Diesel Suburban: Transmission
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In my project to convert a 1999 Chevy Suburban to the efficient power of an Isuzu 4BD1T, one of the most important decisions was what transmission I should use.
The Suburban is a 1500, putting it into the "half ton" class. From what I've read, the body and frame are no different from the 3/4 ton version. The 3/4 ton version, however gets a beefier drivetrain, axles, and springs. They also have lower gear ratios in the front and rear axles -- which is to say that they have higher numerical values, like 4.10 and 3.73. These numbers indicate the number of driveshaft revolutions for every rotation of the axle. Because this project is primarily about fuel economy, not brute power, I purposely bought a 1500 to get a vehicle with lighter drivetrain and to capitalize on the 3.42 gear ratios.
Again, because I bought a 1500, I wound up with a fairly weak transmission, a 4L60E. After fishing around 4BTSwaps.com for information, I found out that some people had used the 4L60E behind the 3.9 liter diesels, but they had issues with durability.
The natural upgrade would be to get a 4L80E and install it. I actually found one on Craigslist for a mere $250 and brought it home. I pulled the pan and cleaned things up. It looked like it was in good shape. Then I started looking at the other components I needed to make it work.
A 4L80E for cheap -- is it cheap to use, though?
The 4L80E is an electronically-controlled transmission. The negative side is that the 4L60E couldn't simply use the Suburban's ECU to control it -- at least not without some major programming changes with equipment I don't own. The benefit is that I wouldn't wouldn't need to repeatedly pull the pan to make valve body adjustments to get the shifts at the right rpms and level of firmness. For these types of swaps, most users will purchase a standalone controller and make these adjustments by reprogramming or twisting knobs inside the cab. This is very cool, but none of the good controllers are cheap!
Then, I looked at the other things I needed for that specific transmission. These included upgraded servos, a different internal wiring harness, and a different connector. It turned out that these were typically pricey dealer parts, and they weren't cheap, either. I found out I'd have $1500+ added to the cost of my "cheap" $250 transmission -- assuming that it didn't also need a complete rebuild.
At the time, IsuzuDieselSwapper.com was the only seller of adapters to fit the Isuzu diesels to the Chevy automatic transmissions. I found out more about their adapters. They maintain the Isuzu's stock flywheel and bolt a puck-like adapter to the aft end of it. Then, they bolt a modified Chevy adapter and torque convertor to that. This was starting to look like it would add more claptrap than I wanted, and I wondered about whether that additional moment arm and second flywheel might get off-center and start wobbling around.
Even if all of this worked OK, there are still issues with automatic transmissions that are designed for use behind gasoline engines. These things don't even come up to a proper working pressure at the low operating rpms of a diesel engine. So, I realized I might have to add some transmission pump modifications to my bill to make things work properly.
Another interesting note is that the stock torque convertors in most diesel pickups have stalls that are higher than the engines' peak torque. If we like torque and efficiency so much, why wouldn't we want a full lock-up at some lower rpms?
Between the cost of the transmission, the parts I would need, a less-than-optimal adaptation method to be used, and reports of people having problems getting even the mighty 4L80E to work right and/or last behind even these small diesels, I started to rethink my approach. Hard shifts were even destroying rear differentials and other components -- after the owners thought they had everything right. I've even seen calls for help by the real experts in this field on the forums. So, I checked to see what other transmissions people were using.
It turns out that I could get an NV4500 from a GM pickup that would actually "fit" to some degree. The GMT400 Suburbans (through 1999) share much of the running gear and components that are used on the pickups (through 1998). A quick look at my firewall and the floor revealed areas designed to be cut out for the clutch master cylinder and for a shift tower. Under the dash, I even found one of the studs used to mount the clutch assembly.
While IsuzuDieselSwapper sells kits that will handle these adaptations, I wanted less claptrap in the approach (though many say that these adapters work great). I called Diesel Tim from 4BDConversions.com, and he convinced me that his approach was the best. It uses an interesting combination of components, but all of them are used exactly as they are designed to be used. The transmission cost me $1,200, and the adapter components (including a machined Isuzu manual bellhousing) cost me another $850. It still wasn't cheap, but I felt comfortable that I made the right decision.
Benefits of using the NV4500 include:
NV4500 with adapters installed, painted and ready to install
Fewer electronic headaches, as I am the only computer in the drivetrain. Note that the 4BD1T is mechanically-injected. The only wires strictly required to make the machine move will be the ones connected to the starter!
Fewer fitting headaches, because they were used in this truck body style (though I don't think you'll find any Suburbans with five-speed manuals from the factory).
The 0.73 overdrive (5th gear) ratio, when combined with a 3.42 rearend and 265/75R16 tires will get the engine down to just under 1900 rpm at 70 mph (If you request it, I can share the spreadsheet--which could be used to for any tire/transmission/rearend arrangement). The best torque and fuel economy with this engine is found around 1900 rpm.
This transmission should be bullet-proof behind a 3.9 liter turbodiesel, as this is the same 5-speed used behind many 5.9 Cummins engines in Dodge applications.
Survivalists will lovethis one: The entire setup is EMP-proof. But, if that's a useful feature we're all in a lot of trouble from nuclear fallout!
As an engineer and a certified Project Management Professional, I believe that the decision-making process is important. However, in my next post on this project, I'll talk about how this went together.