What is manifold pressure?

[gv71]

I was wondering if someone can tell me where and how manifold pressure is measured. I know these guys go WAY past the military emergency setting and I would like to say that 2002 was my first venture to Reno and I'm hooked in a bad way. The people are the nicest, everybody loves to talk to you about there plane, which is a nice change because in upstate NY trying to find someone other than my dad to talk to about air racing does not exist, and I could write a book on how approachable the pilots are!! thanks for the help and great job on keeping this east coaster informed during race week and the rest of the year. GREAT JOB!!!!!!!!!!!




Greg

[AAFO_WSagar]

I'll Take A Shot At IT...

Greg.. glad to hear you're hooked... join the club..

I'll take a lame shot at this, totally based on my knowledge of automotive engine technology (I used to work in the automotive end of things in another life)

Manifold pressure would be measured *somewhere* after the turbocharger, or, as in most Unlimited Racers, supercharger, and before the intake valves or throttle plate. It is measured in "inches of mercury" rather than pounds per square inch.

It would typically be measured by a gauge calibrated in inches of mercury, either plumbed or wired into the cockpit..

Hoping someone else with greater knowledge of the specifics (SlackMan, you listening?) will come in and give a bit better explanation.

Also, you can do a google.com search with the phrase "manifold pressure inches of mercury" and there are some pretty good examples of it there.

Hope this helps, rather than confuses.

Wayne

[shadow]

semantics

Unfortunatly, I think alot of this comes to semantics. In the car world what wayne is describing is called boost. You can go to any turbo or supercharger freak out there and ask them how much boost they are running and they will spit out a number without batting an eye.

Here is where I muddy the waters. I'll have to pay attention to this thread in order to delete this when someone posts an actual answer and this post only serves to confuse people. -

In the automotive world, manifold pressure is the term given to the pressure inside the intake manifold on normally aspirated cars. Yes, it is the on normally aspirated cars where I have heard this term show up. And, as I have heard it used it referred to the vaccum created by the pistons sucking air into the combustion chambers. The higher the reading, the stronger the vaccum, the more air is flowing through the intake manifold to the combustion chambers.

Now, in the aeromotive world, I've heard the exact same thing for normally aspirated aeroengines. The more inches of mercury you are pulling the more power you are making. So I've just assumed it refered to the amount of vaccum in the intake manifold.

ok, now someone correct me

shadow

[shadow]

I was way off

here is a good link describing what is going on.

http://www.rodmachado.com/Product/Books/Excerpts/c3-29.pdf

it talks about normally aspirated engines but you can easily see how boosted engines work from it as well.

wayne, if you could please delete the post above, I was trying to help, but as I thought I just messed it all up.

shadow

[AAFO_WSagar]

Shadow,

you didn't really mess up, I don't think.. it's all still "pressure" just without boost, we're limited to asmospherc pressure as our "boost" stick in a pump, be it a turbo or supercharger and you raise that pressure from ambient to whatever the engine can stand... sometimes, more than it can stand..

As always, despite the differences in uses, engines are engines..

Anyway.... I think we both managed to muddle through a pretty decent answer

Wayne

[FlyKidChris]

Manifold Pressure

Let's see if I can write this without botching it...
On a normally aspirated engine (no supercharging, whether it be via a turbo or mechanically driven), the manifold pressure (MP) will never be more than the outside atmospheric pressure (except for some ram air effect in a properly designed system). The M.P. will nearly equal the outside atmospheric pressure at full throttle, but in any other condition, you have some vacuum (less pressure inside the intake manifold than exists in the atmosphere).
This is where super-charging (compressing the intake air to a pressure higher than static or outside air) comes in. It can be done via a "turbo" (turbosupercharger - exhaust gas driven) or a mechanically driven supercharger.
Of course, compressing the air, causes the temperature of that air to increase, but that's beyond the scope of this explanation (see "inter-coolers").
At Reno Stead, at about 5000' above sea level, the static air pressure will be less than at sea level, perhaps around 25" of mercury (Hg).
The turbos or superchargers increase the pressure above that. A typical value for "War Emergency" power for Merlins was 65" Hg.
The "Super Stangs" run FAR more than that, likely above 120". You might chose to think of that much pressure in terms of "atmospheres" or more than 4 atmospheres!
With that much air, comes a lot of heat, and requires a lot of fuel, too.
I believe it was Dwight Thorn that hit upon the idea of using Allison piston connecting rods in the Merlins to handle the massive pressure.
Someone, please correct me if I'm wrong on any of this...
And to think, we have teams intentionally inducing this incredible stress for purses that often don't equal the cost of those "trick" engines!

[AAFO_WSagar]

Re: Manifold Pressure

Originally posted by FlyKidChris
Of course, compressing the air, causes the temperature of that air to increase, but that's beyond the scope of this explanation (see "inter-coolers").

Ahh... or see the ADI man

We could really get into a lot of trouble here meddling in areas that we're not really expert at (we is me and the mouse in my pocket ) but...

I hear a lot of people talking who seem to misunderstand what ADI is, or at least, what they are seeing trailing behind the racers at power..

What you *see* is spraybar water, which is sprayed on the coolant (in the water cooled motors) and oil radiators (on both water and air cooled motors) to increase the heat transfer beyond what the coolers will produce with just air to exchange the heat..

That is what you see as the mist trailing behind the airplane.. at least, the white mist when everything is going well.. the blue or really black cloud.. bad medicine!

ADI serves a similar purpose as does intercooling in that it cools the combustion mixuture increasing the resistance to detonation and, if I'm not wrong, a cooler mixture is more dense and produces more power..

I'm not sure any of the racers run an air intercooler.. maybe some of the sport guys do but I dont' know about the Unlimited guys...

daDoc is prolly watching this and laughing right now..

Wayne

[Randy Haskin]

Given the discussion of spraybar water, I just wanted to share this photo of Dago going out for the morning shakedown flight on Saturday. Cool view of the spraybar water highlighted by the low angle sun....

[FlyKidChris]

ADI

Wayne - Good point about the ADI and the lack of intercoolers in the Unlimiteds. I was thinking about Greenameyers dual intercooler installation as I wrote that part. Also I was trying to keep the topic as broad or general as possible; back to the automotive powerplant arena, intercoolers aren't unheard of.
Randy - Great photo, thanks
And now for another element, there are "boil-off" cooling systems, too (though not for any induction air, that I'm aware of). Stiletto was perhaps the most noteworthy example of that approach, but I understand Mike Brown is using a boil-off system in September Fury.

[FlyKidChris]

Another form of supercharging

Dare we mention "turbo-compounding?"

[Mayo]

As I understood the merlin supercharger, coolant flowed thru the casing of, and to an aftercooler just prior to manifold and intake, is this not an intercooler?

(taken from a previous post)

http://supercoolprops.eftel.com/ARTICLES/gwhite.htm

[Peashooter]

The links on this "other" post 'splain it all very nicely. Peas

http://warbirdaeropress.com/1forum/viewtopic.php?t=169

[The Phantom Samurai]

Aftercoolers...

The truth is, most Merlin powered Unlimited racers DO run an aftercooler. I believe the only Mustang racer running a "tube" at Reno this year was Dago Red. From what my Merlin friends tell me, when it comes to pushing the really high MP, the tube is the way to go.

TPS

[Air Race Addict]

Originally posted by Randy Haskin
Given the discussion of spraybar water, I just wanted to share this photo of Dago going out for the morning shakedown flight on Saturday. Cool view of the spraybar water highlighted by the low angle sun....

Hey Randy!
How is it that I have exactly the same picture, (only moving, HAH! but not posted yet :-) Hope you had fun out at the Basecamp. You are welcome out there anytime you can make it. You fit in well with the rest of the lunatics, er, uh, Addicts. Okay, I'll let the topic of MP continue.
Bill

[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.
On a turbo or supercharged engine the fuel air charge is being pumped into the cylinder.... the pumping action causes the charge to heat up (undesireable) so the charge may be cooled by an after cooler which is the same as a inner cooler. It is a "radiator" between the turbo/super charger and the engine.
When water is used to cool the surface of a radiator, an interesting thing happens... the water provides directly very little cooling... but when it is vaporized on a hot radiator the "heat of vaporization" comes into play... and that is approximately 1800 times as great as just warming the water one degree on the hot radiator. If an engine is allowed to "boil-off" cooling water, it can also benefit from this effect, but will loose the cooling water in the process. Actually these engines run with pressurized cooling system to prevent boiling and loss of coolant (which may not be water) but that is another story. Howard.