Just read these words from Mr Bob Rawle:

"With regard to the map sensor the output is linear with pressure/vac from minus 1 to plus 1.8 bar, supply voltage is circa 4.98, output in the range of 0.8(8) to 4.8(4) ish.
Atmospheric is 2.24 cvolts dependant on the day"

Could this mean that with the clever insertion of the correct value resistor it may be possible to modify the output of the MAP sensor and so raise the overall boost level (rather than using a dawes type device).

Is this the same reason as folks go for 3 bar MAP sensors?

Questions, Questions, Questions

 

Yes, the ECU MAP value saturates over 1.7 bar. I had a fuel cut lifter circuit which flattened the response after atmospheric which could get you to about 2 bar probably on the standard sensor.

 

can you explain how adding a resistor to the map sensor is going to raise boost?

surely if you artificially raise the output voltage of the map sensor all you'd do is fool the ecu into recieving a higher map reading

I thought the map sensor effectively relates manifold pressure converted from air pressure to voltage to the ecu effectively after the boost is acting

3 bar map sensor is surely only rated 1 bar more than standard to relate higher map levels (from increased boost)

but then i'm stoopid and probably get set straight...lol

edited to say, thinking about it more are you saying the resistor is actually making the map sensor able to relate manifold pressure greater than 1.8bar?, sorry i'm tired

[Edited by jameswrx - 2/25/2003 10:42:40 PM]

 

Resistor or better an op amp with gain or adjustable slopes and knees would allow higher boost since the ECU aims closed loop for a target. Also the MAP sensor is likely to go right to supply voltage, the supply on mine is 5.05V, so potentially 1.9 bar. Compress it by 0.2V and you have something the ECU can work with. 1.9 bar on my rebuilt engine should be rather more interesting than 1.7 bar The lower the MAP voltage the more power I can get out of the ECU as well without saturating the load signal.

 

What I meant by the resistor mod was I think what John's on about (I think) in that you lower the output voltage from the MAP sensor so the ECU thinks the MAP is lower than it actually is.

Does anyone know of some decent documentation that I could get my hands on regarding the boost control system's design and functionality? - I'd really like a better understanding of it before I start monkeying about with it...

 

Or any books I could get hold of??

 

Do an internet search on boost control - nearly every production turbo car works in a similar way with an actuator with a pneumatic feed and a PWM solenoid bleeding off the compressor outlet signal.

 

Is this what you are talking about re resistor http://www.thedodgegarage.com/turbo_map.html

I think i'm missing something about this mod directly raising boost, surely the map sensor only delivers information within it's given parameters ie 2bar or 3bar absolute pressure capabilities to the ecu?, how will it alter boost by upping what would essentially only be it's capability?

I understand that it will be able to read more manifold pressure and relate to the ecu but surely like I say without upping the boost by ebc, mbc or ecu modified to alter solenoid control I don't understand the map sensor mod upping boost.

 

That sort of what I'm on about - There's a trick on the Saab 900T16 where by you lower the output voltage of the MAP sensor thereby fooling the ECU (APC - as it is on the Saab) into beleiving its reading a couple of PSI below what it actuallt is - this raises your boost target (not the fuel cutout boost pressure limit - as this was determined by a separate and adjustable pressure switch).

I may well be barking up completely the wrong tree here - it depends on how exactly the Scoob ECU determines when it reaches it's target - Also there's the possible issue of running lean if the Sccob ECU uses the MAP sensor in conjuction with the MAF to determine the injector pulse periods.

Thats why I just gotta find more about how exactly the Scoob setup works.

One thing I'm totally ignorant of is what the "pressure exchange" solenoid valve does - any one know? - (I'm only used to a setup with a wastegate soenoid valve)

 

You can manipulate the MAP voltage pretty happily and it will use new targets, whether there is enough duty cycle to achieve can be got around by changing restrictors.

pres exchg is for reading atmo pres.

[Edited by john banks - 2/26/2003 10:00:10 PM]

 

Cool - the restrictors, are they inline in the tubing somewhere?

Would the 3 bar MAP sensor have the same effect as altering the 2 bar MAP sensors output?

[Edited by dr_jones - 2/26/2003 10:21:59 PM]

 

sorry John, is this related to the map sensor?, if so I thought the map sensors are designed and made with a sort of 'sea level' atmospheric pressure of 1bar built into them physically so any manifold pressure acting on them is 1bar + manifold pressure.
Does the pressure exchange alter the base atmospheric pressure reading of the map sensor or does it work to relate additional information to the ecu?

 

Info to ECU so it can set boost targets and fuel cuts, not sure if it influences fuel trim or not. MAP is just what it says - absolute, but Subaru ECUs use relative pressure targets by using absolute targets in maps and then compensating with atmo, not always linear either. So it might try to sustain absolute boost up to a certain elevation and then roll it off to avoid stretching a compressor blade.

 

thanks, it's like school should have been, actually wanting to learn

 

I think the MAP sensor reference is actually a "perfect" vacuum.

 

To add after John's words, the ECU will compensate up to 13.1 PSI or ~2.16 V on MAP for atmo's pressures and it will add an extra 1.5 PSI to you held boost. If you go lower than that (i've tried that) then it will not change the standard boost and below 12.5 PSI (atmo) it will actually deduct. But it works with a very simple circuit.

George

 

Blimey! - so you could see a boost level change with a change in barometric pressue! - maybe thats got something to do with my weird boost loss of late.... although I gues it's more likely to be something else