[Gmecm] Example: Simulating the "power valve" in the ECU for PE

[Rick McLeod]

Very interesting table!

Do you happen to have a log graph of some various accelerations of?MPH vs. TPS 
vs. PE ?

My curiosity is what the TPS looks like on acceleratoins from stop, I'd be 
expecting it to drop to PE for a short instant on a loaded pull away from 
a?stop, then come out as I tend to drive (I think, I've never graphed my own 
here) farily constant?foot pressure,?if I'm gonna pull away hard I push down and 
hold, not so much a gradual push to a pont and?hold, so this may be a good 
example of a tune to your foot?

You have me curious now, I'm gonna go back to my log data files and see if I 
have the necessary data points to graph my own foot out.

Great discussion, so glad I got hooked back up to the list, I was missing 
something in my life. Thanks
?
-rick




________________________________
From: Butler James-CJB006 <jim.butler at nsn.com>
To: c.p.e at comcast.net; gmecm at diy-efi.org
Sent: Tue, September 13, 2011 12:45:36 PM
Subject: [Gmecm] Example: Simulating the "power valve" in the ECU for PE


David, et al., just for reference/conversation, I?ll show the changes that I 
made to my stock 1998 L31 5.7L (Suburban) ECU?s ?Power Enrichment Enable % TPS 
Vs RPM? table.
?
In the table, the values were formerly 89.8 everywhere.?? Based on data I logged 
from the ECU/engine while operating in a variety of scenarios, and subsequent 
post-examination of those logs, I modified the TPS entries at and above 1600RPM 
to enable PE at about 27?Hg MAP (I?m in Illinois so my MAP peaks around 30?Hg). 
??Then, after programming the ECU with this change, I validated that PE was 
indeed being effected at approximately the target MAP.
?
I of course made other, typical changes to the ECU?s parameters to enable PE 
without delay; I?m sure most of you know I mean.
?
Clearly these values could not be used as-is on another platform other than a 
stock L31. ?I post these only for illustration.? Some might find these values to 
be much smaller (e.g., 43% TPS at 1600RPM) than they would have expected.
?
?
I can discuss the method of obtaining the TPS values I used in table, i.e., via 
post-examination of the logs, if someone?s interested; it wasn?t rocket science.
?
I?ll note that the ECU does not enter PE often, even with these new values 
(based on logging I?ve done while I?m driving, without a trailer or an atypical 
load on the vehicle), but now PE occurs at a much more relevant, and opportune, 
throttle position.
?
I?ve also made similar changes to my 1995 4.3L CPFI OBDI Blazer, with the same 
benefit.
?
David, any concerns you noted in your earlier posts on this topic regarding 
emissions, etc., are still valid, of course.
?
Jim
?
From:ext Command Performance Engineering [mailto:c.p.e at comcast.net] 
Sent: Monday, September 12, 2011 6:33 PM
To: Butler James-CJB006; gmecm at diy-efi.org
Subject: RE: [Gmecm] Simulating the "power valve" in the ECU for PE
?
Hi Jim,
?
Your words were well put.? I do agree with you.? As you well know, EFI is 
supposed to mimic carburation, but do it better in many ways, including a very 
important one to compensate for elevation changes.? Anyway, a point well taken 
is that GM never incorporated a Power Valve table into the caldron on tunables 
in the ECM.? To deal with transient throttle increases we have the pump shot 
tables to work with and there is the one table that does reference the MAP 
signal to provide a tunable enrichment control that is vaguely related to power 
valve functionality even though it is brief.? Fortunately for us, we have a wide 
degree of control over the amount of fuel enrichment during PE mode tuning that 
the carbureted guys will never have.
?
I guess that ever since I started working with the GM EFI long ago, I have 
learned to tune the PE Thresholds as they are, based on RPM not vacuum? tuned 
together with the normal PE Fuel Enrichment table.? Of course, it is important 
to tune all of your Open Loop and Closed Loop scenarios first before tuning the 
PE portion, due to the fact that when the engine crosses over the threshold into 
PE mode and not at 100% WOT, the calibration program always does a lookup to 
reference the C/L VE Fuel Control table(s) to use as a way to factor an 
appropriate amount of fuel to deliver for any given RPM, referenced to MAP in 
the VE table(s) together with the PE table for that same RPM.
?
Hopefully I explained things better this time.? Thanks for your comments.
?
David Johnson
?
From:Butler James-CJB006 [mailto:jim.butler at nsn.com] 
Sent: Monday, September 12, 2011 4:09 PM
To: c.p.e at comcast.net; gmecm at diy-efi.org
Subject: RE: [Gmecm] Simulating the "power valve" in the ECU for PE
?
By the way, I?m not ?pontificating?, below, but rather I?m testing my own 
understanding and challenging others to ?set me straight? where I run amiss!
?
Jim
?
From:Butler James-CJB006 
Sent: Monday, September 12, 2011 4:51 PM
To: c.p.e at comcast.net; gmecm at diy-efi.org
Cc: Butler James-CJB006
Subject: RE: [Gmecm] Simulating the "power valve" in the ECU for PE
?
David, I?m looking at this now.
?
I?m a tad confused. ??I?ll set the baseline of my understanding, below.
?
Transient fueling needs to mitigate temporary lean conditions which occur during 
changes in throttle position, e.g., at least,
?
????????? to accommodate wet-flow fuel transit latency (as might be more 
prevalent in a TBI setup) relative to throttle tip-in, or 

????????? to fuel any brief, non-steady-state inrush of air into the manifold 
volume 

?
are, as I understand them, accommodated in the carburetor by the accelerator 
pump.?? I?ve assumed transient fueling is accommodated in the GM OBDI FI systems 
through the ?Pump shot? tables (if, of course, we believe that those tables are 
properly labeled according to their function!)
?
Power enrichment, e.g., to achieve the maximum torque output from the engine by 
increasing the AFR, is accommodated in the carburetor by a different mechanism 
which increases the AFR in the steady-state air flow.?? Holley, for one, uses 
the ?power valve?; another, Edelbrock (with their Carter ?AFB? design), uses 
stepped metering rods and a set of pistons which respond to the pressure 
differential between the atmosphere and the manifold (aka ?vacuum?).? In FI 
systems I?ve assumed this is accommodated by the various ?PE? tables.
?
So ?pump shot? applies to transient events and ?power enrichment? applies to 
stead-steady events.? 

?
Are we aligned on this?? A simple yes/no will suffice.? I?ll follow with more 
comments, later.
?
<thanks!>
?
Jim
?
From:ext Command Performance Engineering [mailto:c.p.e at comcast.net] 
Sent: Monday, September 12, 2011 9:48 AM
To: Butler James-CJB006; gmecm at diy-efi.org
Subject: RE: [Gmecm] Simulating the "power valve" in the ECU for PE
?
I?ve tuned more of these ECM?s than I can count in the past 15+ years.? While I 
do bump the TPS Thresholds down somewhat to enable PE as various RPM levels, 
there are two tables in these computers that deal with Acceleration Enrichment 
(AE), simulating the accelerator pump and the power valve in carbureted engine 
configurations.
?
The accelerator pump function is found in the Pump Shot Vs. TPS table, while the 
power valve function is called the Pump Shot Vs. MAP table.? Both tables work 
hand-in-hand and deal with one area of AE more than another, but they do overlap 
somewhat and contribute to the whole picture of what is needed when you are 
accelerating.? These tables add a measured amount of extra fuel (by uSec).
?
The Pump Shot Vs. TPS table should be tuned while accelerating in first gear 
while you are pushing into the gas pedal, usually at a moderate rate.? Watch 
your o2 voltage reading along with your TPS position percentage.? If the o2 
sensor voltage plummets at one or more point(s) in the acceleration phase, just 
bump up the pump shot in that area.? You are looking to keep the o2 sensor 
reading in the 600 ? 800 mV range.? Less than this will be felt as a hesitation 
or bog, while more than 800 mV will cause the exhaust to go too rich resulting 
in failed emissions.? When I adjust the table values up or down to get the right 
feel in the acceleration (while watching the o2 readings) it is by small 
amounts, since this is a trial and error effort.?? Make too much of a change and 
you end up chasing your tail, leaning it out again
?
The Pump Shot Vs. MAP table (power valve) should be tuned similar to what I 
stated above, but while accelerating in usually a high (4th) gear, say 40 MPH 
and higher.? There are tuning breakpoints at 0, 20, 40, 60, & 80 kPa.? It is 
quite common to be cruising in the 40 ? 50 kPa range.? I rarely need to adjust 
the 0 ? 20 kPa areas, sometimes a little at 40 kPa (like 14 -28%), but major 
improvements can be made in the 60 and 80 kPa areas.? The last calibration I 
made last week for an ?87 Chevy ? ton 4x4 Truck with a HT383 engine and a stock 
TBI running 16 PSI of fuel pressure (to get 20% more fuel) ? I needed to 
increase the 40 kPa point by multiplying the stock value by 1.28, the 60 kPa 
point multiplying the stock value by 2.299, and the 80 kPa point by multiplying 
the stock value by 3.335.? That may seem like a lot, but in reality it amounts 
to only an additional 793 uSec of pump shot to the existing Base Pulse Width 
(BPW) at the 60 kPa level and an additional 1709 uSec (or 1.7 mS) of pump shot 
to the existing BPW at the 80 kPa level.? Pump shots do not last very long and 
are indeed considered momentary to cover transitions to increasing load levels, 
just as those components did on a carburetor.? I know that a power valve could 
actually stay open longer in high load situations, but the intent of a 4-barrel 
was to run the primaries lean and the secondaries rich.? So the secondaries were 
the main thing that you would tune for PE at WOT, coordinated with the right 
power valve vacuum value.
?
This kind of detailed tuning will make huge improvements in drivability in 
Closed Loop mode.? They will also help you tune the WOT (Wide Open Throttle) 
phase better, because the accelerator pump and power valve functionality are now 
working properly for you.? Tune the PE phase mainly at WOT (100% TPS) and watch 
your o2 readings to generally run in the 890 ? 900 mV range for TBI engines.? 
885 mV = 12.5:1 AFR.? Port EFI engines can run well at WOT in the 870 ? 880 mV 
range, closer to 13.0:1 AFR, depending on the severity of the load dynamics and 
non-boosted.
?
Once you have tuned the WOT phase, you can play around with the threshold levels 
at the various RPMs to obtain the desired transition from C/L operation into 
PE.? Just remember, if you adjust the threshold values down too far you will 
screw up your fuel economy and emissions.? It is normal to have higher threshold 
values at lower RPM?s and lower values in the upper RPM range.
?
I hope this information helps.
?
David Johnson
?
From:gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf Of 
Aaron
Sent: Monday, September 12, 2011 6:44 AM
To: 'Butler James-CJB006'; gmecm at diy-efi.org
Subject: Re: [Gmecm] Simulating the "power valve" in the ECU for PE
?
Interesting concept.
I?ve never like the power valves in the Holley?s for the same reason. 
Cam choice, throttle body size, boost, driving style, all play havoc with 
vacuum.
The TPS vs. RPM table always seemed a better choice for increasing the fuel 
outside the normal driving.
So when I watch the live data on the TPS vs. RPM table, it?s easy to recognize 
the area or zone were you spend most of your time. Idling thru the city being in 
the lower left, cruising down the road freeway in the middle and rocketing down 
that unused road being in the upper right. That would give me the upper left for 
jack rabbit starts, the upper middle for passing the left lane lounger freeway 
and the extreme right for ?let see what breaks first?. This also gives me a 
chance to take out fuel in the lower right and middle.
For the power enrichment, I?ve always saved for towing, driving in hilly country 
and getting up the boat ramp.
I should also mention, I am by NO means an expert. 
?
From:gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf Of 
Butler James-CJB006
Sent: Thursday, September 08, 2011 6:29 PM
To: gmecm at diy-efi.org
Subject: [Gmecm] Simulating the "power valve" in the ECU for PE
?
Some time ago I begin experimenting with the power enrichment function on my 
1995 S10 to simulate the action of the ?power valve?.
?
I?m wondering if any others have tried the approach I?m about to describe.? It?s 
not something I?ve seen anyone write about, e.g., on Monodax.
?
Most (all?) carburetors enable PE when the manifold vacuum drops below a 
threshold (at least that how it works on my Edelbrock and the Holley ?power 
valve? appears to serve a similar function).? However, every GM ECU that I?ve 
seen (which is only about a dozen) enables PE based upon, at least, a table of 
TPS and RPM, and not MAP.? I assume other vendors? ECUs are similar.? I?m sure 
most of you know all of this.
?
I can see the rationale for not using MAP as a PE threshold in modern systems, 
and I?ll comment on that later.
?
In my case, I always liked the carburetor?s way of handling PE, i.e., when the 
engine approaches maximum load (e.g., 6? of vacuum or whatever one cares to 
define as ?maximum?), enter PE.
?
What I?ve done, in my S10 and some other vehicles since, is adjust the PE ?TPS 
threshold vs. RPM table? to enable PE at some target MAP, e.g., 26? or 28? of 
manifold pressure. ?Of course, I had to have some idea of which TPS values to 
use for each RPM band in the table, so I simply logged a bunch of data from the 
ECU and sorted it to get an estimate of the throttle position/TPS values that 
would yield, approximately, a particular MAP for each RPM range in the table.
?
Upon applying these new TPS values to update the table, the drivability ?fun 
factor? was notably improved, which is why I?ve applied these changes to a 
number of my vehicles.? Configuring them for around 27? of MAP is usually to my 
liking.? I always validate the changes by logging across the RPM band and 
confirming that PE is entered at the target pressure.
?
In the real world a change like can cause at least one problem, i.e., when 
towing, or driving in hilly country, PE can be entered much more easily, and for 
long periods of time, with the typical consequences.? Apart from that downside, 
I find it?s a nice tweak.
?
Jim
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