Manifold Pressure (1 Viewer)

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I'd thought it would be better to discuss pressure units here, rather that off-topic in another thread.

These are not linear units of equivalence, they are national standard units of manifold atmospheric pressure. Some are absolute pressure and some are gauge pressure, notably British Boost. 1 ata is NOT 1 standard atmosphere, it is 1 technical atomsphere.

The relation between units are linear, once the offset points are calculated in (for the units that have it, like british psig of Japanese +mm Hg).

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3) German Technical Atmospheres (ata) = (in Hg) / 28.958 ... not a stndard atmosphere.

4) Japanese / Russian mm Mercury (mm Hg) = (in Hg – 29.92) * (760 / 29.92)

Japanese were using 'gauge pressure', or pressure over standard atmospheric pressure; the Soviets used 'absolute pressure'. Hence + with Japanese, and lack of it with Soviet units.

If you look below on the figure , you will notice 47.25 inches of Mercury (U.S.A. units) is right at 440 mm HG (Japanese units), right where my spreadhseet says it is (really it converts to 440.2 mm HG) ... and it is also right at +8.5 psi Boost (British units) and 1.632 ata (German units). I didn't make this stuff up, but it isn't all that hard to come up with it, either.

47.25 in Hg cannot convert to 440 mm Hg, but to +440 mm Hg - small difference in writting that makes a world of difference in real world.
Othervise i do congratulate to you for this calculator :)

I can post an "extended" chart if anyone is interested, that covers idle to Reno racing levels. The chart below covers from near cruise to upper levels usually attained by radials.

That would be nice.

The Russians typically used mm H2O, not mm HG ... but they DID use mm HG every once in awhile. When they did, it matched the Japanese units exactly, not surprisingly.

Soviets never used mm H2O ("mm bo" (Вода́ = water)), but always "mm of Hg column" ("mm Руть ct.", or "mm pт. ct."). One might check out the tables posted in the Engines sub forum.


More later.
 
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I'm keeping in mind that the Soviets used to run Allison V-1710's that were approved for 58" HG by the USA at 75" HG, so running a Soviet radial at these pressures is NOT a stretch in my book. The stretch would be it holding up for long under that MAP. A few minutes is one thing, 15 minutes is another.

Thanks in advance -Greg

The V-1710 used 100/130 grade fuel for MAP above 44 in Hg (or thereabouts), the CR was 6.6:1 vs. 7:1 for the Shvetsov, liquid-cooled engines tend to handle greater boost better - there is really no comparison with boost levels the V-1710 was capable to achieve vs. the ASh-82.

Isn't a US standard atmosphere for engine purpose 30 inches of Hg even though it's actually 28.5?

The German system is the one that is most sensible.

The 'technical atmosphere' (at) was discarded since it was not a 'straight derivative' (or what is the terminology) of SI units. It have had kp instead of N within itself, hence the discrepancy between the at and a (standard atmosphere). The abbreviation for 'technical atmosphere absolute' was ata, the 'gauge pressure' (ie the pressure with offset point accounted for) was 'atü' - that would be equivalent of British +psi unit.

German system was as good or bad as other systems IMO.
 
The V-1710 used 100/130 grade fuel for MAP above 44 in Hg (or thereabouts), the CR was 6.6:1 vs. 7:1 for the Shvetsov, liquid-cooled engines tend to handle greater boost better - there is really no comparison with boost levels the V-1710 was capable to achieve vs. the ASh-82.



The 'technical atmosphere' (at) was discarded since it was not a 'straight derivative' (or what is the terminology) of SI units. It have had kp instead of N within itself, hence the discrepancy between the at and a (standard atmosphere). The abbreviation for 'technical atmosphere absolute' was ata, the 'gauge pressure' (ie the pressure with offset point accounted for) was 'atü' - that would be equivalent of British +psi unit.

German system was as good or bad as other systems IMO.


My reasoning is that:
1 Absolute Pressure is superior to Gauge Pressure as it provides more information. When working out the increase in power an increase in boost achieves one must use Absolute Pressure not Gauge Pressure. Why use gauge (relative to NTP anyway) when absolute is required.
2 Most equipment is designed for human use in conditions designed for where people live which is more or less NTP (Normal Temperature and Pressure) at sea level. Relative measures, such as the standard atmosphere given an immediate sense of the change that has occurred. A relative measure of a standard atmosphere makes sense. I understand that ultimately any serious mathematical treatment involves the use of newtons/sqm ie Pascals.

I can tell you from personal experience that these things matter: see what happens to the cooling properties of electric motors, electronic drives in say Chile or Peru (mining operations at 3000m/10,000ft). The reduction in air density can also lead to electrical breakdown in the insulation of electric motor windings or power distribution. It's also interesting to note that old style steam locomotives performed very well compared to diesels in these conditions.

The regulation of 'boost' pressure in an inlet manifold was probably too primitive, what was required was a boost limitation that regulated inlet manifold temperature. For instance if preignition was the concern the a DB605A that worked well 1.42 ata in winter but maybe not in summer.

An interesting problem was the effect of low pressures on ignition systems of aircraft designed to operate at very high altitudes. One had to pressurise them to prevent break down of the insulation via tracking and arcing, likewise with high voltage electronics of radars where highly energetic pulses were generated. One needs either a dense atmosphere or a vacuum, not something in between.
 
Since absolute pressure is VERY easily related to gauge pressure, how is it in any way superior? If you know one, you know the other.

I just didn't know Soviet mm Hg were different from Japanese mm HG. Since they both had the same units, I incorrectly assumed they must have the same reference in the absence of any knowledge to the contrary.

ANY unit is OK as long as you know how to convert it to another unit to which you want to compare it with. Meters aren't inherently any better than inches, feet, yards or barleycorns ... they are simply standard in more parts of the world and therefore more widely interpreted easily by the average person. I wonder how "accepted" the meter would be if "meter" meant different lengths in different countries? Perhaps there would be a clamor to "standardize?"
 
Hey Greg, this material was really interesting. Thank you for sharing it. However after reading it and than some other book I came up with a question in regard to emergency manifold pressure settings.
As we all know in emergency situation pilots did not follow the restrictions and were overcoming them, increasing the RPM and Manifold Pressure to the engine limits to gain advantage or run away.
In case of later American aircraft I understood it was achieved mostly with usage of water injections, however there was also on lower settings a so called dry WEP.

Russians had in some of their aircraft like La-5 a button or lever to overcome engine limits.
Same was in fact for the Japanese, let me quote the Robert C. Mikesh publication, "Japanese Aircraft Equipment 1940-1945". On page 40 there is a description of the "Emergency Boost Control" which is explained as :
" Sometimes referred to as a Manifold Pressure Control, Emergency Power Boost, Supercharger Boost Regulator, Automatic Booster Regulator, and other combinations, depending upon the translator, this unit is used in Japanese aircraft in the high power and performance range. To avoid conflicting terms that all refer to this device, "Emergency Boost Control" will be used. This device should not be confused with the Blower or Supercharger Shift Control, which is worthy of a few words of comment.
Engines of higher horsepower had superchargers that were geared to rotate at higher rations than the normal blower impellers attached to the crank case at the read of the engine. These air pumps, if you will, forced more fuel-air mixture into the induction system than was able to be pumped in by the pistons in the engine cylinders, often called increasing boost pressure. This greater volume of air mixed with fuel provided more power to the engine. These superchargers, or blowers, often had two speeds, with a lever or knob for manually shifting the clutch mechanism. The higher speed was beneficial at higher altitudes, where the air is thinner and more air was needed.

The Emergency Boost Control, on the other hand, provided a safeguard against over-boost pressure that could easily damage the engine, "blowing a jug" or cylinder being the common term. This activating lever, which was located in the vicinity of the throttle lever - often below or to the rear of it - was normally in the down or back position (This was a pull-out knob on the instrument panel of the Zero, Paul, and other examples). It functioned through a servo-motor connected to an aneroid near the carburetor. Regardless of how far the throttle was pushed forward, this aneroid would measure the over-pressure limit and prevent the carburetor throttle valve from opening beyond a rated altitude limit for respective engine (For example, the valve setting was plus 25 cm for Zero). With change in altitude, this aneroid would automatically adjust the carburetor for maintaining that limit. This allowed the pilot to concentrate on air combat where maximum power was the norm, and not have to be concerned about inadvertently over-boosting the engine.

In emergency situation such as in combat, more power could make the difference between victory or defeat. By moving the Emergency Boost Control lever forward, up, or pulling a handle, the automatically controlled restriction limit was raised (to plus 35 cm for the Zero), and it was then up to pilot to gamble on the maximum power to apply as viewed on the manifold pressure gauge, and hope that the engine would not fail because of over-boost."

Since in Planes of Fame you have original Zero with original engine I'm sure there is that Overboost lever in yours too :
RoA0cp.png


Did you ever try to use a higher settings for your Zero ? I couldn't come up with something and was wondering what is the practical difference in MP/RPM with throttle wide open and maximum rpm on your Zero with the Overboost lever pulled open and without it.
 
The 'original' Spitfire Merlin engine produced about 1030hp at 6psig (pounds per square inch guage with the introduction of 100 octane the 'boost' went to 12 psig and the power to 1310hp. Assuming one atmosphere is 14.2 psi the calculation that would give us power increase is

12psig + 14.2psi / 6psig+14.2 x 1030 = 1335hp. (which is close to the actual of 1310) These figures are from the Merlins maximum power.

For a DB605A engine the calculation would be something like this the engine was producing 1300ps at 1.30ATA but when boosted to 1.42ATA it was 1475ps. These figures are from the engines sea level performance. (At 2000m the engines power was more like 1530 metric horse power.

The calculation is simply 1.42/1.3 x 1310 = 1430. This is simpler.

The equation is complicated by the fact that permissible engine RPM concurrently went from 2600 to 2800 which is a 7.6% increase the same time. This was due to improvement is spark plugs, an oil deaerating centrifuge that greatly improved bearing life and the introduction of a relatively sophisticated engine RPM and pitch control " computer" that worked even in dives and seems to have given the engineers the confidence to push to the limits a little further since automatic control was gentler on the engine and stuck to the limits more reliably.
 
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Hi Hiromacci,

Our Zero flies for a Museum. I don't even have to ask ... we never use any extra boost over and above normal settings and very rarely get to full power. Sometimes we may get to full power ... my guess is whenever Steve wants to, but mostly our planes are operated at cruise power for good economy and we takeoff at whatever boost Steve Hinton and Kevin Ekdidge determine to be best.

The Zero's engine limits are known to Steve Hinton, Kevin Eldridge, and John Maloney. If I am not mistaken, these are the only people who currently fly it. There is only one Sakae 31 flkying in the world and it gets somewhat pampered since the supply of spare parts is limited to our second Sakae 31 engine. Our second engine is complete, but is in pieces (unassembled).

To put that into perspective with the P-51's, our P-51's usually take off at no more than 900 - 1,100 HP and then throttle back for initial climb out. I have been in 3 private P-51's and all climbed out at about 2,500 feet per minute or slightly more, which is probably 900 - 1,100 HP. Since we aren't in a war and do not have access to even 130+ Octane fuel (much less 150 Octane fuel), NOBODY can run them at WWII power levels or even wants to. After all, WE are paying for the fuel, maintinance, and any breakage.
 
I expected that, after all this bird is preserved for airshows so the next generations can see it. This is absolutely marvelous job you do gentleman :)
Maybe than it would be possible to ask Mr. Hinton what readings is he getting for that mentioned "above normal settings" or "full power" ?
 
ANY unit is OK as long as you know how to convert it to another unit to which you want to compare it with. Meters aren't inherently any better than inches, feet, yards or barleycorns ... they are simply standard in more parts of the world and therefore more widely interpreted easily by the average person. I wonder how "accepted" the meter would be if "meter" meant different lengths in different countries? Perhaps there would be a clamor to "standardize?"

From a practical point of view, and I fly aircraft with different units of measurement, I don't care what units the gauge is calibrated in, or whether it is gauge pressure or absolute. (I couldn't even tell you whether the CJ-6 gauge or absolute) All I need to know is what figures I need to set, and what approximate numbers I should see.
The only time it matters is when trying to compare engines, and you've got plenty of time to convert, which isn't really that hard anyway.
 
Clearly it might not be a good idea to run our only working Sakae at full power. However, hesitantly as I am not an engineer, would it be a good idea to fix accelerometers to the motor and obtain vibrational spectra under the various safe conditions for running it on the ground? The spectra might not tell you anything that a skilled mechanic could not hear or feel but it would be computer readable and it would be a permanent record in case a part were to fail in future. Vibrations might at least show how evenly the charge is distributed amongst the cylinders. A good computer model of the Sakae would enable the power under all conditions to be calculated and prediction of the vibrational spectra would be one test that the model was accurate. We might even be able to see if a particular vibrational mode prevented Nakajima from pushing the Sakae's output any further.

Any comments from anyone who understands piston engines, especially modern design practises?
 
The Sakae has a certin way it runs. Only 3 pilots fly it and all agree to flag anything out of the ordinary. We're not talking about low-time warbird pilots, we're talking about Steve Hinton, Kevin Eldridge, and John Maloney, three of the best warbird pilots in the world.

They have a "feel" for planes that most test pilots would kill for. If they feel a vibration that wasn't there last time, they check the prop for nicks, dress it is necessary and get to the bottom of it if it isn't the prop. None of them are given to accepting changes in feel without investigation of same. If the source isn't dangerous, it may or may not get fixed immediately, but anything that affects airworthiness is addressed.
 
Hi gunbyk,

The CJ is a nice warbird. I love tghe feel of heim joint in the alerons and elevator.

If I were betting, I'd bet it was absolute just because we found out Russian mm HG were absoulte in WWII. Countries typically don't tend to change neasurement references without some pretty big shakeup.

I know the M-14 likes to be shut down in a very specific sequence, but it is a wonderful engine. I have a friend who flies a Yak-52 and loves it. I think the systems in the Yak are a bit easier to access than in a CJ, but neither is exactly easy to work on, is it? and troubleshooting the ignition system is "interesting."
 
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We've got the original engine in ours, the Housai 6a. the more I think about it, the more I'm inclined to say its absolute too. Take-off is about 810mmHg.
Luckily, I've never had to work on it (besides helping to paint strip it for a repaint), but I've never heard anyone complain about working on one, and from what I've heard they are better than a Yak.
I've never had any qualms about flying behind the Chinese engine. Once they are set up right and you have an engineer who knows their quirks, then they are a good reliable engine.
Shut-down is 30 seconds @ 800RPM, and then shut off the ignition and advance the throttle, as it doesn't have an idle cut-off. Varies a bit from most modern aircraft, but the other machine I fly is a Tiger Moth, which has the same shut-down sequence. I'm not sure how this compares with the M-14 shutdown sequence.
 
I thought the originals were mostly Ivchenkos or Zhouzhou HS6A's of about 285 HP US. My friend Robin knows the procedure for the M-14, but suffice to say, following the book usually give best results.

810 mm HG in absolute boost is 32 inches of Mercury. If your engine is not supercharged or has very mild boost, that sounds about right. 810 mm HG in gauge boost or about 62 inches of Mercury and is not all that far from what they run at Reno.

I'd bet on absolute boost for your CJ-6. The engine you name above is a version of the 14 family, so Robin's preocedure are likely about the same. His is a nominal 360 HP, but the Russians usually quote the low end of the power range, and his makes abit more.

Perhaps the difference between his engine and yours is a supercharger ... I'm not too sure. Not being an owner or flying one very often, I haven't really looked into it.
 
Yeah, that's the same engine, HS stand for Housai AFAIK.
There is quite a bit of difference between the two engines. 9 cylinder radial and physical size is about where the similarity ends. Gear ratios are different, and I don't think there are any interchangeable parts. A bit like the airframe, which started out as a license built Yak 18T, but then developed into a different machine.

The HS6a is more expensive too! Last quote I heard was about $115,000. Pretty expensive when they have an official TBO of only 600 hours!
 

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