Top Page | Technology and Tools
I have a 1986 Jeep Comanche. It's a fairly hideous "sport utility pickup". One of the last product lines of the failing American Motors Corporation, which was subsequently bought by Chrysler Corporation, this was a two-wheel-drive "small pickup" which was characterized by unit-body construction, a 60-degree V6 Chevy LR2 engine displacing 2.8L, an A-904 Mopar TorqueFlite transmission with a locking torque converter, and a VaraJet 2-bbl carburetor. It also had a surprisingly large and capacious bed and a rather high load capacity. Mine came equipped with power steering, and a terrifyingly complex emissions control package.
I bought this vehicle used, but generally well-maintained, with above 120,000 miles on the odometer, in early 2001. In general it has been a very good vehicle, and I have hauled many a load with it. But even at the best of times, this was not in any way a powerful nor fast vehicle; it was in fact a small truck. It also has generally had terrible gas mileage, never better than about 18-19 MPG and generally in the vicinity of 12-14 MPG in suburban or city driving. Part of this is due to the gearing, which has a "tall ratio" which is very well suited for three of the four gearing conditions. First is very low and is suitable for hauling heavy loads in the 0-10MPH range. Second is quite low and is suitable, along with first gear, for hauling heavy loads or cruising unloaded over rough terrain from about 0-25MPH. When the torque-converter locks at about 43MPH, and at speeds above that up to about 70MPH, this is an excellent little truck for highway cruising unloaded over almost any grade or hill. With a load of any significant weight, it was a bit underpowered on the highway when I got it.
The so-called "terrible gearing" of the stock A-904 TorqueFlite for that year is the third gear without the torque-converter locked. The torque converter is pretty "lossy" throughout the operating range of about 20-43MPH, which covers probably 90-percent of all rolling. This lossy torque converter was probably selected to make up for the general lack of power of the 173 cubic-inch (2.8 liter) V6 engine, the same one found in mid-1980s Chevy S-10 pickups. If you don't mind the terrible fuel economy, you can move a lot of load with a small engine by using a lossy torque converter as a sort of additional reduction gear.
At about 162,000 miles, the tired old 2.8L engine finally declared its intent to expire with a hammering sound that sounded symptomatic of bad oil circulation. Checking it for a failing oil pump revealed that it had spun a bearing, fairly common as an S-10 engine killer event. As I had just done a lot of work on the truck, and as it was otherwise in pretty good condition, I decided to go with a GM Performance Parts "crate" engine as a replacement. That engine was a 3.4L (207 cubic inch) engine, with a cam and lifters designed for low-RPM torque. Once it had been broken in and the roughness had mostly gone away, this engine still needed some help breathing. The stock VaraJet 2-bbl carb is a 2-barrel progressive, with a very small primary throat and a rather large (around 46-48 millimeter diameter) secondary bore. The problem is that this secondary doesn't even start to open until the primary is wide open and the demand for air far exceeds its small diameter's ability to deliver.
Here are some notes on conversion from the stock VaraJet 2-bbl "progressive" carburetor to a 2-bbl "synchronous" carburetor, the Weber 38 DGES. I should point out that the Weber 38 DGES/DGAS is a fine two-barrel downdraft synchronous (not progressive) carburetor, but it's not "all that" if you are trying to build a "race Comanche". For that, you might be better served with the combination of an Edelbrock specialty intake manifold, and a Holley 390 CFM 4-bbl carburetor.
