Which transmission choice will give me better mpg?

Question:

I have a 1988 Airstream 325 motor home that’s on a Chevrolet P30 chassis. It has a 454 cid engine in front of a TH400 automatic transmission. I want to increase the fuel mileage on the coach and I’m trying to decide between one of two methods.

The first method is to replace the transmission with a 4L80E transmission. Will this transmission bolt up directly to my 454? If not, what’s necessary to mate the two? I thought about using a 700R4 overdrive transmission, but reliability and strength issues convinced me a 4L80E would be the right tranny in a heavy motor home.

What torque converter specs, such as stall speed, would be best for this application? I understand that aftermarket control units are available to handle the shifting in my non-computer vehicle.

The second choice is to install a Gear Vendor’s under/overdrive unit behind the TH400 transmission currently in place in the Airstream.

Which setup do you believe will be most effective and give me the best gas mileage?

Answer:

You will need an adapter for a late model 4L80E overdrive transmission to work with your somewhat vintage 454 cid big block. You can buy an adapter kit from Bendtsen’s (http://www.transmissionadapters.com).

The best torque converter specifications depend on a number of variables. In general, converter stall speed should be matched to the engine’s power curve. In addition to knowing the engine’s power curve, it’s also very helpful to know the vehicle’s weight and the engine’s peak torque when choosing a torque converter. As a good rule of thumb, your converter’s stall speed should be about 500-750 rpm less than the rpm at which the engine reaches its maximum torque. In other words, if your engine reaches maximum torque at 1800 rpm then the torque converter should have a stall speed in the vicinity of 1050-1300, give or take a little bit depending on how much the vehicle weighs. Keep in mind that a torque converter’s actual stall speed will vary depending on the weight of the vehicle it’s in. In general, a lighter vehicle will yield an actual stall speed that’s lower than the converter’s rating and a heavier vehicle will do the opposite.

The best advice I can give you regarding torque converter choice is to gather up all relevant information for your motor home (engine power curve, peak torque specifications, weight, tire size, transmission gearing, and differential gearing) and then rely on the recommendation of the converter manufacturer that you choose.

As far as whether to swap your TH400 for a 4L80E (or another more modern 4speed electronic overdrive transmission) or install a Gear Vendors under/overdrive unit, I think that the Gear Vendors setup will be more beneficial in a few ways. By splitting the ratios between the existing gears in your TH400 it will improve fuel economy and drivability, and also will lengthen the life of your engine and all engine-driven accessories and components. The under/overdrive does that by consistently reducing engine rpm at nearly all road speeds in nearly all circumstances.