(First I'd heard that horsepower was measured at the transmission yoke than at the flywheel.)
In a pre-emissions era, "Gross" horsepower figures were allegedly more optimized for most power on the dyno. Yet there were SAE specs of how that would be done, just as there were SAE specs for the later "Net horsepower/torque" figures.
Remember, too, that until the earlier 1980s or so (possibly later), running an engine dyno was "an art" as the load on the engine was varied to get the horsepower/torque figures. NOT like the newer computerized dynos that do it all by themselves, once the throttle on the dyno control panel is moved to WOT. So two different dyno operators, which respectively different "feels" for the engine, might come up with different numbers. An article at Allpar stated that the Chrysler dyno operators took great pride in getting repeatable and accurate numbers. There's a YouTube video at Nick's Garage when an original 440/375 and carb are both rebuilt to OEM specs and it produces 375 horsepower on his computer-run dyno.
It was known that there was a "window" of power ratings for any given production line engine. Variations on head castings (relating more to combustion chamber size/deck height), minor differences in ports, etc. Cam changes can be tracked via the particular-year parts book and Chrysler service manual. There were also SAE specs of how cam lobes were measured, too! Even different for intake lobes than exhaust lobes! NO "Lift at .050" specs existed in the OEM, just the SAE method of measurement. Yet the "max" number was what was advertised. ONLY factory operatives at the engine plant knew of how wide that window might be, but it related to the numbers a new engine produced after it'd gone through it's 30 minutes of "Run-in time" at the engine plant dyno room. A "stack" of normal build specs could result in an engine that produced 10 horsepower less than advertised, for example. If everything was "dead-on", then advertised power resulted. In those times, these differences didn't matter too much as long as they "ran like we wanted them to run".
The NHRA specs were monkeyed-with to ensure "more even competition". If one brand's engine is rated at 300 horsepower, yet doesn't perform equal to another brand's engine in an equivalent vehicle, they might "factor" that 300 horsepower down to 280, which might put it in a lower class where the car would be more competitive. By the same token, if they observed the opposite, that 300 horsepower could be factored UP to 320 horsepower, which might put it in a higher class and "make it work for its trophy", so to speak. I think the Chrysler 340, in the earlier years, was factored from 275 OEM rating to 290 NHRA rating?
Now, when we got to the NASCAR years, with 427 Fords and 426 HEMI Mopars, it was highly obvious that the factory ratings were "low" for those motors. In the later '60s, there were various formulas the insurance industry used to put rates on "muscle cars". Cubic inches and weight, for example. GM had their internal rule about 400cid being the largest engine in a mid-size car, BUT that didn't keep dealers from putting larger engines in themselves. Other manufacturers kind of tended to follow that rule, but not completely. That tended to generate a LOT of 400cid engines in GM brands, in their smaller cars. As the full-size and larger cars still had larger engines from the same engine families. Tis was also the era that generated the Chrysler 340 and Ford 351 "Cleveland" small block V-8s. NOT related to why the 302 Z/28 Camaros existed, per se. Smaller engines that, with a few tweaks in chassis equipment (axle ratio, tire size, etc.) could perform with their larger siblings on the street or drag strip.
Many OEMs usually rated their engines one way, regardless of the transmission it was hooked to. In the mid-'60s through early '70s, in the World of Pontiac, there were many variations of 389 V-8s whose only difference might be a "smaller 2bbl carb" for allegedly better fuel economy, a manual transmission, or some "economy" calibration. End result was that one engine size and carb type might have 5 or 6 possible build variations AND different power ratings. Otherwise, other OEMs would put a different distributor in for manual transmissions than automatics (different base timing, but very similar total 4000+ rpm total advance specs (base timing + distributor timing) for the same flywheel horsepower.
Now, I've seen where people have obsessed over a 5 horsepower variation from year to year on the same-spec engine. That power difference only happens at 4000+rpm levels, NOT at the more normal 2000-2500rpm levels normally seen on the street or highway. Where throttle response matters much more than sheer power/torque ratings. How responsive the transmission might be to part-throttle kickdown.
Where it might matter is on the measured drag strip. But then, too, if a TorqueFlite Mopar puts 86% of rated power to the ground, that 10 flywheel horsepower becomes 8.6 "ground horsepower". Any differences in performance could well be due to driver skill, track conditions, and how much power might be absorbed by particular tires rather than other tires. When I arrived at those figures, it was in the later '60s when bias ply tires had the vast majority of the tire market.
So, with all due regard, these minor power differences from year to year might make good "bench racing" discussions (in general or in particular situations of particular engines), but for the vast majority of vehicle customers, "not that important" as long as it "drives well". By the same token, there has to be an engineering-based reason for it. Most might well relate to emissions specs for the particular model year, or a body platform that allows for a better under-car exhaust system (in the case of '95+ Pontiac Grand Prix 3.8L cars and similar Buick Regal LS cars with the same engine . . . Pontiac had a dual outlet muffler and got a few more horsepower as the Buick had only a single muffler and a few less rated horsepower, for example).
Now, also be aware that in modern times, if what's advertised is not really there, that OEMs can be fined for false advertising. If 320 horsepower is rated, but most engines only produce 310 horsepower, for example. Happened in the case of 5.0L Mustangs about 10 years ago! Which can be why "SAE Certified" labels are on some engine power ratings now. I suspect there's an SAE spec for that too!
LOTS of things to consider.
CBODY67
