If you already own the 5.2M, then fine, but if not, then your cam choice will, for the most part, dictate your Scr and 9:1 is not gonna fill the bill for city-cruizing.
Thus, pistons may be in your future, and after that, the only advantage to the Magnum is the roller lifters, and the closed chamber heads which I HEAR, are most often cracked.
So then, all-in-all; since, IMHO, you're gonna want to run closed chamber alloy heads, with a tight Q; the LA is once again, on the table.
Furthermore;
245/60-15s come in at about 26.6, and therefore, with 2.93s and .73 od, 65=1760 rpm. Good luck meeting the engines cruize-timing requirements, at that rpm, even with an "RV cam". Thus, IMO;
You could do better, than an RV-cammed Magnum.
But, if you already have it, well possession is 9/10ths.
In either case, with a tight-Q, you will not be needing a Multi-Strike ignition. But could do better with a Single-fire, and a High-amperage coil. I've gone 40 mpgs on an Accel SuperCoil with a factory-type ECU.
The problem in either engine-scenario is, the later that the intake closes, the lower will be the cylinder pressure; which is all the engine cares about....... as to making power. BUT
When it comes to fuel economy, cylinder pressure is automatically adjusted to whatever the load requires by the throttle opening. If the load only requires an effective compression ratio of 5/1, then, you are gonna adjust the gas pedal to get it. Then you are gonna physically build a timing curve, to satisfy the very early timing requirement of that theoretical 5/1 effective compression ratio. and
Then you are gonna physically adjust the fueling to deliver as lean an AFR as the engine will reliably run on.
So, to recap; When it comes to steady state fuel-economy, the engine hardly cares about the Scr.
This is NOT true for stop-and-start city fuel-economy. Here, for accelerating up to speed, you need the Maximum amount of cylinder pressure that your custom built engine can generate, and again, 9/1 is not likely gonna cut it. True, it's better than 8/1. But in either case the tune is gonna make huge difference, especially with a Second-gear Roadgear of just 2.93 x 1.67= 4.89...which is like third gear in a regular trans with 3.55s (1.39 x 3.55=4.93)
There is only one good way to run that trans, and I have run them all.
1) The lowest rpm that you want to run, on the hiway, is about 2000 rpm. Even at 2000, you will not be able to satisfy her cruize-timing requirement.
Honestly, 2200 would be better.
But, for 65=2000 with the previous tires and .73OD, you need 3.34s, rounds down to 3.23, and so, 65=1937.
Guess how much timing your stock style distributor can generate at 1937rpm.
My guess is 38*. and my guess is that the engine, for max economy at 65mph, will want 50* or more. In other words, lacking timing, it often happens that the very same combo will get better fuel economy cruizing steady state, at a higher rpm, in the which the cruize timing requirement has truly been met.
>OK so what about around town?
The starter gear with 2.93s is 9.08, which will get you to 3000rpm=26mph. Then when you shift, the rpm will drop to 1620, and now, your engine has to claw it's way up to say 30/35/40 mph, which, in traffic, is gonna take a fair amount of throttle...... for four reasons;
1) the effective cylinder pressure is very low
2) the mechanical-timing plus Idle-timing is low, and
3) there may be NO HELP from the Vcan, cuz the manifold vacuum is already pretty low.
4) all of these combine to produce a lack of bottom end torque.
As to numbers 2 and 3, there is not much you can do with iron heads, cuz the engine will get into detonation real quick.
As to #1, even 9:1 is not gonna make much pressure.
As to #4, the Second-gear in that trans really sucks with 2.93s. Like I said, I've tried all the combinations of gears that Mopar ever made, back in the Day.
>For Second gear, with a manual trans, What I have found makes good city-fuel economy, is to cruize 35mph at ~ 2800. This will take a roadgear of 6.33. At this rpm you can give the engine the timing it wants, which maybe you can get 50/52*, which will be close. So then, your job is to set the AFR as lean as you can get away with, at 2800 cruize. AND, to also reduce the accelerator pump action, to as small as you can get away with.
Now; with the od trans; 6.33/1.67= ~3.73 rear gears. and
3.73s will get you 65=2175 in overdrive, which is perhaps perfect.(I run 65=2240) But,
the starter gear is 3.09 x 3.73= 11.53 which is pretty deep. Which means that your engine, in first gear, will be up at 3000 real early. (I actually like 3.55s better for in town.
>Now, having dialed the trans in, you gotta build the engine to best take advantage of it.
If your goal is maximize your economy around town, then you gotta go for max cylinder pressure. But, you're not gonna get there with iron heads, which top out at perhaps 165psi with best gas.
Switching to alloy heads will allow perhaps, 200psi. Now you got some torque!.
But, with a stock LA cam, yur gonna need the Scr to be up around 10.6. And the later the ICA is, the higher the Scr will need to go.
>Just to be fair;
Lets say you set the Scr to 9.0, and run the 5.2M cam @Ica of 55*, the pressure will be ~155, which is pretty good for Power, but won't win any awards for fuel economy around town.
But with an Scr of 10.6, that same 55* will get you a pressure of ~ 190psi, which is really good.
The point is that, for city mpgs you need plenty of pressure, so you can run a minimum throttle opening during acceleration.
>Part of Max fuel-economy is which gas yur gonna tune for. IF, you want to run 87 gas, with closed chamber iron heads, you limit yourself to about 155psi, which is about what the factory 5.2M cam will generate.
Switching to alloy heads, I have run them at a tic under 200psi, with a 850TQ/750DP, still on 87E10 gas, at WOT with full-timing (34*). At Part Throttle or with a smaller carb, I bet I could have run even more pressure.
Hope this helps.
