# Manifold Pressure vs BMEP



## Zipper730 (Dec 28, 2019)

I'm curious what the difference between MAP and BMEP is and if there's any way to convert them (either generalized rules of thumb or absolute formula)


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## MiTasol (Dec 28, 2019)

There are a bunch of theory books covering both at Engine design as related to airplane power : with particular reference to performance at varying alt that cover it way better than I can. This is from the first one.


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## wuzak (Dec 29, 2019)

MAP is the absolute pressure of the air in the intake manifold.

Brake Mean Effective Pressure (BMEP) is a calculated value defined as:

"the average (mean) pressure which, *IF* imposed on the pistons uniformly from the top to the bottom of each power stroke, would produce the measured (brake) power output."

Brake Mean Effective Pressure (BMEP): The Performance Yardstick

"Please note that BMEP is purely theoretical and has NOTHING to do with ACTUAL CYLINDER PRESSURES."

BMEP is calculated using power, displacement and revolutions per second. It also factors the number of revolutions per power stroke (2 for a 4 stroke, 1 for a 2 stroke).

So, no, there is no direct relationship between them.

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## Sid327 (Jan 3, 2020)

A simple formula

for calculating BMEP (in psi) that I found some time ago was to multiply torque in lb/ft by 2473 and divide it by the cubic capacity.

As an example, one of the most impressive naturally aspirated production petrol engines ever was the straight six M3 engine (code S54/B32) of 3246cc which was rated at 360hp @ 7,900rpm and 273ft/lbs @ 4,900rpm for the M3 CSL.
(It made more torque per litre than any n/a production V8)

Which meant the BMEP was above the magical 200psi and probably (AFAIK) one of the highest of any n/a production engine.


source: BMW E46 M3 engine | S54 specs, problems, tuning, etc.
BMW Heaven Specification Database | Engine specifications for Motorsport engines
BMW E46 3 Series Coupe M3 CSL Technical Specs, Dimensions


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## Zipper730 (Jan 3, 2020)

Sid327 said:


> A simple formula for calculating BMEP (in psi) that I found some time ago was to multiply torque in lb/ft by 2473 and divide it by the cubic capacity.


Still, is that valid for manifold pressure? Or is that valid for the average combustion pressure in the engine after pressurized air/fuel has been added, then ignited?

The manifold pressure, as I grasp it, is the air at some point in the engine's intake manifold?


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## wuzak (Jan 3, 2020)

There is no relationship between BMEP and MAP. I already explained this.

MAP is the pressure of the air in the intake manifold.

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## Zipper730 (Jan 3, 2020)

wuzak said:


> There is no relationship between BMEP and MAP. I already explained this.
> 
> MAP is the pressure of the air in the intake manifold.


Okay, I was just curious because, what he said contradicted what you said. You're generally pretty knowledgeable about this kind of stuff, but we all have gaps in our knowledge base.


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## wuzak (Jan 3, 2020)

Zipper730 said:


> Okay, I was just curious because, what he said contradicted what you said. You're generally pretty knowledgeable about this kind of stuff, but we all have gaps in our knowledge base.



No, he just showed how to calculate BMEP.


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## Joe Broady (Jan 5, 2020)

Old steam power engineering books give a classic formula for horsepower: hp = PLAN / 33000, where P is the mean effective pressure in the cylinder, L is length of stroke (feet), A is the piston area (square inches), N is the number of power strokes per minute. The formula is a direct result of the definition of a horsepower as 33000 foot pounds per minute.

Steam Power by Dalby

MEP (mean effective pressure) was measured by device called an "indicator" as explained in earlier pages of the same chapter in the above book. The result of the formula was "indicated horsepower," a measure of the power applied to the piston(s) by the steam. The power output at the crankshaft was less due to internal friction. To measure power at the crankshaft in the early days, a mechanical brake would resist its rotation. The torque developed by the engine was exactly equal to the brake's tendency to revolve. The force necessary to hold the brake stationary was therefore a measure of torque. (Old time books illustrate an arrangement whereby a lever on the brake bears down on a scale which "weighs" the torque.) With torque and rpm known, horsepower at the crankshaft was a straightforward calculation.

Later, a water pump or electric generator instead of a mechanical brake could resist crankshaft rotation, but the term "brake horsepower" endures. And that brings us to BMEP (brake mean effective pressure). If brake horsepower is known, you can rearrange the classic horsepower formula to solve for pressure P. Since it's calculated from brake horsepower, this pressure is called brake mean effective pressure. It's the pressure which, if exerted for the full length of every power stoke, would cause the engine to develop the same horsepower, assuming zero friction, and atmospheric pressure in the cylinders for the full length of the non-power strokes. Of course this assumption is not attained in the real world, so BMEP is a purely theoretical value. It does give a general sense of the intensity of combustion in the cylinders.

MAP (manifold absolute pressure) is the pressure in the intake manifold. In American practice it's traditionally inches of mercury, so at sea level with the engine not running it's about 30 inches. In the old days the pilot or flight engineer would note this pressure before start. Later, during run-up, the throttle would be adjusted to the same pressure in order to accomplish certain checks.

During the war I think the Germans measured manifold pressure in atmospheres, while the British used psi. Whether these were absolute pressure or gauge pressure I don't know.

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## wuzak (Jan 5, 2020)

Joe Broady said:


> Later, a water pump or electric generator instead of a mechanical brake could resist crankshaft rotation, but the term "brake horsepower" endures. And that brings us to BMEP (brake mean effective pressure). If brake horsepower is known, you can rearrange the classic horsepower formula to solve for pressure P. Since it's calculated from brake horsepower, this pressure is called brake mean effective pressure. *It's the pressure which, if exerted for the full length of every power stoke, would cause the engine to develop the same horsepower, assuming zero friction, and atmospheric pressure in the cylinders for the full length of the non-power strokes.* Of course this assumption is not attained in the real world, so BMEP is a purely theoretical value. It does give a general sense of the intensity of combustion in the cylinders.



BMEP does not assume zero friction. BMAP is calculated on the brake horsepower.

IMEP, or Indicated Mean Effective Pressure, is calculated from the power produced assuming 0 friction. That is, calculated from indicated hp.

Similarly FMEP is the mean effective pressure related to the friction power.




Joe Broady said:


> MAP (manifold absolute pressure) is the pressure in the intake manifold. In American practice it's traditionally inches of mercury, so at sea level with the engine not running it's about 30 inches. In the old days the pilot or flight engineer would note this pressure before start. Later, during run-up, the throttle would be adjusted to the same pressure in order to accomplish certain checks.
> 
> During the war I think the Germans measured manifold pressure in atmospheres, while the British used psi. Whether these were absolute pressure or gauge pressure I don't know.



British used boost pressure above standard sea level pressure (~14.7psi).

So +18psi would be 32.7psi absolute pressure. Or 66.6inHg MAP.

I'm not sure if it would be classed as gauge pressure, which I would expect would relate to the air pressure at the altitude measured, rather than compared with sea level pressure.

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## Zipper730 (Jan 5, 2020)

wuzak said:


> BMEP does not assume zero friction. BMAP is calculated on the brake horsepower.


Frankly I'm confused. I'm not sure what means what. BHP was what they'd use for the engine power output?

What did they use for steam power?


> I'm not sure if it would be classed as gauge pressure, which I would expect would relate to the air pressure at the altitude measured, rather than compared with sea level pressure.


Doesn't manifold pressure gauge the pressure of the engine while at a given altitude?


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## wuzak (Jan 6, 2020)

Zipper730 said:


> Frankly I'm confused. I'm not sure what means what. BHP was what they'd use for the engine power output?



Brake horsepower (bhp) is for the nett power out as measure by a brake dynamometer. That means the power includes the loss due to friction.

As explained above, bhp is still used even though brake dynamometers are rarely used. It denotes power at the output shaft.

I'm not sure there is an equivalent in the metric system. I think it is just Watts (W) or kilowatts (kW).




Zipper730 said:


> What did they use for steam power?



Steam engine output was brake horsepower, as explained above.




Zipper730 said:


> Doesn't manifold pressure gauge the pressure of the engine while at a given altitude?



Gauge pressure is the pressure above ambient air pressure.

In the case of British boost specifications it is pressure above standard sea level pressure.


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## Zipper730 (Jan 6, 2020)

wuzak said:


> Brake horsepower (bhp) is for the nett power out as measure by a brake dynamometer. That means the power includes the loss due to friction.


Okay


> I'm not sure there is an equivalent in the metric system. I think it is just Watts (W) or kilowatts (kW).


That can be translated into horsepower, so that's fine.


> Steam engine output was brake horsepower, as explained above.


Okay, that's useful


> Gauge pressure is the pressure above ambient air pressure.


Okay, so you simply take the gauge pressure and add atmospheric pressure and you get the absolute pressure?


> In the case of British boost specifications it is pressure above standard sea level pressure.


Is that why the British used +PSI as a measurement? I thought it was just to be simple and easy to understand for aircrew who could vary in educational background...


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## pbehn (Jan 6, 2020)

Zipper730 said:


> Still, is that valid for manifold pressure? Or is that valid for the average combustion pressure in the engine after pressurized air/fuel has been added, then ignited?
> 
> The manifold pressure, as I grasp it, is the air at some point in the engine's intake manifold?





Zipper730 said:


> Is that why the British used +PSI as a measurement? I thought it was just to be simple and easy to understand for aircrew who could vary in educational background...


 People use what they are used to using, there are all sorts of ways to express pressure, it would be best to use the units of the engineers who designed the engine, or supercharger or turbo. On a normal conventionally aspirated engine the manifold pressure will be less than atmospheric pressure, people used to fit "vacuum gauges" to help them drive economically, the closer you have the engine running to atmospheric pressure the more economical it is. With a supercharger or turbo the pressure in the manifold is what that pump can supply.


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## Snowygrouch (Jan 6, 2020)

There are MANY videos on YouTube which explain all of this, in a format which is much easier and better than typing on forums. 

BMEP - YouTube

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## Joe Broady (Jan 8, 2020)

An example may make things more clear. In the Hellcat "Pilots Handbook of Flight Operating Instructions" there's a graph that shows 1000 bhp at 2050 rpm gives 140 psi BMEP. Let's plug that BMEP and rpm into an idealized R-2800 engine and see how much horsepower is generated.

The idealized engine has no friction, no power lost to ancillaries (such as driving a supercharger), and atmospheric pressure in the cylinders except during the power strokes, during which the pressure equals BMEP (140 psi) for the full stroke. In other words, all the energy delivered by the BMEP acting on the pistons is delivered to the propeller.

The bore of an R-2800 cylinder is 5.75 inches, and therefore the piston area is 26 square inches. Multiply by 140 pounds per square inch and obtain 3620 pounds force on the piston during each power stroke. Stroke of the R-2800 is exactly half a foot, so each stroke generates 3620 * .5 = 1810 foot pounds of energy. Total number of power strokes per minute is half the number of cylinders times rpm = 9 * 2050 = 18500. Total energy per minute is 1810 foot pounds times 18500, or 33,400,000 foot pounds per minute. Since one horsepower is 33,000 foot pounds per minute, the idealized engine delivers 1010 bhp. Compare to the 1000 bhp from the graph in the Hellcat manual. Close enough!

Anyone who checks my math will notice a little error here and there. That's because, in the spirit of the 1940s, I did the whole computation on a slide rule just as an engineer might have done back in the day. (Slide rule was a late 1950s Keuffel & Esser 4181, a good science and engineering slide rule but not top of the line, manufactured in large numbers and available for a modest price today.) Also, there's unavoidable error in estimating values from the engine performance graph by eye. I give myself a passing grade for matching the book within 1%.

We should understand that BMEP is less than the actual combustion pressure. A real engine has to fight cylinder pressure during the compression and exhaust strokes, drive its supercharger, overcome internal friction, etc. Thus, the mean pressure during the power stroke has to be well above BMEP.

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## XBe02Drvr (Jan 9, 2020)

Zipper730 said:


> Okay, I was just curious because, what he said contradicted what you said.





Zipper730 said:


> Frankly I'm confused. I'm not sure what means what.


Don't feel like the Lone Ranger, Zipper! In my years of flight instructing this was one of the most confusing topics for students to wrap their heads around. They often left the classroom not truly understanding it and did their early complex airplane work (under the eagle eye of an instructor) by rule of thumb until understanding sank in.
I think your confusion with MAP and BMEP stems from the fact that from a pilot's perspective they are operationally related, whereas from an engineering standpoint they are separate things. Operationally, setting MAP and RPM in the wrong relationship can raise BMEPs to levels that will damage the engine. High MAP and low RPM puts the engine in a "lugging" condition (like driving slowly up a steep hill in top gear with a manual transmission car). Unfortunately this same configuration carried to a lesser extreme gives the best fuel economy, as Lindberg so famously demonstrated to Gen Kenny's Lightning pilots in the PTO. It's a trade-off between fuel efficiency (read: range) and stress on the engine.
MAP in itself can be confusing, as in a normally aspirated engine, the manifold pressure will always be equal to or lower than ambient atmospheric. That's because the engine is a suction pump and any flow restrictions (air filters, throttle plates, manifold plumbing) will reduce the manifold pressure to below ambient and make it a suction, or vacuum value, as pbehn said. (You always just knew normally aspirated engines sucked, you just didn't know why, right?)
Now, just to be sure, understand that all of that bit about RPM vs MAP only applies to an engine with a constant speed prop. With fixed pitch or two position props, throttle controls RPM directly and MAP pretty much follows.
Hope this hasn't been too insultingly simplified.
Cheers,
Wes

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