Hi All, I'm helping a friend diagnose his new turbo setup.

We're dealing with a 1998 Grand Prix GTP which is all stock, except for the
recent addition of:

- Turbo 17 PSI
- Intercooler
- 1.9:1 Ratio Rockers

The engine has a 2 Bar MAP sensor. When the engine runs under full boost,
I am speculating that the stock MAP sensor is not able to read the manifold
pressure properly.

As I understand it, the sensor has a range of two Bar from a reference
point.

IE: - 1 Bar to + 1 Bar

Approximate Elevatoin 1100 ft.
= ~ 0.94 Bar (~28.6 in./Hg. )

MAP at idle = ~ 10 in./Hg.
Gauge Reading = ~ 18.6 in./Hg.


With the engine off and key on, the MAP is reading pressure at our current
elevation which is apporximately ~ 14.2 PSI (~ 28.6 in./Hg.)

So...would 14.5 PSI be considered zero as the reference point of the MAP sensor with a range of 0 PSI to ~ 29 PSI?

or

Does the sensor reference begin at zero PSI with a range of - 15 PSI to + 15 PSI?


If the latter is true, will he max out the MAP sensor and default to Speed Density Mode?

Thanks...and sorry the "math" !

 

Map sensors are absolute pressure sensors so they start out at 0 psi absolute. In your case with the weather conditions you mentioned the map sensor will stop increasing @ 15.2 psi gauge pressure. There is no reason to believe that the engine will stop making power there, you don't necessarily need to continue to add fuel for the last 2 psi. The air may be hot enough that the optimum tune might actually lean out a bit.

Pressure gauges read differential pressure and are referenced off of barometric pressure to set zero. (except liguid filled gauges that are set at the factory's baro/altitude conditions permanently)

 

OK, but if the computer 'thinks' the MAP sensor is out of range and not able
to report properly, will the computer switch to Speed Density mode , or continue
to operate in Power Enrichment mode?

The discussion revolves around getting a 3 Bar sensor (which is better IMO), or editing the SD tables to add more fuel.

Thanks.

 

The sensor isn't going to go "out of range" , its just going to stop increasing in voltage. A 3 bar sensor would be a waste of resolution in this application. The computer has a limited amount of bits to digitize the 0-5v signal into. I do not know whether its 8 or 16 bit, but I'm going to lean towards 8 bit. With 8 bit there are 255 discrete steps for the 0-5v range. Thats .020v per step. The 1,2 and 3 bar sensor has to work within this.

1 bar sensor .057 psi per step
2 bar sensor .114 psi per step
3 bar sensor .171 psi per step

In actuality the sensor does not output a full 5v at max pressure. It is lower than this a bit so the range is cutoff even more. The lower resolution of the 3 bar sensor could affect drivability as the steps aren't as smooth. BTW speed density is a calculation from rpm, air pressure and temperature to determine how much air is entering an engine. That what the map sensor is there for. When the map sensor is being used its in "speed density" mode.

 

Yes, right...I messed up. It's the MAF that I was confusing with the MAP.
I was under the impression that MAP was still used in PE mode as well?

Thanks for the input on the 2 bar, vs. 3 bar sensor. I guess we're better off
using a wide band 02 sensor and adjusting the injector pulse width in PE mode.

 

MAP is still "used" in PE mode, as a measure of engine load, for the timing tables, etc. It just isn't used to calculate mass air flow, because you have the MAF for that. I would think you would still need to program timing vs. MAP vs. RPM with a high boost application.

 

Timing is something I've questioned in another forum and I'm not very clear
with the replies.

Maybe some of you can take a look and explain further (Forced Induction Forum - Timing Curve w/Turbo Engine)?

http://web.camaross.com/forums/showthread.php?t=388224


Thanks.