What is manifold pressure?

[AAFO_WSagar]

Originally posted by Howard
Interesting how what we "name" things so ofter gets screwed up. On a non super/turbo charged engine, the manifold pressure is not a pressure at all... rather it is a vacuum produced by the engine "sucking in" the fuel air charge. The higher the number, the higher vacuum and the higher power setting.

Howard... think about that for a while.. Higher vacuum, higher power setting???

Wayne

[Howard]

Hi Wayne...

Now we are talking non turbo/supercharged.

Well, let's see... what's is high and what is low....

If I go out and look at my manifold gage now it will read about 30. That is approximately atmospheric pressure. This is "higher" than any powersetting that you can pull. There is no vacuum in the manifold. This is a "low" vacuum.
When I start the engine and run at idle the number will go to lowest number you are likely to see at that alitude before you are ready for a major overhaul..... I think about 10. This is a high vacuum and a very low power setting.

Yea, Wayne, I did not go back to read my posting again yet but there is confusion on what is "high" and "low"... for sure....
Howard.

[AAFO_WSagar]

Hi Howard,

For sure, the terminology gets confusing when talking normally aspirated and boosted... but...

If you think of MAP and not vacuum, it makes it easier.. In a non-boosted engine, if you think pressure and not vacuum, then at full open throttle, you'll read somewhat less than atmospheric pressure, which from reading the articles that are available seems to be about 30 inches of merc at sea-level...

What threw me in your message was talking of vacuum and higher readings being higher power, when thinking vacuum, it's the exact opposite as the engine will pull more vacuum with a closed throttle and somewhat close to zero at full throttle...

If we throw vacuum out of the explanation and go entirely with MAP, then it begins to be a "bit" easier to understand...

I think...

I'm just sitting here giggling because The Doctor, SlackMan and others have to be rolling on the floor with all of our struggling with this

Wayne

[Unregistered]

Manifold pressure

Yes the atmospheric engines have vacumn and so do most charged engines at idle which changes to a charged engine as the method of charging becomes efficient enough to exceed atmospheric pressure. The ADI helps in that as air is compressed it heats up and the super charged engines can get the air so hot without ADI, that the heat alone can set off the burn in the cylinder to be an explosion. We actually burn fuel not explode it. When detonation occurs, it is an explosion rather than the controlled burn that we want. It will light off like a diesel engine which is technically a compression ignition engine. Most diesels have compression ratio's of around 34 to one or higher. In a gas engine the supercharger is in effect increasing the comression ratio so high that it may explode well before the piston gets to the top of it's stroke, which breaks rods as it would act as a hydraulic lock. That being leaving no further movement upward because of the explosion, thus breaking a rod and or piston. To allow the higher compression/boost, we cool down the charge by putting in a mix of water and alcohol. Alcohol will not separate once mixed with water and alcohol will be around 160 octane by itself. Octane is the resistance to exploding with the higher number so we want to get that number as high as possible to resist the explosion or detonation so we have the race fuel to give a greater margin before detonation. Additionallly, the water it self helps cool down the mixture so that you may run a more dense charge of air and fuel. Remember that as you heat a gas (oxygen) the given volume will take up more space. As we cool it, we can get more fuel in the same charge which will equate to more horse power. The water in the ADI will also remove heat to allow a more dense charge because it will turn to steam, which again, when heated to a vapor, expands to 1857 times it's volume as a liquid. On the downward stroke when this happens it also helps push the piston down faster than with gas alone. A lot of this can also be affected by cam timing, but that is another issue.

[Howard]

Wayne, you are right for sure..... I guess that forty years as a pilot and a engineer was not helping me much... always has been confusion on this matter but I would like to think I would have it down pat by now....

Back to basics.... push forward to go faster!!! Howard.

[AAFO_WSagar]

Originally posted by Howard
Back to basics.... push forward to go faster!!! Howard.

Yeeee Haaawww!



Wayne

[Howard]

Water injection...
Brings to mind the early Boeing 707. 1961 oe 1962. They used water injection to allow increased thrust on take off... They got rid of those engines as quick as new ones were available. I can not remember all the details, but I remember that they injected so much water for the 90 seconds on take off that if they continued to inject at that same rate for a long flight, the water could weigh more than the aircraft
When a racer uses water/alcohol for injection or to dump on the radiator for cooling the added weight must be a considerable factor. Howard.

[Unregistered]

Right Wayne, high vacuum = low power. Think of it all as PRESSURE. Vacuum is simply nagitive pressure read below zero on the gauge.

Larry

[mayday51]

Manifold pressure

Howard,

........pressure (boost), or vacuum (low pressure), are both
measureable states in the intake tract (post throttle plates,
prior to intake valve)........

.........are we all wading through semantics?..........

......low (numerical eg: 8"Hg) is a higher pressure than a
High (numerical eg 27.5"Hg).

...........yes.....race engines (read automotive,stock car & others)
....have very near O" Hg pressure drop across the throttle plates
..........and only slightly higher drop in the "venturi " area of the carb (under ideal conditions)

..........I have dyno measured either at less than .2"H2O....

.........yep,another term for conversion!

Thanks everyone for the shared info & Ideas!.....


Mayday51
Jim Gallagher

[mayday51]

ADI and Spray bar

....Doc,Sparrow,Engine builders/tuners

.......will someone (a noteable "expert") please clarify theese terms ?......

.....ADI ........injected water/alcolhol mix actually ingested into the engine to lower combustion tempratures (?)(!)(?)

....Spray Bar/spray bar water
.............water/water mix delivered across a radiator
or liquid to air cooler to enhance the transfer of heat to the air (?)(!)(?)


...........ya know sometime the "oldtimer's" sets in and
my mind has fallen and can't get up
(see what happens when you're 52 and fightin')
(that was for Wayne)


Thanks again,

[Unregistered]

Originally posted by Howard
Interesting how what we "name" things so ofter gets screwed up. On a non super/turbo charged engine, the manifold pressure is not a pressure at all... rather it is a vacuum produced by the engine "sucking in" the fuel air charge. The higher the number, the higher vacuum and the higher power setting..

That isn't quite accurate.

Manifold Pressure is usually measured in *absolute* units where only a perfect vacuum reads zero. The "vacuum" in a normally aspirated engine's manifold is lower than atmospheric pressure, but still higher than a vacuum so it is still a positive absolute pressure.

If an engine has a MP guage calibrated in inches of mercury, it will read roughly 30 inches of mercruy (atmospheric pressure at sea level) with the engine SHUT OFF at sea level. Start the engine and let it idle, and the pressure will drop well below atmospheric pressure. Apply boost and it will rise above atmospheric.

Now, the relationship of pressure to power is the SAME in a normally aspirated engine as in a boosted engine also. Higher absolute pressure implies higher outuput power for the most part. It is not true that higher vacuum equals more power, quite the opposite. A normally aspirated engine operating at FULL power will have a manifold pressure that approaches atmospheric pressure. If you close the throttle to idle (minimum power) the pressure DROPS (or "vacuum" increases) not the other way around.

To further muddy the waters, most turbocharged or supercharged cars do not have "manifold pressure" guages, they have BOOST guages that read pressure realtive to atmospheric and are pegged at zero so they don't show vacuum at idle. They are also calibrated in pounds per square inch (PSI) rather than in inches of mercury as most aircraft engine guages are calibrated. So when a turbo car boost guage shows 10 PSI of boost, you have to convert PSI to inches of mercury and then ADD the local atmospheric pressure in order to convert to a true Manifold Pressure reading.

Steve_L