There is simply something about people that become Engineers, that drives them to know with an almost absolute certainty, what the effects of the technologies they are applying to any given problem will be. Cause and Effect down to the atomic level if at all possible, and all Causes and Effect also have to align with each other before they feel comfortable. (pity the EE working on Quantum Computing)
In-turn, this very part of their nature causes them to squirm nervously and look sheepishly when they hear others misrepresenting the facts of any matter. You simply can not have a "general discussion" of any technical subject with good engineers; because it almost always has to be driven to minute technical details, to satisfy their personal quest for "truth" without all possible exceptions being fully explained and understood.
In the early to mid 1950's the American Automobile Manufacturers were in fierce competition for market share with one another. They were making all but complete model changes ever year! In 1952 the Chrysler Corporation had introduced it's now famous Hemi... as an answer to GM's Caddy and Olds offerings. By 1954 Chrysler reported to the public that the upcoming "Chrysler 300" was thus named because its Hemi engine produced 300HP.
If there was a single starting point for the HP Race.. that was most likely it. That announcement however seems to have left several Automotive Engineers squirming in their seats and looking somewhat sheepish. They could not publicly refute their bosses in Sales and Marketing because after-all that's who they actually worked for in the long run. When you get Sales, Marketing and Advertising People together.. new limits to how far the truth can be stretched are all most always found.
So what to do?.... Let's publish a paper through the Society of Automotive Engineers that will divulge the truth to anyone interested enough to find and read it. We also know that none of the Automotive Press nor Magazines would be included in that first group, so good PR can be maintained.
Thus the SAE Gross HP ratings were defined.... and anyone with any technical understanding of the subject would see at once that as written, it left all manor of loop-holes wide open. It is just my personal opinion, but shared by many that it was simply a way for the Engineers to tell the truth... the truth was all Gross HP ratings were no holds barred and lots of marketing hype! Nonetheless the truth was out there... and the engineers could squirm a little less, give a knowing wink and nod to each other.. and go on with their lives with a little less guilt.
All this worked well.... "until".... the Federal Government got involved. Emissions Control because the marching orders of the day around 1964 and by 1967 all automobiles produced for sale in the USA had to start controlling specified emissions.... So standard Compliance Testing had to be established for all makes and models.... thus standards that much more fully defined the test conditions had to be developed. Thus the SAE HP Rating was defined, and it closed all manor of loop-holes intentionally left open in the initial SAE Gross HP rating system....
Once the Federal Government gets involved in anything, there is endless growth ahead... by 1971 the manufacturers knew that the Emissions Standards would be greatly increased in the years ahead, and they started planing accordingly for their 1975 models. With the fuel shortages of 1973/74... they also knew that Emissions Tests would be used to estimate and report fuel mileage to the consumers. What is it they say? "you get what you measure"!...
Thus the SAE Net HP rating standards were developed..... The task at this point was to define a testing standard that yielded the lowest possible amount of the federally controlled emissions, while yielding the highest possible estimated fuel economy. It also had to be rigidly applied across all brands/makes and models!!
"Houston We Have A Problem!" reported the Engineers ..... the criteria that gives us todays goals, will clearly show that the rating systems we had in years past - was a partial or complete falsehood. No problem replied the Sales and Marketing People, the consumers don't remember anything for more than two years... lets tell'em what they want to hear!!... "Our engines are clean and fuel efficient!"
SAE Gross HP - any thing goes. Any grade of fuel, no accessory loss, no water nor oil pumps attached, One Run Wonders OK. Most of the engines used to hit their Peak HP ratings were junk after one run. They were also special built, set up loose etc. Very few of them would have ran over 1000 miles in actual use... But They Did Hit, at least once, the Numbers As Advertised.
SAE HP - required a standard production engine be taken at random off the engine line, equipped with oil pump, water pump, fan, distributor, and generator/alternator. It also had to have standard exhaust manifold and header pipe in place.
SAE Net HP - all the above, PLUS all standard accessories found on the average model the engine was installed in. It had to have a full exhaust system, standard air cleaner in place, coolant in circulation and be measured at standard operating temperatures as found in the average model it was installed in.
In 1970/73 the L24 was rated and reported as "SAE HP".... NOT SAE NET and certainly NOT SAE GROSS.
Today - most stock 240-Z's that have been put on chassis dyno's - in good operational condition will produce numbers between 108 and 120 HP at the rear wheels. Most stock 75-78 280Z's will produce 120 to 130 HP at the rear wheels.
Converting Rear Wheel HP to Crankshaft HP ratings... the rule of thumb varies between a loss factor of 20 to 30%. L24's, 150HP x 0.8 = 120HP and/or 150x0.7=105HP. L28's 165HP x 0.8= 132 and/or 165HP x 0.7=115. (California Models with the L28's had lower reported HP numbers.. tuned for stricter emissions and some came with Cat's)
So why didn't the HP Ratings on the L28 drop as greatly between reported SAE HP and SAE Net HP as most American Cars did - The Muscle Cars from 69/72 - dropped greatly in reported HP by 73/75 (some by as much as 30%!!), the L28's did not however drop from a reported 165HP to 108!.
The reason is - the L24 and L28 between 70 and 78 on average all had the same lack of standard accessories. The average American car had to take the additional loss from PS, PB, A/C, larger Alternators etc; all of which were pretty much put on the average model sold. So we saw far greater HP number loss in the typical American Cars when they started using SAE Net... By 1980/82 more and more of the 280ZX's sold were equipped on-average with PS, A/C and larger Alternators... so they too lost a few more HP on the reported numbers.
We also see large variations in reported HP numbers on the L28's - between California Models and the standard US spec. models - because California models had stricter emissions standards (engines de-tuned for emissions plus some models got Cat's earlier etc).
Redesigned combustion chambers etc also account for some small HP loss in the 280ZX's as meeting emissions standards not performance was the main goal.
In The End:
The nervous engineers, aren't squirming so much now... because now more of the "Cause and Effect" factors align. Ones that they much earlier worried a little about...
It takes a certain amount of Torque/HP to push a certain mass from rest, and then accelerate it to a certain speed, within a certain time, within a 1/4 mile. We all have rules of thumb for that - and in most cases they line up pretty well if we are using SAE Net HP numbers with the weights of todays cars... The engineers knew back in the 50's that things didn't line up well if you were using the SAE Gross HP numbers advertised by the various manufacturers... and that always left them looking a little sheepish.... squirming a little in their seats if HP ratings were being discussed....
Always keep in mind, nothing is perfect. Figures don't lie, but liars figure.
Nonetheless, when the engineers at Nissan reported HP for the Datsun 240-Z it was as they stated in their service manuals, on the cars and in the advertising... it was SAE HP. Not SAE NET nor SAE GROSS.