I am wondering, can g/rev be converted to psi/bar? It would make tuning a little bit more simple if you can just look at you're logs (provided you have a map sensor) and go from there, not that it is difficult but it would help my friend out a lot! as he can't log g/rev at the moment. I think mmcd offshoot does though I'll have to look into it!

notsofastdsm55 said:

I am wondering, can g/rev be converted to psi/bar? It would make tuning a little bit more simple if you can just look at you're logs (provided you have a map sensor) and go from there, not that it is difficult but it would help my friend out a lot! as he can't log g/rev at the moment. I think mmcd offshoot does though I'll have to look into it!it!

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No.

g/rev is the mass of air entering the motor per revolution. Pressure (psi or bar) is pressure which doesn't have anything to do with mass or revolutions or minutes.

If he can log airflow (probably can) in lb/min, you can convert this to g/rev. If LBMIN is the air flow in lb/min and RPM is the rpm when LBMIN was sampled/logged, and GREV is the g/rev, the following equation can be used:

GREV=LBMIN * 454 / RPM

So if you logged 21lb/min airflow at 6000 rpm:

GREV=21 * 454 / 6000

Which gives 1.589 g/rev

I guess you could technically do it, that's pretty much how speed density works. You'd have to know your engines volumetric efficiency curve and some other factors I probably can't come up. There's no real simple solution aside from fixing your logger.

Hey thanks you guys and thanks kenamond for the conversion formula!!! that will make it easier!!!

I think you can estimate your boost pressure based on your g/rev value. In fact, our factory boost gauge operates on this relationship. The factory boost gauge is monitoring your g/rev value and displaying this value as an estimated boost pressure. Unfortunately, this is ONLY an estimate since the relationship varies depending on intake air temperature and turbo efficiency.

zippyshoe said:

I think you can estimate your boost pressure based on your g/rev value. In fact, our factory boost gauge operates on this relationship. The factory boost gauge is monitoring your g/rev value and displaying this value as an estimated boost pressure. Unfortunately, this is ONLY an estimate since the relationship varies depending on intake air temperature and turbo efficiency.

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And intercooler efficiency.

I ran across the following table of estimated g/rev values versus manifold (boost) pressure:

Manifold Pressure g/rev
------------------------------------
22 inHg (vacuum) 0.1
20 inHg (vacuum) 0.2
18 inHg (vacuum) 0.3
13 inHg (vacuum) 0.4
12 inHg (vacuum) 0.5
8 inHg (vacuum) 0.6
5 inHg (vacuum) 0.8
3 inHg (vacuum) 0.9
0 psi (boost) 1.0
1 psi (boost) 1.1
4 psi (boost) 1.4
7 psi (boost) 1.6
10 psi (boost) 1.8
17 psi (boost) 2.2
25 psi (boost) 2.7

Hope this is helpful.

thanks that will be a great help for my friends!

The thing is, you can get 2.5g of air in the cylinder in one rev (2.5g/rev) at 20psi intake manifold pressure at one temperature. But if you have a much more efficient intercooler, cold air intake, more efficient compressor (different turbo), etc., you can have 20psi in the intake manifold but have 3.0g in the cylinder. If air gets hotter, it takes up more room.

A smaller quantity of hotter air (lower g/rev) can give the exact same manifold pressure as a larger quantity of cooler air (higher g/rev). Same pressure, different temperatures, different g/rev.

Also, you can have 20psi in the intake manifold at 3000rpm and get 1.5g/rev and you can have 20psi in the intake manifold at 7000rpm and get 3.0g/rev. That would be a hypothetical example of what you'd have if you held 20psi boost from 3k to 7k rpm. Same pressure, different rpm, different g/rev.

The calculations you'd have to do to compute the g/rev from pressure would be the same sort of thing the ECU does. The MAS has a temperature sensor, pressure sensor, and air speed sensor. These three sensors combined with the RPM value are required to compute g/rev. That's what the ECU does to determine g/rev. You can't get g/rev from pressure alone. You need the temp, air speed, rpm, and some equations like PV=nRT to end up with a load (g/rev) value.

Hi,

I don't think that g/rev can be directly related to psi or psi normalized with respect to pressure in bars. Use:

P = (m*R*T)/(V*A)

where P is pressure (in Pascals), m is mass flow rate (explained later) in kg/sec, R is the gas constant of air ( in J/kg K), T is the temperature of the air from the MAF (in Kelvin (K)), V is velocity from the MAF (in m/s), and A is the cross sectional area of the MAF (in m^2). The mass flow rate (m) is equal to:

m = (g/rev)*rpm*(1kg/1000g)*(1 min/60 sec)

All the units have to be in proper units to cancel out. Once the pressure is found it can be converted to psi (Pa to psi can be found in any text book or at online conversions), and then normalized to psi/bar (so basically take psi value and divide by 14.5). This calculation can definitely be done in English units such as ft/sec and stuff like that as long as units agree and cancel out.

If any questions just ask, sorry if I described this badly I have been drinking all night, haha. Units are often the most confusing part of a calculation like this. Are your buddies familiar with these units and converting to like units.

In engineering calculations it is often easier to use SI units such as I have.

Bill

If you can find someone who has your exact setup, then maybe it would be worth the effort. If not you're wasting your time. Boost is not airflow, its heat.

123bobby123 said:

Hi,

I don't think that g/rev can be directly related to psi or psi normalized with respect to pressure in bars. Use:

P = (m*R*T)/(V*A)

where P is pressure (in Pascals), m is mass flow rate (explained later) in kg/sec, R is the gas constant of air ( in J/kg K), T is the temperature of the air from the MAF (in Kelvin (K)), V is velocity from the maf (in m/s), and A is the cross sectional area of the MAF (in m^2). The mass flow rate (m) is equal to:

m = (g/rev)*rpm*(1kg/1000g)*(1 min/60 sec)

All the units have to be in proper units to cancel out. Once the pressure is found it can be converted to psi (Pa to psi can be found in any text book or at online conversions), and then normalized to psi/bar (so basically take psi value and divide by 14.5). This calculation can defintely be done in English units such as ft/sec and stuff like that as long as units agree and cancel out. If any questions just ask, sorry if i described this badly I have been drinking all night, haha. Units are often the most confusing part of a calcualtion like this. Are your buddies familiar with these units and converting to like units. In engineering calculations it is often easier to use SI units such as I have.

Bill

Click to expand...

And after you do all of that complicated calculation, you'll find that the pressure you calculate will be the same as a barometer sitting in your house.

I agree completely with all of this theory, but all of the values you know about are in the MAS, not after the turbo. So you only find out what you already know; that the pressure in the MAS is barometric pressure (the air speed, temperature, area, etc. are all at the MAS).

If you want to know the relationship elsewhere in the intake, you need all new values for each of those, which you don't have.

OK, thanks for correcting me on that.

But I think that if you knew your Temp in the intake manifold, your g/rev, and engine volumetric flowrate you could probably do it.

Since V*A is a volumetric flow rate, you could use the engine volumetric flow rate (convert cfm to proper units)

Engine Flowrate (cfm) = (RPM*Displacement*Volumetric Efficiency)/3456

So:

P = (m*R*T)/(engine flowrate)

m and R would be the same as before

Would that make sense, you think.

Thanks,
Bill

I think that it is safe to say that manifold pressure has been/can be estimated from g/rev measured at the MAS, since this is exactly what our wonderfully accurate factory in-dash boost gauges do - The ECU drives the factory gauge directly from the g/rev value it calculates from the MAS sensors (intake temp, baro, and volumetric airflow) and engine RPM. Also, users of DSMLink will be familiar with the loggable value "BoostEst". The value of this parameter is also directly derived from the ECU's calculated g/rev value.

However, the accuracy of this estimation is certainly questionable. This estimation must make assumptions about additional variables such as intake manifold air temperature and engine volumetric efficiency. What complicates matters further is that volumetric efficiency varies with engine RPM, which means that the accuracy of this estimate also depends on engine RPM. DSMLink users quickly learn that BoostEst is most accurate at around 5500 rpm or so, when the factory engine is at its peak volumetric efficiency - At RPM values significantly far from 5500, this value is pretty inaccurate. Near redline, BoostEst tends to drop-off because volumetric efficiency drops-off, even though a "true" (not factory) boost gauge does not show any such decrease in boost pressure.

The other variable mentioned is intake manifold air temperature. Since the factory DSM's do not include an air temperature sensor at the intake manifold, the ECU must be making an assumption about this value in order to drive the factory boost gauge. The accuracy of this assumption may also be questionable. The intake manifold air temperature depends on quite a few variables: intake air temperature (measured at the MAS), turbo efficiency, boost pressure (even a turbo that is 100% efficient still raises the air temperature since it is compressing the air), intercooler efficiency, as well as ambient temperature at the intercooler (this may differ from your intake air temperature.) The combination of all these variables probably leaves a lot of margin for error.

And as far as the OP is concerned, the answer to his question is still "no".

I think that we're all in agreement that it is possible to compute pressure in the IM, but you need to measure other data in the IM at the same time in order to do this. You need density and temperature. And the OP doesn't have temperature.

I imagine you can get the mean density:

volume/rev ~= 500cc
mass/rev = load
density = load/500cc

But you still need a temperature.

Oh well it was a shot in the dark anyway, thanks for the responses you guys!

Hi,

Well I'm glad that I was correct in my equation but you guys are definetly correct in that all the assumptions you had to make would leave room for large error. If you had the proper sensors/values I'm sure that you could do it and that it would pretty accurate.

Bill