Join Date: Nov 2000
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Re: TiminMB MeagSquirt data
If you are buying a new 02 sensor, the heated version would be a good idea. I had a practically unused jeep (unheated) sensor lying around so I used it. I left megasquirt at the default setting of no 02 correction until 1200 rpms. With the heated sensor, there is no reason you couldn't lower that value to correct the mixture at idle. My 02 sensor reads rich at idle. How rich, you really can't tell accurately with a narrow band sensor. The standard 02 sensor hits the reference voltage of around 0.45 volts at 14.7 to 1 air to fuel mixture and flutters back and forth when you are near that value. It's non-linear and unpredictable in reading beyond that, so in essence you can tell if its 14.7 to 1 (stoichiometric mixture, in other words chemically balanced for complete combustion to C02 and H20) rich or lean,ie just 3 known conditions. Ok, I'm wondering off topic. If you are buying a new one, you'd might as well spend a few more bucks and get the heated one and see if it works better. It will read more accurately during periods of low temperature exhaust, and will more quickly begin to give usefull info after starting a cold motor. The unheated one, however, doesn't seem to be impairing my performance either. So, take your pick. What I wanted to comment was that my mixture reads rich at idle. I adjusted my idle mixture to get the best lean idle, just like using a carb. If I set it by the unheated 02 meter at idle, and adjust it to 14.7 to 1, the motor doesn't idle as smooth as when I add in a little more fuel. Whether my 02 reading is accurate at that point or not is an important question. Or does a 4.2 with it's varying length of intake runners need a slightly rich mixture at a central source to acheive the minimum a/f ratio needed for the furthest cylinders? Multiport overcomes this dilemma.
For the temperature sensors, I'm using the ones I pulled off the 89 parts car. The CTS is a standard GM sensor with a 3/8 npt thread. I screwed it into the drivers side of the block in place of one of the old CTO switches.
For air temperature, I am using the GM open element sensor I got from the parts car. It's the one that has two exposed wires that form a little shell around the thermister. This is the type of sensor found in the air cleaner assembly, outside of the filter element. I've heard they can disentegrate, so don't put it inside the manifold. There are also GM closed element sensor, similar to the CTS sensor that can safely be placed in the manifold. I believe job of this sensor is to correct for temperature related air density changes. The ideal gas law formula, from Boyle was PV=NRT. In other words the molecular quantity of a gas is proportional to volume + pressure and inversely proportional to temperature. So the hotter a gas is, despite it's pressure, the less molecular quanity there is. So for a turbocharged engine, putting the sensor in the manifold is necessary, as the air becomes heated by the turbo compressing it, and cooled by the intercooler. That's a lot of temp changes. For a normally aspirated engine, you just need to know what temp of air is entering the throttle body. I placed mine just on the outside of my K & N open element filter. Would it have made a difference if I placed it inside the intake manifold. Possibly, as I am using a heated intake manifold. Once again, you are open to experiment. My setup however seems to be working well.
You could order the two temp sensors based on an 89 chevy caprice 305 v8 and you'd have the same two I am using.
Here's some info I've gathered from other users of Megasquirt as to the GM sensors they are using:
Coolant temperature sensor (CTS) GM #12146312
GP SORENSEN TSU81
AC DELCO 213-310
GM intake air temperature sensor (IAT) GM #25036979
GP SORENSEN 779-19001
NIEHOFF IGNITION DR-136W ;
I don't know if the above IAT sensor is the open element type or not. They both read the same values though and could be interchanged by you. The open element sensor has the advantage of responding more quickly to temp changes.