I was looking at a '71 LS6 'Vette, and it struck me how abrupt the horsepower cliff was into the '72 'Vettes, and most all other makes too. The last 426 Hemi was '71, and the last Super Cobra Jet. What happened in 1971 to make all this happen? I know they started rating engines on net HP, but it was much more than that, as the high HP big blocks all disappeared at once. I was around then, my wife and I were married that year, but I was many years away from being able to afford a new car, so don't remember it that well. Any facts here? Thanks.
-
Big Tanks In the High Mountains of New Mexico -
Re: The '71-'72 Horsepower "Cliff", what happened?
I think that you are correct that this was when they changed horsepower ratings to net. There were also air pollution requirements, as well as insurance considerations. I thought that all of these happened over the course of a couple of years, but they could have all happened at once. -
Re: The '71-'72 Horsepower "Cliff", what happened?
Michael------
The most significant thing that caused the drop in advertised horsepower between 1971 and 1972 was the industry change from GROSS horsepower to NET horsepower.
Why did special high performance engines start disappearing rapidly after 1971? Most likely emissions regulations in the "pre-cat" era.In Appreciation of John HinckleyComment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
Two things happened at GM in '71. First, compression ratios on premium fuel engines were dropped about two points to allow operation on 91 RON unleaded fuel. This dropped the torque/power curves about ten percent across the entire rev range. The rest of the industry followed in 1972.
In '71 both gross and net torque were advertised, but only net in '72. Net horsepower uses a lower air density, which drops the rating about 4.5 percent. Then you have the parasitic affect of front end accessories and exhaust pumping power loss. Many muscle cars of the era had restrictive exhaust systems. The Corvette offered one of the better ones. It's not too restrictive with small blocks, but is beginning to become restrictive with big blocks.
All the above could result in net power ratings up to 30+ percent less than gross, but they were a lot more honest in terms of telling us what the engine could deliver installed in the car as delivered.
The affect of front end accessories can be seen in current chassis dyno tests. If the fan clutch does not tighten it only absorbs 1-2 HP. If insufficient external cooling fans allow the clutch to fully tighten it can cost 15 lb-ft peak torque and about 10 horsepower from 3500-up.
DukeLast edited by Duke W.; December 23, 2012, 12:16 PM.Comment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
Yes, thanks Duke, but I guess the emissions things is what killed the SHP LS6 from '71 to '72. The same thing happened to the 426 Hemi, as I understand it.Last edited by Michael J.; December 23, 2012, 02:27 PM.Big Tanks In the High Mountains of New MexicoComment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
The 1970 amendments to the Clean Air Act were the cause.
Everything that was stated above, and much more, were the effects.Comment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
High overlap cam engines are very tough to emission control because they must run very rich at idle and low speed/low load conditions, which creates high HC emissions, so "big cams" were no longer viable in the emission era.
Also, each vehicle, engine configuration, transmission, axle ratio had to be emission certified, so the days or six engine options, three transmission types, and six axle ratios were over.
DukeComment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
I agree. The Clean Air Act targeted tetraethyl lead, NOx, and unburned hydrocarbons (HC). This caused the reduction in engine compression ratios, and the addition of the engine air injection pump (AIR), retarded initial timing and ported vacuum advance, and ultimately the single restrictive catalytic converter.
Around this same time, our country went through the gasoline embargo and gas crisis/shortages. I remember buying and storing gas in 5 gallon cans for use with my cars and motorcycle on the weekends
The insurance companies also contributed by significantly raising the premiums for the high performance cars....especially for young, single men, with large CID engines and light weight cars.
The net result was a sell-off of many of the HP cars by their owners and a push to get everyone into economy sedans. The major car companies went along with this. HOWEVER....some of us resisted
FWIW. LarryComment
-
Big Tanks In the High Mountains of New MexicoComment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
The age of high performance was not THEN, it's NOW!In Appreciation of John HinckleyComment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
All engine output ratings from '72 to today for cars sold in the USA are SAE net.
DukeComment
-
Re: The '71-'72 Horsepower "Cliff", what happened?
Not only that, but now many cars have SAE certified ratings. When this first happened many cars "lost" a few horsepower again.
Vice-Chairman (West), Michigan Chapter NCRS
71 "deer modified" coupe
72 5-Star Bowtie / Duntov coupe. https://www.flickr.com/photos/124695...57649252735124
2008 coupe
Available stickers: Engine suffix code, exhaust tips & mufflers, shocks, AIR diverter valve broadcast code.Comment
Comment