Dictys

Is there a conversion from the absolute MAF output to MAF voltage.

So if I had say 289 and 350 as a absolute MAF output, how would I convert these two figures into MAF voltages (standard MAF).

Or will I have to log again and record the MAF voltage as well as the absolute MAF.

The MAF has been recently calibrated by John.

Speedtoys

Maybe..whats the underlying question? Voltage isnt very useful, it'll cap at 5.1v or something, even at low absolute values.

Dictys

I wish to work out the flow using this graph from John.

Speedtoys

Sweet.

FredR

I dare say John has a conversion curve of the two parameters and I suspect it is pretty much a linear relationship given the log plots of MAF absolute and rpm's form a more or less straight line at full load runs. Without that you need to log both data sets in real time to create a conversion curve or you will need 3 or 4 full load data points and then get Excel to work out a functional relationship for the data set.

If you managed to get a max MAF reading of 340 I might as well go out and torch my 928 [BMW M6 anyone?]! To be fair the max reading I have seen so far this season is 310 and logic suggests that as our temps fall we will see bigger numbers. It would be interesting to know what n/a folks in cold climes see as MAF output at say zero degrees centigrade.

Not sure why the system has MAF absolute values other than comfortably recognisable numbers to work with. Voltage outputs are the same [I presume] just not so easy to visualise a difference between 5.1 and 5.3 volts [for instance] in a quantitative sense.

Regards

Fred

Speedtoys

What have you done to hit 340?

Here is an almost tweaked out N/A S4 Auto:
https://rennlist.com/forums/8525048-post8.html

Thats ~288rwhp territory IIRC back then.

Im pushing 140 at unknown voltage on a Supermaf right now.

Dictys

We got 349 on a S4 Auto with a stock GT engine (23c outside temps) over the weekend.

The 289 is on my stock S4 (34c outside temps).

Speedtoys

Then im missing something significant in how the MAF reads those numbers, because the way I see it as a measure of HP, there's no way.

I need about..4psi boost to hit 340s in an S4. (guessing, but still need _boost_ to do it on a 70f day)

jcorenman

Dictys, a 349 reading for MAF is not possible for a 5.0L N/A engine, with a properly functioning MAF. The plot that Jeff posted a link to is pretty typical, max flow mostly in the 260 row, max of 289 sounds about right.

If you are trying to get to some estimate of hp, then I don't think airflow will get you there- the volumetric efficiency of the engine won't be the same for the S4 and GT (cams). If the cars are both properly tuned, then look at the injector PW at 6000 (pick an RPM) and work backwards to fuel flow and then horsepower -- 0.50 lb/hr per hp is a pretty good estimate for these engines, but again the difference in cams might mess that up.

It's dark in the UK, let's see what John has to say in the AM.

Cheers, Jim

Dictys

Once I get home at lunchtime I'll try and post up the latest maps of both cars, the S4 and the GT.

Edited:

here are the maps for my S4 Auto and the S4 Auto with the GT Engine (both have the highest readings on the right, both MAF calibrated recently, on my S4 the lean stops have now been sorted.)

jcorenman

Thanks for posting the plots, that helps a lot. What's going on is that the GT plot has quite a bit more jitter at high loads than the S4-- probably because the RPM's and loads are both going higher. What you want to do is draw a line through the WOT points in the 5600-RPM cells, to represent the average of the dots (i.e. average MAF load). The differences are going to be an average of 270-ish and pretty flat for the S4, and maybe 300 and rising for the GT (at the top of the 5600-rpm cells, i.e. 5900 rpm). Looking at both plots there are higher-- and lower-- values, but the average is what counts.

So if you look at the average rather than maximum values, then this makes sense-- and those differences are not unreasonable for a S4 vs. GT. The big difference is cams, the other difference is that the GT is pulling to higher RPM's before shifting. I would also guess that the GT might also have its cams significantly retarded by the way that the MAF signal climbs at the top end, but that's reading an awful lot into these particular tea-leaves.

One way to take a closer look at this is to copy the data from those cells into Excel and plot it there, and add a trendline to show the average. It would also be interesting to see what could be done with SP, if you wouldn't mind sending the log files. (support-at-sharkplotter.com). My high-load data is always more sparse, I suspect I am in a lower gear ...

Why the increase in jitter above 5600 rpm for the GT plot? Part of it is because the MAF output voltage is a logarithmic function of flow, and is compressed-- which then gets decompressed by LH and plotted on a linear scale by SP. So SP's vertical scale gets exaggerated towards higher load, magnifying any variation. (I'm changing that on the next version of SP, so that each row is the same size).
It may also be turbulence in the airflow, causing variation in the voltage output from the MAF. The MAF needs its screens (especially the top one), but this still may be an issue for the stock intake. The "split-Y" (EIS-style) is an unknown in this respect.
And there is a difference in how the LH does its A/D conversion on the MAF signal at higher loads, which will introduce more jitter from sample-to-sample.
In any event, if you look at the AFR's, there is not much variation and what there is does not track the variation in MAF load.

John Speake

The A/D convertor tops out at 5.1v and that why "MAF volts" stops there as well. However there are other measurements of MAF volts going on in the LH and that allowed us to map accurately up to over 6v....

Above that limit a SMAF is needed :-)

FredR

Hi Jim,

Excellent analysis as always. I have just ordered one of those honeycomb packs to try in my EIS system to see if I reduce the point scatter which I am not at all happy with. I my try the stock air filter to test the impact to see i my pinging problem attenuates at all.

My MAF was calibration checked by John so I am confident of its accuracy.

Regards

Fred

Speedtoys

So..reading that, at 145 absolute @5.09v...620chp?

John Speake

The graph is an attempt to predict the MAF and SMAF charactristics at flow rates well in excess of what I can achive on the bench. Although the limit value of 6.1v tallys well with our rule of thumb that about about 440HP crank is the limit for a stock MAF, the predicted power values at the higher voltages are over optimistic, I believe.

As previously mentioned in an earlier thread where this chart was shown, if anyone can help with dyno power numbers versus MAF volts, we can then replot the curves...