Is it worth changing gears for better MPG's???

[QUOTE=TurboDiverArt]If I'm understanding things correctly, I "think" we are all talking the same thing but using different or maybe slightly incorrect semantics. You are definitely correct that the shift point is load and TPS based and not strictly RPM based.

One area I think I'm going to disagree with you on is that the stock shift points are where you want them and that lowering them somewhat is not what you want to do. Yes you hold torque multiplication longer by delaying the shift point higher in the RPM range but torque multiplication is power in multiplied by the gear ratio. So, making the shift point above the max output on the torque curve means less power into the torque multiplication so less power out. Now, you do have to shift higher than the max torque otherwise the bottom of the shift will be below the torque curve and the truck makes no power after the shift. I don

The shift points should actually be near the engine's power peak because power is the rate at which work is being done. You want the work to be done at as fast a rate as possible to get down the road as quickly as possible.

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Originally Posted by ronniejoe

The shift points should actually be near the engine's power peak because power is the rate at which work is being done. You want the work to be done at as fast a rate as possible to get down the road as quickly as possible.

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Hello RJ, When you mention power peak, is this torque, or hp, or somewhere in between? In my case peak torque is at 2,200 rpm's, peak hp is at 3,500 rpm's, I'm thinking peak torque, right, or?

Yes I believe you are all correct in that you want the shift at peak torque, in fact you want the engine to always operate at this point. But.... after the shift you don't want to be out of the power range either. If you shift at 2200 RPM

I am talking about power as measured in HP. The engine is most efficient at its torque peak, but gets the work done more quickly at its power peak.

Now in the case of these engines in stock form, the real optimum point is around 2700 - 2800 rpm where the rate of increase of power (second derivative...) drops. On my dyno plots, you can see the power curve trail off at about that point. It keeps increasing all the way to 3500 rpm, but not very rapidly. If I were racing, I would want to shift at that roll off point, or around 2800 rpm.

If performance is your goal, you want to operate near the maximum power point, which will be above the maximum torque point. If efficiency is your goal, operate near the maximum torque point.

I should have an experimental turbocharger installed next week that will let this thing breathe up to 3500 rpm. I think big numbers are possible with good airflow at 3500 rpm. Planning to go to Montana in a coupla weeks...;)

I want to thank all of you for the expert input, mine is just so, so but the rest of you make sense of it.

Will it hurt a high mileage motor (like mine) to run all day in 3rd at 2800 RPM? Mine does seem to pull best like this, but I have avoided it because of fear of blowing it up. BTW, I tow a little less than 9,000 lbs. I am not greatly concerned about gas mileage, but don't want to get stranded on vacation.

As discussed, the 45mph is the lockup point on the 4L80E with 4.10's and 245's.

I like that lockup point and want to maintain it during the swap to 3.42's and 265's (at the same time).

But, one thing I don't like is that once up to speed at say 65mph and above, I don't get a shift out of OD, so passing is done with boost and fuel.

So, my question...

If you use the vss modifier and I leave my VSSB at the stock 4.10 setting, will I still see a no-shift out of OD at 65+?

Hmm.

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Originally Posted by john8662

As discussed, the 45mph is the lockup point on the 4L80E with 4.10's and 245's.

I like that lockup point and want to maintain it during the swap to 3.42's and 265's (at the same time).

But, one thing I don't like is that once up to speed at say 65mph and above, I don't get a shift out of OD, so passing is done with boost and fuel.

So, my question...

If you use the vss modifier and I leave my VSSB at the stock 4.10 setting, will I still see a no-shift out of OD at 65+?

Hmm.

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I'm not sure how the 4.10's work. The 3.73:1's lockup the converter at about 47-48, which is pretty close to the 45's you now have. At 65MPH it'll drop out of OD. it'll drop out of OD at 75 if there is enough load like when you have a trailer in tow up a hill.

Art.

Good Day!

On reflection, I really don't care much about the shift points, I just wish the TC would lockup at a lower speed, & have more hysteresis in that lockup speed.

I sure wish I'd known about these trucks as they were being introduced in '94. I think a guy could have written some killer code to allow changing many different parts of the algorithm to suit the owner. I would REALLY have liked to integrate my Gear Vendors Aux. OD into the PCM.

Blessings!

(signature in previous post)

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Originally Posted by ronniejoe

Boost is the reason you feel the lessening acceleration. The problem is the small housing and nozzle on the GM-4 turbine. Above about 2800 rpm, it simply cannot flow enough to keep up with the engine. The backpressure blows the waste gate open (even against full vacuum straight off the pump...as I've been running for a year now) and the turbine rotor holds constant speed. I've plotted data points on the compressor map and the compressor is following a constant speed line above 2800.

This means that boost falls off even as total flow increases. Power falls as well.

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I thought my turbo was a GM-8? I thought the GM-8 came on the newer trucks. Mine being a 1999 I assumed mine came with it. Maybe the GM-8's only came on the Hummers?

Thanks,
Art.

Art,

RJ's only had experience with the GM4 turbo which was used from 94-95. Being that you have a '99 you should have the GM8. I do personally believe there is a little better flow with the GM8, but the internals of the turbo are almost identical, the change is in the exhaust side. GM5 and GM8 are almost the same turbo too, except for a better wastegate bushing on the '8.

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Originally Posted by john8662

Art,

RJ's only had experience with the GM4 turbo which was used from 94-95. Being that you have a '99 you should have the GM8. I do personally believe there is a little better flow with the GM8, but the internals of the turbo are almost identical, the change is in the exhaust side. GM5 and GM8 are almost the same turbo too, except for a better wastegate bushing on the '8.

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So, has anyone seen a compressors map of a GM-8? In our motors where does the turbo run out of steam? On mine it appears that at high RPM's it

[QUOTE=TurboDiverArt]So, has anyone seen a compressors map of a GM-8? In our motors where does the turbo run out of steam? On mine it appears that at high RPM's it

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Originally Posted by DA BIG ONE

Art, The GM-8 exhaust turbine "exhaust outlet" is some of the limiting factor needing some porting. How much porting? It's more a matter of how much risk one is willing to take making walls too thin.

Some say the elbow after the gate is an issue, then some smoothing out of its inside surfaces are in order.

I really thing wee need those bigger Holset 35, 40 turbo's providing we stay out of the surge line, and keep boost at a level of 15+- psi @ intake.

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If the GM-8 seems to run out of steam at 2800, were does a GM-8 run out with a free flowing exhaust? Mine definitely seems to lose it by 3100, maybe 3000.

Art.

The rotors on a GM-4 and GM-8 are identical. The only difference is a slightly better compressor housing and a slightly better exhaust elbow. The waste gate was moved out of the exhaust housing into the elbow on the 8's and I've been told that the elbow is more efficient. With that said, I don't think you'll see significant performance differences between the two. The wastegate move was to improve reliability of the bushing (mine failed) and had nothing to do with performance.

I have an outline map of a the GM series compressors...it has no detail and IHI will not even respond to my inquiries. The other manufacturers have been very responsive to my questions.

It is fair to take my comments as being applied to the GM-X series in most cases.

My main comment here was to clarify the relationship between peak torque and peak power and to give you some of the rationale behind choosing shift points. The turbo talk was simply to help explain some of your observations.

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Originally Posted by ronniejoe

The rotors on a GM-4 and GM-8 are identical. The only difference is a slightly better compressor housing and a slightly better exhaust elbow. The waste gate was moved out of the exhaust housing into the elbow on the 8's and I've been told that the elbow is more efficient. With that said, I don't think you'll see significant performance differences between the two. The wastegate move was to improve reliability of the bushing (mine failed) and had nothing to do with performance.

I have an outline map of a the GM series compressors...it has no detail and IHI will not even respond to my inquiries. The other manufacturers have been very responsive to my questions.

It is fair to take my comments as being applied to the GM-X series in most cases.

My main comment here was to clarify the relationship between peak torque and peak power and to give you some of the rationale behind choosing shift points. The turbo talk was simply to help explain some of your observations.

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Thanks for the info, it's good information! It definitely seems that the stock turbo putters out in the 3000 range. This is why I thought the shift point should be at about that point. It just seems to take forever to get from 3000 to 3500. Now if you had a turbo that would pull all the way up to 3500 RPM

[quote=TurboDiverArt]Now if you had a turbo that would pull all the way up to 3500 RPM

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Originally Posted by ronniejoe

Hold that thought...

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You just couldn't resist could you!

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Originally Posted by jspringator

Will it hurt a high mileage motor (like mine) to run all day in 3rd at 2800 RPM? Mine does seem to pull best like this, but I have avoided it because of fear of blowing it up. BTW, I tow a little less than 9,000 lbs. I am not greatly concerned about gas mileage, but don't want to get stranded on vacation.

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My experience has shown that egt's run high under those conditions. I prefer to run in OD at higher boost levels where things stay cooler.

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Originally Posted by ronniejoe

My experience has shown that egt's run high under those conditions. I prefer to run in OD at higher boost levels where things stay cooler.

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I'll agree with that! RPM's and low boost equals high EGT's. Before the intercooler install, when the transmission would downshift out of OD and the RPM's would go up to the 3000 range the EGT's would climb dramatically and quickly. Higher boost helps to cool things down but isn't the real solution to the problem.

That was why in a previous post I was suggesting hacking the VSSB to read 4.10 gears when mine were in fact a higher ratio. If setting it to 4.10 gears helps to keep the converter locked and/or keep it from dropping out of OD then I think it's a possible viable modification. I believe one poster above said that with the 4.10 gears it was very difficult to drop out of OD over 65-70 MPH. With 3.73 gears that's about 2200-2400 RPM's. That's a nice RPM range for a lot of boost when towing. On mine if it kicks down out of OD at 65-70 that means like 3100 RPM's and the trucks just not pulling anymore, only generating EGT's.

Art.

It has been suggested that I try and run my burb in 3rd only to see if mpg results get better.

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Originally Posted by DA BIG ONE

It has been suggested that I try and run my burb in 3rd only to see if mpg results get better.

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You thinking you got too little gear? With the oversized tires you're probably at what, 3.08's? Probably real good at 50 MPH but maybe the engine's got to work too hard at highway speeds with wind resistance. What do you get at 65-70? On straight highway I'll get about 18.5 MPG and 14 towing on relatively flat highway.

Art.

WOW!! I just read through this thread to catch up and find myself wondering what turbo selection has to do with a gear swap for MPG??? Unless one has an interest beyond MPG.

On my '93 I started out with stock 245/75/r16 tires and 4:10 gears. That would net a consistent 15MPG around town. When I swaped to 285/75/r16's I gained nothing... probably lost 1mpg truth be known. I always atributed it to added rolling resistance. I've always been under the impression that swaping out gears doesn't have that effect because rolling resistance is the same and the only difference is how long it takes to get rolling. That's what I got out of this thread thus far, am I somewhat correct?

While I agree the turbo stuff drifted a little, it was originally on subject.

The conversation had to do with engine speeds and ideal shift points for economy. There were some questions that ended up bringing the discussion around to performance as well. Then the subject of falling flat above 2800 rpm... That led to turbos... That led to discussions of power... That led back to economy...

You get the drift.:D

Vicious circle ain't it...;)

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Originally Posted by dieseldummy

WOW!! I just read through this thread to catch up and find myself wondering what turbo selection has to do with a gear swap for MPG??? Unless one has an interest beyond MPG.

On my '93 I started out with stock 245/75/r16 tires and 4:10 gears. That would net a consistent 15MPG around town. When I swaped to 285/75/r16's I gained nothing... probably lost 1mpg truth be known. I always atributed it to added rolling resistance. I've always been under the impression that swaping out gears doesn't have that effect because rolling resistance is the same and the only difference is how long it takes to get rolling. That's what I got out of this thread thus far, am I somewhat correct?

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I think as Ronniejoe said the subject drifted a little but did cover a lot of interrelated issues.

I don't think any of the above topics concluded that a gear reduction has an adverse effect on mileage. I think yours is the only one that has stated that reducing the gearing from 4.10 to 3.73 via. tire change adversely effected mileage. I think DaBigOne's poorer mileage may be because he's reduced a little too far and was using an aggressive tread pattern. Tread pattern has a big effect as does the width of the tire. If you go bigger, any benefits may be negated if you use a really wide tire and/or aggressive tread pattern.

I believe comparing stock trucks to stock trucks it's been proven that 3.42's get better mileage than 3.73

I never corrected the VSSB/speedometer, but made the correction in my head. If I wanted to travel 65 mph I would put the speedometer at 60 mph. Whenever I filled up I corrected the number of miles on the odometer to make up for the larger tires. The tread pattern is not that aggressive, the tires are yokohama geolander at+2.

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Originally Posted by dieseldummy

I never corrected the VSSB/speedometer, but made the correction in my head. If I wanted to travel 65 mph I would put the speedometer at 60 mph. Whenever I filled up I corrected the number of miles on the odometer to make up for the larger tires. The tread pattern is not that aggressive, the tires are yokohama geolander at+2.

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Huuumm, strange I think. I would think you'd see a fairly significant difference going from 4.10's to an equivalent 3.73's if you were traveling at the same speed. I'd think with 4.10's you probably don't go long distances at over 65 MPH. You'd probably be screaming along at about 2300-2400 RPM's. I just wanted to make sure you were not calculating mileage at 60 MPH with the 4.10's and comparing it to 70 MPH with the 3.73's. Although the RPM's may be similar you are going to experience a significant amount of additional wind resistance at 70 that you would not have at 60.

I

Hi Diesel Dummy;

I agree that things did "drift" of topic a bit. Any way my thoughts on the worse MPG for larger tires is that with wider tires you have besides more rolling resistance you also have more areodynamic drag with a larger tire face in the slip stream. The other with a "taller" stance on your truck with the taller tires you also increase the "drag" from the under carriage of the truck.
This all adds up.

Holy smokes! Go away for a couple of days and miss 4 pages of posts!

The hot button is DaBigOne keeps saying his shift points changed. I'm saying the way they are computed is not based directly on road speed and they have not changed; the road speed is what changed. I never meant the factory settings were optimal. To each his own.

On that subject, now that he's got them "back where they belong" he's thinking of driving in 3rd all the time. Anyone else see any irony here? ;)

Re optimal shift points, I always thought you wanted to be where the decending torque curve of he current gear crosses the ascending curve of the next gear. No?

If you want to accelerate down a race track as quickly as possible, you want to shift where the power curves cross from gear to gear.

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Originally Posted by ronniejoe

If you want to accelerate down a race track as quickly as possible, you want to shift where the power curves cross from gear to gear.

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I'm sure you are correct, but wouldn't the torque curve give you the strongest acceleration, hence the highest trap speed? (Bear in mind I've retired all my math skills past basic algebra... ;) )

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Originally Posted by JohnC

The hot button is DaBigOne keeps saying his shift points changed. I'm saying the way they are computed is not based directly on road speed and they have not changed; the road speed is what changed. I never meant the factory settings were optimal. To each his own.?

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Perhaps, I'm just thick headed so bear with me will ya?

I changed out to 3.42 gears from 4.10's frt/back, using stock sized tires 245/75/16.

I hack the vssb to correct speedo and shift points/lockup go to higher rpm's and speed points and od/lockup goes from 45 w/4.10's to 55 mph w/3.42's.

Now I only hack the vss only to correct the speedo and the shift points lockup, od/lockup all stay as they were w/the 4.10's, same rpm's same speeds.

So, I guess I'll remain confused on this one until someone can drill it into my thick skull........................

For what it's worth, I'm with you DA BIG ONE!

I'd like to try the same mod, but keeping the lockup at the same speed on the speedometer.

Torque is basically work, or energy...they both use the same units.

Power is the rate of doing work or rate of energy usage. To go down the track quickly, you want to do the work quickly.

You going to be racing that burb down the track Ron?

Biggest lesson learned with much $$$$$ in my gear changeout is that you get good mpg until you start going larger tire diameter & width, then you arrive at the same spot you paid so much to get away from in the first place.

So, perhaps leaving the 4.10's and just doing the tire change I might have been happier.

All, in all the burb is a keeper.

Hum, food for thought. I have a set of H2 rims on the way to me. I was planning to put slightly bigger tires on them and increase the diameter by about an inch and a half. This would not have reduced the gears down to 3.42's but probably to about 3.55's. The width would have increased by 0.4 inches. Now I'm wondering if it's worth it. If I

I'll add that my burb w/it's mods is happy w/3.42's and stocked sized tires. These 6.5 td's aren't like the old 6.2's in that the 6.2 4x4's were ok w/3.08 gears.