john banks

I take it your boost drops if you go up to say 8000ft (2500m)?

I know the ECU reads the atmospheric pressure from time to time. Does it use this to compensate the boost map?

I would be concerned about overspooling the turbo at high altitude by giving it a target boost based only on throttle position and RPM.

Cosie Convert

You're taking this Knockhill thing too seriously

john banks

john banks

A fixed DC does not mean open loop though Theo. It just means a fixed gain on a closed loop system. The restrictor orifice coupled with a recirculating bleed depending on duty cycle controls the amount of negative feedback through the wastegate actuator system, hence fairly small offsets due to load or environmental conditions. The duty cycle is not closed loop, but the pneumatic system is.

The ECU maps for boost apparently use TPS and RPM to give a fixed DC which is then adjusted to target boost.

My system uses TPS and RPM with a variable duty cycle according to what errors there are during spool up and held boost.

If the ECU does not compensate for atmospheric pressure in its maps (I guess it must why else does it read it) then the turbo would work harder at lower atmospheric pressure to make the same boost. After a point is this dangerous, or will the boost just drop like it does at high revs?

Cosie Convert

My understanding was that the ECU only read the MAP at atmospheric during a diagnostic check ?
But I've been wrong before

madou

John

Absolute vs Relative boost targets

http://bbs.22b.com/ubb/Forum7/HTML/000111.html

john banks

VERY useful thread - THANKS. Seems that BAP and MAP are used by the JECS ECU but not by the Link with absolute pressure targets. Of course the BAP could be controlled by my little circuit too. I will point this out to the high altitude guys that may use it and let them sort it out. The idea of relative boost targets is quite nice really.

pob on the job

Knockhill.......at this time of year I would have thought ambient temperature is going to effect you as much as if not more than altitude, and I don't think ambient temperatures are going to be through the roof at this time of year somehow. Throw in a bit of Scotch mist and youre laughing. Swings and roundabouts spring to mind?

Alan

EvilBevel

@ CC

But it's a good question though ... one of the benefits of the standard setup is (alledgedly) that it is closed loop / semi closed loop (whatever you want to call it) so that it can "adapt" to specific conditions, loads, etc.

When you disconnect the standard system, the ECU has no way to adapt to these conditions (like altitude/going up hill towing a caravan).

What are the real consequences of this ?

Although I don't use the Dawes device, I am in a similar position.
The boost control of my Unichip setup is rather one-dimensional as well: DC vs RPM. That's it. No compensation, no closed loop/feedback.

So I should see big fluctuations over the year, but it seems limited to about 0.1 bar (1.25 bar in summer, 1.35 bar in ideal winter conditions). Less than I would have expected. Add a FMIC into the mix, and the difference may even be smaller ?



[Edited by EvilBevel - 2/10/2002 12:02:51 PM]

john banks

All the contributors to this thread have been very wrong before

The plot thickens - I don't know, but the pressure sources control valve does apparently occasionally activate during driving on late models, and you can tell as there is a little blip in the boost at this point. I would have thought the ECU would know when it reads atmospheric and compensate, but it seems not.

The reason I am wondering all this is a few Americans are already building my boost controller - I've asked them too but their brains don't seem to be wired like European brains. Not wrong, just different.

[Edited by john banks - 2/10/2002 12:25:31 PM]