Regular Plus or Premium gas
Thread Starter
Racer
Joined: Feb 2017
Posts: 407
Likes: 12
From: Maywood NJ
Regular Plus or Premium gas
Hi All .I have a question about type of gas you all use in your C3 . Is regular unleaded good enough or do u use the plus { half regular half premium) or is it worth and make a any difference if u use the Premium. Does any also add any gas treatments? Thanks everyone 
Le Mans Master
Joined: Oct 2007
Posts: 7,455
Likes: 977
From: RI, Now Franklin/Nashville TN
Yes...I have an L-82 OEM block bored .030 (355) with a Roller Cam (.525/.525 Lift, Duration 219/225, LSA 110) AFR aluminum Heads and 10.2:1 compression and I use 89 octane with zero ping....The same combo with iron heads would probs require 93 octane...depends on you motor
Racer
Joined: Sep 2012
Posts: 322
Likes: 11
From: Staten Island New York
My '77 with the L-82 runs just fine Regular 87 grade. The only problem I have is when I fuel up at a BP Station, my car doesn't like there 87. For some reason I get Pre-ignition (aka dieseling), when I turn she off. That may sound strange but its the only gas that does it..
Thread Starter
Racer
Joined: Feb 2017
Posts: 407
Likes: 12
From: Maywood NJ
Thread Starter
Racer
Joined: Feb 2017
Posts: 407
Likes: 12
From: Maywood NJ
My '77 with the L-82 runs just fine Regular 87 grade. The only problem I have is when I fuel up at a BP Station, my car doesn't like there 87. For some reason I get Pre-ignition (aka dieseling), when I turn she off. That may sound strange but its the only gas that does it..
Thread Starter
Racer
Joined: Feb 2017
Posts: 407
Likes: 12
From: Maywood NJ
Yes...I have an L-82 OEM block bored .030 (355) with a Roller Cam (.525/.525 Lift, Duration 219/225, LSA 110) AFR aluminum Heads and 10.2:1 compression and I use 89 octane with zero ping....The same combo with iron heads would probs require 93 octane...depends on you motor
Pro
Joined: Oct 2011
Posts: 743
Likes: 71
From: Jarrettsville Maryland
It will say in the original manual what kind of fuel it requires (regular, premium, etc). Just remember that the octane number quoted back then is the RON, and current gas is measured using AKI ((MON+RON)/2).
Most C3s 1971 and newer that is still stock was designed to run on 87 unleaded, but that can change if things are no longer stock. Most performance engines made before 71 had high compression and needs premium (93).
Most C3s 1971 and newer that is still stock was designed to run on 87 unleaded, but that can change if things are no longer stock. Most performance engines made before 71 had high compression and needs premium (93).
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Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
I only run premium in my '79. I started doing that because at one time premium was the only fuel around here that didn't have alcohol in it. Since then they have started putting alcohol in premium as well. Alcohol is hell on our cars. If you can still get alcohol free gas, use it. If you can't, buy the cheapest gas you can on which your car will run well. IIRC your car in stock configuration should have about a 10.25:1 compression ratio. With cast iron heads, I would expect you would need to run premium fuel.
Thread Starter
Racer
Joined: Feb 2017
Posts: 407
Likes: 12
From: Maywood NJ
It will say in the original manual what kind of fuel it requires (regular, premium, etc). Just remember that the octane number quoted back then is the RON, and current gas is measured using AKI ((MON+RON)/2).
Most C3s 1971 and newer that is still stock was designed to run on 87 unleaded, but that can change if things are no longer stock. Most performance engines made before 71 had high compression and needs premium (93).
Most C3s 1971 and newer that is still stock was designed to run on 87 unleaded, but that can change if things are no longer stock. Most performance engines made before 71 had high compression and needs premium (93).
Thread Starter
Racer
Joined: Feb 2017
Posts: 407
Likes: 12
From: Maywood NJ
I only run premium in my '79. I started doing that because at one time premium was the only fuel around here that didn't have alcohol in it. Since then they have started putting alcohol in premium as well. Alcohol is hell on our cars. If you can still get alcohol free gas, use it. If you can't, buy the cheapest gas you can on which your car will run well. IIRC your car in stock configuration should have about a 10.25:1 compression ratio. With cast iron heads, I would expect you would need to run premium fuel.
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
This is what I would do. I would recurve the distributor to have a little more timing at idle and come in full by 3000 rpm. Put the timing at about 36 total degrees with vacuum advance disconnected, then reconnect vacuum advance. This will make the engine feel a lot peppier and make it's best power. This will then require premium grade gas, but will be well worth it.
Mike
Mike
Team Owner
Joined: Jun 2001
Posts: 23,375
Likes: 1,067
From: Virginia USA
This is what I would do. I would recurve the distributor to have a little more timing at idle and come in full by 3000 rpm. Put the timing at about 36 total degrees with vacuum advance disconnected, then reconnect vacuum advance. This will make the engine feel a lot peppier and make it's best power. This will then require premium grade gas, but will be well worth it.
Mike
Mike
Team Owner
Joined: Jun 2001
Posts: 23,375
Likes: 1,067
From: Virginia USA
Race Director
Joined: Apr 2003
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From: Tampa, Florida
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Grade of gas needed depends on the compression ratio of your engine and how well it is tuned.
Engines with 8:1 or 8.5:1 CR do fine with regular unleaded; 9.0-10.0:1 CR usually needs mid-grade; and 10.5-11.5 CR engines need premium fuel.
You might get by with a little lower grade of fuel by retarding the timing a bit, but you will be risking a "pinging" problem with that approach.
Engines with 8:1 or 8.5:1 CR do fine with regular unleaded; 9.0-10.0:1 CR usually needs mid-grade; and 10.5-11.5 CR engines need premium fuel.
You might get by with a little lower grade of fuel by retarding the timing a bit, but you will be risking a "pinging" problem with that approach.
Melting Slicks
Joined: Dec 2009
Posts: 3,233
Likes: 657
From: Thunder Bay
The reason to use higher octane fuel is to prevent detonation. This is where the mixture ignites prematurely under compression. The higher the compression ratio, the higher the octane rating that will be required to prevent detonation. Detonation is also known as 'ping'. It is that rattling sound you hear under heavy load. It will also occur if the timing is advanced too far for the fuel you are using. If you are running aluminum heads, you can usually tolerate about a point higher compression with the same octane fuel. This is all a gross over-simplification, but it should give you a feel for the factors involved. So the bottom line is, if you can run regular fuel without the engine pinging, you would be wasting your money to buy premium or mid grade fuel. The alcohol contamination problem is the only factor I can think of that would change my approach to selecting a fuel grade.