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  #46  
Old 09-01-2004, 06:13 PM
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Okay... I think I may have the missing links. But I'm gonna think outloud as I go here, mistakes and all.

Pressure is basically weight per surface area. Like pounds per square inch. I theorize that if we double the pressure, and the surface area stays the same, then we are doubling the weight of the air... an example would be if our post turbo intake pipe has 100 square inches of internal surface area, and we go from 1 psi to 2 psi, we double the weight of the air in the intake.

This does not mean we have 200 lbs of air in the intake! The weight of a ft^3 of air at sea level is 0.0765716 lbs at 59 F. But the pressure that air creates is roughly 14.7 psi. I would compare this to a 20 lb steel spring that can create 1000 pounds of pressure to hold up a car... oh even better... a few pounds of air in a tire that hold up a 3000 lb car.

Anyway, I'm going to assume that the pressure a mass of air (I'll just use weight) in a volume is linear to the amount of air in the volume measured by weight.

I'll need this in a minute...

Back to John's turbo
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  #47  
Old 09-01-2004, 06:28 PM
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Okay... 46" MAP. Let's start with sea level numbers for comparison.

46 inches mercury (Hg) * 25.4 mm per inch is 1168.4 mm of Hg.

1168.4 mmHg * 0.019337 = 22.5933508 PSI (pounds/inch^2)

Air pressure at sea level is roughly 14.7 PSI. 22.5933508 psi / 14.7 psi gives a pressure ratio of 1.537... 1.5 ratio. I guess I could have just divided 46" map / 30" map to get 1.5, but I think I'm going somewhere with this... I hope.

This is where I plug in my theory about twice as much pressure in a fixed volume is caused by twice as much air... I hope.

A cubic foot of air at standard 14.7 psi sea level weighs 0.0765716 lbs at 29.92 inHg and 59 Fahrenheit. Since our pressure ratio is 1.537, that would give us 0.117687688... lbs per cubic foot.

John gave me 46 inHg, I think at 6000 RPM... at 6000 RPM, 80 cubic inches displacement, 100% efficiency (I know... no 100% exists... more later), then we get a volume intake of 277 cubic feet. This gives us a pounds per minute number of 32.5994896...

Now what did I do with that turbo map?
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  #48  
Old 09-01-2004, 06:51 PM
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Okay... found the turbo map. (Look back to page 1 to see what I'm talking about.)

At a pressure ratio of 1.5 with ~32 lbs of air flowing through the system, the turbo needs to spin at ~73000 RPM. Okay, not going to blow up here!

Now let's jump to 10,000 feet.

46 inHg is... uh... wait, I have it here somewhere... 22.5933508 psi. At 10k ft we would roughly have 20.57 inHg at 23.3 F with a weight of air equal to 0.0507472 lbs. (from a standard atmospheric table... pick a table, any table)

20.57 inHg * 25.4 mm/in = 522.478 mmHg... 522.478 * 0.019337 = 10.103157086 PSI.

Okay 10.10~ PSI atmospheric pressure at 10k ft. Divide 22.5933508 by 10.10~ and you get... 2.236~ pressure ratio.

Now here's a thought... if a certain mass of air generates a certain pressure at sea level, wouldn't it take the same mass of air to generate the same pressure, even at a higher altitude? Whoops... no... different temperature. RATS!

Standard Atmospheric table says 0.0507472 lbs per cubic foot at 10000 ft and 23.3 Fahrenheit.

Pressure ratio 2.236 * 0.0507472 * 277 cubic feet at 6000 rpm = 31.4351~ lbs of air a minute. Wait... that sure looks alot like the the lbs per minute at sea level! Hmmm...

Anyway, where's my turbo map.

Pressure ratio of 2.236, lbs of air 32... Woo Hoo! Right in the sweet spot! 98000 turbo RPM, 73% efficiency.
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  #49  
Old 09-01-2004, 07:18 PM
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Whoops... sorry CC, I hope I didn't come across as harsh.

Okay... back to the turbo map. Something has me concerned. That giant surge line swings waaaayyyy over to the right of the map. What happens if the turbo impeller is building pressure that the engine is not spinning fast enough to suck up?

CAVITATION!

Unless you have a pop off valve and can just say "screw it!" But then the pop off valves are usually set to some "safe" pressure and you would never get to 46 inHg in that sweet spot!

Well... lets see what happens at 4500 RPM. (assuming for a moment I'm not botching all of this!)

Pressure ratio... I'm running out of toes here... ummm, 2.236.

4500 RPM * 80 cubic inches / 1728 cubic inches per cubic foot = 20.833333 cubic feet per minute.

20.83333 cubic feet per minute * 0.0507472 is 1 lb of air a minute... what tha?

OH... go back, its not 20.83333, its 208.333333 cubic feet per minute. duh! this brings the weight of the air intake to 10.57 lbs per minute.

Whoops... forgot the pressure ratio! Sheesh... 2.236 * 0.0507472 * 208.3333 is 23.6397 lbs of air a minute. That's better.

Look up the ol' handy dandy turbo map... 2.236 pressure ratio, 23.6397 lbs of air a minute or there abouts...

AHHHHHH!!!
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  #50  
Old 09-01-2004, 07:51 PM
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Okay... all of these calculations make 2 assumptions.

1. As I said before, I assume that air pressure is linearly related to air mass in a fixed volume. If you double the mass of air in a fixed volume, you double the air pressure... and vice versa.

I don't know if it's true, but it sounds reasonable. We'll need to find out for certain if we want to put any faith in how I'm calculating this.

2. 100% engine intake volume efficiency. This is not as big of a problem. Use whatever intake efficiency you want and multiply it to the lbs of air a minute and you get you new lbs a minute intake. No brainer.

So recommendations?

John, I wouldn't use the 46 inHg MAP number... it's too close to the surge line at 6000 RPM... unless your running 100% intake efficiency it doesn't leave much margin for error. Especially when you back off 6000 RPM without opening the waste gate first.

Cavitation could be a problem, which is what I first summized before running through some numbers... but it doesn't look like you'll get close to spinning this sucker off its spindle!

Maybe I'll try to put this in a spreadsheet.
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  #51  
Old 09-01-2004, 09:19 PM
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If anyone wants the spreadsheet I'm working on... say yeah or nea and I'll try to get it to you tomorrow.

I hope I'm not just annoying everyone with this...
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  #52  
Old 09-01-2004, 09:33 PM
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Thanks for all the work you're doing, MT.
You're certainly not annoying me.

I'm not planning to try 46 MAP at first, at least not at altitude. My plan with this turbo is to start off a bit more gentle and build up once I get a better handle on the turbo map and the mixture and timing settings.

No rush. I dont think I'll be doing a lot of flying in the next few days. Forecast winds are a bit outside my personal limits.
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  #53  
Old 09-01-2004, 09:40 PM
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John:

Understand the safe progression of testing...

That said, count me in on eagerly awaiting word of the climb rate you can get out of that beast....cruise prop notwithstanding.

Teej
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  #54  
Old 09-01-2004, 09:54 PM
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This is one of those projects that I just couldn't sleep until I finished.

Here's the spreadsheet I came up with so far... everything should be self explanatory.
Attached Files
File Type: xls TurboTest.xls (22.5 KB, 42 views)
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  #55  
Old 09-02-2004, 12:31 AM
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That's very interesting, MT. This is data I can USE, provided of course that it's right.

I played some what-if games with it. It says that anything above about 11,000 feet and my airflow is too low to use significant boost - i.e. anything over normalization is no good. That doesn't sound right to me.

What we really need is a graph with MAP on the X axis, altitude on the Y axis and plots of rpm to show the "island".

Ahha - I think I found the problem. Altitude pressure isnt being recalculated as I change height. Oh. I see that's manual. OK. I'll work on it a bit more in the morning.

Anyone else have comments?
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  #56  
Old 09-02-2004, 10:12 AM
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Yes, mplafleur was over last night doing woodwork in my shop and calculating on my bench, think we/he got it down pat.

Mike, where is the spreadsheet?
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  #57  
Old 09-02-2004, 01:24 PM
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dust,

Should have sent you a note earlier. I was writing a response to this thread late last night and was going to attach the spreadsheet when I noticed an error. It was in the barometric pressure at altitude formula. Had a decimal point in the wrong place. That always got me in college too. Well, it changed the pressure lower and it was enough to throw our numbers off.

Still more work to do...
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  #58  
Old 09-02-2004, 06:00 PM
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Quote:
Originally Posted by MarbleTurtle
Okay...

Pressure ratio 2.236 * 0.0507472 * 277 cubic feet at 6000 rpm = 31.4351~ lbs of air a minute.

Wait... that sure looks alot like the the lbs per minute at sea level! Hmmm...

Anyway, where's my turbo map.

Pressure ratio of 2.236, lbs of air 32... Woo Hoo! Right in the sweet spot! 98000 turbo RPM, 73% efficiency.
I highlighted a point (with ) I've been making since this thread started.. Dust and I have disagreed on it until we are both blue in the face. I am of the opinion that using POUNDS PER MINUTE requires no correction for altitude because you are dealing with a set "mass" of air and one prinicpal rule of physics is that "mass is conserved".. I have been making the point that if you use CFM or a VOLUME measurement.. then you have to correct that volume for density at the altitude you are going to use it at..

Dust is of the belief (solidly so) that you have to correct the Mass Air Flow (in pounds per minute) by the compression ratio.. and he's had this validated by someone at the Turbo company (in my belief he's been UNintentionally misled). I am not intending to get in a pissing contest with Dust.. but I'm hoping to go make it down to Ellington and catch up with some of the rocket scientists (from NASA) in our flying club and see if I can get some engineering input on this.

Anyways.. Dust.. not trying to get the last word by any means.. feel free to try and enlighten me, but I'm prolly gonna need to see something in writing to sway my opinion on this..

Dave
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  #59  
Old 09-02-2004, 06:23 PM
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Hmmm... so THATS why Dust doesn't want to use my spreadsheet!

I don't know for absolute certain that my numbers are right, but the results seem to make sense. A certain mass of air in a certain size volume generates the same pressure numbers... whatever altitude. Now the volume does expand ever so slightly at altitude, but I don't think intake hoses / pipes would expand anything significantly to disrupt the numbers.

Also the results point to what seems self evident about generating the pressure. At higher altitudes with thinner air, the turbo must spin faster to compress the same mass of air into a fixed volume... er... spin faster to generate the same pressure.

I know that the numbers aren't perfect because of the assumptions I make about temperature change at altitudes and engine efficiencies (which can be changed on the worksheet to change the results). I think they may be close though.

Anyway, I'm not particularly attatched to them so anyone who has more ideas to throw into the mix, let me know and I'll try to integrate them.
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  #60  
Old 09-02-2004, 06:30 PM
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Quote:
but I'm hoping to go make it down to Ellington and catch up with some of the rocket scientists (from NASA) in our flying club and see if I can get some engineering input on this.
OH! Ask them if they have a formula for a drop in inHg for an increase in Altitude! That's once piece of the puzzle I haven't found yet... so I have the user enter it manually looking it up from an atmospheric chart. I know the calculation is not linear, but it doesn't appear to be an exact geometric function either.

OH! Ask them if they have a formula for pressure based on pounds of air in a given volume. I think its linear but I'd like to hear what they have to say!

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