[Bronze66]

I have been under the assumption that the highest vacuum reading will give the best mpg for fuel economy. I noticed the other night that when I locked my torque converter(manual switch) that the vacuum would drop anywhere from 1-4" depending on speed. My thinking is I'm not getting the economy I could because of the lower vac reading. Puts more load on motor when it's 1 to 1. If that's the case why have a locking converter? The rpm drop when locked is only 200rpms max. Is my thinking right? Maybe to many of the hippie days flashing back!

[Schurkey]

For a given gear ratio, higher vacuum generally indicates more fuel-efficient operation. Change the gear ratio and all bets are off. Engaging the TCC is more-or-less the same as going to a higher gear.

Actually, one of the reasons a Diesel gets better mileage than a gas engine is that there is NO manifold vacuum--the engine doesn't have to fight the vacuum and still make the vehicle go. IF you had an engine of low-enough power to run WFO (zero manifold vacuum) and wasn't jetted rich for power, you'd get better mileage still. Unless you want to add a tiny secondary "cruising" engine to your car, balancing lower-rpm against manifold vacuum is the best you can do.

[Half-Inch Stud]

You CAN calibrate the Vacuum reading in Hg to be instanta MPG by selecting the proper rear gear (final drive ratio)...like a mechanical Car Computer huh. Good example is the range of 2.73, 3.08, 3.31:1 gear with 26", 27", 28"tire.

Too much gear will cause Vacuum readings to appear numerically higher than the numerical MPG. For example 3.54:1 & 27" tire (like what I got) and other over-Revin final drive ratios. HIS

[Bronze66]

"Too much gear will cause Vacuum readings to appear numerically higher than the numerical MPG. For example 3.54:1 & 27" tire (like what I got) and other over-Revin final drive ratios. HIS"

Uh? Do you mean It will show a higher(lack of better words) on the vac gauge than what the better mpg vac reading would be?

This is something I'm playing with this year. I made the car faster and decided to see if I could get some mpg out of it. It should run 13:00s may be high 12s. I currently got 16mpg around town type driving over the weekend. That was with a few gettin on it. Some how the timing got advanced 5* more than I wanted. Turned out to 20 initial and 40 total. It just wasn't running right so I finally looked in to it. Way to advanced and killing power. But I got almost 17 highway that way and only 13.5 around town. So I'm going to have to do some highway running again. I think I can hit 18mpg easy and hoping for 20!

It has 3:55 gears with a .67 overdrive = 2.37 with 26" tires. I was locking the converter before all the time in OD. Now my thinking is not to because of the higher vac reading and the 16mpg I got doing it that way plus the timing retard back to 15-35. I still have to see what works on highway. Locked or unlocked converter.

[Cliff R]

I ran a 4L60 in my 67 Impala SS for several years, racked up just over 35,000 miles on it before we sold it. When the transmission was first installed, I used a custom L/U converter. Fuel economy on the highway ran between 18-26. A few hundred miles into the adventure, we blew up the L/U converter (took the entire trans with it!). I replaced it with a custom non L/U unit. Fuel economy only dropped about 1mpg, still averaged 18-25. I LOVED the new converter, and was getting pretty tired of the L/U unit anyhow. Having torque multiplication with any heavy throttle opening was a LOT better than L/U converter, as it uncoupled anyhow.

We used a TCI wiring kit for the L/U converter, routed thru a vacuum switch hooked to ported vacuum, so it would lock and unlock anytime the vehicle was coasting, or any heavy/full throttle application. The solenoid was grounded on a normally "open" 4th gear pressure switch, so it only worked in OD anyhow. Still didn't like it much, preferred the non L/U converter at every level.....Cliff