MazdaManiac

iTrader: (3)

No, I meant your girlfriend, Flo.
Sorry for the confusion.

HiFlite999

iTrader: (2)

At the risk of setting a dangerous precedent, I agree with MM here.

Part of the general confusion I think is how gauges are labeled. It's important to remember that pressure is measured always relative to another, reference value. Suppose we have an engine that is off and the manifold gauge in use is referenced to atmosphere; the gauge reads "0". This is the normal reference of car people. For airplane drivers, it reads 14 psi (or in our units, 29.92" of Hg at sea level), because the gauge is referenced to not "air" but to the true absence of pressure as in outer space. On Pike's Peak, a couple miles above sea level, airplane drivers read ~19 psi (~20" Hg), while a car guy's gauge still reads "0". With my airplane's manifold pressure gauge and the engine off, I can see immediately that without boost, my engine's only going to make (very approximately) 2/3 of its rated power at that altitude. Car guys can't see that because their gauge is referenced to outside air!

Now, back at sea level, let's start engines (assuming here typical gasoline engines) and let them idle. The car guy (NA) will read ~-20"Hg or -19psi, commonly known as "vacuum". Sorry guys, but this is NOT a vacuum (if you don't think so, try running your engine on the moon). The airplane guy knows this, because his gauge is reading ~10"Hg or +5 psi, ie there is indeed air in that manifold. Complicating the task of understanding for the car guy is the convention of calling the 'manifold pressure gauge' a 'manifold vacuum gauge' to get rid of the minus sign in front of the number being displayed. (I'll leave in the minus sign.)

Ok, let's open 'em up. Ignoring another effect which I'll get to in a minute, the highest pressure either (NA) engine can possibly get to is "0" for the car guy and 30"Hg (14.7 psi) for the airplane guy. The real world is not so kind because there are restrictions in the manifold pipe which cause a pressure drop even at full throttle, so the car guy reads -2"Hg (1 psi) and the airplane reads 28"Hg (13 psi). The car guy wonders how his max power corresponds to the smallest number, the airplane driver gets it, the higher his number, the more power he's making!

Now, moving to Pike's Peak, what happens to the full throttle readings? Car guy, the same -2"Hg and 'wtf-this-thing-is-sure-slow'. The airplane guy reads 18"Hg, compared to 28"Hg at sea level and knows where his power went. This is exactly why "turbos" came first primarily from airplanes and not cars. Some airplane engines are what's call "turbo-normalized". In this setup, the wastegate of the turbo (or supercharger) is controlled, either manually or automatically, so to deliver the same manifold pressure as what can be reached at sea-level (regardless of altitude). For this example, restoring that 18"Hg on Pikes Peak at full throttle to the 28"Hg at sea level, and also restoring the power that would otherwise be lost. The pilot does not think in terms of boost, but in terms of a manifold pressure referenced to a true zero. What the turbo is doing is of a complete non-interest as long has he can achieve the desired manifold pressure. Turbo-normalizing can be done without serious engine mods. Turbo-charging, in other words achieving manifold pressures significantly higher than atmosphere, requires an engine suited to it. Every airplane engine comes with a set of power charts through which the pilot can set the rpm, manifold pressure, and fuel mixture, within certain boundaries given by exhaust gas temps, cylinder head temps, and other instruments not found on cars, to achieve the settings that give the best power and fuel flow for his condition (and don't blow his engine up). While the manifold pressure most definitely does not directly indicate flow, it is related. The engine manufacturer does the dyno runs, takes and publishes the data. From those charts and something called 'density altitude', the airplane driver gets a good idea of what power his engine is delivering based on manifold pressure and rpm. That's why you will always, always, always, find a manifold pressure gauge on piston-engined airplanes (with a possible exception of the latest computer-manged variants.) You will also always find the power charts in the "glove box" of airplanes ... not only a good idea, but FAA required, considering blowing up one's airplane engine can have nasty consequences.

Ok, flow vs. pressure: they ain't the same, and a simple thought experiment will show one reason why. The usual way manifold pressure is sampled basically has a gauge stuck on a small tube which tees into a much larger pipe, aka manifold. Suppose both ends of the large pipe are open to air, and there is no flow, the gauge (referenced to zero) reads 30"Hg (14.7 psi). Now start flowing air. What happens? The pressure drops! Loosely speaking, it's called the Bernoulli effect. Given the same pipe size, the only way to flow more air is for it to move faster. Keep increasing the airstream speed and the measured pressure in this setup keeps dropping. Taking things to extremes, go supersonic and the pressure reads zero. (That's 30"Hg on a car gauge). Now argue that the manifold pressure is relating to flow - can't! What's happening is that the moving molecules have less and less time to move sideways into the pipe leading to the gauge. Of course the typical velocities in an intake manifold are way under supersonic, but the effect is still clear; everyone knows what happens when you open your side window at 70 mph - the pressure in the car drops and for the same reason. Without specifically relating the gauge reading to engine output on a dyno, the pressure relationship to mass flow is only approximate. The MAF sensor is one of the greatest inventions ever.

I hope this serves to clarify some things for some people. Others, sorry for wasting your reading time.

MazdaManiac

iTrader: (3)

What I saw when I read the above:


No. Seriously, thanks for that post. It gives a totally different (and fresh) insight into flow, pressure and their relationship.

Kane

iTrader: (1)

Sarcasm???

MazdaManiac

iTrader: (3)

None whatsoever.
Really, really good post.
Simple and good use of conversational language.

Kane

iTrader: (1)

Between a diver and a pilot - we can get this thing figured out.... LOLZ.

I read it a second time and it started to make more sense to me.

Brettus

iTrader: (2)

funny - i read it like this

MazdaManiac

iTrader: (3)

Well, that is good, because that was the part you didn't seem to get up to this point.

Brettus

iTrader: (2)

I've noticed you don't really ever try to understand where I'm coming from and just assume I have no clue .
BTW read that sentence from hiflite again very carefully .......

MazdaManiac

iTrader: (3)

Yes.

laythor

lawl

Brettus

iTrader: (2)

Here , you don't seem to get the inference so I translated it for you .....

paulmasoner

iTrader: (1)

and how can you relate the two? not by any standard constant value, but you could by a variable based on one or more other pieces of data.

MazdaManiac

iTrader: (3)

How did you get
from
How are you making these incredible leaps? There isn't even a tenuous relationship between those two statements. You just tried to make them sound similar.
This is physics, not homophones.

Brettus

iTrader: (2)

good point PM - how about these variables

Boost pressure at manifold
uncorrected dyno whp
mass air flow
Ambient air temp

now increase boost and what happens

boost goes up
hp goes up
MAF goes up
ambient is the same

Increase ambient temp and what happens

boost goes down
uncorrected hp goes down
MAF goes down


decrease temp

boost goes up
uncorrected hp goes up
maf goes up

MazdaManiac

iTrader: (3)

No. Increase boost and MAF might remain the same. It depends on where the Tout is because of compressor efficiency.

I have hit (on several RX-8s) a "mysterious" point where raising the boost - even a little bit - lowered the MAF.

Watch the live feed from the SoCal dyno day at the end of March. You will see some interesting stuff.

Brettus

iTrader: (2)

Well that certainly is mysterious and one of those occasions I would be thinking pretty hard about why that happened .

a couple of things that come to mind would be :
intercooler is heat soaked
wastegate is blowing open earlier IE more boost early but less at the peak . If you recorded the boost profile it may provide some useful clues


BTW have you ever tried to tune a car with a vacuum leak between the MAF and the turbo ?

MazdaManiac

iTrader: (3)

"Vacuum" leak? You mean an open somewhere after the MAF?
All the time. Its annoying.
BOVs are notorious for that, though I see leaky couplers, bad solenoids and check valves, cracked OMP vacuum distribution blocks, cracked/missing injector "O" rings, bad welds, bad intake manifold gaskets, improperly seated UIMs, leaking IC cores and a myriad of other problems that get blamed on my tuning.
Its a blast.

The dyno does this. If I need that information, its there.

Brettus

iTrader: (2)

Well specifically on the intake side of the turbo .
I used to always wonder why cars you tuned had much higher maf readings than i did yet made less power .
I had a leak on the turbo inlet flange - gave me a real shock when I finally found and fixed it .

MazdaManiac

iTrader: (3)

I'm not sure which cars you are talking about, but I do tune cars to survive and make usable power.
Having one pop would suck.

That said, they all make the usual 1g/sec per crank HP for the most part.

I was just out flogging my own car for a bit today and saw quite a bit of 400+ g/sec around 12 PSI. Wanna guess the dyno numbers?

Brettus

iTrader: (2)

show me the boost profile for the whole run and I'd love to take a stab at it ....

although I would have to know that your engine is a stock renesis

paulmasoner

iTrader: (1)

^^i'm pretty sure you know better

MazdaManiac

iTrader: (3)

Boost profile?
I set the controller to whatever Pr I want and it stays there.
I said 12 PSI. That tells you nothing.
I also said 400+ g/sec. That tells you everything.

Brettus

iTrader: (2)

why I say that is because iv'e seen it on here so many times where guys say x boost "to the redline" and then you actually see the dyno and it is dropping off after about 6500 .
Then you have boost guages that are not accurate - like mine .

12psi all the way to 7500 on a stock renesis ,without meth injection, suggests just shy of 350whp to me .
but your MAF readings do seem to indicate somewhat higher than that .

If your engine is modified - does it ? You may have increased /decreased its efficiency .

MazdaManiac

iTrader: (3)

I typed that while I was looking at it!