[Subscribe to Daily Digest] |
Now wait a minute, using your predicted numbers for extended max MAF frequency and looking at the values from your direct measurement, it is clear that the MAF sensor is not capable, even using your calculated extended-range, of measuring any greater than say, for example, 384 cfm on a chilly 20 degrees F day.
Meanwhile, the volume of an internal combustion engine is fixed, and so the atmosphere entering that same engine at any given rpm is also fixed, volume not mass. Assuming 15 psi of boost, and using ball-park calculations puts that that system inlet in the neighborhood of 480 cfm at 6500 rpm; 384 cfm is passed through the MAF at around 5200 rpm on a 20 degree day. For a modified Viggen, the boost would be more like 19 psi which puts you around 545 cfm at 6500 rpm; and 384 cfm is passed through the MAF at only around 4600. And even if the boost drops to 12 psi at 6500 rpms you'd still be passing 432 cfm across that same MAF sensor that can only read 383 cfm because it's cold outside.
From a controls perspective that doesn't look like a very capable system. And I'll probably run some more numbers to be certain, but I just can't imagine that the inlet charge heating from compression could cause a volume expansion that would equal like 100 cfm, especially after intercooling.
I've tried different flow calculations and all show a reasonable likeness to those generated in the equations I've been using, so I believe the CFM numbers to be reasonable. They still look good comparing CFM necessary to HP production as well.
So where do the mass numbers seen through OBD come from if not from the MAF sensor? At what point do those numbers no longer represent real measured mass values and change to calculated numbers?
At a minimum, during high rpm operation on a cold day, what is controlling the engine output--and especially, what is controlling the engine output on a Viggen that is running modified software, upgraded hardware, and has an engine running even higher VE than stock and pumping even more air? Using even the very rough HP to CFM measurement that you provided for a NON-TURBO car, a 300 whp Viggen would need 520 and 550 cfm to make that power. What in the world is the MAF sensor doing then? Even on an 80F day there would be a 100 cfm shortage in MAF sensor range.
So anyway, it just seems to me that there is something else going-on inside T-7 other than 100% mass-based control and that the OBD mass ouput is probably not worthwhile to look at because it's generated from other inputs, unless you're considering low rpm operation.
In fact, I'll bet someone a dollar that the MAF isn't really considered in the control logic much past what is listed as the effective range within WIS. Of course I'm wrong a lot too. And in this case I wouldn't mind being wrong because I really am curious about how these things work. :)
posted by 67.10.24...
No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.
 |
 |
 |
 |
 |
