six barrel
How to adjust for fuel economy? '70 AAR 6 barrel motor in a 1969 fastback with overdrive 4speed. any thoughts???
You didn't mention your rear gears.
But if you are running the factory 114 LSA cam, and getting 18mpgUSg out of her, that is phenomenal.
The factory 268/276/114 cam on the advertised numbers and in at 110LSA, has only 104 degrees of exhaust duration, which is really lousy for fuel economy. But the compression degrees while only 116,with a true 10.5 Scr , the Dcr is nearly 8.4@800ft elevation, which is really good, if you manage to keep it out of detonation at WOT.
The thing is; that cam likes to be spinning,
at the very least, 2200rpm at your chosen cruising speed, 2400 would be better.
The slower you spin it, below ~2000 with headers, the worse the economy will be. And the faster than about 70 that you drive, also the worse it will be. Taking all those things into consideration, and with the 3.09 first gear, the optimum rear gear will be about 3.55s; for 65= 2120rpm with 27" tires.
3.73s would get you 2230, 3.91s would be 2330, but now the starter gear, at 11.53 is getting a lil excessive.
At these low rpms, it is doubtful that your MJs will be flowing more than a trickle, but since it's a 6bbl, you can lean them out until the you can't any more,lol.
However, the BIG secret is timing. At 2000 rpm it will be impossible to give the engine optimum Cruise Timing.
Even at 2400 it will be tough.
I estimate your engine will like 54* at 2400 rpm.
If you have set your PowerTiming to 34* at say 3400rpm,
And she idles on 18*
And doesn't start to advance until 1000rpm,
And has a single-stage curve;
THEN
the rate of advance will be;
(34 less 18)/(3400 less 1000) x 1/100 = .6* per 100 rpm, therefore
at 2400, the mechanical in the D is 8.4. and so, to that you add the 18 =26.4. to get to 54, your Vcan needs to bring in ~28 degrees, which, AFAIK is impossible.
You can mod almost any Vacuum canister to get 22 degrees, maybe 24 tops, and so if you did that, you would only be short 4* from my estimated 54*.
How much Cruise-Timing your engine actually wants is easy to figure out.
1) determine your cruise-rpm.
2) rev it up to that, and put it up on the fast idle cam.
3) grab the D and start advancing it. you will notice that the rpm goes up. Just keep advancing it until the rpm no longer rises, then
4) put the rpm back to your cruise rpm with the fast-idle screw.
5) now reset the timing to get the highest rpm.
6) adjust the mixture screws for the highest rpm
7) put the Rs back to Cruise-rpm, and one more time; adjust the timing for highest rpm.
8) finally, set your mixture screws 1/4T rich; and read your timing on the balancer. Subtract 3 degrees, and write it down!
9) return the engine to idle, restore the timing to what it was at the first, and let it cool off.
What you have just done, is determine the ideal cruise timing at cruise-rpm at speed. The 3 degrees I took out is for load compensation at speed.
If the mixture screws are too far from in the center of their adjustment range, or the engine will not idle on the new setting, then you will have to fix something.
If your engine truly is at 10.5Scr and has a factory 340 cam, in at +4*, then the compression readings should be close to 170 psi. And that is a great number to make fuel economy with; BUT; the engine has to be reving high enough to be out of reversion. How can you know when this is? But a vacuum gauge on the intake , then slowly rev the engine up until the vacuum peaks, and no longer rises. The lowest rpm that it peaks at should be considered to be your lowest cruise rpm. Just match that to your chosen cruise roadspeed , and yur done. A lil higher is Ok, but slower, will lose efficiency, until you change the Ica, by either changing the Cam, or the installed Centerline.
If your engine was a 360 with a fast-ramp 268/276/110 cam, I'm pretty sure that it would not yet be on the mains at 65 mph. But your 340 might be dribbling some. Since it hasn't maxed out the PMJ; for cruising on, it don't matter what size MJ is in there...... don't even bother with them, for this mode of operation.
Well maybe I shouldn't be so cocksure of myself; cuz one time I got a tank of something that was not 87E10, and I had to stop on the side of the road TWICE, to upjet the primaries, just so it would cruise. Man that was awful.
But I just gotta say, 18 mpgUSg in a 340 is like Mike said
Probably not going to get better than that.
BTW;
For cruising;
If you get the engine revving fast-enough, and the AFR set right, and the timing set right; then it should not matter what kind of carb you are using, nor how many barrels it has, nor how many total cfm it has...... as long as the primaries are sized decently for cruising. How you get to your cruise speed is an entirely different matter,lol.
