Fastest Piston Engined Aircraft of WW2?

Howard Gibson · Nov 16, 2021

tomo pauk said:
Care to elaborate the two sentences?

My reference quotes for the P-51D a top speed of 437mph at 25,000ft. This is done with 100/130 octane fuel. I am not sure what had to be done to a Mustang to take advantage of 150 octane, and I don't know how available it was. WW2 Aircraft Performance's article on 150 Octane fuel is full of memos about how 150 octane fuel is fabulous and that we should use it. That does not mean that they did.

The XP-51G and XP-51F both used the Packard V-1650-7, like the P-51D.

Martin Sharp & Michael J.F. Bowyer's book Mosquito provides the following table on page 448.

Octane	Injection	Horsepower
87		1150
100		1800
100	Water	2000
150		2400
150	Water	2600

Note how supercharging is not important here. Maximum output can be achieved without it, although not at any sort of altitude.

The de Havilland Hornet's Merlin 130/131 engines were designed among other things specifically to run on 150 octane fuel.

tomo pauk · Nov 16, 2021

Howard Gibson said:
My reference quotes for the P-51D a top speed of 437mph at 25,000ft. This is done with 100/130 octane fuel.

There is a lots of tests here showing 445+- mph clocked for the P-51D. WER setting was used, ie. 3000 rpm and up to 67 in Hg boost.

Howard Gibson said:
I am not sure what had to be done to a Mustang to take advantage of 150 octane, and I don't know how available it was. WW2 Aircraft Performance's article on 150 Octane fuel is full of memos about how 150 octane fuel is fabulous and that we should use it. That does not mean that they did.

Basic thing was to change the spark plugs more often due to lead fouling of the plugs, since the lead content was much greater on the 150 grade fuel than it was in 130 grade.

Howard Gibson said:
The XP-51G and XP-51F both used the Packard V-1650-7, like the P-51D.

XP-51F indeed used the same engine as the -51D, but the -51H used the improved -9, that also incorporated water injection feature so greater boost could be used (up to 90 in Hg). XP-51G used the British RR engine, RM.14.SM. That engine was pretty comparable with the V-1650-9 in power. See here.

Howard Gibson said:
Note how supercharging is not important here. Maximum output can be achieved without it, although not at any sort of altitude.

There is always supercharging in a ww2 piston engine. Something needs to provide all that boost at required altitudes, that's what superchargers do. More boost = more power.

Howard Gibson said:
The de Havilland Hornet's Merlin 130/131 engines were designed among other things specifically to run on 150 octane fuel.

Those also worked with 130 grade fuel, max boost limited to +20 psi. On 150 grade, it was limited to +25 psi.

Thumpalumpacus · Nov 16, 2021

Howard Gibson said:
[...] and using 150 octave fuel.

The musician in me is stunned. That is one long guitar neck!

Howard Gibson · Nov 16, 2021

Thumpalumpacus said:
The musician in me is stunned. That is one long guitar neck!

I was bound to do that sooner or later.

Howard Gibson · Nov 16, 2021

tomo pauk said:
Basic thing was to change the spark plugs more often due to lead fouling of the plugs, since the lead content was much greater on the 150 grade fuel than it was in 130 grade.

My understanding from reading up on 150 octane fuel was that Mark IX Spitfires required new throttle quadrants. Maybe the Mustang didn't. The author of one of the reports did recommend five blade propellers on the Merlin Spitfires.

wuzak · Nov 16, 2021

Howard Gibson said:
Martin Sharp & Michael J.F. Bowyer's book Mosquito provides the following table on page 448.

Octane
Injection
Horsepower
87
1150
100
1800
100
Water
2000
150
2400
150
Water
2600

The power numbers seem suspect.

Firstly, what engines were these? Merlin XXs? Merlin 72/73? Merlin 76/77? Merlin 100 series?

No Rolls-Royce built Merlin used ADI in service. Some Mosquitoes used nitrous oxide to boost speed to chase V-1s.

As far as I am aware, no Mosquitoes used 87 octane fuel.

Merlin 66 made ~ 1,800hp in MS gear with +18psi boost using 100/130 fuel. Merlin 72/73 and 76/77 made 1,710hp in MS gear with +18psi boost and 100/130 fuel. The difference in power reflects the different supercharger gearing between the 66 and the 70-series engines named. Also note that the maximum power was achieved in the lower gear.

The Merlin 66 was capable of ~2,000hp at +25psi boost on 150 grade fuel.

100-series development engines were able to run more power than they would be rated.

The RM.17SM was a 100-series development with a larger supercharger (supercharger impeller sizes 12.7/10.7 inches vs 12.0/10.1 inches) . It was tested at ~2,400hp at +30psi and 3,300rpm dry. It was also able to make 2,600hp using ADI, 3,150rpm, +36psi boost and 160 grade fuel (not a standard fuel - 150 grade with extra TEL, I believe). The RM.17SM was rated at 2,200hp MS gear/2,100hp FS gear dry (ie no ADI). It did not go into production and did not get a type number.

Howard Gibson said:
Note how supercharging is not important here. Maximum output can be achieved without it, although not at any sort of altitude.

Supercharging was very important.

Higher fuel grades allowed greater boost and thus more power.

Without extra boost or compression ratio the higher fuel grade is no better than the standard grade.

wuzak · Nov 16, 2021

Howard Gibson said:
My understanding from reading up on 150 octane fuel was that Mark IX Spitfires required new throttle quadrants. Maybe the Mustang didn't. The author of one of the reports did recommend five blade propellers on the Merlin Spitfires.

Not sure about that. The boost control unit had to be adjusted.

don4331 · Nov 16, 2021

Power numbers more/less match what's in RRHS #19.

Differences: They have 100 octane + Water Injection at 2,100hp and didn't define a limit for 150 octane + water injection other than indicating that the RM.17SM had run at 2,620hp for its endurance test at values mentioned.

W wuzak : I was typing at same time, so some overlap here.

No RR built Merlin might have used ADI in official service, but doesn't mean it wasn't tested.

You modify the boost control unit to allow increased boost (at lower critical altitude) and you get more power. RM.17SM critical altitude in low speed being ~1k ft, and ~17k ft for high versus the RM.14SM (the 100 series) being about 15k ft and 30k ft. respectively. As a result, RR was looking at 3 speed supercharger drive to ensure power at 30k' and above for the follow on to the 100 series.

Tweaking the boost controls on the planes was regularly done when mfrs. were attempting speed record runs. Heck, the guys at RR talk about how they would inject a shot of oil into the engine when the RAE technician was distracted, so engine would momentarily produce a little more torque and give better numbers.

wuzak · Nov 16, 2021

don4331 said:
You modify the boost control unit to allow increased boost (at lower critical altitude) and you get more power. RM.17SM critical altitude in low speed being ~1k ft, and ~17k ft for high versus the RM.14SM (the 100 series) being about 15k ft and 30k ft. respectively. As a result, RR was looking at 3 speed supercharger drive to ensure power at 30k' and above for the follow on to the 100 series.

The critical altitude changes with boost level.

The 3rd speed would have been LS gear - for low altitude.

Mike Williams · Nov 17, 2021

Howard Gibson said:
My understanding from reading up on 150 octane fuel was that Mark IX Spitfires required new throttle quadrants.

wuzak · Nov 17, 2021

Howard Gibson said:
My reference quotes for the P-51D a top speed of 437mph at 25,000ft. This is done with 100/130 octane fuel. I am not sure what had to be done to a Mustang to take advantage of 150 octane, and I don't know how available it was. WW2 Aircraft Performance's article on 150 Octane fuel is full of memos about how 150 octane fuel is fabulous and that we should use it. That does not mean that they did.

The XP-51G and XP-51F both used the Packard V-1650-7, like the P-51D.

Martin Sharp & Michael J.F. Bowyer's book Mosquito provides the following table on page 448.

Octane
Injection
Horsepower
87
1150
100
1800
100
Water
2000
150
2400
150
Water
2600

Note how supercharging is not important here. Maximum output can be achieved without it, although not at any sort of altitude.

The de Havilland Hornet's Merlin 130/131 engines were designed among other things specifically to run on 150 octane fuel.

Having looked at my copy of that book I see a chart showing limits for fuel grades with and without ADI. I am unsure whether they are theoretical or tested limits.

Actual service and test Merlin versions are plotted on the graph with a line of best fit.

None of the service models came to the theoretical limit. The V-1650-9 with 150 grade fuel and ADI made just over 2,200hp, compared to the limit on that graph as 2,600hp.

Mike Williams · Nov 17, 2021

Howard Gibson said:
WW2 Aircraft Performance's article on 150 Octane fuel is full of memos about how 150 octane fuel is fabulous and that we should use it. That does not mean that they did.

Please dig a little deeper into 100/150 GRADE FUEL and P-51 Mustang Performance to find documentation showing operational use of 150 grade fuel by the 8th Air Force and RAF.

A few examples gives a quick idea:
Use of 100/150 Grade Fuel by Eighth Air Force Headquarters Eighth Air Force, Technical Operations, 4 April 1945.
Requisition MER/388/43., 24th August 1944, HQ ADGB
316 Squadron ORB
118 Squadron ORB

dedalos · Nov 17, 2021

It appears that very late in the war , y american field mechanics of both p51 and p47 units , were able to over boost the engines. They also provided additional streamlining by using putty and gloss paint. Thus , these types were able to fly rings around the Lw piston engine fighters, and almost closed the gap with the me262. Now the jet , because of the high fuel consumption, could not use full throlle for more than a few minutes. Maybe, in practice, given all the limitations of the me262, very late p51s and p47 had higher "combat" speed

FLYBOYJ · Nov 17, 2021

dedalos said:
It appears that very late in the war , y american field mechanics of both p51 and p47 units , were able to over boost the engines. They also provided additional streamlining by using putty and gloss paint. Thus , these types were able to fly rings around the Lw piston engine fighters, and almost closed the gap with the me262. Now the jet , because of the high fuel consumption, could not use full throlle for more than a few minutes. Maybe, in practice, given all the limitations of the me262, very late p51s and p47 had higher "combat" speed

Seam putty and gloss paint helps but it's not going to buy you THAT much. At Reno we tapped up seams with "100 MPH tape" and got an extra 5 mph in most cases.

FLYBOYJ · Nov 17, 2021

dedalos said:
It appears that very late in the war , y american field mechanics of both p51 and p47 units , were able to over boost the engines. They also provided additional streamlining by using putty and gloss paint. Thus , these types were able to fly rings around the Lw piston engine fighters, and almost closed the gap with the me262. Now the jet , because of the high fuel consumption, could not use full throlle for more than a few minutes. Maybe, in practice, given all the limitations of the me262, very late p51s and p47 had higher "combat" speed

Another thought - I do know that mechanics in the field during WW2 did do tweaks to get better performance from their assigned aircraft, but at the same time there were squadron maintenance officers looking over their shoulders. Then, as now, if you're doing a maintenance process outside of documented procedures and an incident results, big trouble can come your way!

Howard Gibson · Nov 17, 2021

wuzak said:
The power numbers seem suspect.

Firstly, what engines were these? Merlin XXs? Merlin 72/73? Merlin 76/77? Merlin 100 series?

I assume these were generic Merlin engines. Twelve cylinders, 27l displacement, and 3000rpm.

wuzak said:
No Rolls-Royce built Merlin used ADI in service. Some Mosquitoes used nitrous oxide to boost speed to chase V-1s.

As far as I am aware, no Mosquitoes used 87 octane fuel.

No Mosquitos put out just 1150HP per engine!

wuzak said:
Supercharging was very important

Higher fuel grades allowed greater boost and thus more power.

Without extra boost or compression ratio the higher fuel grade is no better than the standard grade.

You mix fuel into your intake air such that there is exactly enough oxygen available to burn everything. You insert the air into a cylinder and you compress it. As per the gas laws you may or may not have learned in physics and/or engineering, the temperature increases. Ideally, it goes up to just below the ignition temperature of the vapourised fuel. You fire the spark plug(s) at exactly the right moment. Under these conditions, the piston does the maximum possible amount of work on its stroke.

The pressure and temperature in the cylinder are the sum of atmospheric pressure, supercharger boost and compression ratio. You can do this without supercharging, you just cannot do it at altitude.

Howard Gibson · Nov 17, 2021

Mike Williams said:
Please dig a little deeper into 100/150 GRADE FUEL and P-51 Mustang Performance to find documentation showing operational use of 150 grade fuel by the 8th Air Force and RAF.

A few examples gives a quick idea:
Use of 100/150 Grade Fuel by Eighth Air Force Headquarters Eighth Air Force, Technical Operations, 4 April 1945.
Requisition MER/388/43., 24th August 1944, HQ ADGB
316 Squadron ORB
118 Squadron ORB

Definitely, it was used, particularly against V1 buzz bombs. Definitely, some of it crossed the Atlantic, where the USAAF tested it. I have not found anything about the US Navy getting at it. Did it replace the 100/130 fuel, or was it handed out to squadrons with high priority missions?

FLYBOYJ · Nov 17, 2021

Howard Gibson said:
No Mosquitos put out just 1150HP per engine!

Not every model...

VA5124 · Nov 17, 2021

FLYBOYJ said:
Not every model...

I'd say the corsair and the P&W R-2800 have to be on the the list over 440 mph maxium speed and over 2,000 hp

FLYBOYJ · Nov 17, 2021

VA5124 said:
I'd say the corsair and the P&W R-2800 have to be on the the list over 440 mph maxium speed and over 2,000 hp

I was referring to the Mosquito having engines of 1150HP per engine and not every model of the Corsair had a top speed of 440 mph

Fastest Piston Engined Aircraft of WW2?

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