Manifold Pressure?

[joker]

Awake!!!! (Yes, I was tucked up in bed while you were all enjoying this one. )

 

And loving the disucssion here...although still a little weary eyed to take it all in!

 

I see 'flame front propagation' mentioned...good! I dunno if 'manifold back pressure' is relevant, but I'm going to mention it first, just in case it is!

 

Joker!

Edited April 19, 2007 by joker

[me shakes fist]

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)

 

and WAKE up joker & preflight that 76 !

 

edited for spellin' ;P

If it's backfiring through the intake then you've got yourself a case of pre-ignition

[joker]

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)

 

 

a lean mixture hotter/faster

 

 

Hold on,

 

Isn't a way too lean-a-mixture a slower buring mixture...the fuel mollecules are further apart?

 

Remember, the fastest burning mix is just an incy-wincy bit richer then the best mix (peak mix). Anything richer or leaner actually burns slower.

 

So what happens is the combustion is still taking place as the intake valve begins to open during the end of the exhaust phase and there is valve overlap. Manifold backpressure sends forces the hot / combusting air back up through the intake valvue, which ignites the fuel air mixture in the intake manifold.

 

This causes a popping through the carb.

 

????? Just a guess ?????

 

Joker

[Guest pokey]

Hold on,

 

Isn't a way too lean-a-mixture a slower buring mixture...the fuel mollecules are further apart?

 

Remember, the fastest burning mix is just an incy-wincy bit richer then the best mix (peak mix). Anything richer or leaner actually burns slower.

 

So what happens is the combustion is still taking place as the intake valve begins to open during the end of the exhaust phase and there is valve overlap. Manifold backpressure sends forces the hot / combusting air back up through the intake valvue, which ignites the fuel air mixture in the intake manifold.

 

This causes a popping through the carb.

 

????? Just a guess ?????

 

Joker

 

 

BINGO ! As the intake valve starts to open tho, the slow burning lean mixture is STILL burning & ignites the fresh charge in the intake system & backfires thru the carburetor.

 

I know this is hard to belive that its so simple. There used to be a jump school here, & on the weekends it would be the norm for each 182 Cessna that they flew jumpers with, to easlily make $10-15 thousand per day. One saturday as i was walking over to the helicopter shop, i hear this 182 backfireing thru the carb & the owner comes running over & askes me to take a look at it. I immediately tell him that it sounds like he has an intake leak somewhere which is causing a lean mixture. We go over, pull the cowl off & sure enuf, an intake hose clamp has fallen off & creating an intake leak. He's back in the air filled w/ jumpers in about a 1/2 hour. NOW here comes the best part, at the end of the day he comes over & hands me an envelope w/ $200 in it not bad for 1/2 worth of work !

[Guest pokey]

here is what wikipedia has to say about it:

 

http://en.wikipedia.org/wiki/Back-fire

 

Backfiring thru the exhaust? no wonder all them Bonanza's backfire on final ! ( learn sompin' new every day)

[joker]

If it's backfiring through the intake then you've got yourself a case of pre-ignition

 

Pokey / 67November,

 

I wonder if you could just clarify something for me.

 

I may be wrong, but:

 

What Pokey and I just described above (backfiring through the carb) and pre-ignition are not the same thing.

 

I was taught that pre-ignition was when there is some sort of ignition before the spark plug fires.

 

This would on the piston's upstroke (compression cycle). It can happen anytime from Bottom Dead Centre (BDC) to Top Dead Centre (TDC) in the cycle.

 

Regardless of when this happens, it is very destructive, as the upward moving piston encounters all the force of the combustion. This can cause destruction inside the cylinder or even in the crank shaft and other mechanical parts of the engine.

 

There are a number of different causes for pre-igntion, ranging from timing problems and incorrect spark plug selection. The most common is due to carbon deposits which form hotspots on the plug or inside the cylinder. As the compression stroke happens, these hotspots ignite the compressed mixture early.

 

I may be wrong, so just want to check.

 

Joker

[Guest pokey]

Yip Joker, that is the way i remeber being taught too. Lets not forget about detonation too ! Pre-ignition can lead to detonation, where the piston is coming up to the early burning charge & then Ka-BOOOOoooooooM ! ( it explodes, rather than burns)

 

Many things can lead to these destructive events, including mixture problems, poor fuel quality defective OR foreign materials/parts in the combustion chamber.

[joker]

Wow! Let's recap. We have mentioned...:

• Backfiring through the carb.
  
• Backfiring through the exhaust.
  
• Pre-ignition
  
• Detonation
  
• Pre-Ignition Induced Detonation.
  
• Manifold backpressure.
  

...all from someone's simple question about Manifold Pressure! I wondered if the original question was actually every answered. I think so.

 

All of these things are great indicators to the health of the engine.

 

I am no A&P, but I have worked on boat engines and bike engines. So by knowing what I'm hearing and seeing, I can troubleshoot engine problems much quicker. By knowing the causes of pre-ignition, I can avoid the likelyhood of my engine self-destructing!

 

Here is a pretty good article (which I just happened to find totally by chance in my computer library on my harddrive ) which explains in quite simple English some of these terms:

 

Engine Article

 

Hope it is useful to someone.

 

Joker

Edited April 19, 2007 by joker

[Guest pokey]

good article Joker, thanks.

 

and a good point on the original question,,, what was it again?

[Fastlane]

Wow! Awesome info here guys, I didn't realize my post would start this type of a discussion, but glad it did!

 

Keep in mind that ALL internal combustion engines fire BEFORE top dead center. It's just physics. Get that fuel burning, the flame front started, so by the time the piston gets just past TDC, you're at maximum cylinder pressure.

 

Along the lines of detonation/pre ignition, this usually happens in a lean condition as well. Don't forget about a "hot spot" in the chamber, such as carbon build up on an exhaust valve. It "glows" red, and as soon as the intake charge hits, BAM! Acts like a diesel engine.

 

FANTASTIC thread we got going here! Glad Joker woke up! LOL

 

So basically, as far as drag racing goes, ditch the MP gauge and get an altimeter. Do some homework, so eventually I'll know at "X" density altitude, I'll need "Y" jets and adjustments for peak performance, regardless of the tracks actual elevation. Right? Or am I barking up the wrong tree?

Edited April 19, 2007 by Fastlane

[Guest pokey]

So basically, as far as drag racing goes, ditch the MP gauge and get an altimeter. Do some homework, so eventually I'll know at "X" density altitude, I'll need "Y" jets and adjustments for peak performance, regardless of the tracks actual elevation. Right? Or am I barking up the wrong tree?

 

yer barkin' up the right tree, kinda. The MP gage & the altimeter work basically the same tho. ( read in the link joker posted earlier about how the altimeter is corrected for sea level pressure)

 

http://helicopterforum.verticalreference.com/helicopterfor...amp;hl=manifold

 

 

Density altitude takes into account: pressure, temperature and humidity.

 

What worked yesterday, may not work today,,,,even at the same dragstrip,,, BUT? it may work tomorow at a different strip.

 

dontcha just love living beneath an ocean of ever-changing air?

 

 

how to compute density altitude? Joker !! wake up !

[me shakes fist]

Yes, pre-ignition is the fuel igniting before the spark hits, which can also lead to an intake backfire. The charge can go off before the intake valve closes and ignite whats in the intake manifold.

 

However, I wasn't thinking of a mixture so lean that it burned so slow as to still be burning during the next cycle. I ASSumed that pokey was speaking relatively lean. In the automotive world you are pretty much always running rich, except at cruise and idle where it is stoich. For example in my car if I was at full throttle and showing an AFR of 13:1 (stoich is ~14.7:1) I'd probably be melting some pistons real soon cause it was too "lean" even though a 13:1 mixture is technically on the rich side.

[Fastlane]

Density altitude: Pressure altitude corrected for nonstandard temperature variations.

 

Basically in layman terms, it's what altitude in a standard atmosphere that you would have to reach to obtain the pressure you are currently at in a nonstandard atmosphere.

 

So what Pokey said is absolutely true. What works at race track "A" today, may not work there tomorrow. But, what I'm thinking is if I figure out what the DA is for that day at the track, write down what jets/tire pressure/shift points works for a given DA. Then, after building up a reasonable database, simply hit the track, find the current DA, look through the database, then set the car to match.

 

Sound good?

[joker]

Density altitude: Pressure altitude corrected for nonstandard temperature variations.

 

And to be strictly accurate, add in 'humidity'.

 

Then, after building up a reasonable database, simply hit the track, find the current DA, look through the database, then set the car to match.

 

This sounds perfectly reasonable, but I don't think it is a new concept. A google search of 'Drag Racing Density Altitude' yields a wealth of sites already talking about the two....or am I missing something.

 

“The most important measurement to drag racers is density altitude as they are able to dial their car off this number alone,” said Christine Munding.

 

I'd love to help further, but I'm not exactly sure what you're looking for now. Maybe a drag racing forum will give you more specific help.

 

Joker

[Fastlane]

And to be strictly accurate, add in 'humidity'.

This sounds perfectly reasonable, but I don't think it is a new concept. A google search of 'Drag Racing Density Altitude' yields a wealth of sites already talking about the two....or am I missing something.

 

“The most important measurement to drag racers is density altitude as they are able to dial their car off this number alone,” said Christine Munding.

 

I'd love to help further, but I'm not exactly sure what you're looking for now. Maybe a drag racing forum will give you more specific help.

 

Joker

 

Thanks Joker! You guys answered my question about 2 pages ago, but just enjoying the conversation in here now.

The DA is not a new concept, as you mentioned. I was wondering if the manifold pressure gauge in a car would help, but that's a different animal. I don't think that will help down on the blacktop. And to be perfectly honest, TRY mentioning some of this stuff in a drag racing forum. LOL Most racers understand the "numbers", but you pilots understand what's actually happening and the concept behind the whole thing.

[joker]

Oh, I see....a case of sort of 'thinking aloud' !

 

OK, well, I am really treading into deep water here, as I admit I know NOTHING about drag racing...but this is fun. What the hell. I'm thinking aloud now. No you've gotto help me!

 

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".

 

Day 1. So if it is a standard day, then your gague reading is showing an accurate representation of your power. You prep your car for the race before engine start, and you note the gague reading of 0" Let's say you wind her up and see a reading of 20"

 

Day 2. On a hotter-than-standard day (or higher track), the density altitude is higher. This time with the engine off, you note the reading is 2". You wind the same dragster up, and this time you find a reading of say 22". Well, the difference between the two is still 20", so the car is performing pretty much the same.

 

The difficulty comes when you have a lower density altitude. Because the needle is bottomed at 0" at 29.92 you have no way of knowing what it should read. When you wind her up, the reading climbs to say 17"

 

You have no way of knowing (from the gague) that if the car is under performing, or the DA is actually 3" below standard.

 

You do a quick calculation (or use your handheld weather station) and find that actually the DA is only 2" less than standard. Interpolation suggests then that a gague that did go negative should read -2" engine off. Thus, you only got 19" of power when you wound her up. Compare this to the other two days and you see that something is wrong.

 

So, if you can calculate the DA, by knowing an accurate temperature, pressure, humidity and track altitude, then you can determine if your car is underperforming or not, by checking your observed output readings with expected ones (from your database) and interpolating for non-standard conditions.

 

An aircraft style MP gague (that reads for lower-than-standard days) would simply eliminate the need for the DA calculation, and probably be more accurate.

 

Engine Idle MP - This is a really important number. Any anomolous reading in the idle MP could indicate problems. e.g. a higher than normal idle MP might indicate an intake leak. Get used to knowing what you should expect in different conditions, and get in the habit of checking this.

 

Joker

[Guest pokey]

came across this:

 

http://wahiduddin.net/calc/calc_hp_dp.htm

 

some handy info there

[Fastlane]

That's kinda cool Pokey! Played with that with some "imaginary" numbers, and it seems pretty darn accurate.

 

Joker, for "not knowing much about racing", I'll hire you as my crew chief any day! I'll just show you which end of the wrench to hold.

 

Now, I gave this thread a bunch of thought at job #2 today, and I think I hit upon something. We're overlooking 1 very important thing. Well, better stated I'M overlooking 1 important thing. As a pilot, you need to interpret this information accurately so you know how your ship is going to perform, and keep yourself and crew out of trouble. Don't "out fly" the weather, so to speak.

 

But in the drag racing world, I need to interpret this information accurately, then tear my engine apart and make the car perform the same, but in a different atmosphere. For those of you that don't know, this is called "bracket racing" and it has a built in handicap so virtually ANY car can compete. It's not necessarily the fastest car that wins. I literally have to predict what my car will run, so does the guy in the other lane. Slower car gets a computer figured head start so that theoretically both cars reach the finish line at the exact same time. If I dial in (racer speak for "predict") my car at 11.06 and it actually runs an 11.09, but the other guy dialed a 15.44 and ran a 15.45, I LOST.

 

So anyways, I guess the point I thought of today was that in both worlds performance is important. As pilots we need to know what/when/where our aircraft is and is not capable of doing. We do NOT tear apart the engine before every flight to MAKE it perform like it did that really nice day last week. As a racer, that's exactly what we do.

 

Make sense guys?

[me shakes fist]

But in the drag racing world, I need to interpret this information accurately, then tear my engine apart and make the car perform the same, but in a different atmosphere. For those of you that don't know, this is called "bracket racing" and it has a built in handicap so virtually ANY car can compete. It's not necessarily the fastest car that wins. I literally have to predict what my car will run, so does the guy in the other lane. Slower car gets a computer figured head start so that theoretically both cars reach the finish line at the exact same time. If I dial in (racer speak for "predict") my car at 11.06 and it actually runs an 11.09, but the other guy dialed a 15.44 and ran a 15.45, I LOST.

It's always those 17 second automatics that do the best in the brackets.

[Fastlane]

It's always those 17 second automatics that do the best in the brackets.

 

LMAO You NAILED it! There was a guy at our local track that literally drove his mom's 22 second Buick LeSabre. Front wheel drive, Computer controlled fuel injected 3.8L, and you COULD NOT beat this guy! He was track champion (in his class) 3 years running! I can't believe you said that, you must have "been there, done that".

 

Where's my new crew chief, mechanic, and Pokey at? I thought I had something for you guys with my new revelation. Or would that be an epiphany? Ahhh, that's a whole different thread.....

Edited April 21, 2007 by Fastlane

[me shakes fist]

LMAO You NAILED it! There was a guy at our local track that literally drove his mom's 22 second Buick LeSabre. Front wheel drive, Computer controlled fuel injected 3.8L, and you COULD NOT beat this guy! He was track champion (in his class) 3 years running! I can't believe you said that, you must have "been there, done that".

 

Where's my new crew chief, mechanic, and Pokey at? I thought I had something for you guys with my new revelation. Or would that be an epiphany? Ahhh, that's a whole different thread.....

Yeah, I've definitely been there. I've only bracket raced once or twice myself but I've seen a few and it's usually some POS that wins it. All that matters with those cars is their reaction time for mashing the gas pedal.

 

As far as the MP gauge goes, I don't think it would do too much for a naturally aspirated car, but then again a regular mechanical vacuum gauge isn't very expensive. You can also get a wideband O2 sensor. This will tell you exactly what your air/fuel ratio is.

[mechanic]

Fastlane,

Seems like your gettin it. I am not a performance engine builder. I have always worked on stock gas/diesel equipment/cars/small equipment. I work for JD and I see alot of plug fouling in the Kawasaki V twin utility engines. Our elev is 348' but with high temps and humidity we get into the 3,500's in summer. I end up having to drop 2 main jet numbers and up 1 on the air jet to get the engines balanced and stop the plug fouling. Most have the problem when they are stopped and started with out long run times to bring the engine coolant up to normal temps.

 

I see some guys run those cool cans, I think thats what they are called, to chill the aircharge to help with the density thing. They put ice in them..

 

 

Later

 

P.S. My bud said his 502BB is in the 700's hp..