Bf 109 G-6AS (ASM motor) vs. K-4 (DB motor) performance.

[MaxPower]

I think I've discovered something but I would like to get some community expert opinions on it before I go through the process of filing a bug report etc.

According the to spec sheets, the Bf 109 G-6AS/ASM has 1800 PS at sea level, a standard weight of 3306 kg with the MW-50 system added, a top speed of 570 km/h, and a max climb rate of 17.3m/s.

According to the spec sheet, Bf 109 K-4/DB has 1850 PS at sea level, a standard weight of 3361 kg, a top speed of 599km/h, and a max climb rate of 20.5 m/s.  The top speed of the K-4 was confirmed by Count Zero's testing.

Both aircraft seem to visually have similar aerodynamics, but not the same.  They both have the smooth cowling, tall tail, and erla haube.  However, the G-6AS has a non-retracting tail wheel, open wheel bays, and there are other minor differences like the exact placement of antennas and the configuration of trim tabs.

On the face of it, the planes seem to have similar properties but much different performance.  There is a 1.6 percent difference in weight, a 2.7 percent difference in power, a 3.7 percent difference in power-to-weight, an 18 percent difference in max climb rate, and 5 percent difference in top speed.  Doing some back of the napkin calculations, they should have more similar climb rates and top speeds.  As I know it, the differences in top speed could be related to parasite drag, propeller efficiency, and exhaust thrust.  I hesitate to share this because I'm not an aerodynamicist... but all other things being equal, I calculate a 9.4 percent difference in parasite drag coefficient.  So, I guess my questions are:

Could the speed difference be explained by the undercarriage fairings?  Can the difference in drag also be used to explain the difference in climb rate?  Did the DB-605DB have much better exhaust thrust optimization?  Was the K-4's propeller a big step forward in terms of efficiency?  Are there other refinements I am not aware of that might contribute to a dramatic reduction in drag?  Or, are perhaps the data I'm basing this post on incorrect?

Anyway, thanks for reading my tedious post 🙂

edit:  I just realized my error with the climb rates.  They are given for combat power and the K-4 has more horsepower at combat settings.  So, that's one question answered, and a major part of the inconsistencies put to bed.
 

Edited March 8 by MaxPower

[sevenless]

This might help:

https://info.eduard.com/en/09-2024-1/unordnung-late-versions-of-the-bf-109-g

[AndreiTomescu]

Weight distribution also has impact. The plane is "floating" in air, somehow like a submarine floats in water.

Just the figures are not enough to compare performance. The final result , by testing the prototype, sometimes surprises (good or bad) even the producer.

[Kurfurst]

The single greatest differentiator is parasitic drag - the K-4 has fully covered wheel well doors, these alone account for +10 km/h at SL; the tailwheel is also fully retractable and covered, this further adds +12 km/h at SL against the short tailwheel and +17 km/h in case of the long tailwheel. When you add this up to the G-6/AS specs, you arrive at the K-4 specs. These drag figures are measured on actual wartime planes by Messerschmitt and increase in speed was cc. +50% by their rated altitude of 6-7.5 km.

Apart from these two major changes, the two planes are aerodynamically very similar - they sport the same supercharger and propeller, the coolant radiators are the same, the oil radiator was originally smaller on the early /AS series but in later production it was replaced with the larger oil cooler of the K series.

The base G-6/AS is lighter but again this largely comes down to the MK 108 being fitted on the K series as standard, so the G-6/AS may be a bit more agile on the same power settings.

Further to that, its worthy to note that the G-6/AS represents an early spring 1944 power ratings of the AS engine, that is, 1.7ata manifold pressure; in December 1944 the engine rates were increased on both the AS and D series, with 1.8 ata (1850 PS - called now ASB and DB, respectively) being the base setting on standard B-4 fuel, 1.90 ata interim setting briefly used in operational trials with JG 11, and 1.98ata for both the AS and D series (now called ASC and DC respectively) when using high octane fighter grade C-3 fuel. In practice, during the winter of 1944/45 most 109Ks are also seen without their wheel covers, either due to mechanical issues or due to snow/mud considerations, so in practice there was very, very little actual difference between late war G-6/AS, G-14/AS, G-10 and K-4 as far as performance went; they all run on the same engine power and in similar aerodynamic configuration until the wheel covers seem to arrive / return to the K series in the spring of 1945.

[MaxPower]

Thank you for the feedback everyone!  I always knew there was a big speed difference between the 109K-4 and the Gs.  I didn't realize they were so close in horsepower.  I actually first realized the similarity when I was looking at the German Aero Engine Development document on enginehistory.org.  I had assumed that the K-4 had a majorly different engine and was surprised to see it they were so similar.  Thanks for the expert breakdown, the resource, and the interesting factors I hadn't considered.

Interesting about the G-6AS weights.  In the game documentation, the G-6AS standard weight is 3222 kg.  The ASM addon is an additional 21 kg, and the MW50 mixture is 63kg.  This adds up to 3306.  The K-4 is 3361.  That's a 55 kg difference.  If you add the MK108, in the game, it is only 4kg.  The difference in weight between a standard K-4 and a similarly equipped G-6AS is 51 kg.  So, not nothing but also not an extreme difference.

In that GAED document I saw that there is an DB605 AS, ASCM, DCM, DB, and DC.  It's not quite clear but it seems like the mix of C-3 fuel and MW-50 in the document is not exactly the same as in the game (it seems to say that the DB engine has C-3 fuel but not MW-50, for instance).  The ASM motor in the game uses C-3 fuel and MW-50 but does not have 1.98 ata.  It sounds like the ASM engine with C-3 and MW-50 would be called ASCM in this document, and later in the war would have the power settings you mentioned.

Here are the pages in case you're curious.
https://www.enginehistory.org/Piston/Daimler-Benz/ged0109.jpg
https://www.enginehistory.org/Piston/Daimler-Benz/ged0110.jpg

[sevenless]

Funny what Grok has to say about the topic. IMHO pretty spot on:

https://x.com/i/grok/share/e9ce746b6e554379be21c80dc3b65e68

Q: Bf 109 G-6AS (ASM motor) vs. K-4 (DB motor) performance

 

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The Bf 109 G-6/AS (often with the DB 605 ASM engine variant, enabling MW 50 methanol-water injection for boosted performance) was a high-altitude optimized version of the G-6, introduced to counter Allied bombers and fighters at higher altitudes. It featured a larger supercharger from the DB 603 lineage, improving the full throttle height (FTH) significantly compared to standard G-6 models.The Bf 109 K-4 ("Kurfürst") represented the final major production evolution of the Bf 109 series, powered by the more advanced DB 605 DB or DC engine family (the "DB motor" in your query likely refers to this D-series engine). It incorporated aerodynamic refinements (streamlined cowling, retractable tailwheel, better wheel covers), structural tweaks for production ease, and higher boost levels.Key Engine and Power Differences

• G-6/AS (ASM motor): Typically DB 605 AS/ASM. With MW 50, output reached ~1,700–1,800 PS (some sources cite up to ~1,850 PS in later ratings). Boost around 1.7–1.8 ata depending on period/fuel. Excellent high-alt performance due to the enlarged supercharger (FTH around 7,800–9,000 m).
• K-4 (DB 605 DC/DB motor): More powerful DB 605 D-series. With MW 50 and C3 fuel, up to 2,000 PS (1.98 ata boost on DC variant) or ~1,800–1,850 PS on DB variant at 1.8 ata. This gave superior power across most altitudes, especially when boosted.

The K-4's engine was a clear step up in raw output and reliability at high boost.Performance ComparisonBoth shared similar airframe refinements (e.g., smooth cowling, tall tail, Erla Haube canopy), but the K-4 had further drag reductions and better power-to-weight.Top Speed (approximate figures from historical data, tests, and simulations; MW 50 boosted where applicable):

• G-6/AS: Around 650–660 km/h (404–410 mph) at high altitude (8,000–9,000 m); lower at sea level (560–580 km/h boosted).
• K-4: Generally faster overall — up to 690–710 km/h (429–440 mph) at 7,000–7,500 m boosted; ~600–615 km/h at sea level boosted. Production figures often cite ~710 km/h max in ideal conditions.

Climb Rate:

• G-6/AS: Strong at altitude but generally inferior overall.
• K-4: Superior acceleration and climb (often described as a "beast" in climb/acceleration); initial rates up to ~1,090 m/min boosted vs. lower for the G-6/AS.

Speed at Various Altitudes:

• The G-6/AS excelled in the high-altitude band (better than standard G models but close to G-10 level).
• The K-4 outperformed it at most altitudes due to higher power and aero tweaks, especially low-to-medium and with full boost.

Maneuverability:

• G-6/AS: Slightly better low-speed turning and handling in some accounts (lighter wing loading in some configs).
• K-4: Heavier overall, worse at very low speeds/turns, but superior sustained turn and energy retention at higher speeds due to power advantage. Often preferred as an energy fighter.

Other Factors:

• The G-6/AS was an interim high-alt solution (mid-1944 onward), while the K-4 (late 1944–1945) was the ultimate refinement.
• In simulations (e.g., IL-2, DCS, War Thunder), the K-4 consistently wins in speed, climb, and acceleration; the G-6/AS holds its own at very high altitudes but loses overall.
• Historical pilot accounts sometimes favored earlier Gs (like G-10) for dogfighting feel, but the K-4's raw performance made it the fastest and most powerful piston Bf 109 variant.

In summary, the Bf 109 K-4 (with DB 605 D-series motor) was the superior performer in almost every category — faster, better climber, stronger accelerator — thanks to more power, refined aerodynamics, and higher boost capability. The G-6/AS was an impressive high-alt specialist and a stopgap upgrade, but it couldn't match the K-4's all-around edge. The K-4 was essentially the peak of the Bf 109 line.

 

And as for G-10 vs K-4

https://x.com/i/grok/share/df3b81f054f94630ad2b78104a368297

 

Q: Bf 109 G-10 (DB motor) vs. K-4 (DB motor) performance

 

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The Messerschmitt Bf 109 G-10 and Bf 109 K-4 were two of the final and most advanced Luftwaffe variants of the Bf 109 series, both powered by developments of the Daimler-Benz DB 605 inverted V-12 liquid-cooled engine (primarily the DB 605D series variants like DB 605DB/DC/DM, with MW 50 water-methanol injection for boost).The K-4 (Kurfürst) represented the ultimate production refinement of the design, entering service in late 1944, while the G-10 was a late-war "standardized" G-series upgrade (often using the same DB 605D family engines) produced in large numbers as a stop-gap high-performance fighter.Key Engine and Power NotesBoth used the DB 605D family:

• G-10: Typically DB 605DM/DC variants, with ~1,800–1,850 PS (PS ≈ hp) emergency power using MW 50.
• K-4: DB 605DB/DC, rated for up to ~1,850–2,000 PS emergency/Sondernotleistung (with MW 50 and higher boost like 1.8–1.98 ata in some conditions), plus better sustained power at higher manifold pressures (e.g., higher 30-min combat power).

The K-4 generally had a slight power edge in sustained/repeated use, though peak emergency outputs were similar.Performance ComparisonThe K-4 was the superior performer overall due to:

• Cleaner aerodynamics (revised cowling, fully retractable tailwheel, better wheel well/leg fairings, reduced drag protrusions).
• Slightly heavier empty weight but better power-to-weight and thrust-to-weight in practice.
• Improved high-altitude performance from the engine/prop combo.

Top Speed (with MW 50 boost / emergency power, typical figures from historical data and tests):

• G-10: Around 690–710 km/h (429–441 mph) at optimal altitude (~7,000–8,000 m / 23,000–26,000 ft). Some sources claim up to ~715 km/h in ideal conditions, with excellent high-altitude performance in Erla-built examples.
• K-4: Around 710–727 km/h (441–452 mph) at ~7,500 m (24,600 ft), often cited as 710–720 km/h in production configuration (higher figures like 727 km/h or 452 mph with optimal/experimental props or conditions).

→ The K-4 was consistently faster (by ~10–30 km/h / 6–19 mph) across most altitudes, especially in level flight and combat cruise.Climb Rate:

• G-10: Excellent, typically ~20–25 m/s (3,900–4,900 ft/min) initial/ emergency, with strong performance at medium-high altitudes.
• K-4: Superior, often 22–32 m/s (4,300–6,300 ft/min) in emergency power configurations, with outstanding sustained climb and acceleration.

→ The K-4 had a clear advantage in climb (especially low-to-medium altitude) and acceleration, making it a better energy fighter / boom-and-zoom platform.Other Aspects:

• Maneuverability / Turn Rate: The G-10 was often lighter (especially clean configurations) and preferred by some pilots for dogfighting, with slightly better low-speed instantaneous/sustained turn in some scenarios. The K-4, being heavier and with more power, traded some low-speed agility for better energy retention, dive, and high-speed handling.
• Roll Rate / Acceleration: K-4 generally better due to power and aero refinements.
• High-Altitude Performance: Both excellent (DB 605D supercharger optimized for higher ceilings), but K-4 edges out due to lower drag.
• Armament: Similar (typically 1 × 30 mm MK 108 engine cannon + 2 × 13 mm MG 131 cowl guns), though some G-10s had optional 20 mm options; K-4 often had heavier wing cannon gondolas in some units, hurting maneuverability.

Summary

• The Bf 109 K-4 was the better performer in raw speed, climb, acceleration, and energy fighting — the pinnacle of the 109 line.
• The G-10 was very close (especially Erla-built examples), often lighter/more agile in turns, and sometimes described as nearly as fast at very high altitudes.
• In practice, the K-4's aerodynamic refinements and slight power/sustained output advantages made it noticeably superior in most flight regimes, though pilot preference varied (some favored the G-10 for dogfights).

These figures come from wartime German documents, Allied tests, and postwar analyses; real-world performance varied with boost settings, fuel quality, maintenance, and exact sub-variants. Late-war fuel shortages and Allied air superiority limited their combat impact regardless.

 

Edited March 10 by sevenless

[[250H]Ursus_]

You said it yourself.
Aerodynamics.
K-4 is more streamlined than G-6AS, wheels are fully covered, tailwheel is not exposed, even with G-10 you would notice that K-4 is better with the same engine (DB or DC doesn't matter). And those have a real impact on the final speed

Anothers examples of this are P-51D and P-51B/C, or La-5FN and La-7, or Spitfire XIV Malcom v Spitfire XIV Teardrop

Same engine but the other has a more streamlined airframe. 
 

Edited March 10 by [250H]Ursus_

[Shinobimono]

10.03.2026 в 17:21, [250H]Ursus_ сказал:

examples of this are P-51D and P-51B/C

Early blocks of P-51B without 85-gallon fuel tank are definitely better than early P-51D. Aerodynamically clean, lighter, much better weight distribution.