How Many “Real” Horsepower Did The 1971 426 Hemi Really Make? A Look At The Gross vs. Net HP Games


(first posted 10/13/2016)      1971 was something of a watershed year for the great performance/muscle car boom that had been building ever since the “horsepower wars” of the mid fifties. Two very large factors were to change the braggadocio that had been building for decades: insurance industry backlash and a switch lower compression across the board for 1972, in order to comply with the mandate that all engines be able to run on unleaded regular gasoline. And then in 1973, the industry agreed to only advertise SAE net hp ratings, which reflect the actual installed output of an engine rather than the SAE gross hp ratings in use for so long, which measured an engine’s potential output without any engine-driven accessories, maximum ignition advance, no air cleaner, open exhausts.

While GM and Ford stayed with gross ratings for that final high-output year in 1971, Chrysler went a different route, and either just didn’t show any hp numbers in their brochures (in the case of Dodge) or showed them both (gross and net) in the case of Plymouth and Chrysler. Which makes for some interesting musings.


Let’s take a closer look at these engines from the 1971 Satellite brochure. What stands out? How about the disparity between the gross and net hp ratings among the various engines. The two extreme examples are the 318, whose net rating is only 67% of its gross, and the 440-6, whose net is a whopping 86% of its gross rating. What does that tell us? That the 318 was over-rated, and the 440-6 under-rated, neither of which are surprising. I never could quite see (or feel) why the 2-barrel low-compression 318 had any more hp than say a comparable 327 Chevy (210 hp rating). Or had 30 more hp than a Ford 302 or Chevy 307. Realistically, the 318 should have been advertised with no more than 200 hp, based on its 155 hp net rating, if not less. Meanwhile, everyone knew all too well that the 440 six pack was a monster, and underrated at 385 hp.

The other equally obviously underrated engine is the 340, whose 235 net hp is a whopping 45 more than the 383 2V, although both are rated at 275 hp gross. That the 340 was underrated was no secret at the time, and this makes it clear just how much.

What is surprising is how low the net rating for the 225 slant six is. 110 hp is pretty modest given that the serious de-smogging era had yet to come. And it had only 5 hp more than the significantly smaller 198 six. As a point of comparison, my Ford 240 six was rated at 150 hp gross, and 129 hp net. But that was in 1966, before any emission controls were required.

Which brings up an important point: emission regulations began in earnest in 1968, and were generally tightened every two or three years, with California having its own more demanding requirements. That means that by 1971, emission regs already had an effect on these engines, generally the result of modified ignition advance curves (generally less advance), as well as running air injection pumps (“smog pumps”) that promoted further combustion of unburned fuel in the exhaust manifolds.

The reality is that many/all of these 1971 engines weren’t making the same power as they had previously. This shows up in comparisons of gross/net power outputs for those where these are available in the pre-smog era. For instance, by 1971, Chevrolet’s 250 six was rated at 145/110 hp. Back in 1966, it was 155/125. My point is that although Chevy did lower the gross rating by 10 hp, Chrysler stayed pat with its gross numbers for 1971, even though they had been similarly affected. It does show up in the net numbers, especially in the case of the 225 six, which undoubtedly had a net output of some 120-125 hp in the pre-smog era. But the gross numbers (“advertised hp” stayed the same for all of these engines in 1971. We will have to speculate how much the 426 hemi really made back in 1966. By 1971, it was 350 hp net, still a considerable number.

Of course, much worse was yet to come: by 1980, the 225 six was down to a paltry 85 net hp. As were all of the other engines, and not just Chrysler’s.


The Chrysler brand also showed both ratings for their big cars in 1971, so here’s a couple of 440s that weren’t on the earlier chart. The interesting comparison is between the 383 4-barrel and the lower-output 440, which although rated at 335 gross hp, made only 220 net hp, significantly less than the 383 with 250 hp. How to account for that? It may mostly be a combination of wanting to show a higher number for the larger engine, as well as the significant effect that an open exhaust has on an engine that comes delivered with a restrictive single exhaust.


One good source of net hp ratings are the Vehicle Information Kits at the GM heritage Center. Chevrolet showed both gross and net ratings, and often dyno charts, for many of their engines, up to a point. meaning that their top output engines were typically shown only with the gross numbers, as the net was probably giving away too much for either competitive reasons or maybe because the gross number were either too high or low relative to the net number. These are from the full size ’65 Chevy kit.



The last year Chevy showed net and gross hp numbers (and a dyno chart) for a top-performance engine was in 1959, when the hi-po fuel injected 283 was rated at 290 gross, 245 net. And the dyno chart clearly shows the key difference: the gross peak happens at 6200 rpm; the net peak at 5600 rpm. The way engines are set up to run on the gross test, with ignition advanced as much as the engine will take, no mufflers, and no drag from driven accessories allows the engine to rev higher, the key to more hp. Unfortunately, the 315 hp version of the FI 283, which appeared in 1960, did not have net hp shown, and for that matter all of the hi-po 348/409/327/396 and 427 engines starting in 1959.


But then Chevy also clearly manipulated the rpm at which it advertised the peak gross hp. For instance, the L30 version of the 327 had been rated at 250 gross/210 net for some years. That 250 gross peak was given at 4400 rpm. For some reason, in 1966 the same L30 engine was got a 275 gross hp rating, although the net was the same at 210 hp. The only apparent difference was that the gross peak was now given at 4800 rpm, although the dyno charts do show the peak at those corresponding rpm.

The same machinations are also evident with the 325/350 hp L79 version of the 327. And undoubtedly others too. It is accepted wisdom that manufacturers just plain lied about their hot engine’s gross hp, as in made up the numbers. They presumably  just chose the rpm level where the engine made the amount of power they were wanting to advertise. Or?


I spent way too much time delving into all kinds of obscurities at the GM vehicle Kits, but here’s just one odd example. The 1969 truck kit shows the 292 High Torque six, with two dyno numbers and ratings, depending on whether they were equipped with air injection systems or not. Both are rated at 170 hp gross, but the net numbers are 125 and 130. Now this is common throughout, and another example of how the gross advertised number was misleading. But that’s just part of the story.


Here’s the same basic engine, but as installed in the larger C50 series trucks. Same 170 hp gross rating, but now the net numbers are 140 (with air injection), and 153 (without emission controls). A little digging showed that back in 1966, prior to any emission controls, the 292 was rated at 170 gross, 153 net, in all applications. Again, this shows that manufacturers were mostly no lowering gross ratings, while emission controls were eating away at the net numbers; 28 net hp in the case of the 292 with air injection. Or the emission control systems were just completely stripped off when the gross dyno test was run.

Well, it would be a mammoth project to to correlate all of the variations between gross and net hp numbers, especially in the period of 1968 through 1971, the last year gross numbers were used, and the game was finally over.