Idea for 2 bar MAP sensor hack

[turbo_bu]

Disclaimer - doing this will result in having to scale portions of the tune which will mean that things will be come relative. IOW - certain airflow / MAP values will no longer line up with what is actually going on with the engine. But, since we have scaled / accounted for that, you can still tune it.

I recently started to play with the MAP sensor calibration after PeteS comments about how to manipulate these values to get different MAP curves (a big thanks to PeteS for providing the spark to do a little playing with my bench 0411 setup!). Anyway, the main reason you would do this is for a boosted setup without having to resort to a custom OS. If you have either EFI Live or HP Tuners, then using one of their custom OS's provides a much nicer solution. BUT, if you don't have those, or wanted to try something on your own, then this will work.

The basis for this comes from the old school 2 bar MAP hack. Back in the old LT1 days (old school LT1 w/ opti spark circa mid 1990's), there really weren't any custom OS's available. For those who wanted to do boosted setups, the go to was a 2 bar MAP hack. This consists of installing a 2 bar MAP sensor and then going through the tune and shifting everything downward. Now anything from 0-50 kPa was really 0-100 kPa. The bonus was that anything about 50 kPa was now really 100-200 kPa. The main downfall of this was that your resolution was cut in half. Trying to tune the lower RPM / MAP cells (i.e. - your driving around stuff) was a little more of a challenge since your had less cells to work with. In the LT1 world, both the fuel and spark were done using MAP vs. RPM tables.

Now, for the LS PCM, you can still do the same 2 bar MAP hack. Only real difference is that your spark is controlled by airflow vs. RPM, but airflow is calculted based on MAP. So in essecence its still similar. The part that I started to play with was how to get the MAP readings to use up more of the cells available in the tables. Unfortunately, this is where it gets somewhat confusing. The trick to keep everything straight is to look at MAP output voltage!

Stock VE table has MAP values from 15-105 kPa. Using a 2 bar MAP sensor would result in the 50 kPa cell being equal to 100 kPa. Anything above 50 kPa is boosted (up to 15 psi), and anything below 50 kPa is vaccuum. Unfortunately, if we have a larger camshaft, then we might only idle with a MAP reading of 45-50 kPa. This would equate to the ~25 kPa cell with the 2 bar MAP sensor. So from idle up to boosted would be a range of cells from 25 kPa through the 50 kPa. As you can see there are not a lot of cells available in this region. But what if we could slide down / scale the MAP value that the PCM "calculates".

Again, the key is to watch the MAP sensor voltage! The sensor will only put out a certain voltage based on the intake manifold pressure. Using the stock MAP scalar and offset values, 50 kPa = ~2.1 volts and 100 kPa = ~4.75 volts. Replacing the 1 bar with a 2 bar MAP sensor will drop the output voltage by 1/2. So, 50 kPa = ~1.05 volts and 100 kPa = ~2.38 volts.

Keeping the stock 1 bar MAP scalar and offset values with the 2 bar MAP sensor shows: 1.05 volts = ~30 kPa and 2.38 volts = ~55 kPa. This means that the PCM will use the ~30 kPa and ~55 kPa cells for idle up to boosted [6 cells in the VE table]. Going into boost will use the remaining cells (60 kPa through 105 kPa) to represent the 0-15 psi cells [10 cells in the VE table].

MAP_stock_settings.png (13.57 KiB) Viewed 4537 times

Ideally, we would want to adjust the # of cells so that more cells are available from idle up to boost with the trade off of having less cells in the 0-15 psi region. You can do this by adjusting the MAP scalar and offset values. I have played with these values and gotten it so that there would be ~8 or 9 cells for idle up to boost and ~9 or 10 cells for 0-15 psi. But you can play with these values to bias more or less to one region of the other. Making a spreadsheet goes a long way to being able to visualize the MAP value the PCM calculates based on different scalar/offsets.

Green cells are idle up to boost, Blue cells are the boosted portion.

MAP_2bar_settings.png (12.03 KiB) Viewed 4537 times

[turbo_bu]

One thing I forgot to explain, the PCM can only "read" up to 105 kPa. This becomes important because as you can see on the MAP vs. voltage table, that after a certain MAP voltage, the PCM MAP value will stop increasing. It will max out at 105 kPa. Under the alternater 150 / -10 column, this occurs at ~3.8 volts (~155 kPa or ~8 psi). So the cells that will work for boosted cover from 100 to 155 kPa. After 155 kPa, the PCM MAP reading will be stuck at 105 kPa. For these areas of operating, you will be on the last column of the VE table and will be making adjustments based on RPM.

[turbo_bu]

If you have followed along so far, the last piece of the puzzle is making sure your using the most of the timing tables. The spark tables are airflow (grams/cyl) vs. RPM. Airflow gets calculated using the %VE. If we simply swapped in the 2 bar sensor, you would have to reduce VE by 1/2. Since you dropped VE by 1/2, you will also need to reduce the injector size by 1/2. Unfortunately, this is where it gets tricky.... We did swap in the 2 bar sensor, but we tweaked the MAP scalar / offset to use more of the VE table. The amount we tweaked it is what we need to scale the VE / IFR by. Example: scalar = 150, the ratio is 150/94.4 = 1.6 ... 1 over 1.6 = 0.62 or approximately 2/3rds. Multiply the VE by 2/3rds and the IFR by 2/3rds. This will get you somewhat close, and then you can see how much the airflow (grams/cyl) calculates out to be. Ideally, at max boost / RPM, the calculated airflow should be close to 1.2 grams/cyl. If it is low, then only scale by say 3/4 instead of 2/3rds. As with any of this hack, it is a balancing act and requires some iteration to get it dialed in. But the benefit is that you can use all of the cells in the tables and have a boosted setup using the stock OS.

[Jim Blackwood]

Sorry it's taken me this long to reply, but this is exactly what I am trying to do. I may have made some mistakes. On the Main VE Table: Going into the XDF Parameters/Rows I changed the row values to a range of 20 to 170 but if what you say is correct it means that the OS will truncate my table at 105. This could be a problem. I don't expect to be into high boost, probably no more than 5 or 6 psi, 7 at the most. I also changed the MAP sensor scalar (C6301) to 188.87 and the MAP sensor offset (C6302) to 20.66, thinking that this would correct the OS readings of the values on the VE table. I went through the other settings (scalars and tables) to match up anything else that used MAP.

I haven't done anything with the Spark High Octane Table (B5913) yet. The calibration in grams/cyl kinda threw me off.

Jim

[NSFW]

I'd like to think that this can be done in a way where that 105 limit is a non-issue.

For example, if the old MAP sensor put out 0-to-5 volts to represent 0-to-105 kpa, and the new MAP sensor puts out 0-to-5 to represent 0-to-210, then...

You'd change the XDF to double the value in the VE table's MAP headers.

The PCM's old 105kpa limit becomes a 210kpa limit.

Any PIDs that were logging MAP would just need to be rescaled to show 2x what they used to.

And any other tables with a MAP axis would need to be rescaled as well. Most of them show up in the XDF files, but there are a probably a couple that don't. That's where IDA / Ghidra would come in handy, to figure out what tables we're missing, and what they're for, and whether they even matter.

[Jim Blackwood]

That is what I am attempting to do by doubling the MAP scalar and MAP offset but I don't know if it will work or not. I am probably another week or two away from starting the car so I can't test it. I could maybe go through the list and copy down all of the scalars and tables that I have changed to match.

Jim

[turbo_bu]

I'd like to think that this can be done in a way where that 105 limit is a non-issue.

For example, if the old MAP sensor put out 0-to-5 volts to represent 0-to-105 kpa, and the new MAP sensor puts out 0-to-5 to represent 0-to-210, then...

You'd change the XDF to double the value in the VE table's MAP headers.

The PCM's old 105kpa limit becomes a 210kpa limit.

Any PIDs that were logging MAP would just need to be rescaled to show 2x what they used to.

And any other tables with a MAP axis would need to be rescaled as well. Most of them show up in the XDF files, but there are a probably a couple that don't. That's where IDA / Ghidra would come in handy, to figure out what tables we're missing, and what they're for, and whether they even matter.

NSFW - the issue is that in the OS the PCM calculates the MAP value from the MAP sensor voltage. Then it does a rationality check (compares it for negative readings and above 105 kPa), and then truncates the reading if it is out of range. This is why the PCM in theory can only "read" 105 kPa. You can play with the XDF to show MAP readings that are higher, but the PCM only knows what to do with values up to 105 kPa.

[NSFW]

What I'm proposing is that we lie to the PCM about what the MAP sensor is indicating. Let the PCM "think" MAP is still just 0-100ish when it's really 0-200ish. The PCM's limit then becomes 210kpa.

You might look at it as changing the PCM's units of measurement. Right now it's KPA. We'll change the units to 2xKPA by changing the MAP sensor and scaling. The old 105 maximum becomes 210 in 2xKPA units. Everything that worked in KPA units now works in 2xKPA units, so tables need to be rescaled accordingly.

[delcowizzid]

You may need to disable baro updates or fueling will change when you hit boost and it does a baro update. probably gonna spit the dummy at sn unrealistic key on engine off baro reading too. We used to do the same mod on the 808s that vl400 made 12p for. Add 2 bar relabel the map colomns fudge the Injector rate for headroom In the VE table. Most people that used that method just hurt engines even 1 bolt on supercharger company sold tunes done that way cause they didn't want to pay for kalmaker hahaha

[Jim Blackwood]

>> "If Its Got Gas Or Ass Count Me In.if it cant be fixed with a hammer you have an electrical problem"

Hmm.. guess my wife has an electrical problem then... Always wondered about that.

I'm not really sure what to do at this point, but I guess as long as I stay out of boost it might be OK? Wonder what doubling the MAP sensor scalar and offset will do to that? Guess I should just see if I can fire it up and find out what happens. The stock timing map looks fairly close. I want to cut it back to in the 34-38 rang on total advance, drop it back in boost, and raise it a little below idle rpm so I think I'll do that first.

The Grams/Cyl axis throws me off a bit though as I'm used to seeing MAP values there and have no real way of knowing where 100KP/a falls on the axis. Guess I should get started on that. Then I'll go through and make a list of the changes I've made related to the MAP 2bar sensor and post them here.

Jim