Does a “de-smogged” L48 need premium fuel?
Thread Starter
Melting Slicks
Joined: Apr 2021
Posts: 2,730
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From: Minnesota
Does a “de-smogged” L48 need premium fuel?
Good evening Corvette forum,
I remember talking to an owner of a ‘77 L48 last fall about improving the performance of their engine. I told them about advancing the ignition timing to 36 total, limiting the vacuum advance, and all the other goodies that make it better. They told me they set their total timing to 32 so they could still use 87 octane. I’ve been thinking about it ever since.
So to those that are more educated on engine dynamics than me, does an L48 with its low static and dynamic compression need to run premium with a proper timing curve? Say 36 degrees total mechanical with 12 in the vacuum advance on top of that.
Obviously it can run 87 just fine with a stock “tune,” but increasing your ignition timing can increase the chance of pre-detonation, which higher octane fuel prevents. Going a step further, how about a car that has things like a different intake and freer flowing exhaust? Assuming the camshaft and compression is the same does that make a difference on an engines resistance to detonation?
Any information is greatly appreciated!
I remember talking to an owner of a ‘77 L48 last fall about improving the performance of their engine. I told them about advancing the ignition timing to 36 total, limiting the vacuum advance, and all the other goodies that make it better. They told me they set their total timing to 32 so they could still use 87 octane. I’ve been thinking about it ever since.
So to those that are more educated on engine dynamics than me, does an L48 with its low static and dynamic compression need to run premium with a proper timing curve? Say 36 degrees total mechanical with 12 in the vacuum advance on top of that.
Obviously it can run 87 just fine with a stock “tune,” but increasing your ignition timing can increase the chance of pre-detonation, which higher octane fuel prevents. Going a step further, how about a car that has things like a different intake and freer flowing exhaust? Assuming the camshaft and compression is the same does that make a difference on an engines resistance to detonation?
Any information is greatly appreciated!
Last edited by Piersonpie; Feb 13, 2026 at 09:40 PM.
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A low compression L48 does NOT need premium fuel.
Le Mans Master
Joined: Mar 2004
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From: Little Rock AR
An L-48 will run on cheap fuel no matter what.
If you get rid of the EGR, the engine will likely run a bit lean. Free flowing exhaust will want a richer mixture as well. Carburetor recalibration would take care of that.
50 year old cars have condition issues. Sticky mechanical advance mechanisms, idle speed screws set too high causing run-on issues, carbon buildup in the chambers, gummed up carbs, clogged catalytic converters, vacuum leaks, etc.
Sounds like fear, confusion, and misinformation to me.
If you get rid of the EGR, the engine will likely run a bit lean. Free flowing exhaust will want a richer mixture as well. Carburetor recalibration would take care of that.
50 year old cars have condition issues. Sticky mechanical advance mechanisms, idle speed screws set too high causing run-on issues, carbon buildup in the chambers, gummed up carbs, clogged catalytic converters, vacuum leaks, etc.
Sounds like fear, confusion, and misinformation to me.
Melting Slicks
Joined: Oct 2003
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From: Chapel Hill NC
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Fairly uneducated here but Id try 87 and move up in octane if pinging occurs.
Pro
Joined: May 2019
Posts: 561
Likes: 262
From: Fountain Hills (Phoenix) AZ
I followed LARS recommendations to re-curve my 1975 L48 coupe. After fixing the stuck mechanical advance, installing the correct original weights and changing the springs, the car ran much better. Car came from the original owner with headers and duals. No emission equipment either, as you don't need that here in Arizona for a "classic" car.
Even with the automatic transmission, I am very happy with the car's performance and I run it on regular 10% ethanol fuel. No pings or knocks. Car has 43K miles on it and the engine has never been out of the car. Even the q-jet carb performs flawlessly. My 2 cents.
Even with the automatic transmission, I am very happy with the car's performance and I run it on regular 10% ethanol fuel. No pings or knocks. Car has 43K miles on it and the engine has never been out of the car. Even the q-jet carb performs flawlessly. My 2 cents.
Le Mans Master
Joined: May 2007
Posts: 5,036
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From: North tonawanda NY
There is zero practical benefit to higher octane fuel in a low compression engine it will run fine on regular 87 with the ethanol but the catch is the ethanol might react with certain rubber components in the fuel system if there are any... if you have an aftermarket carb like a holley or edelbrock the ethanol will damage soft parts over time unless you switch to ethanol safe components. I had an edelbrock polished 650 carb and I found the fuel inside kept turning to clear jelly like substance but I never had the issue on my holleys I typically run non ethanol fuel when I can and 89 or 90 because I have a 10:1 compression engine in my car now as ive removed my l48.
Ive been told there actually more energy in 87 octane fuel than the higher octane fuel so if anything running 87 would have the benefit.The term "premium" is a marketing gimmick to get separate the uneducated from their money.
Ive been told there actually more energy in 87 octane fuel than the higher octane fuel so if anything running 87 would have the benefit.The term "premium" is a marketing gimmick to get separate the uneducated from their money.
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Drifting
Joined: Jun 2025
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a few exceptions
Not ALL OE rpo L48 have low compression ratio, not ALL OE L48 have dished (aka 'sump head') pistons.
rpo L48 first introduced MY1967; but Not in corvette.
Several versions included flattop pistons With small combustion chamber heads.
rpo L48 first introduced MY1967; but Not in corvette.
Several versions included flattop pistons With small combustion chamber heads.
Last edited by Rebelyell; Feb 21, 2026 at 02:08 AM.
Le Mans Master
Joined: May 2007
Posts: 5,036
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From: North tonawanda NY
I think its safe to say this question was in regards to a c3 corvette l48
The original question was answered.
Advanced
Joined: Aug 2021
Posts: 58
Likes: 24
From: Knoxville TN
I'm running a 75 L48 at 36 degrees max mechanical, headers, true duals, no cat, no smog and LARS tune. Never pings on 87 octane and runs great, for a low compression L48, LOL. It does seem to run alittle lean at cruise once warmed up so thinking about either rejetting the QJet or letting LARS tune it. Otherwise it's a cruiser, not a racer and NEVER needs higher octane fuel, LOL.
Racer
Joined: May 2025
Posts: 352
Likes: 45
From: Georgia
There is zero practical benefit to higher octane fuel in a low compression engine it will run fine on regular 87 with the ethanol but the catch is the ethanol might react with certain rubber components in the fuel system if there are any... if you have an aftermarket carb like a holley or edelbrock the ethanol will damage soft parts over time unless you switch to ethanol safe components. I had an edelbrock polished 650 carb and I found the fuel inside kept turning to clear jelly like substance but I never had the issue on my holleys I typically run non ethanol fuel when I can and 89 or 90 because I have a 10:1 compression engine in my car now as ive removed my l48.
Ive been told there actually more energy in 87 octane fuel than the higher octane fuel so if anything running 87 would have the benefit.The term "premium" is a marketing gimmick to get separate the uneducated from their money.
Ive been told there actually more energy in 87 octane fuel than the higher octane fuel so if anything running 87 would have the benefit.The term "premium" is a marketing gimmick to get separate the uneducated from their money.