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Manifold Pressure?


Fastlane
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OK, this one is going to take someone that is a pilot with some reasonable mechanical skills as well. How exactly does the manifold pressure gauge work?

 

Little bit of insight, I'm a drag racer as a hobby, and we're always looking for that extra ounce of horsepower. Recently got into the helicopter world, and it's my understanding that the MP gauge measures "engine performance" for lack of a better term. How is this? Intake manifold? Exhaust manifold? Is it an oxygen sensor, or does is actually read pressure/vacuum?

 

How does this little fella work?

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Thanks for the link Joker. Read that whole thread, and to be honest I'm still a bit confused.

 

BUT, I DO have a sneaky suspicion that after a little bit of stick time, I'm gonna be FASTER on the drag strip! :rolleyes:

 

Basically it boils down to this. It works on an aircraft, can I interpret the information, and adapt it to a car? I think I can! It won't be the "tell all, end all" tuning device, but it sure will help!

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Fastlane,

The mapg is like the MAP sensor on a EFI automobile. The difference is you are just monitoring the available power in the aircraft engine. In Helicopters, you don't adj the mix in most. The MAP sensor on the other hand sends a reference signal to the computor, which it uses to calculate the fuel mixture to barometric conditions, fuel, timing, and quality/performance.

 

When you are tuning the drag engine. Use the manifold gauge at every strip, before every race to ensure you are getting max power for the temps and humitity of the day. You may have to adj timing or main jets to achieve peak power.

 

Later

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I'll try to explain in Car Nut talk (I own a couple of turbo charged supras)

 

In cars, we're normally thinking of power in terms of Vacuum and Boost. so 20" of Vacuum at idle and maybe 2 or 3" at WOT (non turbo). Then "boost" we think of starting at 0 and moving up to X lbs (or bar) of boost. Now 0 on this gauge is going to be standard pressure, 14.7lbs, or 29.92". On non standard days, you'll notice the gauge doesn't sit right at 0 when the car is off. The gauge still works exactly the same way, the numbers are just painted on different.

 

On aircraft, the vacuum gauge (MP gauge) has it's 0 mark at a perfect vacuum, 0lbs of pressure, or 0" Hg. About 29" of vacuum if this was an automotive gauge. Since you'll never get a perfect vacuum in the manifold (you're always moving air), you'll never see a MP gauge at 0. If you shut off the motor, you'll see the gauge read the current atmospheric pressure, 29.92" standard. On the car, this would be 0.

 

Just imagine an automotive vacuum gauge with a different face plate on it, and it might help. They both measure the exact same thing, they just tell you the information using different numbers. MP gauge tells you the pressure, car gauge tells you how much vacuum which is just lower than normal pressure.

 

Clear as mud?

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I disagree with what Fauxz said, if the scale is HG its HG. No matter on auto or aircraft. 0-29" HG is the same is the same. Now where on the manifold you pick it up is different. I have always taken readings after the throttle plate on auto's. I don't work on aircraft, so not sure, but would think they would do the same.

 

The R22 guage reads 5-35"hg. I am thinking my auto guage reads 0-29"hg. Same scale HG, but different range, but still hg. My gauge reads both neg and low pos pressure.

 

Don't even worry about turbo's. Not even close to what we are talking about. We are talking neg press, not pos pressure.

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I disagree with what Fauxz said, if the scale is HG its HG. No matter on auto or aircraft. 0-29" HG is the same is the same.

 

I'm just talking about the units you use to measure that pressure. True, 29" is 29", but it is also 14lbs and 1bar. However, almost all automotive gauges show 29" of pressure as "0." It's just a matter of where you paint the numbers. 10" on a MP gauge is exactly the same as 20" on an automotive gauge. Same pressure, just a different way of looking at it.

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Explain,

If the aircraft gauge and auto gauge both show Hg on the dial face, then how can you repaint the numbers to mean anything other than true Hg? If they do readjust the scale from true then its not Hg.

 

Thats like saying I have a thermometer. It reads a nice 84 degree's when true temps is 94 degree's. Why even call it manafold pressure inchs of Merc if its some off the wall scale. 10" on a car should be 10" on the aircraft using the same scale Hg.

 

scale 0-40" Hg

range 0-29" or 5-35" Hg

 

Later

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So basically we're talking about Density Altitude, right? All kinds of drag racers have their little "weather stations" out at the track, and I walk past laughing to myself, because they have NO idea how to read the information they're looking at.

 

For the record, I'm dealing with a 625 horse, Naturally Aspirated, 347 cubic inch V8 Ford. I just don't understand how manifold pressure is going to tell me anything I don't already know. But if the FAA decided that it needs it's own gauge in the cockpit, well then it does have some merit, right? On my CAR engine, the less pressure, the better. Keep in mind, no turbos, no blower, no nitrous. So now that I'm starting flight school I see this MP gauge. I'm VERY mechanically minded, the type of person that "has" to know how things work.

 

So if I'm up at 1500' AGL, I'm relying on my MP to tell me how all those little pistons behind me are doing. Pardon the pun, but that won't "fly" on the drag strip. Not comparing apples to oranges here, it's all piston powered.

 

Mechanic, I understand what you're talking about, but I have no MAP sensor. This is all "old skool" carb type stuff. No computer, it does what I tell it to do. There's no computer making the adjustments for me.

 

So in the long run, after I figure all this out, are we talking about taking a few density altitude & pressure altitude readings when I get to the track, look at my "cheat sheet", and I'll KNOW what jets to use in my carb?

 

Wow, this has NOTHING to do with helo's, but daaaaayum am I on to something here!

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I'm just talking about the units you use to measure that pressure. True, 29" is 29", but it is also 14lbs and 1bar. However, almost all automotive gauges show 29" of pressure as "0." It's just a matter of where you paint the numbers. 10" on a MP gauge is exactly the same as 20" on an automotive gauge. Same pressure, just a different way of looking at it.

 

 

Exactly! Automotive gauges aren't as "critical" as aircraft gauges. Hit autometer's website, and with a little digging you'll find that they're all set to 29.92 as "zero".

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Explain,

If the aircraft gauge and auto gauge both show Hg on the dial face, then how can you repaint the numbers to mean anything other than true Hg? If they do readjust the scale from true then its not Hg.

 

Thats like saying I have a thermometer. It reads a nice 84 degree's when true temps is 94 degree's. Why even call it manafold pressure inchs of Merc if its some off the wall scale. 10" on a car should be 10" on the aircraft using the same scale Hg.

 

scale 0-40" Hg

range 0-29" or 5-35" Hg

 

Later

 

Not really. NOTHING PERSONAL, but you're thinking like a pilot. You're out there in the 3D world, with an understanding of sea level altitude, density altitude, and pressure altitude. Your average 18 year old hum buying a pressure gauge for his turbo Subaru doesn't have or understand what is actually taking place. The gauge builders understand that, and they want to sell gauges. That's what they do. SOOOOO, you end up with a "normal" zero reading on the gauge, even though WE all know that is NOT the case. To them, "zero" is "zero". It has nothing to do with the elevation of a particular track, even though it's posted at the front gate. Most automotive gauges are based on 14 PSI atmospheric pressure, and put in a chamber at that exact pressure, and set to "zero". You know as well as I do, that is not accurate, and things change!

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Guest pokey
For the record, I'm dealing with a 625 horse, Naturally Aspirated, 347 cubic inch V8 Ford. I just don't understand how manifold pressure is going to tell me anything I don't already know. But if the FAA decided that it needs it's own gauge in the cockpit, well then it does have some merit, right?

 

The manifold pressure gage really comes to light when you see what is happening as you increase the pitch of the propeller/rotor blades. In a car, you have no way of "absorbing" the power---other than the wheels/pavement. With a variable pitch propeller/rotor ( that is independent of the throttle) you are capable of drawing ALL the power from the engine (regardless of throttle position). That was the reason and importance of the manifold pressure gage in a variable-power-absorbing situation.

 

take these for instance:

 

1) high manifold pressure, low RPM-----W I D E open throttle, alot of pitch (power absorbtion), lots of stress on the cylinder walls from pressure

 

2) low manifold pressure, high rpm------closed throttle, no pitch (no power absorbtion),no load on engine

 

 

i used to be a drag racer back when i was in my teens, the reason for the concern about the pressure then was to get the optimum fuel/air ratio---a WHOLE nother can of worms when it comes to aircraft..

 

Lets ask this question now, since we are on the subject of fuel/air ratios: which burns faster? a lean OR a rich mixture & what do we want to avoid in our cylinders?

 

http://www.avweb.com/news/columns/182081-1.html

 

try that link & hope this all helps us all understand a simply difficult concept B)

 

oh and a quick edit here on the pressure?

absolute pressure starts from zero ( as in a perfect vacuum-outer space)

gage pressure starts from 'whatever' reference point you happen to be starting from ( usually atmospheric, terms vary tho)

 

just realised that link was out-dated sowwwy--try this one?

http://www.avweb.com/news/pelican/pelicans...t_194816-1.html

Edited by pokey
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Explain,

If the aircraft gauge and auto gauge both show Hg on the dial face, then how can you repaint the numbers to mean anything other than true Hg? If they do readjust the scale from true then its not Hg.

 

Later

 

Because the automotive gauge isn't telling you inches of mercury, it's telling you inches of vacuum. A scale can say something weighs 22lbs while another says it weighs 10kg and both would be right.

 

As for why we have a MP gauge, and you don't need it in your dragster, think about how the motor is operating. The dragster you know you make 645hp at 7k RPM full throttle. Aircraft don't run that way. They run at a set RPM (most of the time) with comparatively little change in power setting. Imagine driving your car down the interstate, through the mountains, at 4k RPM at all times. You have to give it more gas uphill, and less down hill. RPM's stay constant, but the amount of vacuum you're pulling will change as you adjust the gas. In a car you don't really care, but in an aircraft you want to know how hard you're working the motor as it can give you information about how the engine is running, are you climbing, etc etc..

 

Sorry, forgot to add: In a car, and most certainly a dragster, you're going from idle, to red line and back again in less time than it takes to bring the rotor in a helo from idle to full RPM. The only reason you care what the MP is in a non-boosted car is to make sure you're pulling the right vacuum at idle and the right vacuum at WOT so you know you don't have a restriction or a bad ring. Everything in between is almost worthless on a car with a carb.

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The manifold pressure gage really comes to light when you see what is happening as you increase the pitch of the propeller/rotor blades. In a car, you have no way of "absorbing" the power---other than the wheels/pavement. With a variable pitch propeller/rotor ( that is independent of the throttle) you are capable of drawing ALL the power from the engine (regardless of throttle position). That was the reason and importance of the manifold pressure gage in a variable-power-absorbing situation.

 

take these for instance:

 

1) high manifold pressure, low RPM-----W I D E open throttle, alot of pitch (power absorbtion), lots of stress on the cylinder walls from pressure

 

2) low manifold pressure, high rpm------closed throttle, no pitch (no power absorbtion),no load on engine

i used to be a drag racer back when i was in my teens, the reason for the concern about the pressure then was to get the optimum fuel/air ratio---a WHOLE nother can of worms when it comes to aircraft..

 

Lets ask this question now, since we are on the subject of fuel/air ratios: which burns faster? a lean OR a rich mixture & what do we want to avoid in our cylinders?

 

http://www.avweb.com/news/columns/182081-1.html

 

try that link & hope this all helps us all understand a simply difficult concept B)

 

oh and a quick edit here on the pressure?

absolute pressure starts from zero ( as in a perfect vacuum-outer space)

gage pressure starts from 'whatever' reference point you happen to be starting from ( usually atmospheric, terms vary tho)

 

just realised that link was out-dated sowwwy--try this one?

http://www.avweb.com/news/pelican/pelicans...t_194816-1.html

 

 

Nailed it! Thanks Pokey!

 

To further explain what's flying around in my head for the rest of you, it's quite simple. It's the pitch! When I'm on the drag strip, my Fairlane is at WOT from the word go, with what could be considered a "constant" load. IE the tire/ground contact patch. Slam some gears, and we're at the other end in 10 seconds, over and done with. Not true with a helo, atmospheric conditions, altitude, air density, etc etc all come into play.

 

So the big difference here is the LOAD. In a helo, we can change the load on the engine via pitch, without changing engine RPM. Which is where the MP gauge goes "WTF?"! Now in a car terrorizing the city streets, there is virtually no change in "load", the variable is the throttle.

 

In a long and complicated way, this topic was made super simple. Thanks Pokey!

 

So now, I'm gonna throw THIS at you all. How can I use this information, adapt it to my Fairlane, and make me faster? Or is this just not feasible due to the physics involved?

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So now, I'm gonna throw THIS at you all. How can I use this information, adapt it to my Fairlane, and make me faster? Or is this just not feasible due to the physics involved?

 

Easy. Go fuel injected. You have to tune the car for every pressure and RPM combination. In a car with a carb, the jets are calibrated to cover that whole range and you tune for wide open throttle. You don't care what the vacuum (pressure) is because you're just taking all of it you can get. You care about the density altitude because that's telling you how much oxygen is in the air, and you know how much air per second you can suck in, and that tells you how much gas to add (jets)..

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Lets ask this question now, since we are on the subject of fuel/air ratios: which burns faster? a lean OR a rich mixture & what do we want to avoid in our cylinders?

 

"Lean is Fast, Fat is safe."

 

Not sure how that applies to the sky, but on the ground that statement is paramount. A lean engine ALWAYS out performs a rich or "fat" engine, hands down. BUT, an engine that's running a bit too lean will absolutely "scream" with power, for a few minutes, and then you need to start thinking autorotation! LOL On the drag strip, ya need to think about getting out of your own oil.

 

At 1500' AGL I'll take fat any day!

At 1300' on the drag strip, the leaner the better!

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Guest pokey
"Lean is Fast, Fat is safe."

 

the drag strip, ya need to think about getting out of your own oil.

 

At 1500' AGL I'll take fat any day!

At 1300' on the drag strip, the leaner the better!

 

 

http://www.avweb.com/news/pelican/pelicans...t_194816-1.html

 

the last link & i hope this one works :huh:

 

the question is : lean mixture, burn rate fast oR slow compared to rich (hint: hot/cold) THAT shud be a dead give-a-way ;) ( why does a lean mixture backfire thru the intake syatem)

 

glad i could help w/ the missing link to the understanding fastlane----it really IS a difficult thing to understand

 

i'm kinda surprised joker didn't add his 25 euro's yet :blink: ( he loves this type of discussion)

 

 

and fastlane? dontcha mean at 1320?

 

edit on that link,,, CHT isnt the one son of a !! ( they changed it on me) AGAIN ! ( i dont like change ) :(

Edited by pokey
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I think the closest thing from the auto world is the MAP sensor or Manifold Absolute Pressure which is a combination of intake manifold pressure (due to the changes/combinations between engine rpm and load) and ambient barometric pressure.

 

This makes sense b/c if you look at the MP pressure gauge when you crawl in for your pre-flight it reads the current ambient pressure. (As intake manifold pressure is zero at the time)

 

When the engine starts up, a "vacuum" is created in the intake manifold due to the piston down stroke and the partially open throttle on the other side of the equation.

 

For you EMS guys, think pulling a syringe at a constant rate, lots of suction (a.k.a. negative air pressure) because the you are filling this large cavity of air t(which at a stand still was at ambient pressure) through a small needle.

 

In this example the speed at which you pull back the plunger on the syringe is the RPM and the needle opening is the throttle; the larger the diameter of the needle, the more open the throttle.

 

 

Hover:

 

The throttle is slightly open (small needle opening) and for a constant rpm of 3,000, a very high vacuum is created. Vacuum is negative so you start with ambient air pressure say 29 less some vacuum value, say 8 and you are at 21 inches waiting to hover taxi.

 

Low RRPM and throttle wide open:

 

The throttle is wide open (large needle opening) for the same 3,000 rpm thus there is less of a vacuum and the pressure in the manifold will approach atmospheric pressure and that’s bad.

 

 

It’s a simplified way of looking at it but it helped me understand why I start with MP at 29 with the helicopter sitting by its lonesome and then it’s at 13 or so at ground idle.

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http://www.avweb.com/news/pelican/pelicans...t_194816-1.html

 

the last link & i hope this one works :huh:

 

the question is : lean mixture, burn rate fast oR slow compared to rich (hint: hot/cold) THAT shud be a dead give-a-way ;) ( why does a lean mixture backfire thru the intake syatem)

 

glad i could help w/ the missing link to the understanding fastlane----it really IS a difficult thing to understand

 

i'm kinda surprised joker didn't add his 25 euro's yet :blink: ( he loves this type of discussion)

and fastlane? dontcha mean at 1320?

 

edit on that link,,, CHT isnt the one son of a !! ( they changed it on me) AGAIN ! ( i dont like change ) :(

 

a lean mixture hotter/faster than a rich mixture that burns cooler/slower

 

it's past jokers bed time other wise he'd be right in the mix, pun intended :D

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Guest pokey
a lean mixture hotter/faster than a rich mixture that burns cooler/slower

 

it's past jokers bed time other wise he'd be right in the mix, pun intended :D

 

OK,,,, If a lean is hotter/faster, then why will a lean mixture backfire thru the intake? ( this is one of those "no way-it cant be" deals) :o

 

and WAKE up joker & preflight that 76 ! :unsure:

 

edited for spellin' ;P

Edited by pokey
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http://www.avweb.com/news/pelican/pelicans...t_194816-1.html

 

the last link & i hope this one works :huh:

 

the question is : lean mixture, burn rate fast oR slow compared to rich (hint: hot/cold) THAT shud be a dead give-a-way ;) ( why does a lean mixture backfire thru the intake syatem)

 

glad i could help w/ the missing link to the understanding fastlane----it really IS a difficult thing to understand

 

i'm kinda surprised joker didn't add his 25 euro's yet :blink: ( he loves this type of discussion)

and fastlane? dontcha mean at 1320?

 

edit on that link,,, CHT isnt the one son of a !! ( they changed it on me) AGAIN ! ( i dont like change ) :(

 

 

LOL no, I meant 1300'! Handful of transmission, and the right foot in the water pump! If you ain't getting it done at 1300', it's NOT gonna happen in the last 20. ;)

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Guest pokey
LOL no, I meant 1300'! Handful of transmission, and the right foot in the water pump! If you ain't getting it done at 1300', it's NOT gonna happen in the last 20. ;)

 

 

of korse ! man i miss my drag racing friends from the old days :rolleyes:

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a lean mixture hotter/faster than a rich mixture that burns cooler/slower

 

it's past jokers bed time other wise he'd be right in the mix, pun intended :D

 

Now you're getting into the whole "flame front" discussion. We're talking milliseconds here, but yes a lean condition will burn faster than a rich condition. When the piston approaches top dead center, and that spark hits, a lean mixture will "flash" across the combustion chamber A LOT faster than a rich mixture. The result? Faster combustion, more efficient fuel consumption, more horsepower, but at the expense of a bit higher cyl. head temperature. Remember, blue flames are hotter than orange ones.

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I use the vacuum gauge 0-30" for engine condition only and rarely. I don't use it for tuning purposes. I use a tach. I checked with my bud at work, whom his dad owns Piney Creek Performance. His dad builds some awsome engines. They use a pyrometer to measure the exh heat during a pass. His son told me Ford pistons start melting at 1380 F. They tune for heat build up and varify at the plug tips.

 

His dad built some bad ass engines for a offshore racer. Twin 502's. Had $26 grand in custom built parts alone. His dad even owns a patent on a custom grind cam. Thats only availble from Piney Creek. The offshore boat had to have F16 canopies due to the speed, for safety.

 

Here is a video of my coworkers Trans beside our shop. Is the WS6. This guy also has one of his dads 502's in an 81' Chevy pickup swb.1981 Chevrolet C-10 Torch Red- 502BBchevy/TH-400-9:1 motor-custom grind hyd roller-fully ported gm heads/1.8RR's w/stud girdle-300horse Cold Fusion Plate w/sep Fuel system-High Rise w/950 Proform Carb-Centerline ConvoPros/12 point Cage. Full weight truck-4260lb---10.71@127.56

 

 

Piney Creek Performance site.

Trans making pass, friends car..with simular engine mods.

19 foot 12in Dia MODEL Rocket.. by Randy Ussery, engine builder, middle of the article.

 

I wish I knew as much about engines as this guys dad. His dad loves racing so much he named his first son, Race....who owns the zam in the video.

 

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