Quote:
Originally Posted by 67ragtp
Great info!
I just want to add the gm ecu can be set up to run a custom OS in a speed density mode, not using the MAF. The car can be dialed in using a wide band and run well on its narrow band O2's. There are tuners who have perfected this and I imagine it helps when running heavily modified engines. My cammed N/A ls7 with an e38 is setup this way, I believe it also uses a 2.5bar map.
Rich
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Yes, I've tuned a couple of cars this way when MAF was not an option for one reason or another. It doesn't actually require a custom tune to do so, although sometimes a custom tune that supports real-time updating can shorten the tuning time. But personally, I prefer not to use OLSD tuning - open loop speed density - unless there is no other option.
Some explanation for EFI novices: there are two strategies to determine how much air mass an engine is ingesting, and therefore how much fuel to supply. (Actually there's a third called Alpha/N, which we won't get into). A Mass Airflow Sensor directly
measures air mass into the engine. Speed density depends on the ECU to
predict how much air mass an engine is taking in at a given time, using RPM, Manifold Air Pressure, and Intake Air Temp as primary inputs - speed = RPM, density = functionof(MAP,IAT), hence speed/density. From there, the tuner models the engine's breathing (volumetric efficiency) throughout the RPM and MAP operating range.
OEM applications almost all use MAF as the primary strategy, augmenting with speed density for transient cases. Aftermarket systems almost all use speed density. But almost all aftermarket systems also use wideband O2 sensors and run closed-loop all the time, thereby providing an automatic correction - a safety net - for any minor errors in the speed/density prediction. That's important from my perspective, because truly accurate predictive modeling of air mass is quite challenging.
For instance, the IAT sensor is typically placed upstream of the throttle body. By the time air reaches the cylinder, it will have been warmed somewhat by the intake manifold. How much it gets warmed will depend on the manifold temperature, which in turn depends on the underhood configuration, and how much airflow is passing over the intake. So the OEM GM applications actually have a complex configuration-specific function which adjusts the predicted airmass by up to several percent based upon the car's recent history of road speed! It boggles the mind how much work must go into creating a factory tune.
So I personally prefer to use tuning strategies which account for conditions outside the day on the chassis dyno. In practice, though, tuners tend to be smart, opinionated people who get results in a number of ways
And OLSD tuning certainly gets used successfully. After all, carburetors can be thought of as crude OLSD devices with bad temperature compensation
and those have worked OK for a hundred years or so.