Quote:
Originally Posted by davepl
I see a number of MAF-bashing posts, and it makes me wonder how hard I should be trying to solve this, given it runs well without the MAF.
For a completely bone-stock setup I have to imagine the SD tune is pretty spot on. But I really don't know.
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This might get long, but hell, I haven't written a short post in a long time.
Fwiw; you mentioned that "GM probably got it right"....and thats not wrong, but its the wrong mindset. When GM engineers do calibrations , they are doing so off of hundreds (often THOUSANDS) of hours of logs, step tests, and failure tests. Designing a VE table that works reliably and has very large margins for safety means that table has to work in thousands of vehicles with that engine (whether its an E-rod or any other GM-built motor). GM isn't building those tunes for power, they are building them for manufacturing goals.
Now, how that specifically affects VE; in a stock configuration (non-custom OS) the VE tables are populated based on those thousands of hours of testing I mentioned above. The VE has a limited resolution (standard ve, not GMVE or VVE), which means that the highest # has to be enough for the highest-expected value @ that particular cylinder volume...and that the lowest # has to do the same. BUT of course we all know that differences of up to 50hp (4-9%) aren't unheard of. So, its possible that the VE #'s populating your table are not only padded, but they could be padded even MORE if you got the runt. AND because you have both closed and open loop, your not spending THAT much time running on VE alone.
Seems like I'm saying GM doesn't know what they are doing, but here's the catch. In stock config, the ECM isn't using VE alone; the MAF and the 02's act as an input-feedback loop. Its saying "I did this, so you should have that"...and if it doesn't see that, it makes changes....or throws a code if those differences are beyond what it can account for. Those "padded" VE #'s can be compensated for by that loop, and the engine is happy. To add to that, the IAT is in the MAF (usually) and the IAT data is further used in combination with the MAF reading and the 02 sensor trims to calculate air density. This means that whether your in 112* ambient temps at sea level or 8* at the treeline, your getting an accurate air density calc, and thereby getting proper fuel control.
SO, then why do many of us preach the virtues of thine Speed Density tuning? (aside from just fitment reasons)
In SD, you no longer have the input-feedback loop. The tune tells the engine systems what to do, and they do it, no back talking. This means that much of the safety net is gone, but the user now has a more direct control.
Even in stock form there can be (small) improvements made to trim the fat, or in some cases richen up area's which were set too lean (usually emissions related).
When you start modifying things like cams, heads, valvetrain geometry, forced induction....you effectively change the way your engine moves air. Sometimes the MAF is a choke point, sometimes it simply isn't designed to meter the volumes of air that highly modded motors can move. In the case of FI cars its not capable of effectively metering air going from engine vacuum to 2-3 bar. Changing to SD means your only on those VE tables (with 02 loop if desired, which is a good idea). This fine control means you can run a tighter ship, making more power more efficiently...ideally. For guys running non standard intake types (carbed, ITB, nelson's crazy alien intake thingy), its the only option.
Here's the rub; all of the perceived benefits of SD are also its pitfalls. The amount of time it takes to do an SD tune properly is 2-10x longer than a regular MAF tune. That ultimate control over VE also means that the person pulling the strings BETTER have a good idea what they are doing. A bad VE cell in a MAF tune could be overlooked and "smoothed" over by the trims from the MAF/02 loop....a bad VE cell in an SD tune could mean pegging lean and detonating. If you were overlanding or going on Safari, where baro pressure and temp could swing wildly during operation (like HUGE), then SD would be at a disadvantage over the more flexible MAF setup.
TL;DR
SD can make more power and in some cases burn more efficiently, but is only a bridge worth crossing if you really need to....and the person at the helm needs time, patience, and a slightly higher-than-normal grip on the science going on to do so at peak effectiveness.
Now as for the E-Rod in specific; its tune will be great for the way it was sold, but it was sold as a package with the emissions stuff. It'll still have a VE table built for safety. Which ECM is coming with the E-Rod package? If its running the GenIV or newer stuff it may be using GMVE or Virtual VE (works the same, but way way more tables and a much more complicated multiplier being used). If thats the case, there are a few more possibilities for whats happening.