I am putting a magnum in my 71 Dart, not sure if 318 or 360. It will have the complete fuel inj and 46re, dana 60 with 3.54 gears. I have changed the cam in the 360 to the 318 cam as it has more lift and duration, per the fsm. I will be using a 360 pcm. Any thoughts on the difference in fuel economy between the 2 engines? If only 1-2 mpg, I will use the 360. If 5 or more difference, I will use the 318. I know it may be a loaded question, but this will only be a cruiser. Tires size will be 275-295, if I can fit them under after moving springs inboard. thoughts are always appreciated. rick.
Here is what I have on file for Factory Magnum cams; first the 5.2M, and
following that, are the worked out durations from the various events. Up first is intake, then compression, then power, then exhaust. Then; overlap, Lsa, and finally lifts.
1) 251/125/111.5/264/31.5/113/ .432
2) 249/131/112.0/269/41.0/109/.410/417
If these numbers are correct, then;
IMHO, not a chance would I install the 5.2 cam for fuel economy.
>The following numbers are when installed at or near split overlap.
>The Ica of the 5.2 is 6 degrees later, which will drop the cylinder pressure of a
360@9.2Scr, from 167psi (5.9cam), to 159 (5.2 cam); both at sealevel.
Pressure is heat is torque, and times rpm is horsepower.
That's 8 stinking psi, or about 5%. This is torque you can feel ALL thru the rpm band, whenever you roll the throttle on. This is like changing the rear gear ratio, a half a size.
>On the other side of the coin, the 5.2 cam has just 31.5* of overlap, versus the 5.9 @41.
Now, if you run log-manifolds, this means nothing to you, cuz logs kill the overlap cycle.
But if you run headers; this is a full 10 degrees more, or about the same as 1.5 cam sizes ...... which will make a nice little bump in horsepower up higher. It
has the potential, with headers, to eclipse the 5.2 cam in power, in spite of the lift difference.
>Now, one place economy is made, is in the power-extraction, and in these two cases 111.5 versus 112 in miniscule, so as to the POTENTIAL for economy, that spec is as good as the same.
>As to intake duration,
the 5.2 cam has 2 degrees over the 5.9, which in your case means nothing, whereas
>as to exhaust duration,
the 5.9 cam has 5 degrees over the 5.2, again, meaningless to fuel economy.
Between those two cams, I'll pick the 5.9M everytime;
More torque, more power, and has the potential for more fuel economy.
the smaller lift spec, in this case, and IMHO, is almost negligible.
And the bonus is that with a 20 degree spit between intake and exhaust durations, and especially with log-manifolds, you can play with the tune even more.
Happy HotRodding
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EDIT
If you put a healthy 5.9 in that Dart, with either of these cams, and end up with about 160psi, with a tight Quench, it will gave you GOBS of bottom end torque.
If you really want economy;
>You can run a Mopar, A833od.
The ratios are;
3.09-1.67-1.00-.73 od.
That torquey engine will run down the hiway easy enough as low as 65=1600; which means, that you can run a 2.76 rear, for a final drive of 2.01; I know because I tried it (1.97 ratio).
Your starter gear of 3.09 x 2.76= 8.53, which is the same as running 3.23s with a standard 4-speed. No, this will not set you back in the seat, but you said she's a Cruiser.
Second gear is 1.67 x 2.76=4.61, which is like Third gear with that 3.23, so, you kindof miss having a Second gear.
Third is a straight 2.76 for 65=2230, already pretty low, and a good gear for max fuel economy because ..... you can actually set the ignition timing to optimum. In this case, at 65=1600 (with the 2.76 x .73), it will be impossible to set the cruise-timing to optimum with the factory ignition system, so you'd always be leaving a bit of economy on the table.
However, same gears get you 85=2130, where you may indeed be able to get the timing optimized. This means you effectively have two hiway gears, one reasonable starter gear and no second.
That's just one option.
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From here, it just gets better.
If you're willing to give up that Second hiway gear, for more Second gear for instance, you could do the math with say 3.23s or 3.55s. All you lose is the high speed touring, but even 3.55s , make 75=2420, and that for sure can be fed say 56* of cruise timing. And 3.55 x 1.67= 5.93, a reasonable Second gear.
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If yur strictly an automatic guy, the Mopar Overdrives offer excellent ratios, and .69 overdrive, that when used with a Lock-Up, can get near manual-transmission results.
>With an A500/518, I'd be tempted to run the 5.2M, cuz, with a Torque-Convertor, you don't need the bottom-end torque of the longer stroke/bigger bore; 5.9M
In this case, I'd go with the A500.
If I had a stall choice, in a LU convertor, I'd try a 2400, with 3.91s. or
2600 with 3.73/3.55s; or
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>2800 with 3.23s, but with this gear 65=1800, which, because of lack of optimized ignition-timing, may not get any better fuel economy over 3.55s(1980rpm), or 3.73s(2080)
All numbers listed are with 27" tires.
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Personally, I really like how a 2800 works with 2.76s. I can't really describe it, but I really like it.
By itself; 2.76s will get you 65=2230
I never really found this doggie with the 2800; not a powerhouse either, but:
All I could get out of my ignition was 46* of advance and the engine wanted, 52>56. So I bought me a dash mounted, adjustable, Timing Module, with a range of 15 degrees; problem solved.
