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Old 08-15-2001, 11:12 PM   #1
davidm_sh
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Default Proof on how much of a difference altitude makes =).

Well I had lunch with a mechanic friend today who works for Kleeman's. This is basically a European company that makes very nice supercharger kits for Mercedes. SO he also gets a E320 (V6 supercharged = 330hp) and another one (V8 supercharged = 430hp) as company cars... lucky @#$% [heh].

SO we met up today for lunch and went for some drives BUT on my way home from work I spotted him and we did some "playing". Me being in my WRX and him in the Supercharged V6 (3.2L) = 330hp (at sea level hp rating). Well we only "played" from roles since we couldn't get a light but I was very slowly pulling away from him from 40ish all the way up to 110mph. Not fast mind you. But on our 40-90pull I probably put about 2 car lengths on him (we both started off at the same time). Then the 40-110 pull he got a little bit of a jump while I downshifted but and only had maybe 1 car on him at top speed.

Just goes to show you how much of a difference altitude makes. Becuase he loses 2 psi on his supercharger system (6psi down from 8 at sea level) up here. I can guarentee that if we ran at sea level the outcome would probably have been completely reveresed... but we aren't at sea level are we =).

Cheers.
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Old 08-16-2001, 12:14 AM   #2
DLC
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The pleasures of a indirect system, your turbo can make up that difference, no matter how fast the engine itself is turning.
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Old 08-16-2001, 09:43 PM   #3
TimberBash
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Question ??

More Details DLC. Are you saying that if the airpressure is lower the Turbo will make up for it? Isn't it harder to compress air if there is less to compress? Do our turbos run harder at altitude than at sea level to achieve the 12 psi boost? Or is it truly just boost (e.g., air pressure plus 12 psi)? I would love to hear more on this.
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Old 08-16-2001, 11:09 PM   #4
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I think the gist of it is that the altitude makes it harder for the turbo to hit a given boost pressure because there is just less air to work with. But, I suppose it is possible to get almost the same boost (with more of a delay) as at sea level--I'm just not sure that the WRX turbo is capable of that.

FWIW,
TRS
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Old 08-16-2001, 11:44 PM   #5
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Exactly.

At 4200 feet, or 5200, depending on whether you're in the Wasatch or near Denver, there's less air pressure.

You're turbo MUST spin faster to compensate for the lack of static air pressure, so it is working harder to make the same amount of boost compared to one at sea level.

I'm running a stock IHI RB52, smaller than a 13G even, at 4PSI over it's stock level (13 vs 9). While it might be on the outside edge of its efficiency range, it's still doing a hell of a job.

The WRX turbo is small, a TD04 i think. While it's not big, it's pushing more stock PSI than my car can do without a fuel cut defeat. The headroom in the turbo design is there to let you run the car as well and long at altitude as at sea level.

One thing i'm not as encumbered with is the dizzying array of electronics the WRX has. My OBDI ECU monitors boost, but can no longer directly control it and relies on a fuel cut to keep me from overboosting.

A supercharger, while forced induction, is directly tied into the speed of the engine. The only way it can make more horsepower is to spin faster. No matter how fast it spins, it can't compensate for altitude because of that. This is why turbos are great for altitude compensation.

Make sense?
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Old 08-17-2001, 05:55 PM   #6
TimberBash
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Thumbs up Now I Understand?

Sure, your saying the Turbo is working harder to create the 12 lbs of boost at altitude, and that the supercharger can't work harder because it's belt-drive from the engine.

I was wondering (you can't really tell it from my clueless previous post) if the turbo would also make up for the lost airpressure that you start with. In other words if I'm at an altitude that's 2psi less than sea level will the turbo give me 14 lbs of boost instead of 12. I guess the answer is "no" on that one. It's going for a relative boost not an absolute psi, right?

P.S. I'm at 7500 ft here in Black Forest. The topo line on the map actually goes right through my property.

Last edited by TimberBash; 08-17-2001 at 06:02 PM.
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Old 08-17-2001, 10:09 PM   #7
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Despite the negative effects of altitude, it is great having a turbo here, we're less effected than N/A folks and it shows up here (9200ft). Once the boost hits, I can accelerate up hills and leave people winding out in 3rd or 4th gear like they were standing still.

TRS
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Old 07-10-2004, 07:17 PM   #8
ridebmxsvt
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a stock wrx makes 14-15ish pounds at sea level and a stock wrx at 5600 ft makes 11-12 pounds. there is less air so your car wont make as much boost. whenever i go on a road trip or leave the city for some reason i have to turn my boost down or itll spike into the 20's
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Old 07-11-2004, 02:34 AM   #9
HamFist
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I soooooooooooooooooooooooo look 4w3rd to testing this theory... I'm not flaming anyone, but it'll be nice just to see what a s/c'd scoobie 2.5 can do when it only has to spin a bit over 2 lbs of weight rather than 8 or more lbs of weight like most blower systems.
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Old 07-16-2004, 06:49 AM   #10
WTypeRogerX
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Is there a formula for HP loss for supercharged engines? IIRC, N/A looses 10% for every 1000 ft; Turbos loose 3% for every 1000 ft. What about superchargers?
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Old 07-16-2004, 10:45 AM   #11
Diz
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Quote:
IIRC, N/A looses 10% for every 1000 ft; Turbos loose 3% for every 1000 ft.
I think these calculations are a bit off, specifically the 'NA' one.

From what I recall, at 10,000 feet an NA car is down 27% power. A turbo is down 17% at the same altitude. After 10,000 ft, the power decreases non-linearly as a function of altitude. Up to 10,000 ft, the decrease is linear. (i think)

So, at 6000 feet, an NA car is down ~16%, while the turbo car is losing ~10%. Quarter mile times at Bandimere seem to be consistent with my math.

Unfortunately, I have no idea how the altitude affects the SC guys.

PS - Are you STILL trying to get that thing supercharged, Gary. Sheesh, buy a used WRX already.
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Old 07-16-2004, 11:01 AM   #12
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Quote:
there is less air so your car wont make as much boost.
This isn't quite correct. If this were the case, my turbo legacy wagon would have run less boost than the sea level guys. The opposite was true - it ran more. It would also increase boost, up to a point, as I would go up in altitude.

There are two ways to limit the amount of boost you can run. The first, as described in the the turbo legacy case, is to use an absolute value for the amount of positive manifold pressure. The older turbo legs (91-94) did this. They would increase boost to accomodate (sp?) for the lower atmospheric pressure at altitude.

The other way is to use the concept of Pressure Ratio. This is what the WRX does. Max boost is a function of current atmospheric pressure. So, at sea level, WRXen are getting 13.5-14psi of boost. Because we're at 5-6K, our atmospheric pressure is lower (IIRC, ~85%) than AP at sea level. So, we're only getting 85% of the boost that the sea level guys are.

Make sense?
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Old 07-16-2004, 12:15 PM   #13
HamFist
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Diz-Let's see jooz CAD design a new blower from scratch, build it, source the parts, etc.... .

It's got 250 miles on it now without a burp. 4psi, no intercooler, 40+ over ambient discharge temps. I'll need a J&S timing retard by the looks of things....but so far so good . I have to get the J&S before the headers, new intake manifold, etc...Wanna ride? You still live in Boulder don't you?
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