seems a lot of you are still confused about something that ive replied about before. its why your boost gauge reads different boost than your PFC.

i had a theory right after i got the PFC which i confirmed using calculations and then also when i drove back down to the dallas, tx area from altitude in colorado.

its actually pretty simple.

boost gauge - measures GAUGE pressure. i.e. its reference point is 0.

PFC - measures from the MAP sensor which measures manifold ABSOLUTE pressure. this means that the pressure you see is pressure produced by the turbos minus the pressure change due to your height above sea level. its reference point is 0 at SEA LEVEL.

lets call the difference you see in pressure (boost/vacuum) measurements value X.

(keep in mind im going off memory so im estimating these numbers)

but say youre at 7250 ASL like me. boost gauge says 13 psi, PFC says around .7 which is about 10 psi. so 3 psi is value X.

---go find information on the atmospheric pressure at different altitudes. find the difference between pressure at sea level and that at your specific altitude. this will equal value X.

---go turn your key so that the power is on, but dont start the car. it will say you have a negative value (vacuum). this number will also equal value X.

when driving the car back down to dallas i watched the PFC numbers start to diverge to become closer and closer to the pressure i regularly see on my gauge. obviously this is due to the increase of pressure since i was driving closer to sea level.

in dallas theyre only about .5 psi off while in colorado theyre about 3 psi off.

what to do about this.....im not sure. i just let the PFC do its thing and keep my boost at levels that i see on the gauge. i guess i could just use absolute pressure and tax the turbos more at altitude....but im ok with the loss of power for times when its up here......for now that is.

what do y'all think? is this helpful or am i just repeating something that has been covered before?
it just seems like a lot of people were very concerned and confused about what was going on.

 

Solution:
Get a Defi Boost Gauge. It matches my PFC.

Richard

 

From what I understand the boost kit uses your factory map plugged into the FC as a real time reading for air pressure and fixes this, while the new map plugs in to the old map harness allowing higher boost to be read by the FC. anyone have more info?

 

and you're only at 332 ft ASL so it should be very accurate and close to your PFC readings. i have an apexi gauge for those who are wondering.

 

Hey Nic,

well, I'm in the Dallas area, and there is a 2 psi discrepancy between my gauge and the PFC. I've been using the PFC as the yardstick; My engine is actually seeing 13psi, while the gauge is showing 15.

Rich

 

rich, maybe your boost gauge is off then. i calculated out the diff in atmosperic pressure for both altitudes and everything corresponded. it all fit together and made way too much sense to be wrong. are you still using that POS...i mean PFS gauge?

 

Thats strange. I live at almost sea level a have tried 2 different boost gauges, Autometer and Greddy)they both read the same. My AVCR matched my boost gauges (it has its own map sensor). They all showed higher than my PFC display. Did you see Chucks thread on the PFC display being off?

 

I have tested and compared my old HKS and newer Autometer gauges to the PFC, and the map sensor output voltages as per Mazda manual. I live in Houston. Read my notes under UEGO test.

Unfortunately, the PFC and your theory are off.

 

chuck,

i assume by meaning that you checked map sensor voltages you mean that you checked the sensor. did you also check all of the leads?

maybe this just works for my setup/PFC, but my calculations were very accurate. i cant remember exactly #s from when i compared and the car is back in tx now, but say vacuum said -176mmHg, my calculations were the same down to the last digit - something like -175.

they were like this at altitude and also while in tx. ill check out your post, but what ive found seems right to me.

how much difference are you seeing?? try out the calcs and see what you get. post the results.

im open to the fact that i may be wrong.....but im reluctant to let it go b/c everything matches up so well and makes sense.

thanks

 

My previous gauge was Autometer, it was off by 2 lbs. I put the Defi Gauge in and it is right on with the PFC.

Richard

 

I have a Defi Boost gauge and it reads about ~1 psi higher than the PFC.

 

im going to start a new post and ask that everyone reply with the vaccum that they see with the key turned so that power to the PFC is on, but the engine is off as well as altitude for where this measurement was taken.

so please go do that if youre reading this.

 

my pfc shows boost at about .8 bar ; my gauge shows 13-14psi sometimes. i hope its just the gauge thats off-its an autometer
i think the elevation is only 74' asl

 

Does this mean I can run more boost? Whoopeeeee!

 

Ok, so, the PFC display is off, usually by around 0.1kg/cm^2 it seems.

So, knowing that, the important question is, what does the PFC itself believe the pressure to be? Does the computer itself think it's 0.1 lower than it actually is, and will then pull timing and fuel from the wrong place on the map or (more realistically given the resolution of the map) plug the wrong parametric value into interpolation between cells?

I suppose if it did, it wouldn't be that bad, since it's not off by that much, and the tuning would have been done with the values already being off anyway.

But still. Imprecision like that when it comes to something like this makes me a little nervous.

Maybe a Haltech is the right answer after all.