Does anyone have this for their car, it is useful as a comparison for others who are experiencing problems.


I will stick mine up later for comparison, however mine is not low mileage.

 

OK, tonight was not the best night to try this, it's pouring with rain and I was getting wheel spin/traction control kicking in in 2nd, 3rd and 4th so the results were all over the place.


One thing I was very surprised about was the low peak MAF number, it topped out at 195.5g/s, my gut feel tells me that this is too low.


So I would be very interested in a known good max number for a normally aspirated 4.2.





I will try again when it is dry in a higher gear which will give more data points.


RPM MAF
606 5.4
609 31.4
1804 65.7
2745 78.3
2856 104.3
3776 144.7
4808 175.5
5787 194.1
6088 182.1


It is easy then to convert into an approximate "dyno plot" with BHP and Crank Torque.

 

I guess you would expect the flow to be greater based on the advertised 300HP rating? As you said 195.5 g/s flow works out to about 244HP (195.5/.8=244.4). The wet weather would make for less dense air. It would be interesting to see what you get on a cool, dry day.


Dan

 

As part of getting the twin screw set up, I spent a lot of time fiddling with the mass flow curves. The MAF I'm using has been calibrated on a flow bench, so I can compare the "real" flow to the corresponding voltage output.

What I discovered was that the mass rates that the ECU reports over OBD are bogus and way lower that you would get on a flow bench.

Blue curve is data from the stock MAF as reported by the ECU. Pink is the Mass-Voltage curve I am currently using with the aftermarket MAF where the mass is the bench calibrated value.

On the high end they are different by almost 100g/s for a given voltage output.

 

No more runs yet as its been too damp.

I see you are using the 0.8 ratio of MAF to BHP, this is a commonly used rule of thumb in the VAG 1.8T tuning world and fits the stock 225BHP TT which has a peak MAF reading of 180g/s. That's also what made me think my 195.5 g/s reading was low. However after second thoughts perhaps it is not so odd when you consider the power consumed by the turbo which is considerable.


My TT read a peak of 210g/s with a boost over atmosphere of about 1.35 bar, which would equate to a 1.8*2.35 ~= 4.23l NA engine, but only generated about 265 crank bhp. So where did the rest go, presumably into driving the turbo.


So my new thinking is that the 0.8 ratio does not fit with the 4.2l normally aspirated engine. Perhaps it's closer to 0.66, but we could do with knowing the reading from a low mileage car at 25dec at sea level with a new MAF.

 

Hmm, well that's just odd, could it be that your old maf was just faulty. I have seen a few MAFs that have been exposed to oiled filters that end up reading low.

 

No, it's not that, the sensor was new.

Using the current MAF, where I know the actual flow rate vs voltage, the ECU reads back something much lower in the g/s field. As an example, 4.85V reads as 373g/s on OBD but that is close to 475g/s on the meter calibration curve.

I know that the voltage is being read correctly, the ECU reports that over OBD also, and it's within a few mV of what you read at the meter with a DVM, so it's not a measurement error.

I also experimented with generating my own calibration curve based on the measured data. In trying to use it though, the car was so rich that it would barely run. This corroborates the observation that the reported values from OBD are lower than the actual airflow.

My assumption is that the control algorithm takes the MAF voltage as a direct input and that the g/s value is an afterthought, reported for diagnostic convenience more than anything.

Still, this has no bearing on the original question, which was what a "normal" curve looks like. Whether or not they are correct in absolute terms, they should at least be consistent.

 

Standard obdii meters are not so accurate, you only get few figures per second. Always try to select as few parameters as possible, so you get more per second for those, (ie only rpm and MAF for this one). Better is also to do a run in 3rd gear, as that takes longer you get more readings, and thus more accuracy.

I have used this maf value when I started experimenting in tuning, as I figured this was a way to see if I would have made additional hp, by looking at how much airflow has been gained.

The figure looks close to the rwhp, but becoming less accurate as Charles has said the higher you go (outside the stock values).

The 198 gr/sec would be about 198 rwhp, but at a high error margin, and that is not even taking environmentals into consideration, which has an effect on airflow as well.

So the power has more anecdotal value than true value of engine power, but its good to learn about your car to understand what normal values are to detect if it isn't right anymore (i.e. faulty MAF or air leak after the supercharger for instance).

 

The problem with 3rd gear is that I'll be doing more than 100mph by the time I hit 6krpm, need to use that private road again.

I went out tonight to try again but the roads were still to damp, however I hit another problem, that is how to hold the car in a particular gear.

I thought that by using the J gate in 2nd and not in sport mode it would stay in second, however at WOT it changes back down into 1st.

Any suggestions on how to hold the car in a particular gear?

 

The J gate will keep it from upshifting but it doesn't prevent it from downshifting should the TCM decide you need to.

The only way I know is to roll into the throttle and not completely floor it until about 3500.