I've read a bunch lately about pressure drop usuallytied in with aftermarket intercoolers and CAI's. But what we are seeing lately is higher dyno'd hp and torque #'s with lower pressure due to the pressure drop associated with the performance parts.

Could somebody explain the theoretical relationship between boost pressure, charge density (temp?) and hp and do a graph?

Is there an engineer in the house?

 

Didn't you participate in this thread?

https://www.northamericanmotoring.co...mp;topic=22675

 

Roughly,

PV=nRT

Where:
P = Pressue
V = Volume
n = number of air molecules
R = Constant
T = Temperature

If you increase the intercooler size (volume) thus decreasing charge temperature, you'll decrease charge pressure and/or increase the number of air molecules in the intake charge. It's the greater number of air molecules, not necessarily the boost pressure, that allows for more power

_________________
'04 MCS IB/W on order with the Scorpion exhaust & 16" SSR's wrapped in Yoko's standing by!

 

today I got my in/out intercooler temp gauge and a few fittings so I can hook up my Magnehelic differential pressure gauge. I studied theoretical physics and physical chemistry in grad school, but prefer the empirical method.

 

It is all about Oxygen molecules, good ole' O2. The gasoline needs to react to the O2 to burn. The more O2, the more gas is brought in and the more powerful the explosion in the cylinder and finally the more powerful the output. Therefore to increase power, especially in forced induction engines you need to increase O2.

To take Fueled's equation one more step further the ratio is as such:

n = PV/T,
(ignore R right now, because it is constant and we are only looking at a trend and not real numbers.)

Therefore anytime you increase pressure and volume, or decrease Temperature you will get more air molecules. As long as the overall equation increases, the individual values are not so important. The aftermarket intercoolers, almost always loose Pressure, but the Temperature is lowered by so much that it cancels out the loss in pressure and overall number of molecules increase.

Air is really needed by engines. Just to give you an example, at wide-open throttle the new LS1 in the Pontiac GTO requires 8 cubic feet of air per second.


jlm,

I agree with you. I have had a ton of physic, include Quantum Physics. The equations from physics are only created after lots and lots of tests. Otherwise they are just theoretical and not fully reliable. Most equations are based on closed systems, disregarding friction under SMTP conditions. Once you go beyond these assumptions they becoming increasingly difficult to calculate and it often times is a whole lot darn easier to measure it with a sensor and be done with it. Kind of like the pulley temperature calculations being thrown around in an earlier post.

 

>>Didn't you participate in this thread?
>>
>>https://www.northamericanmotoring.co...=22675<<

Yes, but not at the level you guys did. I saw a log of pressure data and a bunch of temp data but nothing attempting to correlate the two with power output.

I thought YOU might have the equation!

 

dgsz:

when I was in school, the text we used was the classic "Quantum Mechanics" by Powell and Craseman. I was taught the course by Powell and did grad research work with Craseman (who could make a bang up paella!)

 

Love your sig.....RONFLMAO

That was what my garage looked like last June
>>
>>_________________
>>'04 MCS IB/W on order with the Scorpion exhaust & 16" SSR's wrapped in Yoko's standing by!
>>

 

Without a MAF, we don't know EXACTLY how much air is going into the engine, but we can get a pretty good estimate from the speed-density calculation that the DME uses.

Airflow in g/s is a good rough indicator of horsepower, crank hp is about 80% of the peak g/s reading. So, a stock 163 hp MCS should flow roughly 130 g/s.

Since we don't have a MAF, the speed-density formula uses rpm, displacement, and volumetric efficiency to get a "baseline" cfm for a given condition. This baseline is then corrected for conditions like temperature and manifold pressure. So:

airflow = cid x rpm x 0.5 * Ev /1728

To convert cfm to g/s, let's say we have "ideal conditions" of 0° C, 0% Relative Humidity and 760mm of mercury air pressure. The ideal air density is 1292.9 g/m^3 and 1 m^3/sec = 2119 cfm so 1 g/s = 1.639 cfm.

A stock MCS should see roughly 214 cfm.

We know the cid is 97.5. We know the stock hp peak is reached at 6,000 rpm. Let's use 90% as a volumetric efficiency. Plugging this into the airflow equation gets us:

airflow = 97.5 x 6000 x 0.5 x 0.9 / 1728 = 162 cfm.

That way off from the rough calculation of 214 cfm. Why? Well, that doesn't take into account boost. A stock MCS has manifold gauge pressure of about 9.5 psi at 6,000 rpm. That's an absolute pressure that's 1.67 times atmospheric. So:

162 cfm x 1.67 = 270 cfm.

That's closer to the guesstimate but now it is much higher. Why?

Well, for one thing, the engine is really making more than 163 hp, but some of that power is getting used up operating the supercharger. According to the Eaton specs, that's about 20 hp at a blower speed of 12,360 rpm (6,000 engine rpm) and 10 psi boost. So, our 163 hp engine is really a 183 hp engine, but some of the power can't be used by us.

270 cfm x 163 / 183 = 241 cfm

Now, we're getting close. Only 27 cfm to go. Well, we didn't yet take into account how much the air is being heated by the supercharger. According to Eaton's specs, that's about 180F of temp increase at a blower speed of 12,360 rpm (6,000 engine rpm) and 10 psi boost. If our Intercooler is 80 % efficient, that knocks that value down to 36 F. In absolute terms, that's 496 Rankine compared to an STP value of 460 Rankine.

241 cfm x 460 / 496 = 223 cfm

That's close enough to 214 cfm for government work.

 

>>Love your sig.....RONFLMAO
Thanks!

 

>>That's close enough to 214 cfm for government work. <<

Andy - That is beautiful! And while I couldn't generate it I can follow it perfectly.

Now take me the last step.

Let's assume a nice summer day, say 80 degrees F. Cheese's 19% engine creates 180 net hp at 6,500 rpm with an IAT of X.

Cheese wins big at the saturday night poker game and buys the latest and greatest intercooler which comes with the baggage of a 3psi additional pressure drop. How many degrees F lower than X does the new intercooler have to cool the air inorder to equal Cheese's original 180 net hp? And how many hp will Cheese's engine produce for each additional 1 degree F drop in IAT?

Can theory take you to that one or do you just have to build it, log the data and plot the curves?

 

>>>>That's close enough to 214 cfm for government work. <<
>>
>>Andy - That is beautiful! And while I couldn't generate it I can follow it perfectly.
>>
>>Now take me the last step.
>>
>>Let's assume a nice summer day, say 80 degrees F. Cheese's 19% engine creates 180 net hp at 6,500 rpm with an IAT of X.
>>
>>Cheese wins big at the saturday night poker game and buys the latest and greatest intercooler which comes with the baggage of a 3psi additional pressure drop. How many degrees F lower than X does the new intercooler have to cool the air inorder to equal Cheese's original 180 net hp? And how many hp will Cheese's engine produce for each additional 1 degree F drop in IAT?
>>
>>Can theory take you to that one or do you just have to build it, log the data and plot the curves?
>>
>>
>>
>>
>>
This is very late so I will give it a stab. You need to know the volume difference between the two intercoolers. Since a drop in pressure is being caused by an increase in volume.

3psi is 171.3 mm Hg. This is the drop you will see. If n=PV/T, then you will need 151 F to compensate for it. But this isn't a correct answer because the volume would have increased as well, bringing the required temperature significantly down. In a three variable equation (since we are saying n is constant), you need at least two variables. Since P and V are so tightly integrated we need both of those numbers.

 

Pressure drop for instantaneous response is related to volume of the IC, but once the IC is "filled" (well under a second) the pressure drop is related to the IC's air resistance. There are two things that actually matter in the example BlueMCS gave: temperature and pressure in the manifold. The Engine couldn't give a rat's posterior about what is going on in the supercharger, IC, etc. All it cares about is the qualities of the air it sees coming in past the intake valves.

Luckily, absolute temperature and absolute pressure have a linear relationship. Raise absolute temperature by 5% and you'll need to raise absolute pressure by 5% to keep the same number of air molecules in that manifold. So, if mayor mccheese had his stock intercooler and saw:

17 psi gauge (31.7 psi absolute)
150 F (610 Rankine absolute)

Then, he bolted on the captain fantastico intercooler and saw:

14 psi gauge (28.7 psi absolute)
100 F gauge (560 Rankine absolute)

31.7 / 610 = 0.05197

28.7 / 560 = 0.05125

So, there are actually MORE air molecules with the stock intercooler in that example.

There is a big wild card in all of this, and it's not easily calculated. Not only does the temperature of the air affect the number of molecules of air being ingested, but it also has an effect on detonation. The hotter the air, the more likely the engine is to detonate. If detonation is encountered, the excellent knock detection system in the MINI's Siemens EMS-2000 DME pulls back timing to prevent damage. This severely cuts back on power.

The best power output comes from running the engine just below the point where it would start to detonate. If you go over that point, the DME pulls back timing A LOT, like way below the limit. So, don't let the knock sensors control your power output, they are a couple of stuffy, librarian-types who don't want you to have any fun.

I think a reasonable expectation could be that a big intercooler reduces manifold pressure by 1 psi. If that were the case, in the above example:

30.7 / 560 = 0.0548

That's a solid 5% increase in air molecules, which should net AT LEAST 5% increase in horsepower (possibly more if it pulls us well under the detonation limit). jlm's dyno testing showed a 10 whp gain with only the IC changed, that's just over 5% from his baseline.

 

I briefly skimmed this thread (grad school midterms eating my free time) and I see a lot of ideal gas law stuff but no mention of intercooler efficiency. You must account for pumping losses through the intercooler... trials are probably the only way to measure this.




--
Cheese


_________________
Click Here For My MINI Super Spring Cleaning Sale!

 

I just got the Magnehelic and differential temp gauges installed and functioning. My onus will be posted soon. Preliminary, not at WOT, i'm seeing 20" water pressure drop across the intercooler.

I'll be at the Giants' stadium autox tomorrow, looking for some more results.

 

jlm,

Maybe instead you should look for areas without cones.

See ya tomorrow.

 

1psi = 2.036ft of water right?
1psi= 24.432"

so you are seeing 0.818psi drop across your intercooler, if im not mistaken thats not that bad......

I guess it depends on how much you are cooling to....

 

lets us know when you get the full drop, since that was preliminary

 

I'll wring it out. the main reason for the sensors is to measure baseline and compare intercooler performance for some prototype replacements.

 

Oh i know what your up to!! Just curious on what your finding out!!!

 

I have recently logged calculated airflow in my MCS with 19% pulley using prototype logging software. The values exceeded 200 g/s at WOT, redline. That's stock Audi S4 2.7T kinds of values ... about 250 hp. This shows that the supercharger eats A LOT of power with the 19% and high rpm, probably well over 40 hp.

 

Andy - that's very interesting and confirms the dyno results from ever higher reduction pulleys, that being elevated torque and hp at lower rpms but only slight peak increases.

I am suprised that no tuner/mfg has tried using one of the more efficient superchargers.

Do you have any information as to the different levels of hp consumed by different types of superchargers at say 17psi?

 

since andy liked this, i thought I would re-animate:

about pressure drop fue to a larger intercooler: pressure throughout the system will not be constant, due to volume change and temp change. While you might get a lower pressure reading with a larger volume intercooler in the intercooler, what really counts is the pressure in the manifold and combustion chambers. since the larger voulme takes longer to fill, it creates a lag, probably imperceptible.

Oh, by the way, I blew up my magnehelic trying to read pressure drop across the IC. The mag is rated for only 2 or 3 psi full scale, which seems about right to measure pressure drop. trouble is the gauge itself is only rated to max 15 psi vs ambient. It made a pop sound and hissed a lot. I'm going back to quantum mechanics and partial differential equations.

 

...pretty much confirming my thoughts; although a 19% may provide additional net power, it does not leverage itself nearly as well as the number illustrates when the enitre system is analized.

 

so I thought I'd bring it back from the dead for those that weren't around for it the first time...

Matt