The flight dynamics issue that I speak of, regarding performance versus engine settings, with both the P-51’s and AT-6’s, is in free flight mode.
The base/“stock” P-51D’s (generic paint schemes) and base/“stock” AT-6’s (generic paint schemes) have quite accurate to real life performance vs. engine settings. You set 50-55" Manifold Pressure/3000 RPM for takeoff power in the “stock”/generic P-51D’s (standard takeoff power in today’s world), and the level of acceleration and top end speed is very accurate. The same goes for the “stock”/generic AT-6’s, running 32" MP/2250 RPM for takeoff power (standard takeoff power in today’s world), and the level of acceleration and top end speed is very accurate. You can fly these aircraft, using the right true-to-life power settings, and they perform quite accurately in free flight mode.
The inaccuracies arise with some of the racer P-51D’s and all of the racer AT-6’s. Unlike what is portrayed currently in free flight mode, these should all have essentially the exact same level of acceleration and very, very close to the same top end speed as the stock examples, when running the exact same engine settings (within 5-20 mph of each other, depending on airframe modifications). The inherent problem with the flight/engine dynamics of the racing Mustangs (namely “Voodoo” and “Strega”) and the racing AT-6’s (all but the generic/“stock” examples) in free flight mode, is that they are accelerating way too fast and the top end speed is way too high, while using normal power settings (Manifold Pressure and RPM). In reality, the only reason why the racing Mustangs and racing AT-6’s are going at those much higher airspeeds is because the engines have been modified to allow much higher Manifold Pressures and RPM’s. That, however, is not depicted in the flight/engine dynamics of these aircraft. For instance, flying the P-51D “Voodoo”, using the normal top-end power setting of 55" Manifold Pressure and 3000 RPM on takeoff shouldn’t result in much of any difference at all in acceleration or top end speed than what you get in a stock P-51D using that same power setting (the top end speed would be expected to be only slightly faster, due to the airframe modifications) - however, it rockets away, unrealistically, because it has been magically boosted to reach much higher airspeeds, even though this is not reflected in any difference in engine settings. There is some “magical”/unnatural performance boost added to the MSFS flight dynamics for the racer Mustangs and AT-6’s, so that they have a crazy high level of acceleration and top end speed, even though the engine parameters/settings remain no different than the stock P-51D and AT-6. The major downside of this is that, in order to operate these aircraft in free flight mode just for a normal flight, you have to use much lower power settings than is accurate/what you would be using in reality, just so as to keep the acceleration and top end speed realistic/accurate. With the racer Mustangs and AT-6’s, there needs to be changes in the engine parameters so that the speeds we’re getting are only reachable by using much higher engine power settings, as per reality - and, when using stock power settings, they should be performing with very near the same level of acceleration and top end speed as the stock examples. No matter if the aircraft is fully stock or modified in some way for racing, the P-51D’s should all use 50-55" MP and 3000 RPM on takeoff (today’s power settings), and this should result in all of them accelerating and getting up to near the same top end speed, no matter which P-51D it is, and then using 34-36" MP and 2300-2400 RPM for cruise, all getting fairly close to the same cruise speed (with marginal differences based on airframe modifications/differences). Where one P-51 would be capable of being much faster than an another is when the Manifold Pressure and RPM, on a racing-tuned Merlin, is capable of being pushed far higher than stock (up to 145" MP, rather than 55" MP, and up to 3500 RPM, rather than 3000) - this is not replicated in the sim at this time.
The same problem applies to the racer AT-6’s. Because the performance on those has been over boosted, while the engine parameters/settings have not, you have to use much lower power settings than you would in reality just in order to operate them with realistic acceleration and top end speed in free flight mode. In reality, with any AT-6, modified or not, you takeoff using 32" Manifold Pressure (36" MP, if you don’t care about the length of engine life), and 2250 RPM, and no matter what airframe it is, it is going to accelerate and get up to speed essentially the same as any other, stock or otherwise modified. Unfortunately, though, since all of the racing AT-6’s have greatly boosted “magical” performance, without any tie-in to greater/higher engine power settings, you have to use much lower engine power settings with the racer AT-6’s in free flight mode to maintain realistic/accurate acceleration and speeds on takeoff and cruise. For instance, I’ve found that with the racer AT-6’s, to maintain realistic and accurate acceleration on takeoff, the power has to be kept down to 20-22" MP, and then even lower in cruise, and that shouldn’t be the case. No matter if the aircraft is fully stock or modified in some way for racing, they should all use 32" MP and 2250 RPM on takeoff (today’s power settings), with all of them accelerating the same and getting up to near the same top end speed, no matter which AT-6 it is, and then using 24-26" MP and 1850-1900 RPM for cruise, all getting pretty much the same cruise speed no matter which AT-6 it is. Unfortunately, this is not true of the flight/engine dynamics as currently portrayed with the racer AT-6’s and Mustangs. Where one AT-6 would be much faster than an another is when the Manifold Pressure and RPM, on a racing-tuned R-1340, is capable of being pushed higher than stock.