350zDCalb

iTrader: (21)

I think Gurgen is gonna have to get on here and bust out some crazy Gurgen Physics...be warned, the mad scientist is alerted

Robert_K

iTrader: (25)

8 psi is 8 psi no matter what altitude your at. I use to own an Evo an sea level guys would tell me all the time that running 25lbs of boost is too much for the stock 4G63T. But as mention the air is thinner so you can run more boost. The altitude does affect boosted cars just not as much as a N/A car.

JETPILOT

iTrader: (51)

That's where the wastegate comes in. The wastegate won't open as much as it will @ sea level routing more exhaust to the turbocharger spinning it faster making the same amount of boost.

The pressurized intake is a closed environment and has no correclation to what is happening to the atmosphere outside. The wastegate is telling the turbocharger to make 8psi. The wastegates job is to send enough exhaust to the turbocharger to make that boost.

JET

JETPILOT

iTrader: (51)

Wrong and Right. You will not be making 2.2psi more boost for the same power. You will need the same exact psi boost as at sea level. The turbocharger will have to work harder to make it.

Right about the gravity thing. But that's simply too immeasurable and insignificant to be a factor.

JET

JETPILOT

iTrader: (51)

That's inncorect. Altitude does not affect turbocharged motors until an altitude where the weastegate can no longer compensate.

JET

GurgenPB

iTrader: (2)

LMAO! You crack me up, Todd. Give me a little time..I will respond. Just want to read the thread more carefully...so far, I do have a few thigns I want to say to correct some of the things that were posted. Give me some time.

JETPILOT

iTrader: (51)

I'm an airline pilot and endured 4 years of aviation physics not to mention about two thousand hours in a turbocharged Piper Arrow. I think I know how altitude affects turbocharged motors. Jet engines are axial flow turbochargers. The same principles apply to a turbojet as they do to a turbocharger. Last time i checked I had over 5000 hours in turbojet aircraft. The above facts I stated stand true.

JET

Robert_K

iTrader: (25)

I'm not denying your opinon but I'm telling you running 25psi on a stock Evo at sea level is very dangerous. 23lbs is the most sea level owners would run.

JETPILOT

iTrader: (51)

The reason being that high altitude locations such as Denver don't get 93 octane. The highest they get is 91 I believe.

JET

Robert_K

iTrader: (25)

I live in Colorado. 85, 87, & 91 are the main fuels you can find 93.

Eazzy

iTrader: (8)

What about Vacuum operated FI components at Hi altitudes??

JETPILOT

iTrader: (51)

That's a good question I don't have the answer to. I have no experience with vacuum operated wg's.

JET

schweatty

iTrader: (2)

true, but you dont need as high an octane at altitude to get the same performance as sea level.

GurgenPB

iTrader: (2)

OK...here it goes... lets hope i read everything thoroughly enough.

Increased altitude affects the working of an internal combustion engine two-fold (mainly): volumetric efficiency and tubro compressor efficiency. The latter affects NA cars to a larger extent than FI cars, and the latter is MOSTLY relevant to the turbo cars and somewhat to SC cars.

1) Volumetric efficiency.
At higher altitudes, there is less air density available. Hence, two things happen, less oxygen (ratio of which is about 21%) and less backpressure (the latter one is almost never mentioned). Less oxygen effectively decreases the oxygen-based (not total) volumetric efficiency of the engine, directly decreasing combustion output, while the exhuast backpressure in a way works in the opposite direction, allowing for greater efficiency of the oxygen that IS present. Of course, the former beats the latter with respect to to the overall effect on engine output. If you were to magically be able to run an NA car in space, or, say, in a specialized chamber with the intake pipe in an atmosphere filled with air, with the tailpipe outputting into a 0 absolute pressure environment (full vacuum), this would significantly volumetric efficiency, since not only the pistons on their intake stroke would be doing the 'sucking', but also the atmosphere. This is especially true with OUR cars, since the internal EGR (i.e. EGR due to cam overlap) is also affected, allowing
for a greater VE, especially on high overlap cams. Onto turbo cars and VE. The same that i said for NA (which also applies to SC cars) applies to turbo cars, but to a different extent. This is because the backpressure seen at the engine and the effective internal EGR is a much bigger factor, given the high exh. manifold pressures of turbo cars imposed by the turbine blades' load. But, there are other things, discussed next that are a far bigger factor.

2) Turbo Compressor Efficiency
Say you tune a car in L.A. for 0.5 bar boost. That's 1.5 bar absolute, and the absolute and not relative pressure is what matters, since 1) it's the number that is directly proportional to the mass, or, more accurately, moles (# of molecules) of gas (read: oxygen) entering the intake manifold (hence the term manifold absolute pressure "MAP"), and 2) it's the number that's therefore used for tuning. Now, 1.5 bar MAP is the same everywhere...it's the matter in which that pressure is achieved that's relevant. So, you tune your car perfectly in L.A. (by Sam at GTM ), and drive to Albuquerque to party with Todd. When you get there, and you have a turbo car (of course, that's the only way to race against Todd on the way to the vegan restaurant of choice), and not change your boost controller setting, your car will NO longer develop 1.5 bar absolute. If you have a regular analog boost gauge, it will still say 0.5bar boost. However, that boost setting in L.A. meant that you are getting 0.5bar "boost" over the ambient pressure of 1 bar (assuming that's what hte pressure in L.A. was), or 1.5 bar absolute. But your car is not developing 1.5 bar of MAP, but it's developing about 0.5bar+0.816=~1.22 bar. Why? Because the compressor generates pressure according to PR (pressure ratio). It starts with an ambient, and compresses it a certain percentage. When you start with 0.82 bar ambient and compress it X amount (the amount regulated by the tubine's wastegate and consequently by the turbine/compressor spin rate), you get pressure Y, that's PR. This is why the use of the term "boost" is not always accurate. Starting with a lower ambient pressure, is going to get you less pressure in the intake manifold, i.e. lower MAP. Also, to achieve 1.5 bar MAP again you will need to change the boost setting on your wastegates to a higher value, because you need to compress the existing smaller pressure by a greater amount to get back up to 1.5bar. Now, your car was of course tuned for everythign up to 1.5 bar MAP, including 1.22 bar MAP, right? So to get to 1.5 bar MAP you will need to just up the boost setting? Well, no, not exactly. The MAP of 1.22/1.5 bar is THE the same as 1.22/1.5 bar MAP in L.A. and NM only with respect ot pressure and NOT with respect to oxygen content in the manifold, because oxygen content is determined by temp AND pressure. Look at how you develop 1.22 bar MAP at sea level and at 5500'. To get 1.22 bar MAP in L.A. the compressor will have to compress 1 bar 'into' 1.22 bar, that means a pressure ratio of 1.22bar/1bar, equalling PR of 1.22. 1.22 bar at Todd's is 1.22bar/0.82bar equaling 1.49 PR. Now, look on a compressor map, which you will notice are ALL contructed in a PR vs flow, not boost vs flow manner. 1.49 PR will for our turbos mean a lower compressor efficiency. The lower the compressor efficiency, the more of the pressure is 'made up' of by temperature (higher temp meanshigher pressure everythign elese being equal) and less by number of gas molecules. This is why on a turbo car 1.22 bar MAP in L.A. is NOT the same as 1.22 bar MAP in NM or CO, an precisely why baro pressure correction is necessariy. The same is true if oyu try to up the boost back up to 1.5 in a 5500' ft elevation environment. 1.5bar/0.86 bar equals a PR of 1.83 PR, which is higher than 1.5 PR (1.5bar/1bar) that would be required for 0.5 bar boost at sea level. You can see why your tune would be off, or more precisely richer at higher elevations, not because there is less pressure (because you are still tuned for the lower MAP when you tune at hte higher map) but because the pressure is made up of less amout of air and higher temperature. If for example the compressor efficiency is 3-5% less at 1.83PR compared to 1.5PR, and assuming you ARE in fact perfectly tuned with baro press compensation (to fix the AFR), you will still make less power at elevation.

if oyu thought the compressor efficiency was complex, it doesn't stop there. Volumetric efficiency comes into play as well... and i am not giing into that here.... too long/complicated. Just try to integrate the two discussions.

One thing to note, none of this is any longer an issue if you use MAF for tunign and not map, since MAF measures MASS of molecules directly (which is not dependent on temperature or anything else to be extrapolated into number of molecules - which is what's relevant). MAF however has its limitations, mostly range/voltage filtering issues (the voltage output is less stable compared to MAP...hence less resolution), and is just not possible with very high boost cars. But, if you are tuned on MAF, you are tuned period, since, using the above example, it would measure the mass of air going into the engine irrespective of compressor efficiency. The compressor is less efficient at altitude, it will pump less air and more heat...but we don;t care b/c the heat factor is out of the equation.

This gives you an idea how complex car tunign is, and why you shouldn't attempt it on your own.

I am spent. HTH

JETPILOT

iTrader: (51)

The oxygen content of air is constant regardless of altitude. Always approx 78% oxygen 21% nitrogen, and 1% trace gasses. The density is what changes. The comressor has to work harder to comprss the less dense air at higher altitudes to lets say 8psi. But again. 8psi is 8psi is 8psi.

JET

350zDCalb

iTrader: (21)

That's all you had to say in response to that elaborate writeup!?!??!?!

Gurgen, my head is spinning..see guys; beware what questions you ask, you may get a Gurgen response that takes two encyclopedias and a physics professor to understand
Good response Gurgen, as always, you are talking over my head

-TODD

GurgenPB

iTrader: (2)

First of all, it's 78% N2 and 21% O2 not the other way around. And your next air is way too simplistic, which is why i was so detailed in my post.

Second, you are taking what I said out of context. If you read that area of my write up, and always, and once interchangebly, refer to oxygen content not as it pertains to total air content (21%) but as part of the content inside the manifold.

This is what I said:
Note the "inside the manifold part" of my sentence there. What i obviously mean here is that the pressure albeit the same is now the result of higher temperature and lower "n" than it was at the lower altitude (n=moles of air, and it's moles of oxygen that is pertinent). And when I say that oxygen content i mean oxygen content not as % of air (which is fixed, and it's ~20.95%), but that less oxygen is available because there is less total air available. I wrote oxygen because that's the relevant agent as it pertains to combustion. Only sayign "density" like you did is too simplistic.

Correct, and that's what i spent the time explaining, except that it's not that simple.

dirtroad

iTrader: (1)

my brain hurts and my eyes are now permanently crossed.

2fast4thelaw

iTrader: (1)

Somebody is gettin schooled! I love it when two power brains go at it.

Lets cut the scientific mumbo jumbo here for a moment. Bottom line this what is really being asked.

What would be the HP difference between my stock 350Z with a ST TN kit at 5000 Plus elevation vs the same exact car at sea-level? Aproximately.

No I have another question that is somewhat related.

At elevation would I be throwing off my tune by using 93 octane? Would I gain any safety net by running 93 octane?

roneski

iTrader: (3)

I thought that it was a simple question when I originally asked it, lol.