Fuel Mileage?

it is sorry my miss quote. this was before I cleaned it up and made a new spacer   

Edited by toolmanxiii - Sep/03/2012 at 1:31pm

I thought the 6=8 was a Clifford trademark,  not Offenhauser's.

Awesome Doc! I hope to be able to dig up some passion to do something with the Rambler I drive everyday. But on the same respect I REALLY love driving a 1959 Rambler just as it rolled off the assembly line. But I may start playing with it once I get done with my 15 year restoration project in the garage (1965 Buick Gran Sport) as I am down to the wire on it. 

You're probably right on the 10% slippage, and a lock-up preventing that. But the old style converters would slip a bit more when pulling hard, so it's probably more like a 15% gain. It really depends on terrain. Flat terrain won't slip the converter as much as hilly terrain, or pulling a heavier load. The converter slipping is what increases torque.

A typical old style torque converter (US types, say before the late 80s) multiplies torque by 2.5 times. So if you have an engine that puts out 300 ft/lbs it's 750 ft/lbs after it goes through the torque converter -- with the converter slipping as much as it will. Say on a take-off from a dead start with a heavy load. Even then it only slips that much for a short time, and slips less the more up to speed the engine gets. Modern high efficiency converters slip much less even without lock-up. The Jeep AW-4 converter only multiplies torque about 1.2:1 according to the Jeep manual.  That's one reason it has a low axle (3.55 in Cherokees/Comanches). First gear is only 2.8:1. 

What you are reporting with the Pacer, 19-20 mpg on the highway, is about all one could ever expect from a stock car. The lock-up 998 might help a little, but the stock 2.73 axle (1977 models) isn't doing you much good. It will pull you right along at 55-65 okay, but on any car it's been proven that aerodynamic drag goes up exponentially after about 65 mph. A bit less on the Pacer than something like my wagon, but still a lot more. At that speed you need more power, and that means more rpm.

The 2.73 with OD was a mistake IMHO. Almost ALL OD equipped cars have more gear than that, especially for a 0.75:1 OD (25% -- the old Borg Warner units were 30% -- 0.70:1). A 3.15 or 3.31 gear would have made the OD cars much nicer in town and on the highway, most likely increasing gas mileage by 2 mpg all around. Conventional wisdom at the time was the national speed limit was 55 mph, and at that speed 2.73 with OD was acceptable, but even then I'd have wanted a 3.08 or 3.15. 3.08 was optional, don't know why AMC didn't make it standard for OD cars.

The Eagle would do much better around town with lower gears (at least 3.08) as well, but would suffer a bit on the highway at speeds over 55-60. Can't have everything with just three gears! The funny thing is the AMC sixes could have taken a bit more gear spread in the transmission. The cam would have to be tailored to a wider rpm band, but it's there. Just another few points lower first and even spreads between the others would have made a noticeable difference. First for the mid 60s Americans with sixes was usually 2.6:1 with a 3.08 axle (and these got the same gas mileage as your heavier Pacer). Final drive is trans ratio x axle ratio, or 8:1 in this case. A 3.0:1 first would make final gear reduction 9.24:1. Then using a 2.73 axle would bring the final drive back to 8.19:1 -- same first gear, but a higher top gear (top gear is always 1:1 unless OD is used).  OD accomplishes the same thing, but almost always a LOWER axle is used so the engine will be in its torque band at cruising speed. Today cars are geared to cruise about 2000 rpm @ 70 mph. Cars in the late 70s were geared to cruise closer to the 55 mph mandatory speed limit. I've found that AMC sixes (all type and sizes) are factory cammed to produce good torque around 2000 rpm, fall under that and the engine will happily pull along without fuss, but it's working harder and mileage will suffer. You can't tell it's working that hard at 1800 rpm, but a long trip and keeping up with fuel consumption will definitely show it! I picked up 2 mpg all around when I went from a 3.08 to a 3.55 gear in my car with the AW4 OD (early model with 0.70:1 OD). When I was running a manual OD it liked the 3.31 gears I had. When I rebuilt a hopped up 4.0L to replace the 4.6L stroker I chose a cam that produces torque at a lower rpm. It's just as happy cruising at 1800 rpm now as it was at 2000 rpm before. The cam I had in the stroker was a bit more than a late 70s stock 258 cam, but not enough!