dantheman1540

[QUOTE=shakenfake;5514275]Yeah that truck doesn't drive so this is not a good representation.[/QUOTE

Hey now, that tranny hasn’t made any ill noises in two years.


Fo reel tho, it was an excellent trans for the years it was driving.

@arthursc2 I see an old friend has returned here

arthursc2

iTrader: (5)

I have a feeling that gear ratios and torque converters aren't fully understood here

not sure by who just yet

Topfuel67

So what does a TC do? Multiply torque for free? Let's say the engine's output at 1000 RPM is 100 ft lbs. Is one TC going to magically multiply that output at 1000 RPMs to be 120 ft lbs? No, it is going to have a higher stall at say 1200 RPM to get 120 ft lbs at 1200 RPM, which is why there are stall speeds all the way up to 4500 and beyond. For racing, towing or whatever you prioritize your goals. What does this do for me? I am happy with my subs towing capacity. Towing 7000+ lbs she wants to do 65 effortlessly. I had to put on the cruise control to hold her back. Yes, my launches aren't great. But that is not my goal. My goal is MPGs. I will not sacrifice MPGs just to burn the tires. Towing or not I do mostly highway driving. I often find car parts 100 of miles away, drive and pick them up and then resell them. So my goal is to be able to have the sub cruise at 1700 RPM on the freeway as I feel that would greatly improve my MPGs. If I'm towing, I can leave it in say 4th gear if I were to do a NV4500 swap and have 3.21 gears and probably be around the 2100-2200 RPM range she is now. I am fine with getting 11 MPG towing. I don't like getting 11 MPG when I drive her while not towing as it is just a waste. I drive my Silverado instead. However, the sub has leather seats, power everything and is not as beat as my Silverado. Plus I feel when towing, with my hypothetical 3.21s and a NV4500 combo that I can drive in 5th on wide open flat highway. That way she could get even better than 11 towing too. If not, I downshift into 4th. I towed a 5000 lb car/trailer with my Silverado. Most of the time I was able to cruise at 60 in 5th gear on the flat highway. A slight hill came up and I downshifted to 4th. The thing that was great with that, is she stayed in 4th even thought the hill got better and worse. The shift light kept telling me to upshift, but I didn't as she needed to be in 4th. With an auto it will upshift, downshift, endlessly which to me is annoying.
I understand gearing and TCs. I have specific goals. 4.11s or a higher stall TC are not going to improve MPGs in my situation. Keep in mind that the suburban is limited by the trailer hitch. I can't tow much more than 10,000 lbs due to the hitch, so why go crazy on gears?

Bgbldodge

I’ve never heard or read of an 8.1 getting more than about 13mpg. Especially in a heavy pig like a burb. Everyone I know with one reports the same thing you do. Mileage is the same towing or not but it’s never great. It’s 496ci. Lower rpm could also lead to lugging which isn’t great for mileage either not to mention lower rpm in general doesn’t mean you’ll gain mileage. I’ve geared 3 different vehicles lower and they all stayed the same or slightly improved with mileage because there was less load on the engine to maintain speed. Hell my k5 actually gained mileage up to 55mph going from 3.55’s to 4.88’s because the gearing was so far off. And last I checked you can turn off OD so the transmission isn’t constantly shifting or lock it in third but it’s been a while since I’ve been in that year truck.

So either spend the money to convert and hope it pays for itself in 3+ years or just accept you are driving a brick with a big block.

Topfuel67

I'm betting you have some tall tires for 4.88s to improve your millage. 4.88s on street tires wouldn't even drive on the freeway at a reasonable rpm. I did buy the sub specifically for towing. So I am okay with it's mpgs. I would just like to improve it with a manual trans. My LS in the Silverado is designed for higher rpms than the older sbc or bbc. The 8.1 is a lower revving engine. Plus with the aggressive 5.2 starting ratio of the nv4500 it would be less lugging than the 4L80 even with 3.21s. If I just changed the rear to a 3.21 and left the 4l80 all the statements about lowering the gear would be bad would be true.

arthursc2

iTrader: (5)

my post wasn't directed at you. I agree with most everything you said, except that rehearing might actually improve economy by getting the engine into a more efficient VE zone. I don't know the 8.1 well at all, but peak VE is also peak torque. So if you can cruise at peak torque RPM that's where VE is the best, and where the engine is the most efficient

but that is a single data point. Getting the load moving to that RPM and maintaining that RPM are also part of the equation

_zebra

iTrader: (5)

did you use that diablo to reprogram your tire height while you were at it? you mentioned running 275/75-16s, which are almost 2" taller than the stock 245/75-16s and would throw your mileage calculations way off because you're actually traveling faster than the speedometer shows & farther than the odometer reads.

i ask because you said 65mph is around 2100-2200rpm - but even the stock tires would put 65 at 2000rpm, so yours should be even lower... assuming your torque converter stays locked up. if not, then that's the equivalent of driving with the clutch partially pushed in and a result of not making enough torque at the locked rpm yet not deficient enough to downshift (if i remember my automatic trans principles correctly... it's been a long time).

at the bottom, i'll add some shots of a sheet i built a decade or so ago that shows expected road speeds for each gear in 500rpm increments, based on rear end & tire height. bonus on the right is instantaneous rpm for up/downshift to the next gear if initiated at the red rpm (the 4L85 numbers disregard the TC, though). also, since i ain't never seen or heard of a 275/75-16, i threw in some more-plausible other sizes.

reiterates my experience of gaining fuel economy below 65mph by only swapping my 3.73s to 4.30s with my NV3500.

that was another part of my earlier point, too. the only real way to know for sure before doing the work is to dyno the truck & look at its curve (which the boat intake should help a bit based on some other 8.1 builds i've read over the years). the closer you can get to your peak torque rpm without inducing too much drag (usually from the air, but also driveline load), the better fuel economy you'll get. case-in-point is my 408 cummins (because i remember some of the math on it). a stock one gets the best mileage ~62mph & 1500-1600rpm because that's where aerodynamic drag starts to increase more than the torque as you go faster. consequently, if you're below that rpm but at the same load (be it weight, drag, bigger tires, whatever), then the engine has to work harder to produce the same required power.

in your case, if the torque converter's keeping the engine above 2000rpm to cruise, then it might be because it wants to be there to make the torque needed. if your rear end is basically a circular lever that has a short arm now, making that arm/fulcrum even shorter requires more effort to apply the same amount of force on the back end (meaning with the 3.21s, the truck'll have to produce more torque at lower rpm to drive the same as you do now). on the flip side, doing lower gears like the 4.11s equates to making the see-saw longer on your end so that your scrawny butt can lift the fat kid off the ground. what that means for your truck is that even if you're still at 2200rpm at 65mph again after the trans & diff swap instead of 1700rpm, it'll be without the clutch slipping & turning some of your power into waste heat like your TC's doing now.

it's late & i had to type all of it twice because the webpage crashed... so i hope that still made sense.

_zebra

iTrader: (5)

stock 245/75-16



supposed 275/75-16



common 275/70-16



common 265/75-16

_zebra

iTrader: (5)

for sake of comparison to simply keep your 3.73s: the manual's 4th-5th is basically no change from your current 3rd-4th, and the manual 2nd-3rd will be slightly better than your original 1st-2nd. the bonus is that you've also got an extra gear to start moving with the trailer, if you need it, in the nv4500.

ps - and the comment about your trailer getting wiggly... you didn't load it too far to the back end, did you? you always want a decent bit of tongue weight to keep it from trying to wag the truck.

_zebra

iTrader: (5)

also, comparing trans/diff combos in old cars is much different than a tow rig. i'm happy with the 2.66 1st, .5 6th, and 3.42 rear in my vette on 27" tires... but that's because it only has to get out of its own way, which is only 3300lb. i'd contemplated doing a 3.15 rear or potentially the ZR1's 2.29 1st, but my hefty cam moved my torque band a bit higher & would've therefore lugged the crap out of her when starting from a stop or cruising below 85mph.