Can someone explain why the '74 454 has so much less HP than previous years? I know it was done for government regulations, but what exactly is different?

 

Low compression pistons, restrictive heads. (and they changed the net rating a few years earlier, so the numbers just appeared lower)

But for all intensive purposes, its the pistons and heads. I had a stock 72 that came with less than 300 stock. Its the dam low compression pistons and heads. Ive just recently upgraded to a mile dome piston but couldnt budget heads...heads alone on a 454 are good for 45-60 HP.

 

Putting a performance advance timing curve, more initial timing (and HEI swap) will also help wake up the beast! Helps make up for the low comp. and retarded timing for smog control.

 

Hi N74,
Welcome! New Member and First Post!!!!
Here's some info that clearly illustrates the change in hp rating systems that Scott mentions.
This is for the 1971 cars and is taken from Corvette News.
The difference for the 71 numbers was a reflection of ACTUAL changes from the 70 motors AND the change in RATING method.
Regards,
Alan

Note the LS-5 that had been rated at 390 hp (gross) for 1970 was rated at 365 hp (gross) and 285 hp (S.A.E. net) for 71.
Also remember that when the SAE numbers became the 'published' numbers in 72 they had dropped a bit more.

 

yes the primary difference was the GROSS ratings used prior to 1972 and The NET HP/TORQUE ratings use from 1972 until today. The Gross ratings in short are not very relevant to the actual power the engine will make Installled in the car which is the reason the the NET ratings are used even today, starting in 1972. Net HP ratings represent the power the motor will make with a full exhaust, all accessories hooked up and running under the engine's power (not supplied by an outside source) etc versus GROSS which is the engine run on a stand with no exhaust, accessories etc. Almost all crate engines are still advertised as GROSS HP-bigger numbers (marketing) but not really the HP the engine will make installed in a car. A 330 Gross HP crate engine (or a 71 LT-1) on average would make about 255-260 Net HP-BIG difference in the numbers

 

The heads on a '74 454 are OK...they can make incredible power. The real killer comes from a real compression ratio that measures 8.0 theoretically...but in real life often came in lower than that.

Some real pistons and cam change along with some pocket porting in the heads will make a very strong performer.

JIM

 

Alan and jb78L-82, also did'nt GM also advertise the HP rating at lower RPM's? In order to fool industries/govt standards? I thought the LS5 in 70 was a strong under rated engine, but with a gross hp rating of 390, does that mean is was much less in net terms? You always here the engines such as the L71 and LS6, L88 actually made much higher than their advertised numbers.

 

The underrating of the L-88 was definitely true but was really more a race engine than a street engine. As you can see below from the Super Chevy article, the LS6, not the LS5 you asked about, appears to be rated properly since the Gross HP rating was @ 5,600 RPM which looks about right for the highest HP in the engine's RPM range:

"The '70 SS454 Chevelle, with a 454ci, solid-lifter, high-compression, LS6 engine, was rated 450 gross hp at 5,600 rpm and 500 lb-ft of torque at 3,600 rpm. That engine had the same internals (camshaft, heads, lifters, and so on) and 780-cfm carb as the L72 427. Many enthusiasts thought the LS6 was the more powerful engine, but Tom Langdon, a '60's Corvette powertrain development engineer, pointed out, "Increasing the stroke without enlarging the bore doesn't translate into a real increase in power. Some of that extra power is eaten up by increased friction. A good 427 (L88) would put out about 600 horsepower. The 454 pulled more torque, but power was just about the same as the L88." (This quote comes from Classic Corvette-The First 30 Years by Mike Mueller, originally published by Motorbooks.) Langdon's statement also applies to the L72 and LS6. They were identical engines except for stroke-derived cubic inches, and both made 450 hp."


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However, another source indicates the LS6 was also underrated…so not really sure who to believe:

"The Big-Block was expanded again for 1970 to 453.96 cubic inches (7.4 L), with a 4.250 in (108.0 mm) bore and 4 in (100 mm) stroke. The 1970 Chevrolet Corvette LS5 version of this engine produced 390 hp (291 kW) and 500 lb·ft (680 N·m), and the LS6 engine was rated at 450 hp (340 kW). It has been suggested that the LS6 was substantially underrated and actually produced well over 500 horsepower (370 kW) as delivered from the factory, although there is no empirical evidence to support this claim."

 

Nope. These engines were federally tested and emissions certfied. There may have been some media hype from GM directed at the insurance companies, but they weren't hiding anything from the Feds.