Mast M-90/M-120 vs. GM ECM

[67ragtp]

Originally Posted by PTAddict

I use the Mast M90 on my personal car (LS7). I've also tuned a lot of GM ECMs on late models and retrofits.

As to what is "better", it depends. From my personal perspective, I won't use an ECU which doesn't support knock sensing/control, and both of these support it. Both also support drive by wire throttle, if that's what you want. To oversimplify a bit, the biggest difference at a high level is that the GM ECU runs open loop at wide open throttle, relying on the MAF sensor to account for airflow variations. The Mast runs closed-loop all the time, using a wide-band O2 sensor. Wide band closed loop is the optimal strategy for a performance motor, IMO, because it can account for variations that a MAF sensor can't, like the oxygenation of the fuel. But in practice, both can work well in performance applications if carefully tuned.

On that careful tuning part: some ups and downs to both. The Mast is much simpler in terms of the number of tables and interactions you have to deal with, but the (free) tuning software lacks convenience features like table smoothing and interpolation. That makes data entry for fuel and spark tables quite tedious. The GM ECU must be tuned with a package like HPTuners, EFI Live, or SCT Advantage. All of these packages are significant extra cost ($700+) if you obtain them yourself; otherwise you rely on a professional tuning shop, which is recommended in any case if you're not up on EFI tuning. The GM ECUs have a significant learning curve to tune correctly, more so than the aftermarket alternatives.

Another alternative to consider is the latest Holley HP or Dominator EFI system. I've not had any experience with these on LSx motors, but have tuned a couple of retrofit systems using the HP ECU recently. It's a very nice combo of ECU and software.

I'd give you a brief warning about cost assumptions. Depending on your exact combo, by the time you get everything necessary to make each ECU work, the cost differential can end up lower than you might imagine.

All this said, unless you're planning to change cams on your crate motor, I'd personally probably lean to the GM ECU package. Make sure you get it tuned, though - I've tuned three GM crate motors with retrofit ECU package, and each of them was fairly far out of tune, particularly the MAF transfer function.

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

[PTAddict]

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

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.

[67ragtp]

Im in the novice territory with regards to understanding EFI, and trying to understand the technicalities. Im fairly certain that my speed density tune runs closed loop with the short term fuel trims active for idle/cruise normal driving and open loop during WOT relying on Volumetric efficiency tables and PE table. This stuff gets pretty complex, throw in HP tuners

Thanks for the info- Rich

[PTAddict]

Originally Posted by 67ragtp

Im in the novice territory with regards to understanding EFI, and trying to understand the technicalities. Im fairly certain that my speed density tune runs closed loop with the short term fuel trims active for idle/cruise normal driving and open loop during WOT relying on Volumetric efficiency tables and PE table. This stuff gets pretty complex, throw in HP tuners

Thanks for the info- Rich

Yup, you have it right, SD tunes typically run open loop only during PE mode (higher load). And yeah, factory ECUs are very sophisticated and complex now, and that complexity all comes through in the tuning software. Your garden-variety ECU would have been considered a supercomputer 20 years ago.

[BANKO]

PTAddict, I really appreciate all the feedback. It further confirms my assumption that the Mast setup would be a better overall system for a modified motor. Interesting that the GM retrofits were out of tune, I wouldn't have expected that. Thanks for spending the time to explain for us non-EFI tuners!