C3 octane requirements?
Thread Starter
Le Mans Master
Joined: Dec 2000
Posts: 5,386
Likes: 2
From: San Angelo TX
C3 octane requirements?
Ok, so I'm new to the whole C3 scene (C4 guy) but I'm searching for a chrome bumper car these days. I keep reading that 1970 was the last year for high compression engines. Well, what is the octane requirement for these 1970 and earlier cars? Do they require special fuel that I wont be able to find at a present day pump? My town only has up to 92 so it is a concern. Also, what octane do the post 1971 cars require? Do the 1970 (and earlier) cars require a modification to the carb in order to run unleaded fuel? I've searched a good bit and can't find these answers. Any help you can provide would be greatly appreciated.
Team Owner
Joined: Jun 2000
Posts: 38,923
Likes: 1,482
From: Southbound
Cruise-In II Veteran
74 and up C3s have hardened valve seats and will run on cheapo no-lead. They like 91 octane, but they'll run on 87. The earlier cars were designed for leaded fuels and higher octanes but will run on no-lead. My '68 has stock heads and 10:1 compression; she runs okay on no lead. Periodically, I run a can a lead subsistute with a fill up.
You have genuine concerns when you go to the super horsepower cars (L-88, etc) designed to run on 104 leaded.
You have genuine concerns when you go to the super horsepower cars (L-88, etc) designed to run on 104 leaded.
Safety Car
Joined: Apr 2000
Posts: 3,802
Likes: 61
From: Wichita KS
I believe the compression ratio for 1970 and earlier is from 10.5:1 to 11:0:1 except for the extreme engines like the ZL-1 and L-88.
In any case my 1972 454 runs fine with absolutley no pinging on 87 octane because the compression ratio is 8.5:1. I'm pretty sure the 1971 is also 8.5:1.
No modifications to the carburetor are necessary to run unleaded but from 1971 on, the cars were designed to run on lower octane unleaded fuel.
cc
In any case my 1972 454 runs fine with absolutley no pinging on 87 octane because the compression ratio is 8.5:1. I'm pretty sure the 1971 is also 8.5:1.
No modifications to the carburetor are necessary to run unleaded but from 1971 on, the cars were designed to run on lower octane unleaded fuel.
cc
CF JASOC Member
Joined: Feb 2006
Posts: 8,040
Likes: 34
From: Downtown Annapolis, MD. The Future is where we all have to live. Let's not screw it up.
St. Jude Donor '08-'09-'10-'11
My '71 L48 runs great on 87 octane. The '88 is another story. Got to have at least 91 for it.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Instructor
Joined: Oct 2006
Posts: 161
Likes: 1
From: Maple Shade NJ
I always run premium in both cars 71 & 72 ,350 base with 8.5:1 .I also pour in a bottle of NOS (cost about $10) It raises 6 full numbers (60 pts) Runs like a champ, no hes, no ping
Elkhartgreen72 (Dave)
Elkhartgreen72 (Dave)
