Torque converter stall vs. gearing

[dahoyle]

Have a question for the transmission gurus here.

Truck is an 89 Suburban, with a 700-R4 and I use it as my DD, so fuel economy is way up on the priority list for the truck. Any changes made to it have to be neutral, or better, where fuel mileage is concerned.

I currently have the 3.41 gears in it, and am considering swapping to 3.08's in an attempt to improve economy. First, question, will that see any improvement at all as far as economy is concerned?

The second question, is whether or not a combination of a higher stall speed, with those gears, will have any appreciable effect on retaining some of the acceleration. I pay attention to economy, and drive this thing accordingly, so I seldom accelerate hard anyways.

I guess my concern is that I would like a bit better cruise economy, without giving up mileage in city driving, and if I could do that without hurting performance too much, that would be a bonus.

Any ideas?

[darren250r]

I guess my concern is that I would like a bit better cruise economy, without giving up mileage in city driving, and if I could do that without hurting performance too much, that would be a bonus.

Any ideas?

Sounds like you should leave it as is for your goals. Higher gears will make is struggle to accelerate more and be harder on the tranny as well as hurt economy. A higher stall converter will hurt you mileage too. The only point of a higher stall is to get the motor in the power band before the truck moves....not what you're goal is. Get a Honda or something for economy. You have a big suburban.

[dahoyle]

Get a Honda or something for economy. You have a big suburban.

Thanks for most of your input, but these remarks are less than useless. I have a big suburban, because it fits my needs. That doesn't mean that I shouldn't try to get the best economy out of it possible. Somehow, I don't really think a Honda is going to haul all my gear to the various jobsites I travel to, and it won't get to many of them at all, regardless of load.

As to the rest of your advice, well, in the case of a higher stall converter, and taller gears, that was exactly my point, to let the engine run a little higher in the RPM when starting out, so it is exactly my intended goal. A higher stall in a lockup converter will not hurt your cruise economy, unless the lockup is defective.

As to overall gearing, well, I really don't know if it would help or hurt. No question that it would be worse in city driving, but it seems like hiway should be better.

[ybsane]

Darren250r was right, here is what I have: 2007 Suburban 4x4 with 3.73 gears and factory 17" tires, I got a performance tune done and my mileage went up, plus performance when I need to put my foot in it. Try out Black Bear, PCMforless, Wheatley,etc...and give them your information about your vehicle and forget the gears..! you will be happier in the end.

If mileage is what you want spend $2,000.00 and buy a cheap day to day vehicle.

Rob

[dahoyle]

If mileage is what you want spend $2,000.00 and buy a cheap day to day vehicle.

Rob

Thanks for your input, but again, this is an absolutely stupid statement. What I want, is the best mileage I can get out of the suburban. Nowhere in there did I say I wanted a particular number, and I have already stated that another vehicle is not what I need, not what I want, and really not any part of this question. I'm not a F$^*in idiot. I know that a Civic will get better mileage. That is not the point, and has absolutely no bearing on the question.

My day to day vehicle needs to be at least a 4 door, enclosed bed, 1/2 ton or greater, with 4WD. The Suburban meets those needs with flying colors. Do I need those characteristics every single day, no, but for the most part I do. It gets used for work, almost exclusively, and work may be anywhere between a 50-100 mile round trip. I already have a grocery getter for running around on the weekends, and if it was suitable, I would be driving it to work. The fact of the matter is that it is almost totally unsuitable.

I really do appreciate the input on the tune, but the last statement is somewhat condescending and almost insulting, at the worst, and totally worthless at best.

[ybsane]

Seriously..? if you provided the info on your first post that you have on the last no one would have suggested a second vehicle. Its not a stupid statement..you can not make a vehicle that weighs close to 6,000lbs and has aero features of brick to get very good mileage.

Now we know how many miles a day your commute is how about you give us an idea of what your mileage is now and how much more you want to get. If you stop calling us idiots and give us some more information we will be glad to help you...:)

Rob

[dahoyle]

Seriously..? if you provided the info on your first post that you have on the last no one would have suggested a second vehicle. Its not a stupid statement..you can not make a vehicle that weighs close to 6,000lbs and has aero features of brick to get very good mileage.



Rob

What difference does it make whether or not I describe my needs in detail? I asked a specific question about a specific vehicle. That is all the information needed to answer the question. I appreciate the information about the tune, but even that is outside the topic of my question.

As generalities go, I know how to get better mileage, and I know that a Civic gets better mileage than a Suburban. Neither has anything whatsoever to do with my question.

I know there are some trucks out there now which are running double OD, and in essence, going to a higher final ratio would be quite similar, with roughly an 11% jump in final gear ratio. The issue is whether or not I am already at the limit of useful gearing as it relates to my engine.

So, you see, The question on torque converter and gearing, is right on target, but it is becoming apparent to me that nobody here actually knows what a torque converter is for, torque multiplication. The same general characteristics of a converter which influence torque multiplication, also have a pronounced effect on stall speed, and in fact, outside of performance circles, you will almost never hear stall speed in a converter discussion, but rather stall torque multiplication ratios. Generally speaking, higher stall speed(I hate that term because it is so ridiculously vague and non descriptive) equals higher torque multiplication. That is why smaller engines in otherwise identical OEM configurations have higher stall speeds. The 4.3 and 5.7 have a very similar power band, with the 4.3 simply being scaled down. The overall shape of the curve is very similar, yet the 4.3 has a higher stall speed. Do you really believe the purpose was to move stall speed? Somehow, I tend to believe the purpose was to increase the torque multiplication. The increase in stall speed was simply a side effect. Yes, that does indeed generate heat, but that is what they make coolers for.

I apologize for being so cantankerous, but I really thought that my choice of forum, and my specific question made clear that I wanted to discuss torque converters and final ratios, not what other brands of vehicles got for mileage. If anyone wishes to discuss the specific question, then I'd love to hear from you, but I won't respond to anything outside that topic after this. Don't mean to be ungracious, but I do intend to keep on point, and there are hundreds, possibly thousands, of threads which already deal with the other generalities and specifics. Of particular interest to me would be the actual torque multiplication ratio of the V8, vs the 4.3 converters.

For the record, ybsane, I am getting just shy of 14mpg in mixed rural/city driving.

[ybsane]

OK, I see where you are going with it and in theory it sounds good but its not and here is why.

1) Tuning is everything, your peak torque and the RPM you are turning at highway speed have to be close in the torque band, reason.? If you are turning to low of RPM's below your torque peak than your foot will be in the throttle to maintain speed. It goes back to old school of having a vaccum gauge to monitor manifold pressure.

2) The stall speed converter idea even if sounds good is not, because higher stall speed is to get you higher off the line into torque peak. It will create more heat problems for the heavy suburban too.

Before I got my tune done I was getting around 14 city and 16 highway, now I am upto 15.5 city and have it 20 mpg going from Charlotte to Columbia and back. Since you still have computer controlling your Engine and trans, you can get the shift points change and your air-fuel and spark dialed in better, to get better mileage.

I get this mileage from 3.73 gears and stock 17" tires, if I knocked my gearing down to 3.42 or 3.08 it would be below the point of were my motor is making torque and I would be into the throttle more. This way very little throttle input is needed to maintain speed, which with a Suburban you know yourself has heavy mass and needs a certain amount of torque to maintain speed.

Rob

[dahoyle]

Thanks for your input. I think the jury is still out, on useful gearing. I have been looking for a chart which details the torque curve, and BSFC curve for the L05 engine. If it was a Cummins, I would be able to obtain one easily.

There is more to the equation than meets the eye, which is why I asked the question. The simple fact of the matter, is that short of anecdotal information, there is no data which supports one point of view over the other, as near as I can tell. Your assertion about peak torque and RPM are almost correct, but it is in fact slightly misguided. The best fuel mileage will be at the lowest point on the BSFC curve, not the peak torque, and then only if you are utilizing all available power. That is seldom the case. In reality, the power needed to maintain cruise, is much lower than what is needed to rapidly accelerate up to that speed, arguably 25% or less.

I know for a fact that the L05 engines produce peak torque between 2400 and 2800 RPM depending on version and application, and I sincerely doubt that is your cruise RPM. Just guessing, based on your description(don't know your precise tire diameter) I would guess that your are near 2000 RPM at 75mph, which pretty much shoots a hole in your best economy=Best torque RPM. If that was the case, everyone would be cruising at 2500-2600 RPM and there would be no such thing as overdrive. No modern vehicles cruise at their peak torque RPM. It isn't even worth debating. Up to a point, the taller gearing, the better the economy. That is simply the case.

You may well be happy with your overall package, but I can almost guarantee that your gearing is costing you fuel mileage, unless you are running oversize tires, or doing predominantly city driving where you never reach cruise. That may not be an issue to you, depending on your priorities, but it is a fact.

I tend to disagree with the statement about mass being more of a detriment, with the higher gears, as cruise is concerned. Obviously, it will take more energy to accelerate it, with the taller gears, assuming the same rate of acceleration, but in a steady cruise, it will become less of an issue. That doesn't mean that it doesn't influence the mileage, but as speeds increase, aerodynamics take over as the primary detriment, and mass falls into a distant second place. !0,000,000 semi trucks running OD bears that out. Get them up to speed on flat ground, and they don't even notice the weight. What is the reason they run taller gears? The bottom line, increased economy and profit.

As to the additional heat generated by a higher stall speed converter, again, that is more of an in town issue, than a cruise issue. Once the torque converter locks, then stall speed is irrelevant. If I was using it to tow heavy, then the higher stall could push things over the limit of what is acceptable.

Don't get me wrong. I don't have the answers. That is why I asked in the first place. I do have a good deal of knowledge about the mechanics and physics involved. There is no question that you can over do the gearing, in either direction, I just don't know where that point is in my particular case.

[user0186]

i think the easiest way to determine if you need a higher (numerically) gear ratio is if your motor lugs or your trans down shifts on hills and then approximate their slope, follow that up with your knowledge of dynamics and you should be reasonably close.

in my experience gas burner full size trucks need a 3.73 or lower rear end ratio if loaded at all in order perform reasonably, not overheat, and not break transmissions--this goes back to peak torque and also the torque multiplication taking the load of all components in front of the rear end.

your plan for the the lockup tc and higher stall may help, but what if it has to lock and unlock on every slope, that is going to impact life.

plus if you can do all the work yourself and this is a 2wd you are goin to spend around $1000 min and may never recoup the cost in fuel.

good luck

[Fast305]

I can say first off MANY of you have TERRIBLE advice to what the original poster is looking to do. A 3.08 gear and 2,600 rpm stall is a GREAT combination for getting away from a light moderately quickly and cruising down the highway. Cruising at or near peak torque is NOT the way to get the best fuel economy possible. A L05 is perfectly happy cruising at 1,600 rpm doing 70 mph and can actually get decent fuel mileage this way. Consider the General built ALOT of fullsize vans with the 700r4 and 2.73/3.08 gears. They are a box on wheels and had MPG ratings of up to 20 mpg highway. The lower the rpm you can cruise at the lower the vacuum you will have. Vacuum at cruise is harmful to fuel economy on an EFI setup. As long as you have enough power to stay out of power enrichement mode, keep the TCC locked, and stay in overdrive, you are going to get better mileage. That being said, the most efficient combination I have ever had for mileage was a 305 TBI with flat-top pistons, 10.5:1 compression, stock "929" 350 camshaft, 1.6:1 full roller rockers, ported 081 TPI heads, stock TBI intake, ultimate TBI moded TBI, doug thorley tri-y headers, and 2 1/2" true duals with a X-pipe, dual high flow cats, and magnaflow mufflers. It had a S10 2,600 stall converter in the 700r4, 5,500 rpm LT1 F-Car governor, and 3.08 geared 10-bolt. On the highway I was able to pull down 23 mpg running 70 mph @ 1,800 rpm. Ever driven a brick 700+ miles on a 30 gallons of gas? I have and honestly miss that little engine from an economy stand-point. The current engine is lucky to get 19 mpg on the same trip.