P40: The underated underdog, or just behind the times

Clay_Allison · Jul 21, 2009

Shortround6 said:
"The reason Performance didn't change much with the change to the Merlin was because that Merlin was setup similarly to the Allison, i.e., single stage/single speed supercharger."

No, the Merlins used in the P-40 had single stage TWO speed superchargers.

They used 8.15 and 9.49 gear sets and were rated at 1240HP at 11,500ft and 1120hp at 18,500ft using 9lb of boost ( 48")compared to the later P-40 Allisons which gave 1125hp at 15,500ft at 7.25lbs of boost (44.5in)

The Merlin was rated at 1300hp for take off compared to the late Allisons 1200hp.

Going to the TWO stage Merlin design of supercharger would have ment an extra couple of hundred pounds of engine weight NOT INCLUDING the Liquied radiator for the aftercooler. This radiator has to be placed somewhere and will add at least some drag.

THE question is not weither it could be done but weither the improvement was worth the effort and the delay or interuption in production.

In my opinion it definitely was, the two stage supercharger is the only good way to get decent horsepower at altitude. Without that, it's a one-legged man in an ass-kicking contest. I'd throw out the pilot armor plate to make up the weight if it had to be done.

Shortround6 · Jul 21, 2009

Elvis said:
Hey Shortround6, pull down your skirt, your inexperience is showing.

An engine is spec'd so that it will reliabily perform the task for which it was designed, regardless of the inginition system.
Yes, the Brits gain in performance is directly due to the increase of pressurization of the cylinders, but if you know you're going to do that, then you want to make sure the tolerances of the engine's affected parts are "spot-on", not just "close enough".
This is why they were "hand built" and why they were blueprinted.
Why else be so careful about assembly, if you weren't going to take the time to check each and every part.
Large rocker arm clearances are an indication that the engine's builder forsees the engine operating at high cylinder tempretures for an abnormally long period of time. This means the metal parts will expand to the point that they will exceed the normal clearances, so the manufacturer/builder allows for the prolonged heat by increasing the gap.
However, actual practice can sometimes show this is unneccesary and will thus rescind that spec and return either to the normal spec, or a lessened gap that may still be greater than the normal spec.
"Diesel Clatter" has nothing to do with the moving parts of the engine.
Its a condition commonly referred to as "engine knock" and it never does go away, or even lessen.
This condition exists in a diesel engine because it is not a "spark ignition" engine.
Instead, it compresses the air so tightly, it heats up to the point to where it can actually burn fuel, when atomized.
The same condition exists when a spark ignition engine "knocks". The air in the cylinder is actually hot enough to burn the a/f mixture after the intake valve has closed and the piston has begun to compress the mixture, but before the spark goes off.
Cylinder pressure in a spark ignition engine typically runs in the 90-150 psi range, but in a diesel engine, it can exceed 400 psi (thus the astronomical c.r.'s).
This puts a great increase in the load a diesel engine sees, and why those engines tend to be built a lot heavier than a non-diesel engine.

Elvis

You are quite right. I pulled a major mistake and a boneheaded comment about the diesels.

But that is no reason to join me

Yes the British did overboost their Allisons compared to the USAAF and Allison recommendations at the time. BUT the AVG were not British and their engines were not British contract engines. I am not sure how the Allison factory workers who assembled the engines in Indianapolis in the spring of 1941 would KNOW how the engines would be operated by the AVG in combat STARTING in Dec of 1941, 6-9 months later.

THE Chinese engines were "hand built" becasue they were doing things like " fitting steel inserts that were plated to fit oversize tapped holes, connecting rod bearings altered to fit slightly undersized crankshafts, and dozens of other similar fixes were made"

THe point about the Merlin compared to the diesel is that each manufacturer decides what clearences are needed by their engine when running at temperature and how they are going to get them. THe fact that an engine may show different tolerences or clearances when cold compared to a totally different type of engine should not mean that the engine is question is "sloppy" or loose. I may have read more into the statement than was ment but I am tired of reading about how much better American Merlinis were than British bult ones and other such nonsense.

BY the way, are you getting those cylinder pressures with a compresion gauge?

they seem to be just a little low for pressure once the fuel starts to burn.

Shortround6 · Jul 21, 2009

Clay_Allison said:
In my opinion it definitely was, the two stage supercharger is the only good way to get decent horsepower at altitude. Without that, it's a one-legged man in an ass-kicking contest. I'd throw out the pilot armor plate to make up the weight if it had to be done.

Well, the pilots might not agree with you.

The P-40 suffers from two problems when trying to compeate against the 109.

One is that the 109s engine used lower boost to make it's power so that a single stage supercharger can supply enough boost several thousand feet higher than the more highly boosted American and British V-12s.

THe other problem is the much greater weight of the P-40. Even using the weight of the light weight P-40N you have a combat weight of 7,725lbs compared to the weight of a 109 F at about 6400lbs or an early 109G at 6800lbs. Now with 1300-1355hp at just over 18,000ft for the German engines you have power to weight ratio of 4.92-5.02. For the P-40 to equel this it needs an engine that will provide 1545hp at 18,000ft or little above. You need the Allison to provide almost 40% more power than it did originally and to do this several thousnad feet higher than it did. while this performance was avialable later it might not have been available in Jan of 1942. You are going to need every HP of a Merlin 60 series engine.

The Spitfire gained about 900lbs between the MK V and the Mk IX of which only about 250lbs are artributed to the DRY weight of the two engines. Pulling the armour won't come close to cover the weight of the COMPLEATE installation even if there were several hundred pounds not included in the engine change.

Clay_Allison · Jul 21, 2009

Shortround6 said:
Well, the pilots might not agree with you.

The P-40 suffers from two problems when trying to compeate against the 109.

One is that the 109s engine used lower boost to make it's power so that a single stage supercharger can supply enough boost several thousand feet higher than the more highly boosted American and British V-12s.

THe other problem is the much greater weight of the P-40. Even using the weight of the light weight P-40N you have a combat weight of 7,725lbs compared to the weight of a 109 F at about 6400lbs or an early 109G at 6800lbs. Now with 1300-1355hp at just over 18,000ft for the German engines you have power to weight ratio of 4.92-5.02. For the P-40 to equel this it needs an engine that will provide 1545hp at 18,000ft or little above. You need the Allison to provide almost 40% more power than it did originally and to do this several thousnad feet higher than it did. while this performance was avialable later it might not have been available in Jan of 1942. You are going to need every HP of a Merlin 60 series engine.

The Spitfire gained about 900lbs between the MK V and the Mk IX of which only about 250lbs are artributed to the DRY weight of the two engines. Pulling the armour won't come close to cover the weight of the COMPLEATE installation even if there were several hundred pounds not included in the engine change.

That's why in another thread I suggested that the US should have taken over development of the Griffon when the brits dropped it and skipped the Merlin, putting the Griffon in instead with an 11' five-blade prop.

Shortround6 · Jul 21, 2009

Clay_Allison said:
That's why in another thread I suggested that the US should have taken over development of the Griffon when the brits dropped it and skipped the Merlin, putting the Griffon in instead with an 11' five-blade prop.

And do what with it when?

Early Single stage two speed Griffons were rated at about 1730HP at 750ft and 1490HP at 14,000ft. at 12 lbs boost. (54") They weighed about 1800lbs. By the time this engine got to 20,000ft it might give you 1275HP? Merlin 45s in MK V Spitfires could give 1230Hp at 18,000ft and weighed 400lbs less.
THese weights do not include radiators, coolant, oil, oil coolers or propellors.

You would just have to develop the Griffon with a TWO stage supercharger about 6 months faster than the British developed the Merlin with it's two stage supercharger.

By the way, the Allsion -73 in the P-40 K was rated at 1550HP at sea level for WER at 60" of boost.

Clay_Allison · Jul 21, 2009

Shortround6 said:
And do what with it when?

Early Single stage two speed Griffons were rated at about 1730HP at 750ft and 1490HP at 14,000ft. at 12 lbs boost. (54") They weighed about 1800lbs. By the time this engine got to 20,000ft it might give you 1275HP? Merlin 45s in MK V Spitfires could give 1230Hp at 18,000ft and weighed 400lbs less.
THese weights do not include radiators, coolant, oil, oil coolers or propellors.

You would just have to develop the Griffon with a TWO stage supercharger about 6 months faster than the British developed the Merlin with it's two stage supercharger.

By the way, the Allsion -73 in the P-40 K was rated at 1550HP at sea level for WER at 60" of boost.

probably better to dig up the "original" thread than have the same discussion in two different threads.

Elvis · Jul 22, 2009

Shortround6 said:
BY the way, are you getting those cylinder pressures with a compresion gauge?
they seem to be just a little low for pressure once the fuel starts to burn.

Which engine are you referring to? The gas engine or the diesel?

Elvis

Shortround6 · Jul 22, 2009

Elvis said:
Which engine are you referring to? The gas engine or the diesel?
Elvis

Both.

If we are talking about a cylinder pressure of say, 120lbs that can be acchieved by an engine with an EFFECTIVE (after the valves close) compresson ratio of 8 to 1. Not including any pressure rise due to the increased temperature of the gas.
A diesel at 20 to 1 would have 300psi again not including the pressure from the higher temperature.

IF these were the peak presures the engine wouldn't run. The pressure on the piston on the down (power) stroke would only equel the the work needed to compress the the gas (the air, air fuel, what ever non- liquied is in the cyllinder) with nthing left over to overcome friction let alone give you any power at the shaft.
You do know what BMEP and IMEP mean don't you?
And they are far from the peak pressure in a cylinder.

Shall we each lower our skirts now?

Elvis · Jul 22, 2009

Shortround6,

No, we shant.
However, it just occurred to me that the figures I quoeted could be a little misleading, in that we've been discussing aircraft engines this whole time, yet I quote automotive figures (at l;east with the gas engine figures).
Granted, the application shouldn't matter under natural aspiration, however, I did not state that the figures were aggregate for general automotive applications, and in that sense I apologize for not doing so.

...and no, those pressures are not "out-of-line" and yes, they were achieved with a compression gauge.

Elvis

P40: The underated underdog, or just behind the times

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