Someone asked a question about a de-smogged L-48, and it made me wonder if de-smogging it raises the HP and how much. Just curious. ('72)

 

Hello,
I was taught that it's the change in the compression ratio that affected the horse power output to change from 70 to 71. 300 hp to 270 hp.
The 350/270 didn't have any of the air-injection reactor system ( AIR /smog pump).
The change between 71 to 72 (still no AIR equipment) was due to a change in how the engine output was determined.
SAE rating in 72.
Regards......

 

If you are talking a 71-74, low CR L48 with duals, I would not expect much. A distributor curve will help it the most. 10-15HP?

If you are talking about a 75-82, the ones with a single cat conv and backpressure as high as 1/2 atmosphere, then absolutely!
Pulling the cat, running headers, true duals, and turbo mufflers adds like 50HP. It's huge!

At the end of the day, both of the above wind up around 280HP gross.

Almost as good as a 68-70 Hi CR L48, but not quite.

They all run nice that way. But not when all choked up.
Someone tested the exhaust backpressure on a 75 and it was 7psi!!!!

 

I believe it’s my thread you’re talking about (here) and like what’s been said it’s pretty substantial. Even without changing a single thing to the long block of the engine once it’s de-smogged it’s not only more powerful, it gets better fuel economy, throttle response, and runs cooler too!

Now the question is how about an L82?

 

RE: L48. There's No PHYSICAL diff '71 v. '72: same displacement, same cam, same pistons, same static compression ratio.

in 1971, GM PUBLISHED power rating in Both SAE Gross And SAE Net formats (Gross 270 HP, Net 210 HP)
in 1972, GM PUBLISHED power rating Solely in SAE Net (Net 200 HP). Power was likely measured using Both methods, but only published via one.

any power difference is essentially due to TUNING / SETTINGS / ADJUSTMENTS

JME: that TEN HP difference is of no real practical or statistical significance; And can be re-discovered by so-called 'de-smogging', tuning, settings, adjustments.

*due to Some pre-71 L48 higher compression ratio, those motors' throttle response is noticeably more crisp; and more powerful.(although same displacement and same cam as later L48).

**** yay ! Sheldon Creed FINALLY Won his first NASCAR O'Reilly Series race today !
perennial Jeremy Clements again way back.

 

Rebelyell - The higher compression '70 base Corvette engines were not "L-48"'s They were "ZQ-3"s.

 

Never heard of that engine.

 

AI says:

The RPO ZQ3 was the base-level, 350 cubic-inch (5.7L) small-block V8 engine for the C3 Chevrolet Corvette from 1969 through 1974. It was a reliable, street-oriented engine featuring a Rochester "4MV" Quadra-Jet 4-barrel carburetor, 4.00" x 3.48" bore/stroke, and hydraulic lifters. Horsepower ranged from 300 hp (1969–70) down to 190-195 hp (1973–74) due to lower compression ratios and emissions regulations.

JT

 

Thanks for the info. I always thought the 300hp 350 was an L48. I had it in a 74 Nova that had the 350 295 hp. I guess that was a ZQ3 too?

 

Len,
Im not so sure about ALL those those years being ZQ-3 for the Corvette. I think it was over when the CR dropped but that is just a guess.

The '74 was the last year for the L-48 option in a Camaro, but it was only 185 HP by then.

 

Thx for the info.

 

 

Thank you, and
Yes, That's a Fact. And one I've been well aware of; I was waiting with bated breath for someone's admonition of same. I've probably mentioned this same L48-ZQ3 confusion & equivalency in prior post(s).
ZQ3 is-was functional equivalent of L48 (in Most applications). ZQ3 was the General's RPO-scheme designation for 350ci sbc 'BASE' C3 engines thru about MY1973.
Beginning MY1967, there were Several other GM passenger cars which had the 'new' 350ci sbc L48 available. Compared to post-MY1970 sbc, MOST of those were relatively high compression as MOST had flattop piston With relatively small (~ 64 cc) chamber With smaller (1.94",1.5") valves and (again) same smallish cam we've all become so accustomed to finding in, for example, a MY1970 thru 1980 'BASE' C3.

All along, my point in this thread, is that GM did install serial production L48 motors, with relatively higher-compression, in many serial production passenger cars beginning 1967 and thru 1970. Including both L48 and its functional equivalent: ZQ3. And, therefore: Some L48 (and ZQ3) motors' needs may Not be satisfied with 87 Octane. Just sayin'

Why two different RPO designations for what are essentially same motor ? Dunno ! But I can guess. Maybe someone else KNOWS that as well ?

Trying to, but can't recall our OE C3 console engine data plates displaying any RPO; just a marketing dept-approved spec or two.

Len W: I'd owned 'BASE" C3 for several years before I learned its motor was officially a ZQ3. I also learned L48 and ZQ3 are essentially same. I thought my '71 C3 had an L48; when it was actually a ZQ3 (regardless of fact they're essentially the same motor). I suggest You and I and countless others are/were, in that respect, in same boat. I'm neither a collector or judge; instead, I build motors.

FWIW: in late 1966, my late dad purchased for my mom (then also quite lively) a brand new 1967 Camaro SS in Bolero Red w/ white stripe; which had the then-new higher-compression 350ci L48 under its very attractive hood. It also had a Powerglide automatic and AC and AM-FM and power disc brakes. Mom drove it for several years, beat it up then traded it (and dad) for a new Caddy; I drove that Camaro only once (without permission). I promise; its L48 was quite lively as well And it needed Hi-Test ! It was both bought new & traded in SoCal. I last saw it around 1971. I'd rather have that one special car than any of the bumper C3s I've owned.

*** and Yup. ALL of em had their compression ratio's decreased (emasculated ) beginning MY1971

 

I owned a 68 Camaro SS350 4spd 295HP for many years starting in 71. Basically an L48 regardless of what they called it.
10:1 CR definately needed premium gas. Ran terrific! 336 rear. It ran almost the same as my completely desmogged 75 Vette 4spd with headers and duals, even with its 308 rear. It ran fine on regular with its lower CR.

Engine internals were almost functional identical except for the CR. IIRC the 68 may have had a steel crank tho. Both cars could knock down 18mpg on highway with their oem Qjets. The 75 distributor was better tuned.

So forgive me if I call all of them L48s. They are all basically the same. Except for CR.

No idea of the qtr mile times, but either would stay with almost any SB on the street. With only a couple of exceptions.
No need to ask me how I know that!

 

And, your Nova probably Did have an L48; and it was essentially same motor as same era ZQ3. Maybe it was tuned slightly less so that GM marketing could Not abide having the corvette w/ LESS HP. No one, that I know, can feel a 5 HP diff on 300 HP.

 

Go to gmheritagecenter,.com for the official ordering codes and manuals for any gm car including the corvette.

I just looked up the 1974 corvette engine codes and the base engine in the 1974 corvette documents is offically referred to as THE L-48:!

​​​​​​CORVETTE POWER TEAMS Engine, Transmission and Positraction Rear Axle Combinations (Engine hvpewr ratinye ara reflected ar"t" horsepower) POSITRACTIONN REAR AXLE RATIOS Without Ai Conditioning With Air Conditionina ENGINE TRANSMISSION Option Number Description Type (Std or Optional) Optienal Std Std Econ Perf Spес STANDARD ENGINE Standard EightCylinder Engine Ordering Code L48 Turbo-Fire 350-4/DE 8-Cylinder 350-cи-1n displacement 4-barrel carburetor Hydraulic valve lfters 8.5:1 compression ratio Dual exbaust Optional Ecen Perf Spe 4-Speed Wide Range (Std) -M20 3.36 3.08 3.36 3.08 Turbe Hydre-matic-M40 3.08 - 3.36 - 3.08 - 3.36

 

Same thing, but right out the manual

 

1970 ZQ-3 short block shown below. Cast 4 eyebrow flat tops (no dish) with cast iron crank and 4 bolt mains. It originally came with 186 double hump heads on it.

This block has been rebuilt as a solid lifter 383 stroker residing in my '80 Corvette today.


Thanks Lazlo

 

Thanks for the post.

 

Thanks for all of that information.