Novusuhu

Hey everyone

Before I start my question... Here is what I think are HP ratings for the L48 from 1972-1976
1972 - 225 hp ???
1973 - 190 hp
1974 - 190 hp
1975 - 165 hp
1976 - 180 hp

1. What modification did Chevrolet do to the 1976 L48 to get it to 180 HP? I'm asking this because in 1975 the L48 had 165 HP.
Was there really something they changed on the engine to get that 15 HP gain, or was it just GM that cheated?

2. What was the difference between 1972 and 1973 L48? Because there is a 'big' difference between 1972 and 1973?

mobird

I know the answer to #2. Gross vs Net horsepower. Engines were the same, just the way horsepower was rated changed.

#1 is probably just a difference in intake or exhaust (dual snorkel vs single, dual exhaust vs y pipe etc...)

7T1vette

The lower HP ratings during the early 70's is probably the most mis-understood subject by Corvette enthusiasts. Folks think that there were significant design changes that caused the "loss" of power over those years. Actually, the engines were nearly the same between 1971 and 1977. All of them had 8.5:1 compression and essentially the same components.

Starting in 1972, the SAE rating system for HP was changed from GROSS horsepower to NET horsepower. Gross power refers to the total power coming our the rear of the engine; NET power subtracts power that was absorbed by accessories (alternator, pumps, transmission, etc). The '71 and '72 engines actually put out the same amount of power; but the rating system used drastically changed the rating from 270 to 225 HP.

The explanation for further "losses" over the years was also due to the way GM rated their engines. In that era, the insurance companies were raising premiums on vehicles with engines having more than 200 HP. So, GM decided to artificially de-rate their engines by altering the rpm level used in the rating. Obviously, if you measure HP at a lower rpm, the power will be lower than if measured at a higher rpm. 1971 - 270hp @ 5000; {SAE Net HP rating change};1972 - 225hp @ 4400; 1973 - 190hp @ 4400 {some actual power losses from emissions changes}; 1974 - 195hp @ 4400; 1975 - 165ph @ 3800; 1976 - 180hp @ 4000; 1977 - 180hp @ 4000. As you can see, most of the differences in HP were due to what rpm was used for the power rating during that model year. Engines from all years would easily pull through 5000 rpm.

Basically the L-48 engines were very similar over those years and actually generated close to the same actual power. If those engine were tuned similarly, they would all perform similarly.

Dynra Rockets

Not a thing. Long blocks were same.

ED DINAPOLI

[ the engine toQUOTE=Novusuhu;1597714730]Hey everyone


2. What was the difference between 1972 and 1973 L48? Because there is a 'big' difference between 1972 and 1973?[/QUOTE]

The difference in hp between the base engine in 1972 and 1973 was 10 hp. The horsepower rating for the base in 72 was 200 hp. The LT! hp was 255
Ed

7T1vette

Sorry. I had an error in my data for '72. The base L-48 engine was rated at 200 hjp...not 225. LT-1 engine rated at 255 for '72.

85nova

It's interesting, this website suggests considering the weight gain during this period, as well as power. Apparently, '78 C3s were the heaviest but the power to weight was still lowest in '75. Apparently even lower than the original '53 which is really hard to believe.

https://anotherapex.com/making-it-th...c1-c8-engines/

derekderek

from 71 on all automakers were trying to make the EPA happy. you retard the spark you get a cleaner idle. you also lose power. 4 degrees less advance at full advance can be 40 to 100 hp. 100 is an extreme engine. 40 is a normal production car running 32 instead of 36 at 3000. in 76, the quickest production vehicle was the dodge lil red express truck. why? different emission specs for pickups so it could e tuned to run instead of for emissions. get the advance right. do a little carb jetting. these are all 200+ hp motors. just remember the other tune you need when-if you hafta visit the emissions guy..

jb78L-82

I read through the comments and was surprised everyone missed one very obvious reason in the NET HP differences between 1972-1980 (and before BTW from the Gross Ratings as well) on the L-48 base engine (and the 1971 LT-1/1972-1980 L-82's as well).

All of these engines were essentially the same except for timing adjustments, carb limiting mixture screw adjustments/jetting AND MOST IMPORTANTLY, the use of a true dual exhaust on both the 1971 LT-1, L-48, and L-82 engines from 1971-1974. After 1974 and starting in 1975 the true duals disappeared and were replaced by the 2-1-2 exhaust Y pipe and the dreaded awful pancake catalytic converter which prevented all these engines from breathing freely, choking them, and causing the NET HP numbers to plummet.

The best example of the use of the restrictive exhaust effect is the 250 NET HP rating of the 1974 L-82 and the reduction in NET HP in 1975 on this same engine with the 2-1-2 catalytic converter exhaust to 205 NET HP-That 45 NET HP loss is previously why on the later 70's L-82's using headers and a true 2.5 inch exhaust with zero additional changes to a stock motor produces about 50 more NET HP. l made this conversion to my OEM L-82 back 20 years ago when the engine was OEM and yes it made a big difference in power...Hope that is helpful.

Jebbysan

If you pulled every L-48 out of each respective year and put it on a dyno with headers, they all would make about 245-250 horsepower with a proper advance curve. The 350 Goodwrench/Targetmaster engine makes exactly that too with a low rise L-82 intake, same pistons, heads...etc.......
GM's power rating were and are crap......all the way til now.
A 2000 Camaro SS or WS6 T/A was rated at 305hp. They would put that to the tire. The 87' Grand National was rated at 245hp and it would throw 275-280 to the tire on a cold day.
Now on the C3 there were air cleaner and exhaust differences, but I do not consider these things as applicable because NOBODY keeps that pathetic exhaust system on the car if they know whats good for them.
The converters set grass on fire, would plug up in about 70,000 miles, and reduced power ratings so much that it was like driving a different car.
A 79' L-82 will run right with a 73' if both cars have long tube headers, duals, and a proper timing curve.
The 79' T/A 400 4 speed with a curve and headers/duals picks up a whole second in the 1/4 mile.....a whole second.....think about that.
The mysticism on GM's power ratings is ink.....and nothing else.
It is hard to believe now that they sold cars like this in the 70's........the Big Three got caught laying down and it took until the mid 80's when EFI became the magic bullet.

Jebby

jb78L-82

The topic of weight comes up often but the facts are that all the C3's from 1973-1982 weighted on average about 3300-3500 lbs with the 78/79 the heaviest (35XXlbs) and the 82's at 32XX lbs...not a giant difference once you throw out the 1982's weight from the numbers. All of the C3's with relatively little effort on the owner's behalf can lose a couple of hundred pounds very easily like my 78 L-82:

My 78 L-82 weighs about 3250-3300 lbs today by replacing some of the heavier parts with lighter weight components:

1980 Clamshell Seats
aluminum cylinder heads
aluminum Radiator
Aluminum water pump
Light wieght exhaust headers versus OEM cast iron manifolds
no AIR pump and hardware
L-82 aluminum intake manifold (versus L-48 cast iron intake)
Lighter weight aluminum aftermarket 17 inch wheels
Fiberglass/Composite rear transverse leaf spring
No catalytic converter
Lighter weight aftermarket mufflers
lighter weight modern battery
No Heavy AC compressor
etc

Getting the weight down to 3,100-3,200 lbs is relatively easy ^^^^^^^^with these and a few other mods but get under the magical 3,000LBS mark requires a lot more modifications to the basic C3 design and framework......

dochorsepower

1975 was also the first year of the catalytic converter.

L-46man

Delving into history and my vast C-3 library, some things to mention.

Chevy frequently changed;
Configuration of air cleaner
Cold air intakes
Fan size and pitch
Size of Cat
Size of lead-in pipes to cat
size of cat exits.

Besides all the emission tuning details.

Chevy Engineering had stated that the Dual snorkel air cleaner was responsible IN AND OF ITSELF was worth 5HP. A decade later they claimed that the 1982 L-83 Fuelie engine (200HP SAE NET) and the C-4 1984 L-83 engine (same engine) EXCEPT for the FAN netted out 5HP JUST FOR THE FAN CHANGE.

I find it VERY interesting that if you look at 0-60 times published by R&T over that entire DECADE (with their very heavy test equipment back in the day) O-60 remained for over the decade at about 7.3> 7.5 seconds.



Unkahal

jackson

For 1971, GM advertised specs for both Gross & Net ... see pdf attached below ... page 118 of 165

1971 L48 = 270 GBHP @ 4800 RPM ... and ... 210 NBHP @ 4400 RPM

FWIW, I've had two 1971 L48 and both had OE Dual-Snorkel air cleaner assembly ... sold Bridgehampton Blue coupe ... current War Bonnet Yellow roadster

https://www.gmheritagecenter.com/doc...t-Corvette.pdf

jackson

Also, 1971 begins a drastic change ... noticeably reduced compression across all GM passenger platforms.

jb78L-82

78/79 L-82 4 speeds documented at 6.5 sec 0-60 MPH and 6.6 for the auto L-82....Just removing emissions hardware and adding a true 2.5 inch dual exhaust with free flowing mufflers on the 78-80 L-82's, particularly the 4 speed cars, and the 0-60 MPH drops to the low 6's, easily, which is very close to the traction limited early C3 BB's and VERY CLOSE to the L-46/LT-1's, traction limited back in the day as well.....with crap poly tires..