Airflow measurement/calculation (Cobb AP3)

[Fordman]

It was recently appropriately pointed out in the S280 dyno thread that the AP3 airflow calculation isn't extremely solid, as I expected it to be as I assumed it was coming from the MAF hotwire. To minimize discussion to intent of the thread where this came up, I'm starting this discussion after digging in a bit. Thanks again to those who chimed in pointing this out. This motivated me to understand the comments from my perspective. I'm attaching a pdf that summarizes the detail I used to get comfortable with what is in the airflow calc. I could have gotten pretty far down a path using Airflow and not really understanding what was involved. The attached pdf write-up is a bit lengthy, not all the detail is required for me to summarize: there is significant uncertainty in the airflow calc, not in a way that leads to control or tuning issues, but may matter when making comparisons of mods, etc., using airflow. I feel better having gone through this so that I can now determine for myself when it makes sense to use airflow for comparisons. The FiSt community has a wealth of knowledge that I enjoy and am glad this was flagged early.

I'm assuming anyone who wants to read the pdf has capability of downloading and opening (if not and desired PM and I'll PM back the content).

Feel free to chime in to the discussion if desired.

 

[4sfed4]

Great stuff @Fordman! but, we are all expecting nothing about that from you as of late

I've been thinking about the mass airflow deal for a couple of minutes and I've come to the conclusion that, based on the reasonable accuracy of Vdyno, I'm just going to use that to assess improvements, with occasional verifications on a real dyno. In the end, mass airflow is just a proxy for power anyway, with power being the desired and result. Using mass airflow as the proxy seems like it would introduce other variables such that it cannot really be used as direct relationship with power anyway (as fueling and timing changes we needed to be accounted for). And wed also need to know VE (and I dont know how we could measure that across the rpm range).

 

[Fordman]

Agree, I wrote my own Vdyno routine to check and understand what was going on inside Vdyno, how it worked. It's a great piece of software. I ditched using mine after I was done checking and understanding, it was cumbersome to run compared to Vdyno.

The reason I was using airflow (and would still prefer if it was accurately measured) is because it represents potential... what happens with the tune for a given amount of flow defines power. I wanted to take out that piece and look at spool solely based on power potential (airflow). This was really only about evaluating spool. Simply how fast is the Turbo producing airflow in the same gear. I've been sticking pretty much with 3rd gear, would prefer 4th but it's tough in my hood. 4th is going to look a bit better on spool when comparing against RPM.

I'll start showing Vdyno results soon. Maybe on Rev1b or Rev2 (whatever Jason uses), I'll show a few runs comparing both Rev's. Over on the Dyno thread of course.

 

[4sfed4]

I should have waiting to completely read your pdf file before commenting.......you came to the same VE conclusion as I mentioned.....only you did it far more elegantly and with backup data.

I agree that it is a total bummer than MAF is not available to read, esp with that sensor just sitting there :-(

The more I learn about the FiST control system, the more I vacillate between "wow...this is cool how complex they made it" and "wow, this is silly how complex they made it".

The complexity theyve seemingly added appears biased towards safety (compared to the older tech stuff I am used to), but it also makes it harder to mod the car for sure. Im longing for the old days of grainger valves for boost control (they worked AWESOME) and adding fuel via RRFPRs (or myriad other ways) as it was easier for the homebrew mechanic to make changes. I guess I need to join the 21st century.......