Slight Knock When Shutting Off
Thread Starter
Instructor
Joined: Mar 2012
Posts: 183
Likes: 0
From: Centreville VA
Thanks, I will get the timing checked out, it is idleing around 7,000 rpms. I was wondering about the octane, when I first got the car it had been sitting so I put some 93 octane in it, then back to 87 last fill up.
I was waiting in line for my saftey inspection over the weekend and I read the owners manual, the owners manual actually says to use 91 octane....so I am kind of confused on what gas is best.
I was waiting in line for my saftey inspection over the weekend and I read the owners manual, the owners manual actually says to use 91 octane....so I am kind of confused on what gas is best.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
'75s were built to run on what's called 87 octane today. The '91' octane in your owner's manual is in the 'old' rating system which changed a few years later.
Today's 93 octane is equivalent to about 98-99 in the old system.
Today's 93 octane is equivalent to about 98-99 in the old system.
Last edited by Mike Ward; Apr 4, 2012 at 09:58 AM.
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
Mike is right. It should run on 87. I would run most of the gas out, fill with the highest octane you can buy and adjust the timing per the sticky on this forum.
If your car is idling at 7,000 as you stated it should last about 5 minutes.
You mentioned the car sat for a while. what is a while (10 years?). Without stabelizer and a low fuel tank you could have some water condensation in the fuel which would cause a knock when you turn it off.
If your car is idling at 7,000 as you stated it should last about 5 minutes.
You mentioned the car sat for a while. what is a while (10 years?). Without stabelizer and a low fuel tank you could have some water condensation in the fuel which would cause a knock when you turn it off.
Thread Starter
Instructor
Joined: Mar 2012
Posts: 183
Likes: 0
From: Centreville VA
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
Actually, inginition timing doesn't have anything to do with run on, as theres no spark at that point. Run on, is usually a combination of low octane fuel, and engine carbon deposits still glowing red. A fuel cut off solinoid will help, but you'll still be feeding from carb bowels for awhile. Maybe see if you can somehow clean up the internal carbon deposits. I have an old Gumout kit, that actually runs the car on a can of Gumout for a few minutes, but not sure how well it works.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
10 Things C8 Corvette Owners Hate (But Won't Tell You)
Joe Kucinski
10 Best Corvettes Coming to Barrett-Jackson Palm Beach 2026!
Brett Foote
Every Corvette Grand Sport Explained! (C2, C4, C6, C7, & C8)
Joe Kucinski
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
7 Bolt-On Upgrades From Extreme Online Store to Level Up Your C6 Corvette
Pouria Savadkouei
Thread Starter
Instructor
Joined: Mar 2012
Posts: 183
Likes: 0
From: Centreville VA
Mike is right. It should run on 87. I would run most of the gas out, fill with the highest octane you can buy and adjust the timing per the sticky on this forum.
If your car is idling at 7,000 as you stated it should last about 5 minutes.
You mentioned the car sat for a while. what is a while (10 years?). Without stabelizer and a low fuel tank you could have some water condensation in the fuel which would cause a knock when you turn it off.
If your car is idling at 7,000 as you stated it should last about 5 minutes.
You mentioned the car sat for a while. what is a while (10 years?). Without stabelizer and a low fuel tank you could have some water condensation in the fuel which would cause a knock when you turn it off.
Thread Starter
Instructor
Joined: Mar 2012
Posts: 183
Likes: 0
From: Centreville VA
Actually, inginition timing doesn't have anything to do with run on, as theres no spark at that point. Run on, is usually a combination of low octane fuel, and engine carbon deposits still glowing red. A fuel cut off solinoid will help, but you'll still be feeding from carb bowels for awhile. Maybe see if you can somehow clean up the internal carbon deposits. I have an old Gumout kit, that actually runs the car on a can of Gumout for a few minutes, but not sure how well it works.
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
[QUOTE=Vette5.5;1580465311]Actually, inginition timing doesn't have anything to do with run on, as theres no spark at that point.
Not true. Timing has everything to do with knock. If the timing is way off it builds combustion chamber terperatures to the point of ignition at the wrong time without the spark. A car can even overheat if the timing is off. Likewise higher octane gas helps to keep cylinder temps low because high octane fuel burns slower and more evenly allowing for more power, less stress on the reciprocating parts and lower temps.
Not true. Timing has everything to do with knock. If the timing is way off it builds combustion chamber terperatures to the point of ignition at the wrong time without the spark. A car can even overheat if the timing is off. Likewise higher octane gas helps to keep cylinder temps low because high octane fuel burns slower and more evenly allowing for more power, less stress on the reciprocating parts and lower temps.
Burning Brakes
Joined: Mar 2005
Posts: 1,231
Likes: 34
From: Ontario
[QUOTE=08vycpe;1580468443]
The OP was asking about engine run-on , not spark knock .
Actually, inginition timing doesn't have anything to do with run on, as theres no spark at that point.
Not true. Timing has everything to do with knock. If the timing is way off it builds combustion chamber terperatures to the point of ignition at the wrong time without the spark. A car can even overheat if the timing is off. Likewise higher octane gas helps to keep cylinder temps low because high octane fuel burns slower and more evenly allowing for more power, less stress on the reciprocating parts and lower temps.
Not true. Timing has everything to do with knock. If the timing is way off it builds combustion chamber terperatures to the point of ignition at the wrong time without the spark. A car can even overheat if the timing is off. Likewise higher octane gas helps to keep cylinder temps low because high octane fuel burns slower and more evenly allowing for more power, less stress on the reciprocating parts and lower temps.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Hey 75 Stinger- what speed does your engine run at when you unplug the anti dieseling solenoid?
Safety Car
Joined: Oct 2000
Posts: 3,593
Likes: 103
From: Southern Indiana
Run a trickle of water down the throat of the carb and it will clean the pistons off.
Up either you are running crap gas or you need to bump up the timing 2 degrees until it stops running on or until its hard to start.
Worst case is the timing chain just stretched and your timing retarded a few degrees.
Up either you are running crap gas or you need to bump up the timing 2 degrees until it stops running on or until its hard to start.
Worst case is the timing chain just stretched and your timing retarded a few degrees.
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
Here's a quote from The Service Advisor "Octane, by definition, is the resistance to burn or detonation. The higher the rating, the slower the burn when ignited during the compression burn cycle of the piston. The higher octane allows for better control of burning for high compression engines. So we want to match the correct octane rating of the gasoline to the engine design to ensure complete burning of the gasoline by the engine for maximum fuel economy and clean emissions."
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
I agree about the amount of energy released. I wasn't implying it had more energy. It's the same energy but allowed to be released slower instead of a sudden explosion but disagree about the speed not being slower. It is slower and more even. Thus the piston is pushed down and not hammered down.
Here's a quote from The Service Advisor "Octane, by definition, is the resistance to burn or detonation. The higher the rating, the slower the burn when ignited during the compression burn cycle of the piston. The higher octane allows for better control of burning for high compression engines. So we want to match the correct octane rating of the gasoline to the engine design to ensure complete burning of the gasoline by the engine for maximum fuel economy and clean emissions."
Here's a quote from The Service Advisor "Octane, by definition, is the resistance to burn or detonation. The higher the rating, the slower the burn when ignited during the compression burn cycle of the piston. The higher octane allows for better control of burning for high compression engines. So we want to match the correct octane rating of the gasoline to the engine design to ensure complete burning of the gasoline by the engine for maximum fuel economy and clean emissions."
"NOTE: I do not proclaim to be an expert in these matters, but am only presenting an overview of what I have discovered in my work in this industry with the different auto manufacturers.
Copyright, 1997, J. Daniel Emmanuel"
Well there's an understatement
A 'slow burn' could not possibly affect or cure 'detonation'. It's been explained and beaten to death a hundred times here, please read up.
Even Wikipedia got it right
http://en.wikipedia.org/wiki/Engine_knocking
Last edited by Mike Ward; Apr 4, 2012 at 07:48 PM.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
No, actually "08vycpe" brings up an important point. He's saying, if the ignition timing's way off with the engine running, the combustion chamber temperatures could be way higher than normal, when the engine is shut off.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
The mid 70s cars were deliberately 'timed' to run as high a combustion chamber, coolant and exhaust temperature as possible for pollution reasons. The resulting tendency for run-on or dieseling was dealt with effectively by closing the throttle completely after the ignition was turned off. That's why I've asked (now for the third time) about the anti-dieseling solenoid.