The pinnacle of slant six fuel economy was the 1976 Plymouth Feather Duster / Dodge Dart Lite, which was a lightened 2-door coupe (150+ lbs lower weight with some aluminum components) with low rear axle ratio (2.76:1 for automatics, 2.94:1 for 4-spd OD manuals), high-flow exhaust system, and a specially tuned 225 cubic inch slant six engine. The EPA city/highway ratings of the Feather Duster / Dart Lite were 22/31 mpg (7.6 litres/100 km highway) with the automatic and 24/36 mpg (6.5 litres/100 km highway) with the aluminum case 4-speed OD manual transmission.and many owners of these cars with automatic transmissions report highway fuel economies of 31-32 mpg. These cars also had EGR systems to control NOx, which also helped to reduce engine knock. The Feather Duster distributor used the PN 3874795 & 3874796 distributors
From previous experience, I found that my highway fuel economy is best with a fuel mixture that is just rich enough to not have lean surge. It is generally accepted that the optimum total mechanical (initial + centrifugal) advance on slant sixes is 30-32°, all in by ~3000 RPM. Some have found that the best fuel economy is 44-45° of total advance (mechanical + vacuum) while others find that the best slant six fuel economy has 45°-50° of total advance. Since I had my initial timing set to 8° BTDC, I think there is still room to improve my highway fuel economy. The old Scamp distributor (PN 3656859) had worn bushings that caused my timing to vary so I replaced it with a Direct Connection electronic distributor (PN P3690788).
|3874795 & 3874796
1976 Feather Duster
|Initial Timing BTDC||2.5° @ 550 RPM (D)||TDC @ 750 RPM (N)||12°@ 750 RPM (N)||8° @ 600 RPM (D)|
|Advance Starts||1° @ 475 RPM||0.5 - 4° @ 550 RPM||1.5-3.5° @ 650 RPM|
|Intermediate||---||8 - 10.5° @ 800 RPM||---||---|
|Full Centrifugal Advance||12° @ 2200 RPM||14° @ 2000 RPM||5.5° @ 2300 RPM||11° @ 800-1200 RPM
(single red spring)
|Vacuum Advance Starts||5 - 7 " Hg||7" Hg||7" Hg|
|Full Vacuum Advance||7° @ 15" Hg||8.5° @ 11.5" Hg||12° @ 8.5" hg||7° @ 13" Hg
PN 3755073 or 8755073?
|Total Advance @ 2500 RPM||28.9°||23.9°||39.2°||44°|
|Maximum Total Advance||41.1° @ 4400 RPM||29.9° @ 4000 RPM||43° @ 4600 RPM||44° @ 2400 RPM|
With the low-ratio axles, engine RPM at highway speeds would be nowhere near the 4000-4600 RPM centrifugal all-in. With my car equipped with a 2.76:1 rear end and automatic transmission, 100 km/h (62 mph) translates to around 2500 RPM. With the Feather Duster distributor, I calculate that I would have about 39° of total advance at 2500 RPM and 43° of total advance at 4600 RPM & over 12" Hg of vacuum. Even with the Feather Duster distributor, it looks like there are opportunities for improved fuel economy with a recurved distributor.
There were several options for vacuum advance but Standard Motor Products has discontinued many of their products so you might need to go to eBay to find a specific component. Dan Stern provided much of the following list in Distributor vacuum can specs (list). Since the VC-187 cross-references to both slant six and small blocks, it looks like Mopar vacuum advance canisters are generally interchangeable.
Mopar Vacuum Advance Canisters
|Standard Motor Parts PN
|VC-93||4.9" - 7" Hg||5.25° - 7.5° @ 13" Hg|
|VC-173||7" Hg||5.3° - 7.8° @ 10" Hg|
|VC-184||7" Hg||7° - 10.5° @ 11.5" Hg|
|VC-185||9" Hg||7° - 10° @ 15.5" Hg|
|VC-187||10.5" Hg||1° - 4° (7° ?)|
|VC-189||7.0 " Hg||0.5° - 2.5° (11° ?)|
|VC-208||6" Hg||10° - 12° @ 9" - 11" Hg|
|VC-239||7" Hg||7° - 10° @ 11.5" Hg|
|VC-244||9" Hg||7.5° - 9.5° @ 6" - 8" Hg|
With 8° of initial, my PN P3690788 distributor provides 30° of centrifugal advance (all in by 2400 RPM) and 44° of total advance. I bought a VC-208 (11° distributor advance) vacuum advance pod (used on the Feather Duster) to see if I can run a lean fuel mixture that requires 52° of advance. The HEI system upgrade is to ensure that these lean fuel mixtures reliably ignite to avoid a lean surge.
For more information about adjusting Mopar vacuum advance, see How To Limit and Adjust Chrysler Vacuum Advance Cans.