How does the PCM calculate horsepower?

[sandrock]

On the newer PCMs (We'll say after 1997), there are some calculations that you can access via a scanner or a capable scangauge. The scangauge II can, if the PCM is programmed to monitor it, display that data. I have programmed my SG2 to monitor HP, Torque, trans fluid temp, and actual timing. The 2005 Impala PCM that's in my SSE can display both HP and torque, the 2006 GP and the 08 Impala SS only displays the HP.

So far, HP numbers on both the GP and the SSE are about equal, showing about 240 horses at WOT in 1st, give or take about 5. Torque numbers are close too. But the SS?? When cold, WOT in 1st got me 408 hp today...once it warmed up, it cut it back to 288 on average, though a few times I could get 305-310 hp.

So...I know the SG2 doesn't have accelerometers or anything like that...it just takes the data coming from the PCM and displays what it reads. What I'm wondering is just how the PCM calculates the hp/torque numbers, and if anyone else with an SG2 thinks their numbers are on the money, or if they seem a bit exaggerated.

[WhiteArrow3800]

I'm curious about this one as well.

Brad, You remember looking over mine last year? 614 ft.lb. FTW!
Literally double what the actual number likely is. It's odd though, the HP numbers I get aren't that ridiculous at all, they're around 300, which is reasonable.

[J Wikoff]

Commanded fuel injection rate and an assumed heat rejection are probably all that go into it.

[sandrock]

I just read up on one of the forums that SG2 takes data and turns it into gross HP, and it's indeed fairly accurate. Figuring 20% parasitic loss, the 240 HP turns to 192, the 408 turns into 326.4 HP...fairly reasonable numbers at the wheels.

[95naSTA]

My pcm also calculates this. I've logged it via EFI live and also wondered about it.

[Larz01]

It doesn't. Dynometers do. Pcm just calculate torque steps.

[95naSTA]

What are the torque steps? And hp is only a *rpm/5252 away.

[Larz01]

What are the torque steps?

I do not know how the pcm calculates them but it's used in torque management to determine; timing, traction control, up/down shifts.

[mtcrss2006]

I've had some SG2 issues like that. What did it for me was when I told the gauge I filled up and it said 2 gallons, and I assumed it meant how many I put in so I went up to the number on the gas pump, and the HP number went up to around 800. I now believe it means how much was left? I don't really know and haven't had time to read the manual and play with it. I just reverted to default settings and never tell it when I fill up. Now my HP is accurate :-P

Threadjack awareness:
The main issue I have with the ScanGaugeII is that it tells me my KR is crazy... sometimes 4, sometimes 62, sometimes 39... And IIRC, the L67's PCM can only do a max or 25.5* of KR. I have rechecked the numbers programmed in for the Xgauge and they are all right. WTF
/rant

[sandrock]

That's because of a glitch. If it reads 25, it's supposed to be 2.5.