Which Octane?
Thread Starter
Drifting
Joined: Mar 2011
Posts: 1,693
Likes: 144
From: on Barnegat Bay Joisey
Which Octane?
I'm getting ready to pull my '74 L48 M20 out of the garage and one of the first things I need to do is fill it up. I purchased the roadster last summer and this will be my first gas stop. The only thing available here is E10 in 87,89 and 92 octane. What fuel would you recommend I use? The engine is stock original with 37K miles on the clock. It runs strong with no noticeable smoke or ping. Thanks in advance.
Le Mans Master
Joined: May 2006
Posts: 5,814
Likes: 414
From: Pittsburgh suburbs Pa.
I'm getting ready to pull my '74 L48 M20 out of the garage and one of the first things I need to do is fill it up. I purchased the roadster last summer and this will be my first gas stop. The only thing available here is E10 in 87,89 and 92 octane. What fuel would you recommend I use? The engine is stock original with 37K miles on the clock. It runs strong with no noticeable smoke or ping. Thanks in advance.
Team Owner
Joined: Jun 2000
Posts: 38,923
Likes: 1,481
From: Southbound
Cruise-In II Veteran
Race Director
Joined: Oct 2000
Posts: 12,657
Likes: 3,117
From: Shamong, NJ
Starting in 1971, Chevrolet reduced the compression ratio on all of their engines, to allow them to run on regular unleaded. A stock 74 L-48, should run just fine, on any of today's regulars.
Drifting
Joined: Sep 2014
Posts: 1,430
Likes: 59
From: Long beach NY
St. Jude Donor '15-'16, '18
Put in the 92, really how much are you driving the car that a little extra savings at the pump is going to make a difference. I think you will notice the difference in performance with the 92 anyway.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
Le Mans Master
Joined: Jun 2013
Posts: 7,039
Likes: 942
From: Colorado
2025 Corvette of the Year Finalist - Unmodified
2024 C8 of the Year Winner - Unmodified
2024 Corvette of the Year Finalist - Unmodified
2023 C7 of the Year Winner - Unmodified
2022 C7 of the Year Finalist - Modified
2020 C7 of the Year Finalist -- Modified
I too say go for the lowest. When I bought my 78 I made the mistake of putting 91 in it, and it ran like crap. Followed a friend's advice and used 85 (regular in Colorado), and it runs like a charm. 
Drifting
Joined: Sep 2014
Posts: 1,430
Likes: 59
From: Long beach NY
St. Jude Donor '15-'16, '18
Thanks
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
He has a low compression engine so low octane gas is all that is required. Older hipo cars had a lot higher compression and required high octane. Todays hipo cars are a whole new breed with lots more power, higher compressions like a lot of older hipo cars but can run on 91 octane.
Drifting
Joined: Sep 2014
Posts: 1,430
Likes: 59
From: Long beach NY
St. Jude Donor '15-'16, '18
He has a low compression engine so low octane gas is all that is required. Older hipo cars had a lot higher compression and required high octane. Todays hipo cars are a whole new breed with lots more power, higher compressions like a lot of older hipo cars but can run on 91 octane.
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
Octane rating is simply the fuels ability to resist igniting under pressure before the spark plug fires.. { DETONATION} 93 Oct produces no more power than 87 octane .. It only allows high compression pressures without detonating before spark. Putting 93 in a car designed for 87 will not improve performance or cleaning , engine life or anything else .. Its a MYTH
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
On the flip side to the above, if you put 87 in an engine that requires 93, then you will get ping. On newer cars, the engine knock sensor will detect the ping and retard the timing i.e. loss in power.
Pro
Joined: May 2013
Posts: 672
Likes: 31
From: Rapid City South Dakota
Pro
Joined: May 2013
Posts: 672
Likes: 31
From: Rapid City South Dakota
Octane rating is simply the fuels ability to resist igniting under pressure before the spark plug fires.. { DETONATION} 93 Oct produces no more power than 87 octane .. It only allows high compression pressures without detonating before spark. Putting 93 in a car designed for 87 will not improve performance or cleaning , engine life or anything else .. Its a MYTH
Race Director
Joined: Sep 2005
Posts: 18,640
Likes: 121
From: Farragut,TN
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
The OP stated it had not been.. and if it is modified ,then its no longer as designed, hence the word MODIFIED ... I stand by my statement, if the engine is designed to run on 87 octane, there is absolutely no benefit to using 93 octane. None. Its a myth 
Instructor
Joined: Jun 2008
Posts: 233
Likes: 9
From: Marietta Ga
I'm getting ready to pull my '74 L48 M20 out of the garage and one of the first things I need to do is fill it up. I purchased the roadster last summer and this will be my first gas stop. The only thing available here is E10 in 87,89 and 92 octane. What fuel would you recommend I use? The engine is stock original with 37K miles on the clock. It runs strong with no noticeable smoke or ping. Thanks in advance.