JustDSM

iTrader: (28)

Hmm.. What is your elevation?

Fourdoor

iTrader: (11)

My mechanical boost gauge, and the boost measuring equipment on Godspeed's dyno read exactly the same, but the AEM EMS has always read 2 psi less than the mechanical guage or the dyno equipment.

Keith

EVO8LTW

iTrader: (41)

I'm not sure about my elevation. How can I look that up?

EDIT: Looks like I'm 350-400 feet above sea level.

Fourdoor-- I just came across an old post of yours at aempower.com on this very topic. I also found a few others posting about the MAP reading around 2 psi below a regular boost gauge. It seems that the discrepancy lies in the difference between absolute pressure and gauge pressure (boost pressure over ambient). That's all fine and good, but which is the relevant measure for setting boost? If I want to run "20 psi," should I aim for 18.8 (20-1.2) on this absolute pressure gauge? Does ambient pressure vary with weather? What is the relevant metric for detonation -- absolute or relative pressure? I'm so lost...

Coolguy949

iTrader: (10)

That's about the same as the Kavlico MAP sensor for my Zeitronix reads with the car off.

JustDSM

iTrader: (28)

That's not much of a difference from sea level.. Reason I asked was...

I'm also using a AEM 3.5bar and have it mounted in the MDP port with a RRE adaptor (similar to Buschur's piece) and log it through my engine management setup. The software for my setup asks for the elevation in order to properly display appropriate gauge pressure. Due to my elevation of 6000-6100ft ASL the difference between the displayed gauge pressures (sea level vs. 6K+) are quite dramatic due to the ambient pressure being ~23inHg vs the ~29inHg you might see.

Fourdoor

iTrader: (11)

I personally think that the calibration on the AEM sensors is crap. The Godspeed shop in Michigan is at about 900 ft elevation, and my home in Indiana is at 495 ft elevation. According to this site:

http://www.uigi.com/Atmos_pressure.html

I would have less than half a lb pressure difference, and I would have to be at 4000 ft to get a 2 lb pressure drop.

In short, the AEM pressure sensor reads low. Everyone sees it and everyone blames the mechanical gauge for being wrong becaue it references atmospheric pressure instead of absolute pressure. That accounts for about half a psi difference. The other 1.5 psi is because (in my opinion) the AEM MAP sensors are poorly calibrated from the factory.

I gave up trying to get a solution on the AEM pages because they are full of "know it all's" who don't bother to do any research and just go on the hearsay that it is always the mechanical gauge that is f'ed up.

Keith

PS: I always trust my mechanical gauge and run around 22 psi by it (20 psi on the AEM).

nils

iTrader: (56)

great thread..... I just installed a AEM 3.5 bar map sensor off the intake manifold. After reading this it does make sense it is reading low, atleast according to my 'butt' sensor.

n

Fourdoor

iTrader: (11)

If your read the thread then you would know that is should NOT be reading low unless you are at high elevation!

If you are at 2000 FT altitude it would read 1 psi lower than a good mechanical boost gauge.

If you are at 4000 FT altitude it would read 2 psi lower than a good mechanical boost gauge.

If you live in Denver CO it would read 3 psi lower than a good mechanical boost gauge.

Most of us in the United States live somewhere between sea level and 1000 FT altitude, so the AEM should read somewhere between 0 psi lower and 0.5 psi lower than a good mechanical boost gauge.... but they CONSISTANTLY read 2 psi or more lower than a good mechanical boost gauge.

Keith

JustDSM

iTrader: (28)

Interesting resluts you guys are having. Mine does not display a difference of more than .1 and occasionally .2 psi/inHg diffrerence. I'm highly interested in what might be going on here.

nils

iTrader: (56)

Las Vegas Elevation: 2175ft (650m)

n

Tim@AMSperformance

iTrader: (34)

Did you choose the aem 3.5 bar map sensor in the ems setup? If not it may think you are using another sensor

nils

iTrader: (56)

fyi - I bought the zeitronix from RRE. They were out of the boost sensor for the kit. The sales guy on the phone told me specifically to get the AEM 3.5 bar map sensor and told me that is what they now use with their zeitronix unit.

I ordered it about 1 month ago from RRE.

n

EVO8LTW

iTrader: (41)

Thought I'd revive this from the dead with an update. I have found that my AEM 3.5 bar MAP sensor reads -1.1 psi when the car is off, which is 1.58v on the sensor. I ordered another identical sensor from the same mail order house to see if maybe I got a bad one. Well, it reads EXACTLY the same. -1.1 psi with the car off and 1.58v, so the sensors are dead consistent.

What voltage are others seeing on this sensor when the car is off? I'm wondering if it just consistently reads 1.1 psi low.

I'm basically at sea level where I live, so altitude isn't causing this.

Thanks.

TrinaBabe

iTrader: (3)

I dont know anything about any of the actual physics behind this debate but it is VERY simple in my eyes. Who cares what the boost is truly at? It is another relative topic like a dyno sheet. Run whatever you can. Its not like people are going to say your car is so much more balsy because it is at 21psi as opposed to 20. And even if they do... great! So you can tune your car a psi higher then them.

Just in my own little physics ideas though:

Boost pressure changes on many things... not just altitude. Temperature is the other huge thing I would assume.

And yes, psia and psig will always be different. It is up to you to decide which you want to claim as your 'boost' level. If it is 95 degrees outside and I set my boost at say 20psi... when it turns into night at 65 degrees the boost will change a decent amount.

The actual number doesnt matter at all. As long as you have an idea and you can see any anomolies with it it works fine. The AEM (Kavlico) map sensor is incredibly accurate. In that I mean that it will always read relative to itself. It wont throw numbers around for no reason... if it says one day 22.7 and another at 23.4 those are the numbers it is reading.. it is not off by .5psi. The only reason you need an accurate map sensor is one, to tune a map very precisely, and two, so you can see small changes, see re-spool characteristics, etc... Who cares if it matches the other guages.

The other thing is how do you know if you are reading the boost guages correctly? When I look at mine from different angles it can seem to be plus or minus 2 psi depending on where I look at it from. Anyways, ultimately it still doesnt matter.

I run my car at the highest boost level that i can and still hold 'normal' timing. If I could run 30psi every day without worry than I would. Fact of the matter is whatever you can get away with do it. If it reads 12psi so be it. If it reads 35psi so be it. The point is + or - a few psi doesnt really matter in most cases. The difference in one cars 20 to anothers 21 is what.. at most 15 hp.

EVO8LTW

iTrader: (41)

TrinaBabe, I agree with everything you said...but I still want to know how much boost my car is running. If my car is particularly knock prone and I'm keeping the boost low or running superconservative timing for my boost level, then I want to know that. Conversely, if my car is somehow not knock prone and I can get away with more boost for a given level of timing, then I want to know that too. I guess knowing the boost level will give me some indication of whether my car is acting normally for an Evo and help me to know that I'm in the right ballpark on timing/boost. But you are right, if I were a master tuner, I wouldn't need to know or care.