Overdrive Swap Economics

[JCampbell]

Starting to get serious about putting my dually back together. I've been focusing a lot on my options regarding the transmission. I've been seriously contemplating replacing the TH400 with a 4L80e to gain a little highway MPG, but now I'm wondering if the headache and expense will be worth the hassle.

It's not a daily driver. I'd be surprised if it'll see 7,000 miles/year, and 90% of that will be city driving. The only steady highway mileage it will see is the occasional 2,000 mile trip from SoCal to Washington and back. I don't do much heavy towing...maybe 10,000# (max) 2-4 times a year. It's really more of a heavy cruiser than anything else.

The truck was getting 11MPG before the rebuild with a Quadrajet. The new engine will be a TBI unit, so I assume the mileage will stay about the same. Anyone done this conversion before? What are your thoughts?

There's a link to the build thread in my sig.

[SCOTI]

Originally Posted by JCampbell

Starting to get serious about putting my dually back together. I've been focusing a lot on my options regarding the transmission. I've been seriously contemplating replacing the TH400 with a 4L80e to gain a little highway MPG, but now I'm wondering if the headache and expense will be worth the hassle.

It's not a daily driver. I'd be surprised if it'll see 7,000 miles/year, and 90% of that will be city driving. The only steady highway mileage it will see is the occasional 2,000 mile trip from SoCal to Washington and back. I don't do much heavy towing...maybe 10,000# (max) 2-4 times a year. It's really more of a heavy cruiser than anything else.

The truck was getting 11MPG before the rebuild with a Quadrajet. The new engine will be a TBI unit, so I assume the mileage will stay about the same. Anyone done this conversion before? What are your thoughts?

There's a link to the build thread in my sig.

I went through this exact scenario when swapping the bbc in my dually (is it worth the investment given the limited use/range to swap the th400 for a 4l80e OD trans while things are apart?). Even w/the 4l80e already in my possession, it was looking to be about a grand to do the swap in a user friendly configuration. Factor in freshening-up the trans & it was quickly getting expensive.

From my point-of-view, you're 100% dead-on:

• OD has minimal to zero impact for city driving.
• At a 1-2mpg gain, how many miles would be required before recouping the initial expense for the swap? Something that sees limited seat time takes that much longer to recoup the investment. Until that point in time, it's just an added expense.

Obviously, the one absolute 'pro' for a mild TBI motor would be the rpm reduction while on the open road. Quieter is better when driving mile after mile IMO.....

I decided to stick w/the th400. I am still considering future options though & one is a company that builds 700r4's to live in HD truck apps (I found the info on this site in another thread). To me, a 700r4 built to survive a bbc/torque environment would be the best option as it gives the benefit of the deeper 1st gear for getting these fat f@#kers moving (3.06 vs 2.48); the ease of swapping (multiple swap kits w/o the need for electronic management), & a better OD ratio (.70 vs .75).

[JCampbell]

Got any info on that 700R?

[Tx Firefighter]

My 454 TBI truck with factory 4L80E and 4.10 gears never once exceeded 10 miles per gallon ever. That's 100 percent perfect tune and driven quite conservatively. You're already ahead of the curve at 11 mpg. Without putting pencil to paper, I'd estimate you'd need to drive it about eleventy trillion miles to recoup the conversion costs with what you have now. Even if it got you a solid 2 mpg increase, how long would it take to get your money back ?

[JCampbell]

Originally Posted by Tx Firefighter

Without putting pencil to paper, I'd estimate you'd need to drive it about eleventy trillion miles to recoup the conversion costs with what you have now.

Yeah...that's kinda how I'm leaning.

[SCOTI]

Originally Posted by JCampbell

Got any info on that 700R?

http://performabuilt.com/heavy%20hauler%20700-60E.html

[jimbonice]

Throw some 3.54's in the back and you'd be good. I've got a 700r4, and I may still put some taller gears in it. Never been a fan of 4.10's or lower.

[Chevyman350]

There are alot of factors that go into the fuel economy.

The most significant of which is putting the engine RPM at cruzing speed at the torque peak of the engine. This is the spot where BSFC for the engine is the lowest (best spot for economy).

My 1987 1 don reg cab dually would knock down 11 mpg with a 454 tbi highway speeds of 70 mph with a th400 and 3.73 gears although I had smaller than stock rear tires at the time.

I have heard from many people that after they do a over drive swap that the OD is to tall and the engine bogs. This statment makes me bieleve that you should look at the hole system to optimize drive train for the engines powerband inorder to get the best Fuel mileage.

For refrence most of the diesel (cummins) guys the peak torque rpm is aproximately 1800 rpm so they gear to have close to that rpm at cruse speed to maximize fuel economy. I know a BBC has a much hirhger (RPM) torque peak than a cummins so draw your own conclusions.

My .02

Hope this helps.

As stated before the ecomnomics of the change are dependant on the reasonably expected increase in fuel economy so if you were getting 11 mpg before what new mpg would you need to pay off the conversion given the highway mileage you use and the time you want to recover your money in (at current fuel costs), this is (ROI) return on investment.

Now that I have stated the obvious are you looking for what others have gained in fuel economy to help you calculate your ROI?

Wow that was alot longer than I thought when I started this reply!!!

[JCampbell]

Originally Posted by Chevyman350

My 1987 1 don reg cab dually would knock down 11 mpg with a 454 tbi highway speeds of 70 mph with a th400 and 3.73 gears although I had smaller than stock rear tires at the time.

What size tires?

Originally Posted by Chevyman350

As stated before the ecomnomics of the change are dependant on the reasonably expected increase in fuel economy so if you were getting 11 mpg before what new mpg would you need to pay off the conversion given the highway mileage you use and the time you want to recover your money in (at current fuel costs), this is (ROI) return on investment.

It’s not so much a "return on investment" that I’m looking for, in as much as I just want to get this thing back on the road without having to reinvent the wheel. For me 1-2 MPG isn’t enough of an increase to suffer through the added time/cost/headaches.

[INSIDIOUS '86]

You guys also got to look at the amount I wear it cuts down on an engine as well. Lower rpm on the hwy is less engine wear. IMO if the trans don't pay for itself still its at least worth the comfort to me just knowing that driving across state lines won't cost so much
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[68Timber]

I'm struggling with this on my 79, rebuild my TH350 or upgrade to a built R4. I'll be at $450 or so just on the converter either way. I only drive this truck for fun, weekends mostly, and don't need it to go down the highway. I want an R4 but don't need one.

[JCampbell]

The other thing I considered is that the cost to rebuild a 4L80e is substantially higher than a TH400.

[68Timber]

Originally Posted by JCampbell

The other thing I considered is that the cost to rebuild a 4L80e is substantially higher than a TH400.

And the 400 will likely only need to be built right once in the next 20 years.
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[Firebirdjones]

Originally Posted by BRUISER

And the 400 will likely only need to be built right once in the next 20 years.
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Yup, 400's are cheaper to build.

I decided to bite the bullet and do the swap but went a much easier route.
I had the 4L80E here and decided to do a gear vendor instead. You can figure about a 30% increase in mileage, not alot if you are getting 11 mpg now. But I also like the RPM decrease, less wear and tear on the engine, less noise.
It also turned the trans into a close ratio 6 speed, great for pulling the 10,000 lbs trailer from stoplights, and it can be engaged while towing on the highway to drop rpms as well, something that you can't do with the 4L80.

Besides, you'll find once you get into the swap (because I did) that installing a fresh 4L80, controller, driveshaft, torque converter, you'll be almost exactly right on the price of a gear vendor unit, and still only have a 3 speed trans, with an overdrive you can't use when you tow.

[SCOTI]

Originally Posted by BRUISER

And the 400 will likely only need to be built right once in the next 20 years.
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A 4l80e & th400 would be the same as far as longevity after rebuild since they're basically the same trans (minus the electronics & OD).

Originally Posted by JCampbell

It’s not so much a "return on investment" that I’m looking for, in as much as I just want to get this thing back on the road without having to reinvent the wheel. For me 1-2 MPG isn’t enough of an increase to suffer through the added time/cost/headaches.

Exactly why I opted to stick w/the th400 (not enough difference to justify the expense of swapping). I also looked into the GV unit but decided against those as well because of cost & complexity for uneducated users (RideTech used one on their 48hr Camaro & destroyed it by simply overlooking it being engaged when someone put the car into reverse. It mucked something up & ruined the unit).

What I do like about the GV was the under & over drive capabilty. But if car guys can mess one up like that, it makes one hesitant. Cost of a new unit is on par w/the 4l80e rebuilt so they're a wash. Now, if you could find a GV unit that's used but in good shape, it might be a worthy swap.

[Firebirdjones]

Any overdrive swap done correctly gets pricey.

Just depends on how much you use that particular vehicle as to whether it's worth the expense for anyone.

I never was looking for gas mileage increases to pay for the swap I just wanted less wear on the engine (engines are getting expensive too ) Less noise makes the drive pleasant, but I think I'm starting to get old.

It doesn't hurt that I can get close to the mileage my neighbor gets in his 03 duramax in my dinosour 79 1 ton with a 502, so I'm not complaining.

[Chevyman350]

Originally Posted by JCampbell

What size tires?




It’s not so much a "return on investment" that I’m looking for, in as much as I just want to get this thing back on the road without having to reinvent the wheel. For me 1-2 MPG isn’t enough of an increase to suffer through the added time/cost/headaches.

225 75 r 16

If your looking for the least trouble then you have your answer. I guess my guestions was what MPG would be your cut off for worth while at this time?

[68Timber]

I know everyone says the 4L80E is a TH400 w/overdrive and I'm sure it's stout but I'm not convinced it'll give the same years of service with zero trouble. I had a 4L80E once in a 1999 truck that started acting up. I googled the symptoms, it seemed I needed to replace one of the shift solenoids. It was only $20 or $30 at the dealership but I traded the truck without ever swapping it. I own my first truck with a TH400 now, it's a 1985 model and likely as not it hasn't been touched. YMMV, I really don't trust GM tranny's anymore. I've been burned on a few factory R4's that gave up before they should have. Everyone but GM knows how to cure the problems. It's probably the biggest reason I won't buy a new GM truck anymore.

[JCampbell]

Originally Posted by Chevyman350

225 75 r 16

If your looking for the least trouble then you have your answer. I guess my guestions was what MPG would be your cut off for worth while at this time?

That's what size I'm running.

If I could get into the 14-15 MPG range, I'd consider it.

[Firebirdjones]

Originally Posted by JCampbell

That's what size I'm running.

If I could get into the 14-15 MPG range, I'd consider it.

That's fairly easy, I'm doing better than that with a 502.

Lots of other things to consider though before dropping the overdrive in. Lots of ways to find mileage. Narrow tires, lightweight wheels and driveshaft, a very good tune with a wideband and someone that knows what they are doing goes a long way, a good intake and exhaust system to free up some power helps mpg, rid yourself of the mechanical fan and go electric will give close to 1 mpg alone.
These tricks done to mine got me 11-12 mpg around town and 14 mpg highway (without the overdrive ) Running 3.73's and a 400 turbo.

It can be done with a little work and patients.

[KQQL IT]

My vortec 7.4 does 10/12 454 4L80 4:10 and 31" tires. Crewcab short bed single rear.
Years ago did a doug nash o/d on a '86 1 ton with 454 and a quadrajet weighed 8500# before we "over"loaded it and it did 10/12.
We also had an 89 with tbi and it got worse mpg's than the 86 did same application.
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[Redcap]

I'm planning on using one of these Ranger Overdrive units in front of the SM465 in my K20.

[cal30_sniper]

I can shed some practical light on this topic, as I've been working over the same issue during the last year, and have actually had time to really weigh some of the benefits/detriments of an overdrive swap in one of these trucks.

I've got an '86 C20 Suburban, that was a factory 454/TH400 combo with 3.73 gears. I've been running 265/70/17 all terrain load E tires (32.7" tall). After a good tune and clearing out all the emissions junk from under the hood, I pulled down a best of about 12.5 mpg at a steady 60-65 mph cruise. The factory TH400 was doing just fine, but I had a stout TH700 in my wrecked '90 burb, and figured I'd try the swap, since it would only cost me a couple hundred bucks and time, I might pick up some gas mileage, and I knew the history of the trans. I wasn't hoping for much mileage, and my main motivators were to drop engine RPMs on the highway, add the lock-up torque converter, and switch the 75k mile TH400 out for a 20k mile TH700 with a +3 qt pan, B&M Traveler torque converter, B&M shift kit, B&M lock-up controller, and some other goodies that I put in it.

When I did the swap, I also added Doug-Thorley tri-y headers and had a 2.5" h-pipe dual exhaust system and magnaflow mufflers built for it. After some tuning (with a wide-band O2), I got a best of 14.2 mpg at a 70 mph cruise. I normally average around 13mpg with mostly highway driving, and 10-11mpg with mostly city driving now. That's a benefit of about 15% increase in fuel economy at a faster speed. With the TH400, every mph above about 65 rapidly ran my mileage into the toilet. I can easily cruise 75-80 mph now and still average around 12mpg.

The gas mileage increase is nice, but dropping several thousand dollars on a gear vendors or 4L80e setup will never pay for itself. As far as engine wear, keep in mind that even though your spinning the engine slower, it's under more load. Furthermore, the overdrive adds in a much larger danger zone for pinging. Low engine speed and high load is very conducive to detonation, and it can do severe damage to your engine. You have to be conservative with your timing and watch your AFR in situations where the torque converter is locked and you are below 50mph or so in 4th gear. On a separate note, even the warmed over 454 can be a little lacking in the torque department below 65mph. In 4th gear, with 3.73s and a 32" tire, you're only turning about 1800-1900 rpm with the converter locked up. The motor is much happier in the 2200 rpm and up range, but you start dealing with significant wind resistance at those speeds, which is always going to kill mileage in these trucks.

I might see a little better mileage by swapping to 4.10s, but it wouldn't ever pay for itself in cost and labor to do that. When the 454 I have starts giving me trouble, I plan on building a mechanical fuel pump Gen VI block with some good heads and a very mild roller cam. That should take care of my torque problems down low and bring the highway mpg up a bit.

I did the TH700 because I had one already set up for a suburban, and it was a fairly easy swap. The lock-up converter drops trans temps a lot, and with a good shift kit and top quality fluid/cooling, the TH700 will live up to any reasonable driving expectations behind an engine that's not too wild. I've beat on mine pretty hard behind the 454 before, and never experienced even the slightest hint of a problem from it. That low 1st gear also works great for crawling up really rough mountain roads.

I don't think I would even consider the 4L80e swap. I don't like the whole idea of the electronic controller. Sure, it's nice for tuning, but it's one more black box to go wrong that I wouldn't be able to fix on the side of the road. Ditto on electric fans/relays. I like the simplicity of the TH700. A TH400 with a gear vendors would do the same thing, just with a bit less overdrive and no lock-up converter.

Sorry if that's a little long-winded. Hopefully you can pull some good info out of it. If you're looking for it to pay off, don't do it. If you're looking for increased driving pleasure, I recommend it.

[KQQL IT]

^^^ smiles per gallon. Always thought that was the reason we do this.

My 73 with a 850 double pump holley big cam and a sloppy 4k converter 3:82 geared is by far my favorite to drive...
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[Ford Assassin]

I get 13-14 mpg in town, 18-19 mpg highway in my K5, I have a built 4L80e w/a billet 3600 stall converter and its been hassle free from the word 'Go'.

Keep in mind this with a supercharged engine driven by a guy with a heavy foot.

Details in my sig....