OK now on to Plan D 1986 Bonneville GS

[Fast355]

Lotsa flat roads in your area? I have a friend in Nebraska and another on OK that report getting very high mpg results and I believe it's the long, flat stretches. Trying to duplicate that tuning in hilly New England results in lean cruise with frequent jaunts into PE. End result is much smaller gains.

Actually lots of rolling hills here but lots of open roads to run steady speeds on. That old 305 had enough low end grunt to pull most of the grades at 70-75 mph in overdrive even with the 3.08 gears. Had a few that it would downshift to 3rd on along the way. There were a few longer grades that were maybe 1-2 miles long that the cruise would force a 4-3 then a 3-2 downshift on and into the secondaries to hold 75 mph. Biggest killer of MPG in my area is often the 20-30+ mph winds we get. That being said my old 305 used to pull about 10-12 in/hg vacuum at 1,800 rpm at about 75 mph on fairly flat road. The factory power piston enriched at about 5-6 in/hg. Most of the time at cruising speeds even uphill the metering rods were burried into the jets. The cam I ran for the longest was a crane 274H06 and had the Q-Jet on a 7104 performer rpm. Great combination with the flat top pistons and 53cc 601 heads on a 305. After running the factory tubular manifolds for several years, I put throley tri-ys on that old van. The tri-ys woke it up even more. On level road that old G20 would run ~100 mph on the primaries and not exceed 3,000 rpm. The factory engine made peak torque at like 2,000 rpm. The intake, cam and headers pushed it up to more like 3,000-3,500 but it easily made more torque than the factory cam even down below 2,000 rpm.

This is the article years ago that made me decide to run the performer rpm on that old 305. My stock aluminum manifold started leaking and the coolant ports were heavily corroded. Chose the performer rpm over the performer which ended up with me having to later go to a Weiand 7525 when I failed smog for lack of an EGR valve. One of my customers had a few El Caminos and Monte Carlo SS 305s and suggested the 274H06. Swapped that cam in and it was like having 2 stock 305s under the hood. Little 305 really loved that little 218/218 @ 0.050, .450 lift, 106 LSA cam. Low-end power stayed pretty close to the stock power above the 1,500 rpm stall speed, but pin the throttle down and get it up to the 3,000-5,200 rpm range and it pulled like crazy for what it was. Later I played with the governor in the 700r4 and had the trans shifting 5,800ish in Drive. Old 305 didn't like that too much and after about 50K of being spun to 5,800 several times a day with that cam spun 2 rod bearings.

https://www.hotrod.com/articles/ccrp...-engine-build/