Author Topic: Boost VRS MAP (Read 379 times)

F4UDOA · « **on:** June 25, 2003, 12:58:21 PM »

Simple question.

If a Spitfire has +16 Boost how much MAP is that equal too and if a P-51 has 61"MAP how much Boost is that?

And why? What is the conversion?

F4UDOA · « **Reply #1 on:** June 25, 2003, 01:01:40 PM »

Also which Spit used the Merlin 1650-3 or -7?

The Spit designations for Merlins are different I believe.

swoopy · « **Reply #2 on:** June 25, 2003, 02:14:42 PM »

MAN on the 51 is in psi i think, boost is in bar i think

swoopy · « **Reply #3 on:** June 25, 2003, 02:16:39 PM »

ah no spits boost is in Ibs

heres what they called some of the engines

Single Stage, single speed supercharger
Merlin I
Merlin II - Spit I
Merlin 45/46 - Spit V or IV

Single Stage, two speed supercharger
Merlin X
Merlin XX

Two Stage, two speed supercharger
Merlin 61/64
Merlin 66/67/76/85
Merlin 100 series
Merlin 130
Merlin 140

Nashwan · « **Reply #4 on:** June 25, 2003, 02:21:12 PM »

16 lbs boost is about 63 in mercury.

61" manifold pressure is 15lbs boost, or very close to it.

1 lb boost = 2.04" manifold pressure. However, the US measured absolute pressure, Britain measured boost as a figure above standard sea level atmospheric pressure.

So add about 14.8lbs to the British figure, then multiply by 2 to get an approx figure in inches of mercury.

The equivalent of the 1650-3 was the Merlin 63, which was used in the Spit F IX, F VIII, and probably a few others. The AH Spit IX uses the earlier Merlin 61, which was limited to 15lbs boost, or 61" HG.

The 1650-7 was equivalent to the Merlin 66, which was used mainly in the Spit LF IX and VIII.

flakbait · « **Reply #5 on:** June 25, 2003, 02:36:12 PM »

Good question. The conversion rate is: 1 PSI = 2.0360 in Hg @ 0° C (32ºF) or 0.4912 in Hg per PSI.

61" Hg = 29.9632 PSI
16 PSI = 32.576" Hg

If you add in the 29.92" Hg at sea level, and subtract 14.69 PSI, you get...

62" Hg for the Spit
17.88 PSI for the Pony

Obviously it'll change somewhat based on altitude and temp, but that's the basic conversion. The reason for adding 29.92" Hg is because manifold pressure is actually a measure of vacuum, not pressure. Most pressure gauges (ie. car tire gauge) leave it off because they measure positive pressure. Brits (being backwards

) also leave off atmospheric pressure when measuring pressure.

In the US, like Nashwan said, they measure absolute pressure. So you subtract 14.69 PSI to get the Brit equivalent PSI figure.

Converting between the two (US and Brit) means...

US to Brit: -14.69 PSI
Brit to US: +29.92" Hg

http://xtronics.com/reference/convert.htm
A good link for converting all kinds of pressure measurements to almost any other pressure measurement.

-----------------------
Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High

funkedup · « **Reply #6 on:** June 25, 2003, 03:25:40 PM »

Some of this has already been covered, but I want to make sure everything gets explained correctly:

Both types of gauges measure absolute pressure in the intake manifold (downstream of the supercharger(s)) using the same transducer, an aneroid. The measurement is not relative to the outside air pressure, it's relative to the constant pressure inside the aneroid capsule. It's an absolute pressure measurement, because absolute pressure is what matters as far as power output and mixture and detonation, and therefore the boost controller needs to know absolute pressure. I believe there is a section in America's Hundred Thousand that discusses the aneroid and boost control.

Anyhow, the only difference between the US Merlin and the RAF Merlin is how the dials for pilot are calibrated.

The USAAF gauges give absolute pressure in inches of mercury (in-hg). They'll read 29.92 at sea level with the engine shut off.

The RAF gauges give absolute pressure minus standard sea level atmospheric pressure (not the current ambient pressure), in units of pounds per square inch (psi). So at sea level with the engine shut off, they will read zero.

To go from MAP in in-hg to boost in psi: Subtract 29.92 and multiply by .491.

To go from boost in psi to MAP in in-hg: Add 14.69 and divide by .491.

These relationships are valid for any altitude.

F4UDOA · « **Reply #7 on:** June 25, 2003, 03:42:29 PM »

Nashwan and Funkedup great answers thanks!

This chart is the reason I ask.

I thought the climb of the P-51B/C/D (1650-7 is usually the C or D)looked interesting but I didn't know how to read it.

F4UDOA · « **Reply #8 on:** June 25, 2003, 03:44:52 PM »

Here is the corresponding speed chart

Lazerus · « **Reply #9 on:** June 26, 2003, 01:34:33 AM »

My head hurts.