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3-Bar MAP and Alky at High Boost

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mgmshar

Active Member
Joined
Feb 20, 2002
Messages
825
Hope someone can answer this one...

I'm not currently running alky, but I'm thinking about getting one of the kits with a progressive controller. I'm hoping somebody can answer this question.

I currently have a 3-bar MAP in my car for recording boost. I'm using a boost sensing harness to read boost through my DirectScan. At full-tilt, my car runs about 25 psi. I've noticed that right around 25 psi, the reading on my DirectScan from the 3-bar MAP "tops-out" and won't go any higher. The reading on my DirectScan actually "rolls through" to a vacuum reading under these conditions! I don't know if this is a limitation in the MAP sensor, or a limitation in the ECU/DirectScan.

My question is simply this: what does the progressive controller in these alky kits do when the 3-bar MAP is maxed-out? Can someone confirm at what boost the 3-bar max's-out?

Thanks,
 
The 3 bar sensor will max out at 29.4 PSI. It works on a 0-5 v ref. Actually probably .3-4.8. 4.8=29.4 PSI. Zero PSI =1.6vdc.

Now what happens is when you hit 29.4 you have 4.8 out of the senor. You put 35 PSI you still will have 4.8 out of the sensor. What happens is if you run the 3 bar, once you hit 29.4 it will not continue to add any more alky. Kinda like a stock MAF not being able to read past 255 GM's. There are 4 and 5 bar sensors... but the resolution goes down. So when running up to 32-33 PSI, I stay with the 3 bar. Once your wanting to cross 35-37 PSI, then the 4 or 5 bar is whats used.

The DS uses the MAT input through a resistor adapter.. your issue lies with the capability of the MAT input on your stock ecu. Its not a MAP issue.

HTH
 
That explaination helps a lot. Thanks for the feedback. I had a feeling that the issue was with my ECU. I only calibrated my 3-bar MAP to about 25psi, since that's where the DirectScan reading maxed-out. My recolleciton was that the sensor was putting out around 4.5 volts (my data is at home). I didn't think to calibrate higher than that to see if the voltage keeps going up.

Since I don't currently see my car going much over 25psi in the near future (certainly not over 30), I think I'm fine.

Hmmmm, I'm wondering if I put a resistor in parallel with the MAP sensor's output signal if I could drop the voltage enough so that the reading in DirectScan wouldn't max-out. I could always use a second sensor to run the alky kit. Hmmm. Sometimes I wish I was an electrical engineer, rather than mechanical.

Thanks again for the help...
 
mgmshar said:
That explaination helps a lot. Thanks for the feedback. I had a feeling that the issue was with my ECU. I only calibrated my 3-bar MAP to about 25psi, since that's where the DirectScan reading maxed-out. My recolleciton was that the sensor was putting out around 4.5 volts (my data is at home). I didn't think to calibrate higher than that to see if the voltage keeps going up.

Since I don't currently see my car going much over 25psi in the near future (certainly not over 30), I think I'm fine.

Hmmmm, I'm wondering if I put a resistor in parallel with the MAP sensor's output signal if I could drop the voltage enough so that the reading in DirectScan wouldn't max-out. I could always use a second sensor to run the alky kit. Hmmm. Sometimes I wish I was an electrical engineer, rather than mechanical.

Thanks again for the help...

Drop me a PM......

Bob
 
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