Differences between 71 l48 and 75 l48?
Thread Starter
Heel & Toe
Joined: Apr 2026
Posts: 16
Likes: 0
Differences between 71 l48 and 75 l48?
I have a 1975 l48 that the original owner deleted the catalytic converter and put headers on there making it have true dual exhausts. As far as I can tell, the catalytic converter and more limiting exhaust manifolds are the only differences between the 71 and 75 l48s (and the rpm’s that they tested the horsepower at), right? So my engine should now make about 280 hp and 360 ft-lbs of torque at 4800 rpms because I also have a holly 4 barrel carb on there. I guess there’s no real reason I’m asking this, just a shower thought that I thought I’d confirm.
Melting Slicks
Joined: Apr 2021
Posts: 2,684
Likes: 2,134
From: Minnesota
Not sure how having a different 4 barrel carburetor than stock would make a difference, but the short answer is yes. The long block design for the base model engine remained relatively the same from 1971 to 1980 as far as camshaft, cylinder head volume, and potential output. Here is a really good thread that talks about the intricacies of the L48.
Thread Starter
Heel & Toe
Joined: Apr 2026
Posts: 16
Likes: 0
Not sure how having a different 4 barrel carburetor than stock would make a difference, but the short answer is yes. The long block design for the base model engine remained relatively the same from 1971 to 1980 as far as camshaft, cylinder head volume, and potential output. Here is a really good thread that talks about the intricacies of the L48.
Drifting
Joined: Jun 2025
Posts: 1,692
Likes: 590
'71 L48 rated at 270 sae Gross HP; it has OE Qjet carb, true dual exhaust with No cat, dished pistons and larger 76cc combustion chambers, valve lift .390/.410
I've owned & worked on & rebuilt TWO base '71 C3
Differences inside motor among '71-up L48 aka ZQ3 amount to carb tuning and ignition curve+ignition timing.
I've owned & worked on & rebuilt TWO base '71 C3
Differences inside motor among '71-up L48 aka ZQ3 amount to carb tuning and ignition curve+ignition timing.
Team Owner
Joined: Sep 2006
Posts: 31,235
Likes: 4,326
From: Westminster Maryland
Hello OC,
A pretty exciting car for you to be working on.
For some years Chevrolet had a 300 HP small block engine that was put in various cars in its line up. It was typically either 327 or 350 cubic inch deplacement.
It was a popular 'kinda perfomancy' engine that was extremely easy to live with day to day.
The listed hp rating changed over the years because of several things.... the method for determining the hp was changed, the compression ratio was changed/lowered, meeting emission standards required various changes to intake and exhaust systems. (As Ry mentioned above.)
That's why you see the hp rating change/drop as the years went by.
The Holley Spreadbore carburetor was a popular swap for these engines.
Good Luck!
Regards....
A pretty exciting car for you to be working on.
For some years Chevrolet had a 300 HP small block engine that was put in various cars in its line up. It was typically either 327 or 350 cubic inch deplacement.
It was a popular 'kinda perfomancy' engine that was extremely easy to live with day to day.
The listed hp rating changed over the years because of several things.... the method for determining the hp was changed, the compression ratio was changed/lowered, meeting emission standards required various changes to intake and exhaust systems. (As Ry mentioned above.)
That's why you see the hp rating change/drop as the years went by.
The Holley Spreadbore carburetor was a popular swap for these engines.
Good Luck!
Regards....
Last edited by Alan 71; Apr 17, 2026 at 09:22 AM.
Pro
Joined: May 2019
Posts: 559
Likes: 261
From: Fountain Hills (Phoenix) AZ
My 1975 stock L48 is original and never removed from the car. Previous (original owner) put headers and duals on it and I recurved the distributor per Lar's recommendations. It easily runs as good as my 1967 300 HP C2, even though the '75 fitted is fitted with a TH400. Torque is amazing going up a long steep hill near my residence.
I cannot comment on real HP/torque values but seat of the pants feel is pretty strong. And, it still has the original Q-jet.
My 2 cents.
I cannot comment on real HP/torque values but seat of the pants feel is pretty strong. And, it still has the original Q-jet.
My 2 cents.
Race Director
Joined: May 2006
Posts: 18,710
Likes: 2,574
I’m not really sure on how the holly is better either. that too, was done by the original owner, but I have heard people say that a higher quality carb like a holly would add 10-15 hp but not sure on the legitimacy of that. Thx for the thread btw, that has been helpful.
Drifting
Joined: Jun 2025
Posts: 1,692
Likes: 590
when optimized, smaller carbs can make big power
Lotsa circletrack 350-358 sbc w/ flat tappets & flattops & 23 degree Production iron heads making 360 SAE Gross HP + more with just a (race legal) TWO barrel 500cfm Holley or Big Rochester 2GC. Yes, spec race gas but No extra Oxygenation (Or DQ).
Those carbs are tightly-regulated by tracks' tech men (tech-police). Venturi size, booster height etc is measured & held within spec Or DQ.
* there are means to cheat here & improve flow; but those means not practical for street use.
Those carbs are tightly-regulated by tracks' tech men (tech-police). Venturi size, booster height etc is measured & held within spec Or DQ.
* there are means to cheat here & improve flow; but those means not practical for street use.
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