I read somewhere that F4F Wildcats on Guadalcanal had the superchargers set to

maximum boost at all times and they could only get 60 hours before an engine change. It may have said they had the wastegate wired shut, although I wasn't aware that supercharged engines had waste gates, I thought that was a turbo charger thing.

Is there any truth to this?

Sounds very suspicious to me, How do you land at full boost? Cut the ignition?

Shortround6 said:

Sounds very suspicious to me, How do you land at full boost? Cut the ignition?

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No, not full throttle, full boost for the supercharger. According to what I read, they were running the supercharger at the maximum setting from the time they took off until they landed. I guess they were running it in high blower (did Wildcats have 2 speed blowers?) even at takeoff to improve performance against the Zero at the cost of engine life. Sort of what the Russians did with the P39. Has anyone else read or heard of this?

Some Wildcats had two speed blowers and some had two stage with the second stage not even engaged at low altitude.

It still sounds bogus. While you could get more performance you are really gambling with the engine life. It is not just short time times between overhaul but over boosting can cause detonation, which if bad enough can hole pistons, bend connecting rods and even blow cylinder heads and complete cylinders of the crankcase.

Using the the higher gear at low altitude takes more power to drive the supercharger, heats the intake air more making it more likely to detonate, and the hotter air is less dense making less power for a given manifold pressure. Some low level mechanic may have thought he knew more than the factory engineers but this has too many strikes against it to actually work.

P-39 used a single gear and just closed the throttle part way at low altitude to control the manifold pressure. Opening the throttle further increased the airflow and pressure without changing the speed of the supercharger impeller so power needed to drive the super charger changed very little and the intake charge temperature also changed very little. You could still hit detonation limits but now where near as quick.

I wish I could remember where I read that. I go to bookstores and browse alot. I will read one or 2 articles out of several books or magazines and of course then I dont remember where the info came from. Maybe it is a completely false story, but I hope some others comment on it. I'm just curious. I agree on engine life and everything you said it could do, but on the other hand, a Zero pumping 20 mm into you from behind won't help it any either.

If you could be sure that just swapping the engine out back at base more often would take care of things then go for it. Pulling away from the Zero only to have engine pieces come flying though the cowling while the Zero is still in sight didn't really gain much.

If the engine is good for 1200hp in low gear (or first stage) it takes 50-100hp more to drive the supercharger in high gear or with the auxiliary supercharger engaged. That takes the engine DOWN to 1100hp at the same manifold pressure so you need almost 10% MORE pressure to get the Same power to the prop. Except that about 30% of that power to drive the supercharger turned into heat in the intake charge, making the charge less dense so you need even more pressure to get back to the power level you were at. Intake temperature is now going through the roof.
I repeat, you can get more power but you are flirting with disaster every second that this goes on.
This was the whole reason behind two speed superchargers in the first place. To get more power for take-off and down low while still keeping the engine within allowable limits.

Shortround6 said:

If you could be sure that just swapping the engine out back at base more often would take care of things then go for it. Pulling away from the Zero only to have engine pieces come flying though the cowling while the Zero is still in sight didn't really gain much.

If the engine is good for 1200hp in low gear (or first stage) it takes 50-100hp more to drive the supercharger in high gear or with the auxiliary supercharger engaged. That takes the engine DOWN to 1100hp at the same manifold pressure so you need almost 10% MORE pressure to get the Same power to the prop. Except that about 30% of that power to drive the supercharger turned into heat in the intake charge, making the charge less dense so you need even more pressure to get back to the power level you were at. Intake temperature is now going through the roof.
I repeat, you can get more power but you are flirting with disaster every second that this goes on.
This was the whole reason behind two speed superchargers in the first place. To get more power for take-off and down low while still keeping the engine within allowable limits.

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I understand. Is there anything that could be done that would increase horsepower? Supercharged engines don't have wastegates do they?

Supercharged engines had throttle linkage that wouldn't allow the throttle plates to open all the way at sea level even though the throttle in the cockpit was fire walled. This was to protect from over boosting and detonation at low altitude. As the aircraft ascended the throttle would open more until it was wide open. So, what I assume was happening here, if true, was that this linkage was disconnected. Then the pilot would be responsible for maintaining a safe manifold pressure but would have extra power available in an emergency at the risk of the engine.

The R-1830 engine used in the F4F's used a manual supercharger switching system that enabled the pilot to switch off stages and change blower speeds. A schematic of the supercharger control system, taken from the R-1830 manual, is attached. The R-2800 as used in the F6F, F4U, and P-61A/B used the same basic approach.

So it was quite possible for the pilot to select the 2nd stage blower at a lower altitude than it normally would be used. I have seen very little mention in the literature about the workload this imposed on the pilot, and this may be because the Japanese tended to fight at low to medium altitudes, lacking any two stage supercharged aircraft except in trivial numbers very late in the war.

An engine manifold pressure regulator would tend to defeat lower altitude selection of the blower 2nd stage to a degree, by limiting the amount of boost that could be achieved. It is quite possible that the manifold pressure regulators were modified in the field to enable more boost than normally would be allowed with the factory settings.

But not all engines even had manifold pressure regulators. The V-1710's used prior to the P-40M did not. It is common to read of an RAF pilot getting in a P-40 or early Mustang and complaining how much trouble it was to have to watch the manifold pressure gauge to prevent overboosting the engine. It was not even unknown for a RAF pilot to climb into an early P-40 and be stunned at the remarkable performance of the airplane when he shoved in the throttle all the way for takeoff - but then had the Allison blow up soon thereafter.

Ironically, when they added manifold pressure regulators to the later V-1710 engines this caused problems. The P-38 already had a regulator for the turbo interconnected with the throttle and the two systems would fight each other, resulting in erratic power levels. For some recon F-5's the new manifold pressure regulator was removed, since they could not tolerate such surging while trying to take pictures.

And we know of one case where the ability to manually engage higher boost levels was added in the field. The Merlin P-51's had an aneroid device that automatically switched the supercharger from low to high speed at around 18,000 ft. The problem was that just below the switch point the engine was then laboring to handle the thin air. And the Japanese liked to fight at that altitude, not only because that was where their airplanes performed best but because it put the Mustangs at a disadvantage (some Germans figured this out as well). Now, the P-51D had a switch that enabled the high speed blower to engaged, but it was spring loaded so it was for only momentary use. One fighter pilot on Iwo Jima asked that a non-spring loaded switch be installed so he could use the high speed setting below 18,000 ft. The Packard field rep said, "No! You'll blow up the engine!" But they did the mod anyway and some Zero pilots over Japan got surprised by a P-51 that was faster than it was supposed to be.

In Roger Freemans book of the Merlin Mustang, there is a pilot anecdote:
"Speaking about other narrow escape, 'Barry' Corfman commented:" I got jumped by a "Tojo" Mk4 while on the deck strafing a locomotive - don't know what happened to my top cover at 10,000 ft. To save my hide I went through the "War Emergency" safety wire and held the high blower switch in "High" position. The Rolls drew 96in of mercury for 4.5 min and got me away from that Jap doing "Lazy Eights" on my ass. I got out of that with only a few holes and when my crew chief checked the oil cuno strainer he could find no metal in it. Now that's a strong engine!"

Timppa:

Interesting story and that confirms the other piece I read. Note that he said that he had to hold the high blower switch in the high position - that is because it was spring loaded to the automatic postion. That means he had to take his hands off the throttle, and I suppose that if he had it firewalled "through the gate" I guess that enabled him to do that.

I also recall reading where a P-40 pilot, scrambled during the initial Japanese attack on the Phillipines, firewalled his throttle and stayed at low altitude to get away. He then was horrified to see he was pulling only 10 inches of Manifold pressure. He decided his engine was malfunctioning and pulled back on the throttle - and saw the gauge go back down to to 60 inches! He had pulled so much power that the gauge went past the 60 inch mark and kept right on going on another revolution.

As engine power and supercharging increased they had to use gauges that read higher. I have one WWII vintage manifold pressure gauge that tops out at 50 inches. I have another that goes all the way to 75 inches.

And by the way, I once had a guy in Finland claim that you could not hold in the P-51 High Blower switch because you could not be allowed to exceed 60 inches per the aircraft manual - and that if you did do that it would actually cause the engine to develop less power since the supercharger would consume more than added boost would add to the engine power. I quit posting at that website - I refuse to argue inanities with non technical people. Glad to see that not everyone in Finland is so uniformed!

Very interesting information gentleman. Would anyone else like to speculate if the original question I asked was true or not? Is it possible that the F4F's on Guadalcanal had been modified by the mechanics, and if so, would it or could it have increased their performance?

pinsog said:

Very interesting information gentleman. Would anyone else like to speculate if the original question I asked was true or not? Is it possible that the F4F's on Guadalcanal had been modified by the mechanics, and if so, would it or could it have increased their performance?

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Look at the diagram posted above until you find a gizmo labeled Aux. Supercharger Regulator and see that it connects to and controls a throttle plate in the air intake. Disconnect this link and wire the throttle plate open and you have what you are asking about.

Was it done? I have no idea.

Perhaps if we talk about the certainly employed Russian modifications in Lend-Lease planes we can have an answer...

Please see this article for an explanation about using high gear at low altitude.

Superchargers

Thanks to Tomo for the Article.

Engaging the the auxiliary supercharger would have a similar effect. Pilots and mechanics may have done it but they weren't getting the power they may have thought they were getting. Performance may have improved but an F4F pulling 60 in at low altitude was not getting 25% more power than one pulling 48in of pressure.

I remind you again that the P-39s and P-40s did not have a high gear or second stage, so opening their throttles to get higher manifold pressure did not cause the rise in temperature and drop in air density that the two speed or two stage engines suffered from.

Shortround6 said:

Please see this article for an explanation about using high gear at low altitude.

Superchargers

Thanks to Tomo for the Article.

Engaging the the auxiliary supercharger would have a similar effect. Pilots and mechanics may have done it but they weren't getting the power they may have thought they were getting. Performance may have improved but an F4F pulling 60 in at low altitude was not getting 25% more power than one pulling 48in of pressure.

I remind you again that the P-39s and P-40s did not have a high gear or second stage, so opening their throttles to get higher manifold pressure did not cause the rise in temperature and drop in air density that the two speed or two stage engines suffered from.

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Shortround,
Sounds like using stage 2 down low wasn't what they did.
Is there any other engine mod they could have done that would have increased performance?
I sure wish I could remember what the article said they did to increase performance, then you guys could immediately call BS on it or not.

The discussion about using the high gear down low reminds me about two well known fighters, the MiG-1 and Spit V (with normal impleler). The superchargers were designed/set to max out between 15000-20000, and the planes were lacking performance under 10000 ft (= supechargers sucking too much power there). So the MiG-3 received a supercharger with lower critical alt (but having greater power there), while Spit Vs were produced with engines having cropped impeller, and had to wait until two stage engine was introduced, in order to have good all-round performance.
There were good reasons for making two-speed engines.