What octane are you running?
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Melting Slicks
Joined: Jul 2010
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From: SUFFIELD CT USA 2023 C8 CORVETTE UN-MODIFIED FINALIST
2023 C8 of the Year Finalist - Unmodified
What octane are you running?
I tried search w/ no luck although I'M sure this has been discussed ad nauseaum! My manual for my 1977 says use "UNLEADED". What do you all use?
Instructor
Joined: May 2010
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From: Shelby Twp. Michigan
The higher the octane, the slower the burn. The slower the burn, the more compresion you need to squeeze it.
The lower the compresion, the lower the octane. With these stock mid 70's motors at like 8-1, 87 is just fine.
Safety Car
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From: Wichita KS
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Instructor
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i got a 77 L-48 87 oct she pings like a kid playing the drums so tried 91 and all was fine but i just put the good stuff in her if i am spending the price for 91 not much more for 93 lol. also noticed that sunoco gas with a good 10% of ethenol in all there lines i ping on all but there 93 or 94
Race Director
Joined: Jul 2001
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'Old' rating 91 is roughly equivalent to 'new' rating 87. Our high octane of 93 today would have been rated around 98-99ish under the old system.
This change of rating system is what makes a lot of people think that gas 'in the good old days' was much higher octane.
Le Mans Master
Joined: Aug 2006
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
I Hate Change!
Didn't everyone fill up at SUNOCO with 102 on a Friday night. Lead was good back then.
Why did everything change? Tires were GR70-15 and took 20 lbs of air.
What's next? Changing oil ratings?
Why did everything change? Tires were GR70-15 and took 20 lbs of air.
What's next? Changing oil ratings?
Race Director
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Team Owner
Joined: Aug 1999
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From: Ontario
Yes, that's correct- but the octane rating system domestically changed in the mid-late 70s. The old system used the straight RON number. Europe still uses that system. N. American gasoline is now rated by (RON+MON)/2, sometimes called the AKI.
'Old' rating 91 is roughly equivalent to 'new' rating 87. Our high octane of 93 today would have been rated around 98-99ish under the old system.
This change of rating system is what makes a lot of people think that gas 'in the good old days' was much higher octane.
'Old' rating 91 is roughly equivalent to 'new' rating 87. Our high octane of 93 today would have been rated around 98-99ish under the old system.
This change of rating system is what makes a lot of people think that gas 'in the good old days' was much higher octane.
Your Corvette is designed to operate on unleaded or low lead fuels of at least 91 Research Octane.
I did a little research on RON, AKI, etc. and you are absolutely correct: things did change in the 1970s. Live and learn! I have consistently used 91 but I will give 87 a try.
Melting Slicks
Joined: Mar 2007
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From: Frederick Maryland
My '69 427/435 gets 93 Octane. It was originally 11:1 compression but the previous owner had it lowered to 10.6:1 when he had the engine rebuilt and bored .030 over. With the solid lifter L88 camshaft I'm not sure I could hear pinging if it does.
I have another car with a GM 350HO crate engine...9.1:1 compression. The manual that came with the engine specifies 92 Octane minimum to use. It gets 93 Octane as well since that's what's sold in my area.
I have another car with a GM 350HO crate engine...9.1:1 compression. The manual that came with the engine specifies 92 Octane minimum to use. It gets 93 Octane as well since that's what's sold in my area.
Race Director
Joined: Jul 2001
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From my 1974 owner's manual:
Your Corvette is designed to operate on unleaded or low lead fuels of at least 91 Research Octane.
I did a little research on RON, AKI, etc. and you are absolutely correct: things did change in the 1970s. Live and learn! I have consistently used 91 but I will give 87 a try.
Your Corvette is designed to operate on unleaded or low lead fuels of at least 91 Research Octane.
I did a little research on RON, AKI, etc. and you are absolutely correct: things did change in the 1970s. Live and learn! I have consistently used 91 but I will give 87 a try.
Maybe you remember dates a little better. I bought a new car in the fall of '75 and was the first I'd seen that had the 'new' restrictor plate for unleaded fuel nozzles. I never had any problem finding fuel, so availability was already pretty wide spread. Do you remember when unleaded first became available?Back then everyone thought that unleaded was the beginning of the end for the car hobby and would destroy engines, pretty much like people thinking that E10 is the same thing today.
Le Mans Master
Joined: Apr 2001
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From: Minneapolis, Mn USA
2001 LS1 engine with 10.1 compression and I can run either 87 or 91. Knock retard built in, so either runs fine, so just depends on how rich I feel when I pull up to the pump. Runs beautiful either way.
Don't have to worry about oil ZDDP either, as its a roller engine!!
These above points really made the decision to do this swap. I believe I wiped a cam lobe on the CFI motor in its final days.
Don't have to worry about oil ZDDP either, as its a roller engine!!
These above points really made the decision to do this swap. I believe I wiped a cam lobe on the CFI motor in its final days.
Le Mans Master
Joined: Apr 2007
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From: Graceland in a Not Correctly Restored Stingray
Actually, octane ratings by RON (research) will almost always be higher than by MON (motor), the difference sometimes referred to as a gasoline's heat sensitivity. Unfortunately for us, some refirneries take advantage of the fact that it's cheaper to pump up (RON+MON)/2 octane ratings by boosting RON without regard to MON, when it's MON that really matters most under your hood out in the real world.
I'm not going to take it on myself to point any fingers, and I'd hope some of the suspect companies have improved their products since, but David Vizard published a chart not too many years back which plainly showed that not all brands' octanes are created equally. Bottom Line: You could very well run fine on Brand A's 91 while Brand X's 91 pings like hell when you even think about accelerating.
FWIW, my DD gets just about enough better mileage on premium to offset any savings at the pump which mid-grade might seem to offer.
TSW
I'm not going to take it on myself to point any fingers, and I'd hope some of the suspect companies have improved their products since, but David Vizard published a chart not too many years back which plainly showed that not all brands' octanes are created equally. Bottom Line: You could very well run fine on Brand A's 91 while Brand X's 91 pings like hell when you even think about accelerating.
FWIW, my DD gets just about enough better mileage on premium to offset any savings at the pump which mid-grade might seem to offer.
TSW
Last edited by TheSkunkWorks; Jul 19, 2010 at 06:30 PM.
Race Director
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.