much that even enriching with mixture lever wouldn't maintain the same mixture?
And on P-40, doesn't the enrichment seem a bit excessive? (Especially compared to Spitfire which seems to lean out rather than enrich at all.)
And yeah, these a calculations of mine are very synthetic. I wouldn't have made them if I hadn't had a hunch that something might be wrong before hand. (I noticed that low-power cruise on Spitfire was rather high, compared to how low fuel consumption is during climb.)