Better gas mileage and performance.

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L31MaxExpress

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Picking up on the idle issue. What does the IAC valve do ie open or close to say, increase idle speed (against a load) and are the injector durations altered? Curious as my LPG system is entirely independent of injector durations (not all LPG systems are) and my idle is as stable as can be asked for. When I've noticed the odd tremor idling it has been when running on gasoline but as not up to temp in that situation, I'm reluctant to condemn it as unstable.

The PCM first alters the timing, then if the error is too great for the timing control it moves the IAC. The 02 sensors provide feedback for the PCM to continuously vary the air/fuel ratio. The AFR swing from the factory is pretty aggressive, where I have it down to roughly a 0.02 lambda swing now rich and lean. 0.98 to 1.02 lambda.
 

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Do tell!

That's of interest to me - especially the 2nd to 3rd shift.

Depending on the year you have to use the software that matches, but the things such as ignition timing, fueling, and transmission are all adjustable.

Tunercats does not have a parameter list for the Black box but the list is very similar to the OBD1 LT1 list.

 

0xDEADBEEF

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Unless your truck takes 5 minutes to accelerate to the desired speed (or you are constantly stopping), I don't think it makes a huge difference.

If you do the math, it takes more power to accelerate quickly to a given speed than it does to accelerate slowly. Can that extra power come only from efficiency gains? I don't know, but I'm leaning towards no. I would think the car makers would work that into their fuel saving strategies if it were true.
 

L31MaxExpress

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There is a possible explanation for this. First though why an improvement can be had on higher octane fuel. If the ECU can look for detonation and not find it, it can advance the timing and the engine be the better for it. On my smart it took around 4 tankfulls to get to the point where it no longer tentatively probed but went straight to the new timing confident that there'd be no detonation. (MPG went from 45-48 (54-57mpUSg) to 50-55 (60-66mpUSg) and the throttle response was much improved).
If the ECU will not do the above then no gain and, if the higher octane fuel is slightly slower burning - which is possible as it is less explosive, less detonation prone - then the ignition timing can then be too slow and driveability suffer.
On that same token, most GM vehicles have a low octane mode and can learn to remove the added timing without continually relying on the knock sensor. If you reset the PCM completely and drive it, monitoring knock retard you will see the commanded timing values drop with elevated knock counts. Good fuel prevents this from happening.

On most newer GM PCMs, GM actually programs the PCM with the MBT timing map already in it. I have found on good fuel the under load portion of the MBT map usually works very well. The very low load and idle MBT programming has some crazy numbers that could never be used. I generally keep the idle and coasting along timing the same as GM made it but transition to the MBT values as the engine starts to get a slight load on it. When I get some time I will show some screen shots.
 

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The PCM first alters the timing, then if the error is too great for the timing control it moves the IAC. The 02 sensors provide feedback for the PCM to continuously vary the air/fuel ratio. The AFR swing from the factory is pretty aggressive, where I have it down to roughly a 0.02 lambda swing now rich and lean. 0.98 to 1.02 lambda.
The AFR swing cannot happen on mine though when running on LPG. The LPG system is a simple single point mixer - akin to a carb/venturi (it's fuel supply though, is under O2 sensor feedback but via its own ECU). So that leaves timing and IAC valve control for idle stability - which cannot be faulted. FWIW, I have an AFR gauge on the dash and can see the AFR fluctuate during idling but the idle is always rock solid stable. The GM PCM sees those fluctuations also (O2 sensor is wired in parallel to both ECUs) but cannot influence AFR.
Assuming the IAC valve is moved, opened or closed to maintain a given idle rpm when an increased load is encountered? In early days of electronic fuel injection it used to be closed and the richened mixture (inj duration held constant) countered increased load. Still the same? Or opened and more fuel supplied (as would be the case with my LPG system)?
 

L31MaxExpress

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Unless your truck takes 5 minutes to accelerate to the desired speed (or you are constantly stopping), I don't think it makes a huge difference.

If you do the math, it takes more power to accelerate quickly to a given speed than it does to accelerate slowly. Can that extra power come only from efficiency gains? I don't know, but I'm leaning towards no. I would think the car makers would work that into their fuel saving strategies if it were true.

If you do the math it takes the same power thus fuel to accelerate a given weight from a stop to X speed. To get the best fuel mileage you want to accelerate at an rpm and load where the engine best converts fuel to useable energy. It does make a noticeable difference. In the least amount of words that will always be heavy part throttle not using fuel enrichment with the RPM near peak torque. With a stock 350 vortec you want the engine around 2,800 rpm, plus or minus maybe 500 rpm. That is why I have long felt a S10 converter that stalls around 2,500-2,800 rpm is near perfect for these trucks. It easily keeps the engine in its sweet spot even at 0 mph while accelerating. With the shift points set correctly the engine hangs at 2,500-2,800 rpm while accelerating briskly as if the truck has a CVT transmission.
 

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Unless your truck takes 5 minutes to accelerate to the desired speed (or you are constantly stopping), I don't think it makes a huge difference.

If you do the math, it takes more power to accelerate quickly to a given speed than it does to accelerate slowly. Can that extra power come only from efficiency gains? I don't know, but I'm leaning towards no. I would think the car makers would work that into their fuel saving strategies if it were true.
L31MaxExpress touched on this in an earlier post when he mentioned BSFC (best specific fuel consumption). In layman's terms - bang for your buck. If the engine is closer to BSFC, for the same fuel burn as at a lower SFC, there's more power. That gives the extra acceleration without consumption penalty. BSFC is invariably at the point of maximum torque and as (typically) half throttle will enable that, getting the throttle open and letting the engine breathe is the route to essentially free horsepower.

(BSFC is expressed as mass of fuel consumed for an individual unit of power over a specified time period, eg lbs/hp.hour, kg/kW.hour etc. It is the absolute arbiter of engine efficiency. When you are achieving 0.2kg/kW.hr - you're doing well).
 

Pinger

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If you do the math it takes the same power thus fuel to accelerate a given weight from a stop to X speed.
To be pedantic, it takes the same energy.
Power is energy divided by time so to accelerate at a faster rate does require more power - but developed more efficiently and for a shorter time period - which the rest of your post elaborates on.
 
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