# ATS-V LF4 Boost and Baro

I have been thinking about boost and barometric pressure, as what I see in the scans confuses me. Complicating the issue is source of the signal, meaning there are multiple sources of related information, and the poll rate or how often that source updates.

Basic Issue snapshot:

At a specific scan snapshot I see this:

Boost Math 17.7 — this is a parameter in hptuners math that takes MAP minus Baro
Boost / Vacuum 18.7 psi — this is a sensor that compares total boost to vacuum (baro)
Absolute boost (not referenced to baro) 32.7 psi
MAP 31.6 psi
Baro (SAE) 13.49 psi
Baro (mfr) 14.43 psi
Baro (SAE) chart update 13.69 psi

First, let me say I love the detail that we can see in the scanner.

I do get how each of these numbers should relate — for example, absolute boost minus barometer should be what Boost/Vacuum is; that’s the definition. But the math does not quite work. There are 3 baro readings, and they differ slightly, so there’s that. But none of them fit perfectly. There are 2 total boost readings, MAP and Boost Total, so there’s that, but neither of them fit perfectly with any of the 3 baro readings to result in the parameters shown.

Example: Abs Boost – Baro (sae) = 32.7 – 13.49 = 19.21 psi

2348 Boost Math says it uses 50032, the Generic Sensor for Baro, and 50030 the MAP.
Boost Math scans at 17.84 psi. 50032 Baro reads 14.06 psi in that instant. The MAP reads 31.90 in that instant. 31.90 – 14.06 = 17.84, so that works but depends on recognizing the EXACT baro and MAP used in the instant. So Boost Math is Manifold Absolute pressure minus the current (instant) baro reading.

The “Baro SAE” on the hptuners chart at that instant reads 13.49. But when I ‘chart’ baro sae it shows the 14.06 psi at that instant. I suspect the difference is the refresh rate for each.

Boost/vacuum in the chart at that instant reads 18.75 psi, which is almost 1 psi higher. In the channel list it shows at 18.34 psi, which again I suspect is refresh rate. Notionally it is reading a similar thing — total MAP boost minus baro. MAP at that instant (chart) is 31.9 psi versus the channel shows MAP 31.6 psi. We know the source is not the same as boost math, since they differ. So what are boost vacuum’s source values? I observe that Boost Pressure sensor at that instant is 32.84 psi; if we subtract the chart instant baro of 14.06 we get 18.78, which gets us close to 18.75 psi, although not exact. So Boost/Vacuum appears to be the Absolute Boost sensor minus the current (instant) baro reading.

SO which is more accurate, Boost PSI (boost math) or Boost/Vaccum? The difference appears to be the difference between the 32.84 psi Boost Sensor and the 31.9 psi Manifold Absolute pressure. Why do these two sensors for notionally the same thing vary, and which is ‘more accurate’? My intuition is since there IS a Boost sensor, to use it for boost. That makes the Boost/Vacuum the “correct” boost versus the Boost Math.

Changes: Overspeed limit max increased from .5 to 1. Waste gate open boost increased 2 pts. Peak torque and max torque redone as 1.2x stock tunes. Fast torque exit set to 50/25. Boost max limit added 1. Increased knock airmass. Further reduced main spark map, high octane to address KR. Transmission: increased TCC desired pressures, regulator gain, regulator offset, reset TCC desired slip to match Corvette, changed TCC adapt torque min. Disabled TCC in gears 2,3,4.

Conditions: 81F, engine Baro 12.235 inHg 74% humidity; correction 1.037 SAE J1439.
Output: Measured max 466.7 hp at 5315 rpm, 467.4 Ft lbs torque at 5198 rpm so
Corrected 484.0 hp at 5315 rpm and 484.7 ft-lbs torque at 5206 rpm.

Goals: Eliminate KR. Reduce TCC slip rpm. Reduce throttle opening.

Results: KR was reduced at WOT pressures to under 0.5 (but not eliminated); good progress. TCC still showed slip even though TCC was not applied in gears 1-4. Unclear if that helped or not; reset to stock for next tune. Still throttle closing; going to try Sam’s driver demand map which further increases driver demand, and makes more linear. Keeping an eye on misfires; going to go a bit richer next tune, from 12.7 to 12.5 AFR (14.113/12.5 = 1.12904 PE value). Fuel pressure holding fine so far. Boost rose to 19.2 PSI then fell to 18.5 before the throttle opened around 5800 rpm; not sure why. Still seeing a flat HP curve from 5300 rpm to 6200 RPM instead of a steady increase; not sure why.

Thoughts: reduction in spark to reduce KR at high pressures may be limiting hp at high rpm. Consider a richer PE and attempting to restore spark and still avoid KR or misfire.

# Cadillac ATS-V LF4 Tune E1

Purpose: Keep the throttle closed again after boost ramp up. Eliminate TCC slip. Study KR vs PE by going richer, so closer to tune c2.

Plan: Raise PE to 12.7 to try to address KR; no change to spark map for this run. Address TUTD shift RPM to try to enable automatic shift up in 1/2/3/4 not 5/6/7/8 so can hold those if needed for dyno. Adjust Turbo Knock Mass to raise desired boost level to try to close throttle. Study scanner with car to understand missing channels. Modify TCC pressure apply ramp to reduce TCC slip. Rescan.

Conditions: 76F, 29.83 baro, 87% humidity. SAE J1349 correction 1.0052575076413.

Data:
2nd gear 456.8 hp @ 5838 rpm / 458 torque @ 5126 RPM with 17.4 psi boost at peak hp
3rd gear 464.8 hp at 5320 rpm / 469.5 torque @ 4660 RPM; with 19.4 psi boost at peak hp
Corrected 3 gear: 467.2 hp / 471.9 lb-ft torque

Result: Still getting KR at WOT at 12.7:1 PE and current spark map.
Throttle opened at 5718 RPM in at 71 mph on one test, stayed closed in another. Desired boost just before 34.5, actual boost 34.4 psi. Then desired boost falls off and throttle opens. 2.3 degrees KR showing at that point. MAT 133F. Boost 19.1 psi. Cyl airmass 1.34. fine on boost max limit but pinched on knock airmass. So further knock airmass adjust?

Comparative results from last 3 scans on TCC slip error; still needs attention. For the gallery images, right-click then select view in new tab to see full page versions.

Missing parameters: Connected to vehicle, then repolled vehicle for parameters. Now everything is showing again. I think this happened after I changed from stable release to beta release, but beta release is seeing all the parameters after repolling.