Allison V-1710-39 power output at sea level as installed in P40E

[Venturi]

Hello all,

I am in the midst of a heated debate regarding the usage and durability of the Allison V-1710-39 without manifold pressure regulator as installed in the P40E, used in wartime in the North African campaign and in the Soviet Union for lend-lease purposes, as well as in regular use by USAAF squadrons.

The Allison V-1710-39 engine utilizes a single stage supercharger which has a critical altitude approximately 14,000' or so, which obviously means it produces additional boost below this altitude, which must be managed by the operator's reducing throttle as altitude decreases.

It becomes clear that the supercharger could thus produce high levels of boost at sea level, far above the rated levels, and further it seems this was easy to accomplish by simply advancing the throttle to achieve the mechanical limits of boost, which increased as aircraft altitude decreased.

The approved WEP rating for the -39 engine was approximately 45" for 5 min, according to Allison manuals of the time. My question is this, does any evidence exist that these relatively low boost levels were exceeded in combat use by pilots, if so when, where, for how long, and what were the adverse effects on the engine? I have seen some source documents which state that the British were removing the regulators on the -39 Allisons installed in the P-51 and were running greater than 70" without harm, I have seem quotes from Vees for Victory book which indicates that the -33 engine as installed on the earlier variants of the P40 were being used up to 58" as early as Jan-Feb 1942, but I am wondering if any further information of any kind is out there.

Any documentation to this effect would be most welcome.

 

[GregP]

Hi,

I worked an an Allison engine overhauler for 2 years and we had a lot of WWII vets come through. Among them was General Davey Allison (no relation to Allison engines). He said he used to demonstrate then at 75 inches for 10 - 15 minutes at a time with no ill effects. His contention was that if your life was in danger, you used whatever power was necessary to prosecute the fight and he says they regularly exceeded the factory-recommended limits. He is NOT the only guy who said that, but most stopped at about 65 - 68 inches of MAP. Late war they were cleared for 70 inches by the USAAF, so it isn't all that big a stretch.

According to General Allison, the extra power would "wake up" the P-40 airframe. Relative to 45 inches in the same plane, I'm guessing that's true beyond a doubt. Relative to a Japanese fighter, I have no opinion as I do not know. More power is good but sometimes can be destabilizing.

Today, when Joe Yancey overhauls one, he usually doesn't get past 45 inches to seat the valves, and only briefly at that to check for good operation for a few seconds, but he has a few customer who have tried them at up to 3600 rpm and 75 inches on Reno with no bad effects. I watched John Paul's son hold off a Corsair at Reno one year ... he was at 3400 rpm ... somewhat to the dismay of his father, John Paul, and the Corsair could NOT catch him. This was the Bronze class, which is basically stock warbirds.

The V-1710-39 was an "F-series" engine, not one of the delicate "long noses" units, so it was basically pretty strong. The main issue would have been the early intake manifolds.

My spec file has the V-1710-39 (F-3R) at 1,150 HP for takeoff at 44.5 inches of MAP and 1,470 HP at sea level and 56 inches of MAP. I am showing the V-1710-39 as used in the P-51, P-51A, P-40D (1,150 HP), P-40E (same), XP-46, XP-46A .

A lot of people tends to forget that the plucky P-40 gave a pretty good account of itself in combat. It probably wasn't the best anywhere if flew, but it did OK with good pilots and good tactics.

 

[Venturi]

Thank you for your authoritative and thorough reply.

 

[tomo pauk]

This should be helpful: link
Also: link
Basically - Allison acknowledges that in the theaters of war the engines were overboosted to the point of abuse (up to 66-70 in Hg), but neither them nor AAF ever rated the -39 engine, officially, above 56 in Hg of manifold pressure.

 

[Venturi]

First off, thank you. The more info the better.

I have seen the letter from R. M. Hazen and also a letter from an USAAF source, which appears to be from a general detailing the same practice on the -39 engine as installed in the P51A. In that letter there is a request for official approval to remove the MAP regulator which was installed on the Allisons in that airframe, so as to allow overboosting. The general related that "the British" had been running their Allisons for up to 20min at boost levels in excess of 70" MAP without damage. I figure that if an official request was made from that level, then the crews were probably already doing it in the field. "The Brits are doing it" may have been a safe way to justify official permission, or at least turning a blind eye, without getting folks in hot water for a practice already rampant. #33 and #36 in the last page of the attached files detailing that letter.

Also in Vees for Victory as I mentioned earlier, there is mention of overboosting happening in P40 squadrons immediately post Pearl Harbor. I believe pg. 124.

I really appreciate the info guys.

 

[tomo pauk]

Glad you find the info posted here useful
BTW - the -39 was installed in the P-51 (no letter), the P-51A got the -81 (talking about production machines; there was some engine swapping for flight tests). The difference was not just in dash number, since the -81 featured the 9.60:1 supercharger gearing, vs. 8.80:1 in -39 and similar engines. Net gain was increase in power above ~14000 ft (no ram) and aircraft speed; loss was low altitude power (felt under 7-8 thousand feet).
The official permission for war emergency rating was issued for engine type that passed the test, it came from manufacturer and air force in question down to the units and individual pilots, not the other way around. In the US ww2 practice, the engine type was 1st to pass the 7 and a half hour test for the future 5 minute long war emergency rating, obviously using the wanted/needed RPM and manifold pressure.
The important exception from this was the practice with P-38 in UK in 1942, where the brass there encouraged upping the engine limits and actually aproved and issued the more agressive ratings for the P-38F.

 

[Venturi]

I realize that the official operating limits were as posted. It is interesting however to note the linear increase in rated power of the -39 engine over time.

I'm more interested in the wartime settings that were actually used in extremis so to speak, and also the actual mechanical limitations of the engine (which after all, is what the pilots were dealing with when in combat), rather than the manufacturer - approved nominal values. Obviously Allison, from their standpoint, felt abuse might cause subsequent failure of the engine and they do not wish to be held liable. Of course, they weren't getting shot at.

It's interesting to note in Hazen's letter from Allison that there is the line "this company has agreed to 60" boost for WEP" - and the following discussion regarding the philosophy the company had regarding rating the engine power; also the note in it regarding pilot use of the practice of overboosting the engine.

You're right about the P51A and the higher supercharger gearing. I was wondering if you were aware of any other changes to the engine mechanically besides that?

The above letter does mention it is regarding the Mustang I in the header, which is the P51 no letter variant, and that was powered by the -39 Allison.

 

[tomo pauk]

Thank you for reminding me that Allison agreed in the letter about the 60 in Hg limit.
There was also the introduction of V-1710-73 (F4R) engine, that featured strengthening of engine components, gaining 35 lbs in process. Making it easier to aprove for 60 in Hg WER, as noted at Vee's, pg. 153? Take off power was also increased to 1325 HP.
Coolant was now 30% glycol, 70% water, vs. 97% glycol on F3R, that would also make cooling of hot spots easier.

 

[Venturi]

Oh yes, I never quite understood the fascination with running even pure glycol. Really hampers heat transfer. Molality is sufficient in a mix to prevent freezing and water has a much higher specific heat. Detonation / preignition a real problem of course at high pressures and RPM control is even more important. Not denying that the safety margins are decreased and of course Allison knew that too.

 

[GregP]

More glycol results a WAY less internal damage to the water passages. If you want a study in why water isn't particularly good for an Allison or Merlin, just take apart one that has been used in a boat where the lake or seawater was used for cooling. It isn't pretty, and sometimes results in extreme corrosion to the point of going through to the cylinders.

When Steve Hinton was attempting to fly Glacier Girl across the Atlantic to Duxford, the thing that caught his attention and caused the turn-around was coolant from the stacks. VERY evident once you have seen it and know what it is. That's via Steve, not me. He shut down that engine and flew to an alternate field.

Somewhat later I believe it turned out to be a cracked cylinder liner.

 

[AMCKen]

I would imagine the amount of boost you can get away with depends a lot on the fuel octane you are using.

 

[XBe02Drvr]

AMEN! It's all about detonation.

 

[XBe02Drvr]

And the effective anti-knock value of your mixture as it enters the cylinder after compression, intercooling, carburetion(or injection), and manifold distribution.

 

[Shortround6]

Small variable is the inlet temperature of the air.
And you are limited by either the hottest cylinder in the engine or the cylinder with the leanest mixture not all cylinders were cooled equally and not all cylinders got exactly the same mixture.

 

[Venturi]

Small variable is the inlet temperature of the air.
And you are limited by either the hottest cylinder in the engine or the cylinder with the leanest mixture not all cylinders were cooled equally and not all cylinders got exactly the same mixture.

Wasn't the carb upstream from the supercharger?

 

[GregP]

Air entered the carburetor and was fed into the supercharger impeller. The fuel line went to a nozzle with a little round disc that had small flutes cut into it, and that made a circular fountain of fuel aimed right at the impeller about an inch away from the impeller. So, air-fuel mixture was injected directly into the impeller, was then pressurized and fed a large intake tube that went halfway forward in the valley between the cylinder banks. It then split into two vertical stacks going upward (one for each cylinder bank) and split again fore and aft and was fed into two manifolds that each fed 3 cylinders ... 3 front and 4 aft.

It's tough to find a pic looking down between the cylinder banks. Here is one from a quarter view.

You can see the 3-cylinder manifolds sticking up in the middle of the cylinder banks on the intake side. The part that splits the air-fuel mixture into 2 streams is called a baby's butt because that is what it looks like. The center pieces seen above with rubber couplers and hose clamps are rams horns. Each end of the rams horn feeds a manifold that serves 3 cylinders.

From the decal and the smooth valve covers it is a Joe and Pat Yancey engine. Nobody else spends the time to smooth out the cast Allison valve covers except Joe. I did some 12 sets myself when I worked there. It took several hours per valve cover. The paint is polyurethane. Joe fabricates the air intake boxes on top of the carburetor. His engines always look like show pieces, but they run great. Ask any Joe Yancey Allison owner that has purchased a completely overhauled engine. He also does repairs, and minor jobs upon request but unless he gets to do the overhaul, he can.t really vouch for the rest of the engine. Want a full warranty? Get the full overhaul.

Looks like original wood exhaust blockoffs!

Can't tell if this one is a display engine or a runner. What makes me wonder is the ignition harness. When Joe and Pat do an Allison overhaul, Pat makes up new ignition harnesses and polishes them like Navy brass. The harness on this one doesn't look anywhere near Pat's standards. Look at the rest of the engine and then at the ignition harness and you will notice it doesn't seem to match well at all.

 

[Venturi]

That is really great Greg. Having built quite a few engines myself I can see all the work in it and the attention to detail, and we're just looking at the outside! Are they balanced/blueprinted on overhaul, and how is the valvetrain designed?

Also, can you tell me what differentiates say an V1710-39 vs something like the above? What parts were weakest on the V1710s - IE what did they improve on as the models went by - I know sometimes forgings are improved, sometimes an intake runner is redesigned or the block has different strengthening/oiling improvements, this has been my experience on other engines as the production run goes on, and usually these are denoted by revision numbers or some such.

Any details would be great and also really hard to find in a book or elsewhere - I understand if any are trade secrets.

 

[GregP]

Joe typically balances the pistons to within 0.5 grams and the rods to the same. Then he typically puts the rods and pistons / wrist pins and keepers together and balances them all so the heaviest set is within 0.5 grams of the lightest set. If requested, he can get them even closer. He polishes the cranks and checks them for true to Allison factory spec. When he does the main bearings, he scrapes them manually and is the only guy who works on Allisons who has the factory crank tooling to check for bearing fit. When he is done, they plastigage to factory minimum tolerances on the mains. You may have a failure, but it won't be a heat failure from mains that are too tight; they fit. His oil pressure readings are always right there with factory numbers at idle and at cruise power settings.
Joe is an old-school mechanic and overhauls them the right way. If anything is out of spec, he causes it to be fixed or replaced with something that can meet spec. When he gets old mains from an old, derelict Allison, he cleans them and sends them to specific people to be de-plated, and re-plated to Allison factory spec with lead and silver.

A really great trip for an engine guy is to get a tour of the Yancey engine shop, particularly when a newly-overhauled Allison is ready for run-in and valve seating. I worked there 2 years and still love going there and harassing Joe ... not that anyone can cause him much harassment. If anything, it's the other way around. I have great pics, but have been asked not to post interior shots of the shop, and so I don't do that anymore.

 

[tomo pauk]

Hello all,

I am in the midst of a heated debate...

A question - is the debate on-line?

 

[kool kitty89]

More glycol results a WAY less internal damage to the water passages. If you want a study in why water isn't particularly good for an Allison or Merlin, just take apart one that has been used in a boat where the lake or seawater was used for cooling. It isn't pretty, and sometimes results in extreme corrosion to the point of going through to the cylinders.

Lake, sea, or even tap water are far different from distilled water used for engine coolant, so most/all of the corrosion issues there won't apply. (cast iron and carbon steel will still rust somewhat with pure water, but to nowhere near the same extreme and will lack the even more problematic salt/mineral deposits and etching, not to mention potential hot spots and coolant flow issues caused by mineral deposits inducing boiling and steam gaps in the coolant -this is worse for calcium/magnesium deposits in typical hard water than it is for saline solutions devoid of bicarbonates, as calcium and magnesium bicarbonate turn into solid carbonate scum/scale at high temperatures)

Corrosion inhibitors help more, but for the most part, pure water shouldn't be particularly harmful to the engine (and far less harmful than pure glycol on certain seals and rubber components). The early Merlin, Peregrine, and older Curtiss V-1570 all had problems with sealant falures and glycol leaks when using 100% glycol. The shift to 70/30 water/glycol was one of the big steps that took the Merlin beyond the problems the Peregrine was suffering (the older Kestrel had avoided it, normally using pure water or just a very slight amount of additives for antifreeze -more like German engines during the war, though pressurized unlike early and pre-war German engines). I don't think any production versions of the V-1710 were intended to run on pure glycol and had already targeted a water-glycol blend. (I vaguely recall some tests being performed on 100% water, but I might be mistaken there)

Pure (or anything over 50%) propylene glycol is also highly flammable, especially when hot, so elliminating that fire danger was obviously a benefit for using the blended coolant (on top of solving the leaks, being cheaper, and having a higher heat capacity). Incidentally, the 70/30 blend also avoided all the flammability related concerns the US Navy had, though perhaps slightly late to make liquid cooled engines significantly appealing. (plus there was still the logistical issue of storing sufficient coolant reserves on a carrier, aside from adopting a modified coolant scheme using pure water generated with shipboard stills)