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Resistance is futile Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Thu, 25 Jun 2009 04:02:20
In Reply to: 87 9000T AMM and timing for Performance and Ca Smog, Waveman, Thu, 25 Jun 2009 00:31:50 Members do not see ads below this line. - Help Keep This Site Online - Signup |
I never thought that would apply here, but it does.
The resistance of the AMM means basically nothing. The AMM outputs a voltage proportional to airflow, which is read by the LH. Adjusting the AMM changes the gain - how many volts per pounds of airflow per minute. OK, what's the right gain?
In a normal, unmodified engine, you should adjust the AMM so that at idle, the mixture is at the stochiometric ratio of 14.7:1. Monitor the O2 sensor output - it normally swings between 0.1 volts (lean) to 0.9 volts (rich). 0.5 volts is the 'perfect' fuel ratio. However, to make a 3-way cat work, the LH computer actually swings the ratio between slightly lean and slightly rich, about 1 a second.
So, monitor the output of the O2 sensor with a meter. Have the engine at idle, and fix all vacuum leaks you can find. You want to adjust the AMM so that the O2 sensor output swings between about 0.9 and 0.1, back and forth, about 1 per second. Ideally, spending 50% of the time above 0.5 volts, 50% below. But getting the signal off the 'stops' and swinging back and forth, about centered, is just fine. If you use a digital voltmeter, it can be hard to get it just right.
OK, so what's the magic of 380 ohms? That is just a starting point. The designers did the math, and for almost all the cars out there, if you set it to 380 ohms, it's 'good enough' that the car will at least start and run. THEN you can adjust it. You can't adjust the AMM if the car won't run. Whatever resistance you end up with is the right resistance, be it 100 ohms or 700 ohms.
That should give you the right mixture at idle. This adjustment allows you to null out various AMM differences, engine differences, etc. Set the timing at stock (16 degress BTDC, I think.)
OK, how about smog testing? If they just do a smog test at idle, you should be OK, assuming the cat is OK, you don't have any hot spots in the cylinders (creates NOx), are burning oil, etc.
OK, how about above idle? All bets are off. You've got fuel flow to beat the band, and I'm sure the LH computer is hanging on for its very life. Under acceleration, the LH computer doesn't use the O2 sensor, but uses maps of engine speed, throttle, and airflow to schedule fuel flow. In general, to run higher performance, you want to run a little rich. But for emissions, you want to be right on, not to rich (high HC) or too lean (high NOx). To pass smog, you might want to make the AMM run just a little on the lean side, because your set-up is probably running rich - high fuel pressure, big injectors, etc. To run lean, you go to a slightly lower resistance. This causes the AMM to output less voltage for a given airflow, fooling the LH into thinking there is less air than there really is, so there is less fuel. But obviously, that screws up the idle setting.
When it comes to performance, you want to be running a little rich and as advanced in timing as you can before knocking. For smog, you want the timing spot on, maybe a hair retarded, and a nominal mixture.
I'd say your best bet is to find a shop with an exhaust analyzer and play. You'll need to know at what points the CA test is run. I suspect you'd be hard pressed to get anywhere near the performance the engine is capable of, and still get anywhere near the emission standards, all with the same settings. Heck, getting a 22 year old car in good shape unmodified to pass emissions is a challenge - you're dumping in more fuel pulling away from a stop sign than a Smart car uses in a week.
The LH 2.2 isn't particularly sophisticated. It can close the loop around the O2 sensor, but only at steady state, such as idle or constant speed. The rest of the time the system is Open Loop - it measures airflow, and based on engine speed and temperature it guesses at fuel flow. And it sets that fuel flow by adjusting how long the injectors are open. The amount of fuel going in is going to be a function of the flow capacity of the injectors (a lot), and the pressure difference between the intake pressure (boost level) and FPR output. So when you crank up the FPR, you get more fuel for a given injector opening. But remember, the maps in the LH were based on stock injectors and stock pressure, so when you go open loop, you get A LOT more fuel than stock. You've got an intake and exhaust system to provide A LOT more air, but I'll bet it doesn't match up great.
Again, for ball park, adjust the AMM to get the mixture right at idle - that's about all you can do. Beyond that, find someone with an exhaust gas analyzer.
posted by 192.249.47...
, Thu, 25 Jun 2009 04:02:20 <-- Viewing This Message
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