Which tranny should i run?

[GreaseDog]

The 3 best trans guys on the forum (Greasedog, oneluckypops & HotRodPC) have all posted on this last page and you may not realize that fact. You should drop all your old wive's tales you've been holding onto and listen to what they have to say. An engine has an optimum operating range around 2,000 rpms, depending on how it is set up. Lower rpms wont improve mpgs, but lower rpms will hurt your trans. Personally if I sense attitude from someone I'm trying to help I drop the thread like a hot steaming pile of ****, because that is what it will turn into, and they can go find help somewhere else.

Thanks for the props, but I'm by no means up on the working of an automatic Trans, though I've been around these trucks for many years, I do know what it takes to keep one alive. The biggest thing is to not spin the Trans too slow, you are right, 2000 is right about the sweet spot for a stock engine. When you change engine parameters, you want to match them to the rpm range you will be most commonly operating in.

I could go on for days about matching the entire drivetrain and setting it up for maximum drivability and economy, but I'll spare you guys that novel, and just leave a few tidbits.

Operating at least 2000 RPMs promotes good fluid flow, and cooling, which will promote good Trans life.

Gearing to put your most common driven rpm range close to peak torque has a few advantages, the first being fuel economy. The second being the elimination of lugging the engine, which will provide a smooth flow of power to the engine. Think of this as pulling a load, which its going to be easier on equipment? Pulling another vehicle with a chain? Or the same vehicle on A trailer? Lugging the engine, it actually dies for a second, and then starts back up, providing a "hit" in the drivetrain, which is not good for durability on any part of the drivetrain.

 

[Driver4r]

Thanks for the props, but I'm by no means up on the working of an automatic Trans, though I've been around these trucks for many years, I do know what it takes to keep one alive. The biggest thing is to not spin the Trans too slow, you are right, 2000 is right about the sweet spot for a stock engine. When you change engine parameters, you want to match them to the rpm range you will be most commonly operating in.

I could go on for days about matching the entire drivetrain and setting it up for maximum drivability and economy, but I'll spare you guys that novel, and just leave a few tidbits.

Operating at least 2000 RPMs promotes good fluid flow, and cooling, which will promote good Trans life.

Gearing to put your most common driven rpm range close to peak torque has a few advantages, the first being fuel economy. The second being the elimination of lugging the engine, which will provide a smooth flow of power to the engine. Think of this as pulling a load, which its going to be easier on equipment? Pulling another vehicle with a chain? Or the same vehicle on A trailer? Lugging the engine, it actually dies for a second, and then starts back up, providing a "hit" in the drivetrain, which is not good for durability on any part of the drivetrain.

We advanced my cam 4 degrees to bring the optimum rpm range down a lil bit to make it more responsive on the street(this engine has been in 2 other vehicles now(72 lamans,73 monte,my truck) There is no lugging or hesitation in my drivetrain. And I run about 2.1k-2.5k rpm on average anyways. Befor i was around 3.1k-3.75k

 

[HotRodPC]

We advanced my cam 4 degrees to bring the optimum rpm range down a lil bit to make it more responsive on the street(this engine has been in 2 other vehicles now(72 lamans,73 monte,my truck) There is no lugging or hesitation in my drivetrain. And I run about 2.1k-2.5k rpm on average anyways. Befor i was around 3.1k-3.75k

We get all that. You've compensated the motor for the tall ratio, but still the transmission is feeling the strain. Again, I've used this example 100X over and will again. Go get you a 10 speed bicycle and put it in 10th speed. That's the big sprocket up front and the little sprocket in the back. Take the bike for a short ride. Better yet, take that bike like that for a ride on a road that has just a little bit of an up incline and feel the strain you're putting on your legs. This is what you're doing to your transmission. No matter the motor, the transmission is still feeling the strain. All the good power in the motor means, is your able to put the trans under more strain to make it take off like you want it to.

Let's take a look at the mechanical, or engineering type charachteristics of what's going on, or the mathematical charachteristics which ever you'd like to call it. Keep in mind, no matter how much torque a motor has, what matters is what you get to the ground. Drive wheels are what put the torque to the ground. To make this easy to grasp, with 4.10 gears, let's call it 4.0. That's your motor's torque multiplied by 4. Now you've changed to 3.08's let's call it 3.0. Now you're only multiplying your torque by 3. So what you've done is had a 25% decrease in torque multiplication. This means EVERYTHING throughout your powertrain in front of the rear axles is now working 25% harder, and has 25% more strain put on it. That's 25% more strain on the U joint's, the transmission, the motor and all. This is also why lower geared vehicles can carry more weight. With more toque multiplcation, they can carry more weight.

In your case, I know what's up here. Just pointing out, changing the power curve on your motor, doesn't reduce the strain on the transmisison 1 bit. The transmission still feels the strain put on it The ultimate set up will always be a gear ratio in the 4.'s and an overdrive gear to compensate that ratio for highway driving. For a half ton and OE tires, 3.42 is the bare minimum, giving almost 3.5 x torque multiplication that I'd consider baseline. If towing a small boat, step down to 3.73's for 3 3/4 torque x. Tandem axle trailer and maybe a car, go for 4.10's, and if running a taller wider tire, I'd compensate that with 4.56's.
I'm hoping to run 35's in tires on one of my 4x4's. When I do, I'll be using 4.56 gears and hope to be using an overdrive tranmission. But honestly, with 35 in tires, you don't have to have OD with 3.73 or 4.10 gears. I know tires are expensive too, and I know the 3.08's were at hand at no charge, but changing to 35in tires would have lowered your rpms from OE tires size quite significantly. I myself would rather go taller tires than taller gear ratio. Granted, the tires are heavier and more mass to turn, but you still have torque multiplication x 4 to compeansate for that. This is why you should always consider all aspects when making a combo decision.

 

[Christian Nelson]

I hate to be the disagreeable one here, but the rpms on my Suburban with the 4l60E are right around 2,000 at 75.. Going 55, its at like 1,200..

Biggest difference I've noticed between modern trucks that get better gas mileage than older ones, is OD, and lower cruising rpms. EFI doesn't get you as much from what I can tell as the higher gear ratios. Lower rpms, higher gear..

THat said, one DOES need to be sure enough fluid is being pumped to keep the tranny cool, etc.

I've seen guys drop 5speed manual tranny's in place of 3 speed auto's, and get sometimes almost double the gas mileage, no other changes made.

What can I say?

If you live in a hilly area, or do a lot of towing, you will see less improvement, but if most of your driving is highway, and relatively level ground, I think it will be ok.

You may end up feeling a bit doggy.

The advantage of more gears is you can start out with a lower 1st gear.

I am not sure though if the 700r4 indeed has a lower 1st than the th350, etc.

If it did, you might end up with a nicer setup with the tall gears, since you won't be lugging as much off the line.

Eventually though, the aerodynamics kicks in, and the square isn't a real winner there, too much above 70mph, and you are just gonna suck gas, regardless of the gear ratio.

I'd figure out your cruising speed, and design the restof it for ideal rpms at that speed, and figure out whether you even need the OD.

 

[HotRodPC]

So what are you disagreeing with?

 

[crazy4offroad]

Ever get the feeling you're...

 

[GreaseDog]

Lets give them Trans replacement tips instead...

 

[1973 Chevy]

Lets give them Trans replacement tips instead...

It did get a little of course didn't it but you can learn some stuff now.

Got my hands on one of those adapters for $35 from a friend. No trans yet hopefully next paycheck.

 

[Christian Nelson]

So what are you disagreeing with?

Well, I must have read wrong, but I for some reason thought people were saying that dude should gear it so he's cruising at 3,000 rpms, which I consider to be not very good if you are looking for gas mileage.

I've seen the argument before on the slant 6 forum, and a harley shovelhead forum. Some people swear that one should cruise in the middle of the power band (3,000 for most motors, in general) and others argue that one should cruise at the begining of the power band, and downshift if they need to hit a hill or what have you for the best gas mileage.

My experience, and looking at newer cars tells me that cruising at the begining of your power band is better than middle high end of the power band, if you are looking for best fuel mileage for highway driving.

Example, my saturn, little four banger 70mph is only turning 2.5K.. It has some pep if you need it there, but if you really wanna pass someone fast, you just drop it into 4th, and hammer it, and you've got all the power the little dear can muster. I get 35-40 mpg out of the little thing, and I am positive that most of the reason is the high gear low rpms. My 77 Honda for example, 70 mph was 3.5K it got only 30 mpg years ago. I tink you could put the gears from the saturn on that honda, and it would have gotten similar gas mileage. My 80 toyota with the 4 speed was even higher rpm, and got only 25 mpg, but MAN did that thing fly!! I could leave rubber for a city block with that little car, wish I had kept that one..

But, maybe I am just agreeing with y'al, sorry to get all ready for an argument and ending up just agreeing with ya..

 

[crazy4offroad]

Yeah, it doesn't have anything to do with those cars having 1/2 as many cylinders and 1/3 the weight as our trucks.

 

[HotRodPC]

No, I think we all agree the cruising RPM being lower is better. Now that doesn't mean LOWER is better, just lower than 3000. There is no pre determined set rpm to be. All things need to be considered, like Cam, Compression ratio, gear ratio, weight, wind drag etc. An RV cam will actually get better mpg at a higher rpm that a stock OE cam for example. Like say 2200 rpm for example, where the OE cam would probably do better around 1600-1800 depending on how wimpy it is.

 

[Christian Nelson]

It's all about weight to horsepower ratio. If anything, the square has ponies to spare in that area.. Bottom line a given engine has a specific volume in all it's cylinders, it would stand to reason all other things being equal that lower rpms would reduce fuel consumptyion in a given engine. It's not like I am saying you'll get 40 mpg from a square with a v8, but i think one can do better than 10, maybe even knock on the door of 20 mpg.. not easily, but possible.

Yeah, it doesn't have anything to do with those cars having 1/2 as many cylinders and 1/3 the weight as our trucks.