At Part-Throttle, massive cylinder pressure is your friend. Cruise at the lowest rpm that your cam gets onto the highest vacuum plateau.Then crank up the timing and start leaning her out, PLUS put a throttle-stop on your carb, so that the rpm remains constant regardless of the terrain. A slippery shape and small frontal area are the finishing touches. Badaboom! Say hello to unheard of numbers.
The biggest sins I see to mileage are mostly not enough timing.
____________________
At 65=2250, my combo likes timing deeeeep into the 50s, even low 60s. How can you get there with the factory D? If your D rolls in 10* by 2250 and you start with 16, that totals just
26*. So you better have a working V-can. And it better be a massive one. But you are not likely to find much over 16*anymore, so now you are up to 42*, easily 10 to 18* short. I and others have found that you can mod your Vcan to about
22*,( perhaps 24 tops). So now you are up to 26+22=48*, still ~ 10* short. So what do you do? Well with an automatic and a Hi-stall you can start with 18 and let the D roll in maybe 18 and that will get you 18+18+22=58, and now you are there. Well except if the stall is 3600 and yur running it at 2250,lol.
But your 3.55 by 2.66low 4-speed car is not gonna like 18 initial, at low rpm.Nor is it gonna like power-timing of 36*@ 2250. So either the 3.55s (or less) gotta go. Or you are gonna need to figure something else out.
My solution was a dash-mounted, dial-back, timing device, with a range of 15 degrees.
I set it to 8*retard, 7* advance. I run initial of 14* to soften the pressure spikes, and run ~.78* per 100 rpm topping out at 28* at 2800, with another 4 coming in by ~3200. So at 2250 the engine is seeing 14+22+22=56* To which I can add upto 7 for a total of upto 63*.
At the other end, I can subtract 8 from the 14 to get 6* to idle around the parking lot at 550rpm=4 mph with 3.55s and the Commando-box with it's 3.09 low gear. That's how I cover the bases. With the previous 223* cam, vacuum peaking earlier, and a final-drive ratio of 3.55x.71x.78=1.97, I could tell you I once got 32mpgUS over a 600 mile trip, point to point; but some say that's impossible.
It ain't rocket science guys; low rpm, lean it out until it pukes then add timing, repeat until it burns the valves.Increase the lash for more seat time, then lean and advance some more. Add coil power if/when you have to
for best results I recommend aluminum heads at 185psi, tight-Q, and a minimum coolant temp of 207,lol. The 185psi will allow running about one cam size smaller for an earlier vacuum peak, without losing power compared to iron heads at 155psi, both of which should run 87 gas. At 207*F, I had to add some skirt clearance to my hypers.
IMO;this should work on any engine,but perhaps a lil better on the smaller bore/longer stroked ones.
IMO, for this type of combo, the 360 is as close to ideal as you can get. For example;
Build for build, at a 185psi limit, the 360 will pull ~10 index points more VP than a 340. That's usable torque that if you wanted to,your 360 could give away to make power.That is about exactly one cam size you can give to the 360 or take from the 340, to make same VPs.
So.......... now giving the 360 one cam size, she has 20 cubes and one cam size on the 340, and is making about the same strong bottom end ........... I know which combo I want for this application.
Of course VP is only important to manual trans cars and low-stall combos with low cruise rpms.......... which is, I think, what you would use for power with economy combos .
lol
'Course the 383 will out-VP the 360 by a good 10 points, build for build. So you could.................
Read about it here;
V/P Index Calculation
