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Old 02-10-2002, 04:41 AM   #1
john banks
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Default Atmospheric pressure at altitude and boost compensation?

In the UK we don't really have to worry much about altitude and its effect on boost, which is why I thought I would ask you guys...

What does the standard WRX do with the boost if you go to high altitude - say 8000 feet or whatever?

Does the ECU compensate for this in an active way?

I know the ECU measures atmospheric pressure with the pressure sources solenoid valve attached to the MAP sensor.

In terms of fuelling the MAF sensor would presumably still read sensibly since it estimates mass not volume.

In terms of boost I understand that there is a TPS vs RPM map to look up duty cycle for the solenoid and target boost. Anyone know if there is an altitude combination?

I am concerned that with my electronic boost controller design (or with an MBC for that matter) that at high altitude it would still try to make target boost by increasing the duty cycle. Presumably the turbo could then run very fast, hot and outside its efficiency map as it still tries to compress like it would at sea level.

Can anyone advise? Or tell me what happens with an MBC setup at altitude.

If necessary I could measure atmospheric pressure and compensate for it - if so what compensation is required.

Many thanks.
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Old 02-10-2002, 11:26 AM   #2
MJSpiers
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John,

Why are you concerned? UK doesn't ever get above ~3000ft...

I'm an ex-pat living in Colorado at 6000ft.... My observations:

Dis-regarding efficiency and temps., I could up the boost more than at sea-level to compensate for the reduction of ~2psi in ambient pressure. BUT, the stock TD04 turbo is not the ideal partner for this excercise as it's already too small.

Stock boost pressure appears to be similar to sea-level dweller's reports. ~13-14psi max. So, I would say that the stock ECU does compensate (somewhat) for altitude.

The problem arrives (I think) when you start at, say, 6000ft and then drive up the nearest mountain at, say, 14000ft. If you're using the stock ECU boost control, the system will not choke and your max boost pressure will be ~11psi.

If, however, you use some other method (TXS Unichip ABC, MBC, EBC, etc). then you will very likely cause a CEL for overboost... I don't really know why, but it happens to me and others I know wih WRX's...

I think that the reason is that the car is turned on at 6000ft and takes a snapshot of the absolute ambient pressure. Drive a further 8000ft higher and the ECU wants the pressure to drop in accordance, but is unable to control the boost now.... You get the picture.

People around here turn their boost down when they know that they're going up the mountains.

HTH!

Mike.
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Old 02-10-2002, 11:32 AM   #3
john banks
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I am not concerned for me but those using my EBC design say in the US at high altitude. Apparently this is a problem with the Link ECU and EBCs.

The best solution I think is to use RELATIVE rather than ABSOLUTE boost targets. The ECU uses that pressure sources solenoid control valve to read atmospheric pressure.

So a modification for altitude users of my EBC would be to control the solenoid if they want automatic compensation for altitude in the design, but it won't be necessary in the UK.
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Old 02-10-2002, 12:11 PM   #4
Jon [in CT]
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John Banks wrote:
Quote:
I know the ECU measures atmospheric pressure with the pressure sources solenoid valve attached to the MAP sensor.
This may be true for old WRXes, but not for the New Age WRX, which uses a different ECU. The New Age WRX has a barometric pressure sensor mounted right on the main circuit board inside the ECU box.

There was a recent discussion about this here: http://www.i-club.com/forums/showthr...hreadid=140354. The New Age WRX factory service manual suggests that the ECU will try to maintain a certain absolute manifold air pressure (supercharging pressure) regardless of elevation/altitude. This would imply that boost pressure must rise when altitude is increased. However, WRX drivers in Colorado report the opposite - that boost falls when altitude increases.

So that seems to leave two possibilities:
1) The ECU operates as described in the service manual and boost gauges under-report boost pressure at altitude.
2) The service manual is wrong.
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Old 02-10-2002, 12:14 PM   #5
snookem
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John

You lose 1psi for every 2500ft but the standard ecu makes up for this till 5000ft. Only trouble is,myself and others have seen things differently with our cars.

Please read this post to the bottom of the page and it brings up some interesting facts.

http://www.i-club.com/forums/showthr...hlight=alitude

Hope this of help
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Old 02-10-2002, 12:17 PM   #6
snookem
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Jon [in CT]

Beat me to it
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Old 02-10-2002, 12:54 PM   #7
hotrod
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Default critical altitude

I have a Blitz DSBC and did some test runs last fall on Mt Evans ( 14,264 ft Highest altitude you can drive about 14,100 ft).

The critical altitude for the stock turbo charger is about 13,675 ft.

At and above that altitude ( about 50 deg F, barometer this day 29.93 in hg, 24.59 in hg abs source NCAR) the turbo can no longer achieve 1.0 Kg /cm2 boost.

I next set my boost control to a limit of 1.20 and 100% boost ( as I understand this would make the waste gate remain completely closed until boost limit is reached ). I then ran three tests at the following altitudes.
All tests were run uphill at full acceleration from near 0 to between 45 and 65 mph as allowed by the local road conditions..

altitude max boost
14,000 0.90 Kg /cm2 ( Last clear straight away near summit)
13,320 1.11 Kg /cm2 ( up from the most southern switch back)
12,920 1.22 Kg /cm2 ( up the first clear straight away after
crossing the marsh area [ road too bumpy across the marsh for any
speed])

Hope this helps ?
Larry
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Old 02-10-2002, 03:29 PM   #8
ellisnc
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you know the efficiency on the turbo is horrible when you're making it run pressure ratios that high
you'd probably get more power at 0.7 bar manifold pressure at those altitudes
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Old 02-10-2002, 06:11 PM   #9
ToddStratton
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I agree with WRX_nut_co and MJSpiers. As another 9200ft resident, when stock I saw at most .9 bar and normally only .7-.8 near my house. As I get down to Colorado Springs (~6500ft) I see an increase of about .1 bar across the gears. With stage one unichip, I can use the ABC to maintain about 1 bar up here at home and 1.1 down in the Springs.
My guess is that the ECU does try to maintain an altitude correction, but it just doesn't seem to be able to keep up at higher altitudes.


TRS
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Old 02-25-2002, 08:16 PM   #10
pace
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You'll also throw a wastegate solenoid malf. CEL and experience a boost cut at lower indicated boost pressures, as you move to the higher elevations. I live at 8000ft ASL and will experience this up here once I boost to around 16psi. Even accounting for error in the boost gauge (I've used 2 different gauges) and the tap location (BOV), this is still lower than folks see at the lower elevations.

MJSpiers - I'm also a UK ex-pat, now CO permanent resident.
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Old 02-26-2002, 08:45 AM   #11
MJSpiers
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Pace,

It's funny how the 'Best of British' finds itself in one of the most beautiful places in the World.... God, but I don't miss that greyness....

So, which part did you hail from? How long you been over here?

I've been over here since '97 - came from Kingston-upon-Thames, Surrey. I took one look at the Rocky Mountains, fell in love, and decided that tower blocks (even IF they are registered monuments now) just wouldn't cut it.

Anyway, we'll have to meet up some time.

TTFN,

Mike.
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Old 02-26-2002, 10:07 AM   #12
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MJSpiers - Sorry to cut in and all, but I noticed that you used to live at Kingston-Upon-Thames while you were back in England. I used to live in Ewell, Surrey which was about 20mins drive away on Kingston Road. I moved to Canada in 1996, but still have the house back in England. How are you finding it in CO? I live in Ontario, Canada in the town of Collingwood (home of Blue Mountain skiing resort), and am still trying to get used to the cold, and the amount of snow that falls during winter. O well...Maybe one of these years
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Old 02-27-2002, 09:27 AM   #13
MJSpiers
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Quote:
MJSpiers - Sorry to cut in and all, but I noticed that you used to live at Kingston-Upon-Thames while you were back in England. I used to live in Ewell, Surrey which was about 20mins drive away on Kingston Road. I moved to Canada in 1996, but still have the house back in England. How are you finding it in CO? I live in Ontario, Canada in the town of Collingwood (home of Blue Mountain skiing resort), and am still trying to get used to the cold, and the amount of snow that falls during winter. O well...Maybe one of these years
Jigga,

I too kept my house in England for 2 years, renting it out... But I got fed up with the hassle & worry over it, and sold it in '99.

I love it in Colorado - should have been born here I'm lucky in one respect though.... I married an American, so visa/green card wasn't too much of an issue.

My Mum and sister live just north of Toronto in Newmarket. My sister's lived there for about 15 years and is used to the cold and snow, but my Mum only moved there last year.... This is her first Canadian winter, eh?

Hope you continue to enjoy things - you should check out Colorado's snow for skiing...

Mike.
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Old 03-01-2002, 01:23 AM   #14
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MJSpiers - Actaully, Newmarket is about an hour and 20 mins from where I am! Haven't had the opportunity to visit Colorado yet, but hopefully will do so in the near future.....perhaps when I get enough of an incentive to grin and bear the cold for long enough to learn how to ski properly

Take Care!
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Old 03-04-2002, 01:30 PM   #15
pace
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Quote:
Originally posted by MJSpiers
Pace,

It's funny how the 'Best of British' finds itself in one of the most beautiful places in the World.... God, but I don't miss that greyness....

So, which part did you hail from? How long you been over here?

I've been over here since '97 - came from Kingston-upon-Thames, Surrey. I took one look at the Rocky Mountains, fell in love, and decided that tower blocks (even IF they are registered monuments now) just wouldn't cut it.

Anyway, we'll have to meet up some time.

TTFN,

Mike.
Been here since late '97 also. Before that I spent half my life in various London suburbs, and half near Shrewsbury, Salop.

I couldn't make it to the D&B meet, but I know both RockyWRX and Tronner from I-club, and am also acquainted with JaMa and Phast (who I believe is from down your neck of the woods).

Would like to meet up sometime.

-Pace
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