Does anyone have an idea of roughly what the BTU output would have been in an early A body (65 Valiant)

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

I have/had been doing auto "climate control" for more years than I care to remember. I don't rember ever seeing BTU specs of OEM systems. I have seen and used specs of "aftermarket" systems For what they are worth. Aftermarket systems for Large equipment seem to have fairly reliable specs.
It would be interesting to see specs.
One thing to remember, the BTU could vary depending on what temp "T" stat was used. Higher temp T-stat, more BTU.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Would the size of the engine have any bearing on BTU? I know larger engines do warm up faster and maintain steady temps better.

Certain smaller modern engine actually have problems generating enough heat to maintain proper operating temperature at idle in cold weather. Try telling someone that just spent 40K on a brand new car that their heater will always suck and that if you don't drive with the vehicle in "sport" mode that it won't get hot at all in sub freezing temps. I frequently see cars drive up in the winter with ECT 1(block) temps of 130F and ECT 2(lower radiator) temps of air temp (40ish). There are a few things that we do that require getting ECT 2 above 175F so hooking up a scanner is the first step.

I was just looking at some aftermarket / auxiliary heaters and seeing BTU's from 13,000-40000. Just was wondering about what my car would have produced.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6

Not directly. The BTU is based on How much heat the heat exchanger can transfer. 1 BTU = temp rise of 1 lb of water 1 degree. A larger engine "can" generate more heat energy, but if the heat exchanger is too small to transfer that heat energy from water to air you do not gain any increase in BTU, unless you raise the water temp (IE hotter "T"- stat)

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I think not. The limiting factor would probably be the heat rejection from the heater core. The heater core does not know or care what kind of engine heated up the coolant running through it; the factors would be how hot the coolant is, and how much of its heat gets transferred to the air flowing across it. That, in turn, would be influenced by the size of the heater core and its tube and fin configurations. Airflow across the core could theoretically be a limiting factor, but it seems unlikely to be one at the practical level.
What makes you say you know this? I don't agree with either of these statements, categorically. A bigger engine — say, a V8 rather than a 4-cylinder — has more cylinders putting heat into the coolant? Sure, but a bigger engine is also bigger; there's more cold metal sucking heat out of the coolant, too. And a larger engine usually contains more coolant than a smaller one, and more coolant requires a greater energy input than less coolant to reach a given temperature. This was specifically mentioned in the engineering literature about the aluminum 225 engine; its smaller coolant volume meant faster warmup.

There are a lot of factors at work other than coolant volume and engine size, enough of them that I think it's much more likely that the only categorically true statement that can be made is that some engines warm up faster and maintain steady temps better than other engines.
This goes back many, many decades before the advent of what we consider a small engine these days. People who live in cold places have been blocking off their radiators in winter for most of a century, now. I watched the temp gauge of my (then dad's) '62 Lancer (w/aluminum 225 and 180° thermostat) drop from Normal almost all the way down to "C" while the engine idled on a much-below-zero night while we were out photographing Comet Hyakutake in the early 1990s.
Again: not new.

Also, read the paper linked in this Diesel Slant-6 piece (the one titled "A Passenger Car Diesel Engine for America" — click the visible first page to get the whole thing). You'll read about how they had to deliberately add exhaust backpressure to get acceptable cabin heater performance…in the late 1970s.

As to the original question about BTUs: it's an interesting one, at least academically. I've never seen figures for an OE passenger car system.

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