I've searched here but can't find the answer, so hoping someone will help. I have hooked up my upgraded ScanGuageII to the SRT. It seems to be working just fine. I cleared the code & CEL I had (PO505). It is showing me MAP,Volts,Coolant Temp.,I.A.T.,TPS,Ign.Timing,load,brake horsepower,MPG,MPH,RPM, numerous fuel/trip data, economy, etc. More info. than I'll ever use. But here is question. The MAP reports in P.S.I. (although you can change to kilo-pascals if you wanted) and I wondered what the conversion formula is (if there is one) to figure what BOOST is. I saw on another thread someone mention that MAP=Boost+14.7 But 14.7 is just the "stioch" AF ratio,and that can't be right. When I'm at W.O.T. I can see 28-29p.s.i. when I'm brave enough to avert my eyes to it. And I know I'm not producing 28lbs. of boost. I know there are variables(Temp.Elevation,etc.) but generally speaking can this be approximated w/o having to get a stand alone boost gauge or Zeitronix setup like a lot of you have? This is my 1st supercharged engine and just trying to understand it better. Bob

 

MAP-14.7psi= boost
28-14.7=13.3psi boost

 

Boost is measured in PSI also... but you have to remember to subtract one atmosphere(about 14.1?) from the absolute pressure reading to get your boost number.... someone will correct me if I got it wrong...

 

Thank you for that. I just looked up atmospheric pressure, and for every 1,000' ascended, it drops 4%. Only 13+lbs. of boost; no wonder everybody wants a pulley.

 

I believe down is right mike and atmospheric is 14.696 so .7 haha

 

He was on the same street---just the sidewalk!!!!

 

Hey, I know how to 'boost' my numbers.......subtract less=more boost


I keep tripping the 20psi limit, so who cares???

 

At sea level the calculation is MAP-14.7=BOOST (well MAP-14.69xx...LOL), but most of us aren't at sea level. If you want the true boost based on the MAP sensor signal, you really have to use MAP-BAR=BOOST. As pointed out, the higher up you go, the less the atmospheric pressure. So if you're in Boulder at 5K feet, 1 Atmosphere will be 20% less than sea level (4% per 1K feet) and 1 Atmosphere (BAR) is only ~11.76.

It sounds like you can see a lot of the ecm sensor inputs with that tool. See if there is one for the Barometric sensor (BAR) and that will tell you what the atmospheric pressure is where you are currently located. Subtract that number from the MAP and that gives you the actual boost number.

Or you can just install a boost gauge...LOL!

Jon

 

rudy had me set my calculations for boost on my dashdaq in -14.7 to overcome atmospheric pressure. so my boost reads negative until im into the throttle then changes to positive # quickly

 

We live close to sea level, so it's real easy now that I know the formula. I will try to program in the BAR reading (it is capable of that). Would be good, and kind of fun to know, when we're up in the Sierra's(Tahoe Area).

 

How do you upgrade the Scanguage II? I have one and would love to be able to use it on the X-fire

 

Call them(Linear Logic)in Arizona@ (888) 433-5664, Tell them you would like the free upgrade offer to the 3.17MB version (mercedes benz).

 

This thread seemed recent enough and most appropriate to post this... I recently obtained an OBDII interface for my laptop, partly for curiosity but also in hopes of trying to diagnose some moodiness that my car seems to exhibit. I've already had my IC pump replaced, and a number of threads suggested that the Intake Air Temperature sensor might be to blame for that symptom, if not the IC pump.

So I put the laptop in the passenger seat, and had it record Engine RPM, Vehicle Speed, Intake Air Temperature, and Intake Manifold Absolute Pressure so I could get some real numbers for comparison. I wanted to familiarize myself with the output before doing any real test runs, so I had the car in automatic the whole time and wasn't exactly racing. When I was done, I started analyzing the data, half-hoping to see improbable numbers on the IAT graph. Well, that one looked fine to me, but the Intake Manifold Absolute Pressure graph has me confused.

According to this thread and others, I should be able to subtract 14.7 from the MAP value and get boost, right? As you can see in the screenshot, MAP peaked at 50 psi, which when you subtract 14.7 gives 35.3psi of boost, which is ridiculous isn't it? Am I reading it wrong, completely misunderstanding something, or perhaps the victim of a faulty MAP sensor? Or is my engine about to explode...

 

Probably not the boost spiking, judging by slope of the vehicle speed line when the MAP "spikes" . The slope should get steeper and steeper as the boost goes up, and the boost doesn't correspond to the engine speed like it should. Superchargers don't usually spike boost, I think the problem is the sensor.

 

If that's what the sensor is reporting, shouldn't I be getting a CEL like in this thread, or do those people have even higher boost?

 

I believe the map sensor on our SRT6's is a 250KPA sensor which translates to 2.5 BAR. That means the highest MAP pressure the sensor can read and output is 36.75 PSI (2.5x14.7=36.75 - but remember, we have to subtract atmosphere from that to get boost, so total boost it can read is only about 22 PSI).

It really looks like you have your logging software configured incorrectly, and it almost looks like it thinks the sensor is a 500KPA sensor. Is there a way to change the scale for the logged items?

I don't have the actual specs for our MAP sensor, but based on other boosted applications that I've dealt with, the sensor range is generally 0-5 volts. That voltage range is usually scaled something to the effect of 0V to ~1V is vacuum and ~1V to ~4.5V is scaled linearly for 0-22 psi of boost. So if you can log MAP Volts, you should see less than 1V when in vacuum (basically at idle and decel), about 1V when just going into boost, and if you are stock (no pulley mods), about 3.22V at wide open throttle (based on a linear scale, a range of 1-4.5V gives you a scale of 3.5V for 22PSI, or .159V for each PSI of boost - 3.5V/22PSI=.159VperPSI, so stock boost of 14PSI x .159V per psi=3.22V).

So again, I would check the setting of the MAP logging to make sure it's set for 250KPA or 2.5BAR, and if you really want to validate your MAP sensor output, log the true voltage the sensor is outputting and see if it falls in the ranges I've described above (remember - I don't have the actual sensor specs, so the values above are guestimates, but should be close enough to determine if your sensor is acting funky).

Good luck!

Jon