Scott Jackson inspired a thought.
I read the thread by Scott Jackson. He discussed using a bypass tube around the air flow meter and using different sized injectors to spoof the ecm and a possible mechanical device to change the relationship of the crank sensor to retard the timing. He then reposted and reported VW guys talking about letting the knock sensor handle the retard. (That sounds funny.) One of the goals was to not "peg" the air flow meter.
This inspired some thoughts. First timing retard. A mechanical device would be clumsy and prone to wearing out. An electronic device which would do the same thing is very doable. I'm not sure I can do it. But it may have already been done. MSD makes a device which retards timing if you have a distributor. I wonder if the same device would work on the crank position signal. You have a signal going in, the manifold pressure goes over 1 atmosphere and the device starts retarding the signal. All is well. The concept of making one isn't that hard. You need a 2 BAR sensor, when the signal raises above 1 BAR, it starts retarding. A simple linear ramp would do. If you used a simple microprocessor, you could have a fancier curve.
As to pegging the MAS sensor. Who cares. You will run out of injector duration around one BAR anyway. If you go too big on the injectors, even if you are spoofing the ecm, the idle will be ruined. A friend of mine tells me the fastest car he was ever in was a supercharged Hemi '67 Chrysler. It used two Hobbs switches and electric fuel pumps for enrichment on boost. You can do the same thing with some Hobbs switches, the methane, or E85, or some NOX solenoids and propane.
So, we run the stock injectors, or maybe one size up (the computer could probably adjust for that), use some device to spoof the crank sensor into retarding the timing, and have a non-computerized enrichment system to handle the extra fuel while the boost is on.
I like it.
Thanks Scott Jackson.Joe B
'70 RS/SS Camaro '89 325i '94 525i '04 Rainier
Re: Scott Jackson inspired a thought.
I'd just rely on the knock sensors and listen for any detonation. If it don't ping and the plugs look good, it should be fine. As for a simple mechanical fuel enrichment under boost, the old style simple method is to simply plumb in an extra injector or two into the fuel line and have them squirt into the intake pipe. They are simply triggered to open (and hold open) with 12V by a hobbs switch. I suppose you could get one switch to turn on one injector at maybe 2-3psi of boost and then another injector hooked to a hobbs switch that turns on that injector at 5psi or whatever.
It's not a very accurate way of doing it, which was why I liked the larger MAF with bigger injectors idea. I'd just toss a Megasquirt setup on mine if I didn't have the auto trans. The last couple days I've been measuring and figuring up all I need to do in order to just drop in my ford 351C. I know it's an "archaic domestic POS engine" but it's hard to argue against a warmed over 4V 351C once you've been in a car powered by one. So my turbo project has been pushed back on the 540i as I've acquired another farm truck ('90 F150 4x4) and am about done with a 351W for another truck I bought ('91 F250 4x4). So many projects, so little money, so little help...