# Manifold Pressure to Horsepower



## Zipper730 (Feb 22, 2019)

How do you convert these two?


----------



## krow113 (Feb 22, 2019)

Zipper730 said:


> How do you convert these two?


Manifold pressure is the amount of vacuum (negative pressure) in the intake tract.
Horsepower is a rating of the power developed by the engine.
What are you trying to convert them into?


----------



## MIflyer (Feb 22, 2019)

Manifold Pressure can be either positive or negative. These gauges both read about 30 inches, our current manifold pressure is 30.18.
Normally aspirated engines will have negative manifold pressure (left side of the needle). Supercharged engines will be develop higher than ambient manifold pressure. 

The HP developed versus Manifold Pressure will vary with the engine.


----------



## Shortround6 (Feb 22, 2019)

Zipper730 said:


> How do you convert these two?




trying to convert one to other only comes close to working on the same engine (displacement) running at the same RPM and using the same supercharger drive ratio. 

I say comes close because pressure is not the true weight (mass?) of the air. Pressure will rise with temperature and raising the pressure raises the temperature (hold onto barrel of a bicycle air pump to feel this one). so air at 60in of MAP is not quite twice the weight per cubic ft as air at 30in of MAP. By volume you need something like 9000 times the amount of air than you need of gasoline ( from an old book could be off a bit) so the amount of air is the controlling factor. The higher pressure air could be hundreds of degrees hotter. 
depending on how you get your boost (manifold pressure over ambient air) you have to take out the power to drive the the supercharger. 

Look at the power figures for just about any two speed supercharged engine. In low gear the engine always makes more power at the same RPM and manifold pressure than in high gear.


----------



## Zipper730 (Feb 22, 2019)

krow113 said:


> Manifold pressure is the amount of vacuum (negative pressure) in the intake tract.
> Horsepower is a rating of the power developed by the engine.
> What are you trying to convert them into?


Since I only know the horsepower of the engine under certain conditions, and I know what the MAP is under that condition. The idea is I want to compute the MAP across the listings.


----------



## wuzak (Feb 23, 2019)

Zipper730 said:


> Since I only know the horsepower of the engine under certain conditions, and I know what the MAP is under that condition. The idea is I want to compute the MAP across the listings.



Why?

Charts usually have boost (pressure above standard sea level pressure) or MAP along with power.


----------



## wuzak (Feb 23, 2019)

MIflyer said:


> Manifold Pressure can be either positive or negative. These gauges both read about 30 inches, our current manifold pressure is 30.18.
> Normally aspirated engines will have negative manifold pressure (left side of the needle). Supercharged engines will be develop higher than ambient manifold pressure.
> 
> The HP developed versus Manifold Pressure will vary with the engine.
> ...



I think Zipper is after MAP (Manifold *Absolute* Pressure), which is always positive.

Those gauges seem to show positive pressure only.

Reactions: Like Like:
1 | Like List reactions


----------



## MIflyer (Feb 23, 2019)

Which gauges? The ones I showed? To the LEFT of the needle is below atmospheric pressure. And you cannot measure air pressure below zero.


----------



## tomo pauk (Feb 23, 2019)

From anything i've read, a linear rise of manifold pressure will give about a linear rise of power.


----------



## Shortround6 (Feb 23, 2019)

tomo pauk said:


> From anything i've read, a linear rise of manifold pressure will give about a linear rise of power.



You are pretty much correct. 
This assumes a constant rpm and a constant rpm of the supercharger impeller (for a near constant supercharger drive cost) and a constant altitude with only the throttle opening being the variable.

This introduces pumping losses (and heat issues) if the altitude is low and the throttle closed down. See Merlin III making 880hp at sea level at 6lbs boost vs 1030hp at 16,250ft at 6lbs boost.


----------



## MIflyer (Feb 23, 2019)

Shortround6 said:


> Look at the power figures for just about any two speed supercharged engine. In low gear the engine always makes more power at the same RPM and manifold pressure than in high gear.



Interestingly enough I have a postwar P-51D manual that says that the maximum range will be attained by climbing to high altitude and then putting the supercharger Hi/Low switch manually in Low. Of course, manual selection of High normally is not possible because the switch is spring loaded to keep you from doing that, except for some P-51's that were modified on Iwo Jima to enable the pilots to select the High position at below the normal aneroid setting of 18,000 ft. Later the airplanes were modified to have the High and Low selection done based on ram pressure rather than altitude.


----------



## pbehn (Feb 23, 2019)

Zipper730 said:


> How do you convert these two?


With an internal combustion engine. Without a temperature and a volume(rate of flow) the manifold pressure means nothing at all


----------

