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Old 07-29-2009, 09:51 PM   #1
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Shifting points formula "project" : Your opinions?

Hi, I'm a new user of GasSavers.org, and I recently bough a ScanGuage too. I hope sharing informations with the members of the forum combined to the ScanGuage utilisation will make me a good hypermiler.

How about my idea about the "shifting points"? I'll explain :

Using the ScanGuage as a tool to monitor how much Torque in lb-ft the cars need to accelerate, I hope it will be possible to help me making my "in-town" accelerations making better fuel economy.

Here's the torque/hp dyno chart of my car from the manufacturer (my car is completely stock) : http://www.gmperformanceparts.com/En...263&engCat=rpo

As you seen, the chart is not very accurate, except for sure the maximum power and torque inputs, but it could be a good device to know the best shifting points for a specific car.

My "formula" idea is that :

The test with the scanguage to determine the best RPM for torque effiency could be done by trial an errors, starting with a RPM shift point that seems to be the best, according to you. All RPM shift points are determined by the most important thing : The RPM *AFTER* the gear was shifted, assuming the fact that to get better mileage, we must shift the lower RPM as possible without lugging the engine that will results in probably a too rich fuel mixture. So the best thing to do is to shift at lowest possible RPM and determine at what RPM in the new gear "after you shifted the previous gear" it will develops just enough torque for the % of the throttle that will be pressed. And to determine at what RPM to shift to have the right RPM on next gear, it's just trial and errors too, but really easy to do. After you are knowing for example that shifting the 1st to second gear at 2500 drop your RPM to 1500 on next gear (for example), and you know that thing for all the speeds, so you know your car perfectly.

The first step is to determine what's the best acceleration for the conditions, basing on TPS reading on the ScanGuage. For example, if it's an acceleration in town and you want to make a reasonable acceleration without making other drivers crazy, and on flat ground, the TPS value could be, for example, 30% ; If you are alone and driving like a grandma, maybe 20%; If it's an acceleration to uphill, the value could be 45%, etc... The % acceleration could be find by approximately shift as you think it will be the more effiency, then do trials and error to optimize. THE "BEST VALUE" , JUST USED AS AN EXAMPLE FOR THE DEMONSTRATION WILL BE : 33% TPS

The second step is to determine the amount of torque required to make the acceleration at the requested % of TPS, without making the engine lugging, but without revving the engine too much for the pressure of the accelerator you need, because it will burns too much fuel for nothing. To have the Maximum Effiency, we must just to be perfectly in the limit of engine lugging, by using numbers that the scangauge tell you. It will permit to use EXACTLY the maximum torque for the current RPM and throttle position. So, for the second step, we use the TORQUE (in lb-fts) value in the scanguage. Oh, i know the scanguage is not very accurate with this, but anyway, all this post is just an idea that I had and I want to share it with you. THE VALUE FOR THE DEMONSTRATION WILL BE : 40LB-FT

The last step is to read the dyno curve : http://www.gmperformanceparts.com/En...263&engCat=rpo
So : We want to press on the accelerator at approximately 1/3, for sure just after shifting the gear and during the acceleration too. As you seen at the example, our TPS requested value is 33% (1/3 pedal). When holding accelerator steady at 33% on an approximately RPM we approximately guessed before, we develops 40LB-FT of torque according to the scanguage, always according to the example i said before. But this 40LB-FT is developped only at 1/3 throttle at the given RPM. So we will multiply the 40LB-FT per 3, giving us a 120FT-LB of torque.
NOW, we could read the dyno curve and just have to search approximately at what RPM is the 120FT-LB : Approximately 2000RPM (I know, this chart is not really accurate).

So now, we have the essential information : 2000RPM. As you will shift with THIS car (an ecotec 2.2), when holding a 33% TPS value when accelerating, we will need to start at the minimum RPM before luggage, in that case 2000RPM. So, the best shifting points for this kind of acceleration will be :
1 to 2 : Must starts accelerate to 2000RPM in 2nd gear
2 to 3 : Must starts accelerate to 2000 RPM in 3rd gear
Same for the other speeds.

But I want your opinions about all this.
Is the maximum torque in a specific RPM is really divided by 3 in this case, or it's not like that we calculate?? In other words, Is the torque numbers is linear at a same RPM for different throttle pressure, or it could be exponential?

Thank you very much reading me and i'll like read your opinions.
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Old 07-30-2009, 04:46 AM   #2
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Lugging the engine and running rich are not as great a concern with modern fuel injection systems.

In the old carburetor days with accelerator pumps you could make the engine run rich at low RPM, because the accelerator pump put out a specific amount of fuel based on the change in throttle position alone. Modern fuel injection is completely different. It only goes into an enrichment mode when you go to wide open throttle.

Ideally you will get your best mileage by accelerating at a low RPM. The specifics of how low you want that RPM will vary from engine to engine, but as you stated you also want to avoid becoming an obstacle to the flow of traffic. This contradicts the attitude of some hypermilers, but I share your consideration for others and the flow of traffic.

I don't have a scangauge. but I understand they are great tools for teaching drivers to be efficient. This is my method.

Your throttle input percentages sound like they are well thought out. What I try to do is move with the flow of traffic, or choose routes where traffic is no a real concern.

For best efficiency you want the highest gear that will give you the required acceleration.

This will also give you a very low manifold vacuum reading, and it will actually make little difference if you do change the throttle position, because if you want to accelerate at a greater rate, you would actually have to downshift. Even if you dramatically increased the throttle position percentage, your rate of acceleration would not really change due to the higher gear chosen.

You are actually choosing the rate of acceleration with the gear choice, not the throttle position, trying to consider that your objective is to get to speed with the least number of combustion pulses you need to accomplish that goal.

When you understand that lowest gear and the least amount of throttle percentage, that achieves the lowest possible manifold vacuum, while interacting acceptably with other traffic, that is the essence of my formula for mileage.

After that basic operational strategy has become ingrained in your driving routine, it becomes secondary nature.

Then you should concentrate on some of the details that contribute to an improvement, such as lower speeds, DFCO, hill driving strategy, warm up strategy, and other refinements that will add to your mileage.

"Lugging" and engine used to have a different meaning from what it does today. The best way I can describe it today is to consider that there is a minimum speed that the engine will provide decent power without vibration or loss of induction harmonics. That is where your scan guage will come in handy.

The rate of acceleration actually has little effect on mileage as long as you get into the highest gear as quickly as conditions allow.

Think of it as counting the number of times the injectors deliver fuel to the cylinders and you are trying to get to your desired speed with the least number of cycles. Getting there faster will get you to your best mileage sooner. Getting their slower will get you to your best mileage slightly later but will also gain more distance in getting there.

Lowest throttle for lowest vacuum in highest gear considerate of other traffic.

That about sums it up for me. This applies to transmissions where you can select the gear, not automatics where you can not put the transmission in a higher gear than the automatic tranny chooses.

regards
gary
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Old 07-30-2009, 05:33 AM   #3
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I have heard several theories over time, and I'm convinced of none of them.

One theory is that maximum efficiency is at 70% (or some other percentage) of maximum HP RPM. In other words, if your maximum HP is at 4000 RPM, your maximum efficiency is at 2800 RPM. Another theory is that the maximum efficiency will occur at the same RPM as maximum WOT torque. I say that both of those theories are garbage, because there are too many factors about engine design which affect maximum HP / torque, which have marginal or negligible effect of efficiency under "regular" driving conditions.

What you really need, IMO, is a set of torque/HP curves at, say, 25% TPS. It's actually quite easy to do this on a chassis dyno, but most people just "go for the gusto" on a dyno. If you can combine some chassis-dyno time with some scangauge readings, I think you'll really be getting some good data. (But of course chassis dyno time costs $$)

Then, once you have chassis dyno curves at 25% TPS, you could use excel and include your gear ratios to determine your optimum shifting points for each gear.


Quote:
Originally Posted by R.I.D.E. View Post
In the old carburetor days with accelerator pumps you could make the engine run rich at low RPM, because the accelerator pump put out a specific amount of fuel based on the change in throttle position alone. Modern fuel injection is completely different. It only goes into an enrichment mode when you go to wide open throttle.
Of all of the injection systems I've programmed, I have never seen that to be the case. In fact, in addition to adding enrichment when the throttle moves, some systems automatically add enrichment when the MAP increases (even if the throttle doesn't move, for example when a turbo starts to spool up). I'm sure there is less enrichment in an FI system compared to a carb, since there is much less wetted manifold area, but it is still there. Perhaps a direct-injection system does not require any acceleration enrichment.

But in a non-turbo application, I would promote a steady throttle position as most efficient, to negate the effects of "acceleration" enrichment (or, more correctly, "more acceleration" enrichment). Since that is quite difficult to avoid in a manual transmission car, slow pedal movement will give you the least "enrichment" when shifting.

-BC
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Old 07-30-2009, 06:08 AM   #4
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Welcome. That's a lot of number crunching! Most people just drive every day, change up their style after a few days, and watch the trip average on the ScanGauge to see what works best for them. Your approach is more work but should yield useful research that isn't commonly done.

What do you drive? Based on your engine and the way you talk about the transmission, I'm going to guess a Cobalt XFE. Do you really have such long ratios that you can shift at 2500 and enter the next gear at 1500? I'm jealous if you do...I have very close ratios and I run out of gears by the time I hit 30mph.

The chart you posted is too crushed to see well, it looks like the lines are almost flat. I've stretched it so that it's easier to read; the data is still charted properly but the scale is changed to make the curves more curvy. Of course, it's still showing WOT instead of partial throttle, which may not be what you need for this purpose. You may want to try to find a BSFC map for your engine, which may have more useful data too.



Quote:
Originally Posted by R.I.D.E. View Post
You are actually choosing the rate of acceleration with the gear choice, not the throttle position, trying to consider that your objective is to get to speed with the least number of combustion pulses you need to accomplish that goal.
This is my strategy, and it's pretty effective for me. My car does well with WOT and me choosing my acceleration rate by my shift points.

Quote:
"Lugging" and engine used to have a different meaning from what it does today.
IMO, the old definition should remain in place, since the stigma remains. Lugging, in the old definition, means severe predetonation that results from low RPM heavy loads on old non-computerized cars. It is described as sounding like "metal sh*t exploding under your hood". Modern cars with knock sensors and computer-controlled ignition timing either run or stall, but don't lug.

What is popularly called "lugging" now is just when the engine grumbles instead of sounding smooth. There is no damage done by that, although I suppose that making a habit of it might cause some kind of premature wear.

Quote:
Originally Posted by bobc455 View Post
Perhaps a direct-injection system does not require any acceleration enrichment.
Direct injection definitely changes a lot of stuff like that.
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Old 07-30-2009, 10:25 AM   #5
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On that torque curve, I'd go 80% throttle and shift at 4500, until @ target speed then change to cruising gear, might mean a 1, 3, 5

Don't actually like that dip in the HP curve between 1000 and 2000 RPM... unless that's ECU dependant (Meaning it has lower fuel delivery there despite demand and ability to burn it)
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Old 07-30-2009, 10:27 AM   #6
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In fuel injected engines the mixture ratio is maintained at 14.7 to 1 unless you go to wot or very near wot. The oxygen sensor feedback is the final arbiter of mixture.

70-80% of full load does not mean 70-80% throttle position. It means the effective compression is near maximum. Enrichment is necessary to compensate for higher effective compression, and it will always show as very close to 0 vacuum with the least amount of throttle applied in the highest practical gear.

It's easy to test, when you add more throttle nothing really happens to your acceleration unless you downshift.

This will put you at best BSFC for your engine as long as you don't go to low with the RPM. My VX could pull away easily in 5th gear at 30 MPH and 1000 RPM. That is what it was designed to do. The same engine without VTEC-E controls would not work well at that low RPM, because it was not designed to run that low while pulling a heavy load.

Every engine has its sweet spot, usually between 1200 and 2400 RPM and with just enough throttle application to produce a minimum manifold vacuum.

High performance high revving engines will produce power at higher RPM, but the same applies to them as it does to low RPM high torque engines.

Power is produced by compression and expansion. High manifold vacuum means less compression and less combustion pressure, as well as lower efficiency.

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Old 07-30-2009, 10:40 AM   #7
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I'd say it's maintained at target ratio, whatever target ratio is, it might be 14.7:1 or 16:1 or 12:1 depending on the fuel tables the ECU is applying.

Basically it's down to the least number of bangs per mile which, is IMO, least number of bangs to get to speed, followed by least number of bangs to maintain optimum speed, which neglecting air resistance, the choice of which might make a +/- 30 percent or so difference depending where you are then on the BSFC vs load table. Then you have complications about maximum throttle angle allowed to allow lean burn etc depending on the vehicle, and a whole lot of wierdness around what is the aerodynamically most efficient design speed and whether the motor and gearbox match it.
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Old 07-30-2009, 10:44 AM   #8
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Quote:
Originally Posted by RoadWarrior View Post
On that torque curve, I'd go 80% throttle and shift at 4500, until @ target speed then change to cruising gear, might mean a 1, 3, 5
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Old 07-30-2009, 10:56 AM   #9
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Brisk acceleration is the only way to load the motor effectively.... apart from sawing off all but one cylinder, because when it comes down to it the motor is 4 or 5 times too big...
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Old 07-30-2009, 11:34 AM   #10
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Hello again and that you for all your answers and opinions.

I was reading again my formula today, and I was thinking about it :
When I talked about putting the TPS value in the formula, I think that's possible it was wrong. Because of that : To see the optimal throttle position for the right acceleration we want, we need to try this and check on the ScanGauge. But the information we are requesting is the right and minimal RPM to have enough torque to do the task. If, for example, I try by trial and error with any RPM : If the RPM is lower than the optimal RPM to start the acceleration in the new gear, the engine will not have enough torque to do this with a minimal pressure of the throttle. So you will need to press the throttle more than supposed. That will be a wrong TPS value, and also wrong TORQUE value on the scangauge because the TORQUE value will be lower than the optimal value, because the engine will be not able to produce enough torque to show the right value. If, by trial-and-error, I try to start a new gear when the RPM is higher than the optimal RPM to start the acceleration in the new gear, it will show wrong results too. Because the acceleration will tends to be "greater" than the optimal acceleration, because the RPM is higher than the optimal RPM we are requesting, so it produce more torque for the same pressure on the throttle. If we try to lighter the throttle position, it will give you the right acceleration, but you still dont know the right optimal RPM to do the task with the best fuel economy.

But the new idea is to use a G-Force Meter, maybe something like the G-TECH gadget, if anyone knows. It calculate the horsepower of the car and the time to do 1/4mile, probably using the G-Force numbers to calculate this. So, if we use a G-TECH and a Scanguage together, I think we could have better results. Unfortunetely, I don't know if there's a G-Force value on the G-TECH, but maybe someone here knows.

By using the G-Forces required to do the acceleration you are requesting, you just have to shift at an higher RPM than you think need for optimal fuel economy. So you are SURE that the car produce enough torque to have the right readings on the scanguage or the G-TECH. At an higher RPM that needed, we will make the acceleration we need for the desired task (uphill, hypermiler style driving, normal driving, etc). Now, during the acceleration or, if possible, use the "freeze frame" mode on the scanguage and the G-Tech, we will be able to analyse all the numbers.

Now, using the G-TECH, we check the G-Force of the acceleration we need, and we compare it to the TORQUE ratings on the scanguage, or maybe the G-TECH, is possible. So we now have the good numbers : We know the G-Force of the acceleration we need, and we know the Torque required to do the task. After, we decide what % we want to press the TPS to do the task. It could be 33% TPS for example.

Now, we are ready for trial-and-error test with the scanguage and the G-TECH together. We just needs to try some RPM shifting points. How? By accelerating at the desired throttle pressure for the task we need, and checking the G-Tech G-force value in same time, if that exists. The best key to find the optimal RPM will be to check the TORQUE value on the scanguage and the G-Force value on the G-TECH at beggining of acceleration after shifting the gear, at different RPM points. We need to keep the same G-Force at beginning of each shifting accelerations, and keep the same throttle position too.

To find the optimal shifting points, you should, by trial-and-error, Match this 3 things at the beggining of the acceleration after shifted the gear :

1-The G-Force must be the same as decided before, when you tested the wanted accelerating with the G-Tech.

2- The TORQUE value from the scanguage should be the same that it was when we do the acceleration test with the G-Tech before (when we had more RPM than needed to ensure the engine have enough torque the amount of torque needed to move the car at the desired G-Force for a specific task (hills, flat ground, etc). So the torque value must fits as close as possible with the RPM at this time and the throttle position, checking the Engine torque curve. For example, if the desired throttle position is 33% and on the engine torque curve it say that the engine develops 120FT-LB at wide open throttle at 2000RPM, The torque value on the scanguage must be the same that the 1/3 of the wide open throttle value at this rpm, because in this case, we want the throttle position only at 33% for acceleration. If the torque value on the scanguage don't fit, so maybe the tried shifting point RPM is not ok the the TPS pressure, or something like that.

But it's just an idea like this, the formula must be optimized more than that.
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