If fuel economy is important to you;
With 3.73s, in overdrive, with 27" tires; 65= 2100 in LU.
That's great for a factory-cammed 318. But;
Good luck getting any fuel economy out of a carbureted SBM with a nasty cam. The problem, mainly, will be lack of optimum cruise-Timing, unless you lock yur timing, and modify the heck out of your VA. And then you gotta figure out how to make it not detonate around town.
I mean maybe fuel-economy isn't important to you, IDK.
I would like to point out that fuel economy is in the eye of the beholder, first of all. So if you are saying you will never see 20 mpg, you nailed that. If you are saying you’ll never make it out of the single digit mpg, you have been tuning the wrong way.
In my 1972 Duster 340 with spirited driving, I see about 6 mpg. If I just drive it more sensible but not babying it, I can see about 12 mpg. Additionally, I am able to drive it thru town at an idle without any issues. And the setup is as follows.
340 with 284/484 camshaft (10:1 pistons)
750 Edelbrock Performer
LD340 dual plane intake
Points ignition
Distributor curved for 20 advance at idle (1000 rpm), 34 all in by 2500 rpm
(Only using mechanical advance, vacuum advance disconnected)
3200 stall FTI torque converter
727 non lock up transmission
3.91 Sure Grip 8-3/4
27” tall 255/60R15 rear tires
Why I think it gets 12 mpg combined highway and country roads. At 70 mph it’s turning 3500 rpm… in the powerband of the camshaft. Therefore no downshift required to pass. Also, 3500 rpm is above the 3200 stall rating of the converter, so most efficient. (Losing about 200 rpm to non lockup).
Now, any detonation problem is a fuel based problem. I run 93 octane to keep “run on” at shut off at bay. If I use 87, it will “run on” at shut off, that’s the definition of detonation. There’s the easiest test to determine if you have detonation and simplest way to solve it.
I do not understand this “optimum cruising timing” you speak of because timing is not where I had any of my issues with cruising thru town… it was a carburetor tuning issue as I was still learning how to adjust things and had the idle circuit set too lean. I understand timing has benefits but how would that affect mpg performance that much. Am I immune to this because I am avoiding the vacuum advance screwing with my timing and relying on only the mechanical advance?
In order to not hijack this thread, you are doing great work 1975mopar. Remember to have fun while spending all of your money cause that’s what it’s about in this hobby. I too had considered the 8.8 but in the end used the B body 8-3/4 that was already swapped in, which was a regret to not finding an A body 8-3/4 because it’s hard to find 15” wheels I like in enough backspace sometimes….
