273, or 318?
I’m thinking the 273 will not be turning enough rpm to keep it in the power band.
I'm not understanding this statement;
If you're talking at cruising speed, w/3.73s and 27s, 65=3020 in Drive/2080 in overdrive. So that's plenty of power there to cruise with.
The thing is, it takes a specific amount of power to cruise at any given speed, in terms of pounds of fuel per horsepower. So then you can gear any of the small-blocks and with a bit of tuning, get about the same mpgs.
The problem without an overdrive, comes with the take-off power and Second gear performance, with that hiway gearing. This is where the bigger engine comes in handy.
But with the overdrive, which works well with 3.91s to 4.30s, this punches the take-off power back up.
You have to scrutinize this, by converting your various Torque multipliers to Roadgears. If you are willing to purchase a stand-alone timing computer, and program it yourself, then mpgs are just a matter of reducing the cruise rpm until it takes WOT to maintain your cruising rpm, lol. No but I mean
the keys to getting great fuel economy are a tiny engine, operating in a high efficiency mode, at a low rpm, with the exact right amount of cruise-timing.
To cruise at 65mph, usually requires about 35hp. At 2400rpm that requires 77ftlbs. But at 1600 rpm, it requires 115 ftlbs. I think any SBM can spit out these numbers. The trick is to achieve the other parameters, and still have reasonable performance under other driving circumstances, without running a nasty camshaft.
Cruising at under 2400rpm, it gets harder and harder with a stock type distributor, to achieve optimum ignition timing..... which is essential for fuel-economy. And thus the reason for a Lean-Burn Computer for cars that ran the 2.45 and under rear gears.
Your 3.73/.69od comes to a Final Drive of 2.57 and 65mph=2080 with 27" tires. That's a really good cruise-number, but will require computer-timing to be fuel-efficient. And you cannot run any old hi-performance cam with it either, cuz that type of cam is NOT fuel-efficient at cruise-rpm, unless you build it absolutely just right. To operate at 2080, the cam can NOT have any significant overlap, nor any late closing intake valve. and the Power-stroke has got to be long enough to extract as much energy from the expanding gasses as possible, without asking the pistons to actually pump those gasses out. This is very torque-limiting, which usually leads to installing a bigger engine for reserve-power.
However, in your case, gearing and stall can overcome that with a Lock-up Convertor.
Let's look at your gearing in terms of Roadgears, which is simply the transmission gear multiplication times the rear gear. So, the A500 has ratios of 2.74-1.54-1.00-.69od. with 3.73 rear gears, the Roadgears are
10.22-5.74-3.73-2.57
Now;
The 10.22 is plenty of TM with which to get rolling
The 5.74 is a bit long for a Second, but doable
The 3.73 Third is just a gear to get to overdrive, in most cases or a cruising gear in the City, so almost any roadgear will work.
Lets look at the range from 30>50mph, which can make or break any combo.
In Second gear with a Roadgear of 5.74,
the rpm window is 2140 to 3570@ zero-slip, say 2320>3880 thru the TC @WOT So then, whatever engine you install, this is gonna be it's life-long handicap, so it's gonna need to have a natural propensity to produce torque in that Second gear range, else you will forever be downshifting to get some.
Say you have one each of the LAs, and each one is able to output an average of .90 ftlbs at WOT, in the range of 2320 to 3880. The horsepowers would range from:
108>182 for the 273 to
126>212 for a 318, to
135>226 for a 340, to
143>239 for a 360.
Now remember, these are theoretical average WOT numbers.
So then you can imagine that the 360 will NOT need WOT to match the 273 performance. This is always a welcome situation.
However, each bigger engine will have a larger power loss due to internal friction; but, each bigger engine will tend to produce more torque than the percentage of incremental increase in size. That is to say, with all engines built identically, and the 273 at .9 ftlbs per cube, each bigger engine will have the potential to produce incrementally more than .9ftlbs per cube. Say, by the time you get to a 360 it is up to 1.05ftlbs per cube; now the WOT power in the window is up to 167>279 horsepower, outstanding!
Ok so now, you can see, how a specific Roadgear affects the combo.
and if you are happy with the modest WOT performance of the 273, in a Roadgear of 5.74, then build it. If it turns out Not to be enough, well just install more rear gear until it is. But that will mess up your fuel-economy a bit and you can't get it back. Whereas, a bigger small-block engine will hardly affect the economy yet instantly jump up the WOT performance, and you know, you don't have to step on the gas as hard or as long, to achieve the same rate of acceleration.
With a given Cruising Roadgear, the downsides of a bigger engine are; a slight loss of efficiency at cruising speed, and a slight loss of power to friction. However, those losses can be overcome, by cruising at a lower rpm........ and the bigger engine will still maintain it's comfortable power edge over a 273.
AS for me:
1) I would be willing to give the 273/A500 a chance, but maybe Not with 3.73 gears. I would be prepared to install a lil more gear to satisfy my need for acceleration in Second gear, and/or less gear for fuel-economy.
2) I have already built a hi-compression, low-performance 340, which I know is a stinking blast to drive, and so
3) my feeling is that a hi-pressure 318 would be a reasonable compromise, and if it turns out that with 3.73s it more than satisfies your need for acceleration, then there is no good reason to Not give some up, by decreasing the cruise rpm with less gear.
4) I know I could be happy with a 318/A500/3.73s, maybe even with 3.55s;
but not sure I could be happy, with a 273/A500/3.73s. I know the 273 would be just fine in First gear. But I think for me, that 5.74 Second Roadgear would be a tad disappointing at 30mph/"hammer-time". And the downshift into First @30mph is going to 4200, so can't be in First very long until it's time to shift.
But I might be happy with a 273/A500/4.10s. That would make 65=2290 which is now on the lower limit of timing for a stock-type distributor, so yeah, for me, I can see 4.10s being a good compromise. And 30 is now, mathematically ~2600/2700 in Second at WOT..... which is considerably better than 2320.
5) Aw, who am I kidding; that 273 is getting moth-balled, in favor of a high-efficiency, high-pressure, modest-power, 318/A500/3.23s tuned to cruise at about 65=1800, and the timing computer is pretty much a must-have. I'd be running a step-piston, tight-Q, with closed chamber alloy heads, headers, an aluminum 180*intake, a spreadbore carb, fresh air, and a Tight-LSA , all tied together with a very modest solid-lifter cam. Badaboom!
Hey waitasec, I bought this Dual-Port a few years ago; hmmmmmmmm.