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Discussion Starter · #1 ·
I will preface this thread, my ng900 is currently at sea level (where I've always lived) and now I'm moving to Montana 3800-6000ft avg.:lol: Okay so I've done my research and I realize there's pretty much two steps in turbo upgrades, IMO. Upgrading to a TD04-19t or a Big T-28 in the first step as a stock turbo upgrade. Or jumping to a gt2876, 2871, 3071, 3076, hx35, hx40, bigger turbos in general. Now I've done extensive research and calculations on the web, on saab central/saablink, and using T5 Suites features. I've found that when I do the calculations for a turbo at 5000ft above sea level the pressure ratio climbs dramatically and only larger frame turbos fit my flow requirements to make make power at elevation. Recently I stumbled upon this website that gives an engine flow rate calculator and will generate a graph for you to analyze what your engine needs so you can find a compressor map to fit it. This proved worthless when I discovered there was no adjustment for altitude. The I moved to the squirrel performance website where they give a calculator with all sorts of adjustment. What was key was their pressure reference for altitude changes. Now when using their website and doing my one calculations, I found that at ~5000ft above sea level (roughly 12.2psi Abs, remember ~14.7 at sea level) my pressure ratio to make even 18psi of boost was ~2.5. For 22psi boost, ~2.8. These ratios are easily past the capabilities of a stock t25 or 15t. Now to even hold these pressures for a mild upgraded turbo such as a 19t or a T28 they're pushing the limits of the T28 (IVE READ THAT A T28 CAN BE COMPARED TO THE GT28RS) but the 19t seems like it would fair better based on compressor map analysis. Using the Squirrel performance website I concluded that an upgraded turbo (19t/t28) would be needed to compare with upgraded t25 levels at sea level. Therefore to make more power (300+ range) I would need to upgrade immediately to say a 2876 to compensate for the elevation change.

The real thing I'm asking here is how turbos can cope with such extreme altitudes? It doesn't make sense that WRX's with such small turbos can even run 16psi at 5000ft?! When my t25 will barely be able to hold 18psi to 4000rpm. Most smaller frame garrett maps that Ive looked at max out at a pressure ratio a little over 3. Which at 5000ft will allow me 24psi MAX. I just want a turbo that will suffice around 300bhp but provide me room for improvement. Instead of buying the T28 upgrade and realizing I can only run 18psi because of the limits of the turbo at altitude.

I'm looking for experience with turbos at altitude. How'd you fair? What did you change? Was there specific tuning that you did so compensate? Did you skip the 19t/T28 jump and just go to a 2871 or 2876 instead?

The more info the better! Thanks so much!;ol; Please correct any of my data!

THIS LINK PROVIDES THE TURBO MAPS I'VE BEEN REFERENCING.

I've been reading a lot on this forum and most of the input is gold and supportive. :D
 

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Discussion Starter · #2 ·
I think I would've gotten replies if I posted incorrect information instead of all proper information..
 

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I feel your pain, but that is a really complex question. There is a guy on the forums named Jon Z. Williams who has some NG900 1/4 mile time records (JZW tuning) done in Denver in mile high altitudes. He might be the man you should call. Look him up (plenty of videos on YouTube), I'm sure he will have some info for you. I think he bases his tunes on E85, for what it's worth.

And good luck with the move!
 

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As you've figured out, the turbo really functions by "multiplying" pressure rather than just "adding" a fixed amount of pressure. At high altitudes your maximum output pressure will be lower than at sea level, and this will certainly affect power. The question is, are you looking to hit a particular power number for any reason? Upgrading to a larger turbo will make the car more powerful no matter what the altitude (if done correctly of course :p), does it make a difference if you don't hit the same power numbers as what you would see at sea level?

I'm curious where you read the factoid about the WRX turbo. I am not familiar with their turbocharging systems at all, but trying to compare our engine with its particular turbo and a totally different engine with a different turbo really is difficult (and in my opinion futile).

Also, it is very important to watch the units when reading pressure values. Using your numbers from above, and looking at the wrx example, 16 psia (absolute) is only about 4 psig (gauge, or pressure over atm, aka BOOOOST) which is not very impressive. 16 psig (boost) out of the wrx turbo is still only a pressure ratio a little above 2 which still isn't that far fetched.
 

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ah fluid dynamics.... Love IT;ol;

You are missing part of the equation though. The power calculations are also affected by the exit pressure, which will 'give back' some of what the high altitude has taken away by making the exhaust flow better (less pressure on it). The power calculations are much more complex. You can't say, if I have 10psig (boost over atmospheric) I will have x horsepower. You also can't say, if I have 24psia (absolute pressure) I will have x horsepower. Now the absolute pressure is the much more important number, because determines the mass of the air charge (and most importantly the amount of oxygen being feed into the cyclinders) which allows more fuel burned yielding more power. aka, how a forced induction system works... more air means more fuel means more power.

However, that simple look ignores a fact. Decreased atmospheric pressure, also reduces the pressure being exerted at the back end of the exhaust. The air moves through the exhaust because of this pressure difference between the high pressure air moving out of the cylinder and the lower atmospheric pressure. Higher altitude = better flowing exhaust...

My point, well points actually... First, it isn't nearly so simple... What to make your head really hurt, start thinking about temperature differences between your two locations too.. Second, It sounds like you are getting stuck in the details, and a "I want my car to have XXX horsepower", and lets run the numbers to get there. While it is possible to run those numbers and get a fairly accurate result, the more important # how much you are adding...Third, those simulators are all estimators, and their margin of error is well above any variance you will see because of altitude changes. What they are VERY good at is estimating how much more power you will be adding, and is a much better metric anyway unless you are talking about doing some serious competitive racing.
 

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Discussion Starter · #6 ·
I'm not looking for the "I want this horsepower number". But I just want a turbo that will give me a fair amount of leeway instead of upgrading to the Big t28 and having it not be that big of an improvement as I thought because of the altitude change. But rather a turbo that will still retain "decent" spool but have some room for more boost in the future.
Recently I've been researching Hx35's Hy35's and hx30 for their high pressure ratios. Diesels are huge out here in Montana so the availability of a holset would be likely.

Sorry about the unit discrepancy. For the WRX reference I meant 16lbs of boost
 

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Shouldnt the car make relatively the same power even at higher altitude? the turbo will probably take a slight bit longer to spool but it should still make the same psi in the long run therefore providing the same amount of air to the motor.
 

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Shouldnt the car make relatively the same power even at higher altitude? the turbo will probably take a slight bit longer to spool but it should still make the same psi in the long run therefore providing the same amount of air to the motor.
Depends on the Turbo, and the tune.... A turbo can only 'boost' so much at a given flow rate before it runs out power, they of course have limits. You also need a tune/ecu that is able to use that extra turbo when needed (this means under normal conditions you would not be using your turbo to its fullest extent, or leaving some power on the table). Think you will find most custom tunes, don't leave the extra turbo capacity in reserve...

Great example of this is the 03 up 93SS that run the T8 engine software and a stocktune. The T8 is a complete drive by wire system that works on the concept of a 'Torque Request'. So if you are cruising along, and suddenly stomp on the gas pedal the ecu determines how much torque you are asking for, and given current conditions what is the best way to achieve that torque. How much boost, fuel, etc does the computer ask for given the current conditions like temperature, pressure, ect. It doesn't really matter if the car could give you more power if you are already at the max torque for a given gear... The ecu will limit you to that gears max amount of torque.... The upside is the car drives very close to the same under most conditions (within limits of course) as the ECU will use the turbo to make up the difference.
 

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I don't know that my car was making much less power at altitude, so much as it was taking a few hundred RPM more to spool. I forget by how much, back when I was way into it I had all kinds of data but I don't even own a Saab anymore. I was constantly tuning, collecting acceleration curves, tuning etc. and timing 60-90, 60-100, 60-110 pulls. Somewhere along the line I found out that my 60-90 time was virtually the same at 4500-5000ft as it was at 900-1000ft.

The T28 and 19T will give a noticeable bump in power but if you're looking for something that just makes ridiculous amounts of power go for an HX35, or better if you have the funds to build the head and take advantage of the top end it will provide. An HX35 will flow well beyond the ~7k rev limiter if you've got the breathing room.
 

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Discussion Starter · #12 ·
I don't know that my car was making much less power at altitude, so much as it was taking a few hundred RPM more to spool. I forget by how much, back when I was way into it I had all kinds of data but I don't even own a Saab anymore. I was constantly tuning, collecting acceleration curves, tuning etc. and timing 60-90, 60-100, 60-110 pulls. Somewhere along the line I found out that my 60-90 time was virtually the same at 4500-5000ft as it was at 900-1000ft.

The T28 and 19T will give a noticeable bump in power but if you're looking for something that just makes ridiculous amounts of power go for an HX35, or better if you have the funds to build the head and take advantage of the top end it will provide. An HX35 will flow well beyond the ~7k rev limiter if you've got the breathing room.
This is exactly what I was looking for. The real results.
This is very helpful. I was actually thinking of buying an HY35 with the 9cm exhaust housing instead of the HX35's 12cm housing. From what I found on forums and YouTube it won't be as laggy but still will provide a good boost in power but I haven't found anyone that has put an hy35 on a saab.
 
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