my friend has an 86 chevy celebrity(i'm currently trying to find out how much horsepower it has, it's a 2.5L fuel injected 4 cyl, though i thought it was a V6 originally) and her step dad tells her to fill it with premium because the engine knocks with anything less. i doubt that this thing has high enough compression that it would require 92 octane, so what could have happened for it to require it? any ideas?
what causes engine knock?
- Thread starter toph99
- Start date
The most obvious culprit would be the ignition timing although I'm not familiar with that particular automobile.
N8Magic
Lifer
- Dec 12, 2000
- 11,624
- 1
- 81
If the engine is knocking on 89 octane than the car does require 93 octane, unless you retard the timing/reduce the compression.
Short version: engine knock occurs when more combustive force is exerted on the piston than can be controlled. Contrary to popular belief, your car will have more power on 87 octane as long as it does not cause knock/cause the car to reduce its own timing (OBDII).
Parts needed:
Some cash for gasoline, or a gas can filled with a few gallons of 87 octane.
Ratchet wrench, sockets (standard and metric, most likely standard needed), and ratchet extension (Might need, might not, depends on the car), flex attachment (Again, might need, might not)
Timing degree ruler (Auto parts store)
Marker that is not the color of the car's engine. I like red.
To fix the car so that it will run on 87 octane, run the car (nearly) empty, then you need to find the distributor, and retard its timing quite a ways since you won't know its original setting from the factory. The distributor is the piece that all of the spark plug wires run into. Fill the car up with 87 octane and take it back home. Use a timing degree marker to mark off a few degrees plus and minus above center. Advance the timing forward some, rev the car. Repeat until you hear knock (Then stop immediately), then reduce the timing two degrees (To give allowance for the lower octane you will eventually have without the 93 in the tank). Give the car full throttle. If you get no knock, you have cheaply fixed your car and prolly gained some torque/HP in the process.
Short version: engine knock occurs when more combustive force is exerted on the piston than can be controlled. Contrary to popular belief, your car will have more power on 87 octane as long as it does not cause knock/cause the car to reduce its own timing (OBDII).
Parts needed:
Some cash for gasoline, or a gas can filled with a few gallons of 87 octane.
Ratchet wrench, sockets (standard and metric, most likely standard needed), and ratchet extension (Might need, might not, depends on the car), flex attachment (Again, might need, might not)
Timing degree ruler (Auto parts store)
Marker that is not the color of the car's engine. I like red.
To fix the car so that it will run on 87 octane, run the car (nearly) empty, then you need to find the distributor, and retard its timing quite a ways since you won't know its original setting from the factory. The distributor is the piece that all of the spark plug wires run into. Fill the car up with 87 octane and take it back home. Use a timing degree marker to mark off a few degrees plus and minus above center. Advance the timing forward some, rev the car. Repeat until you hear knock (Then stop immediately), then reduce the timing two degrees (To give allowance for the lower octane you will eventually have without the 93 in the tank). Give the car full throttle. If you get no knock, you have cheaply fixed your car and prolly gained some torque/HP in the process.
i know how to retard the timing but i don't see WHY a car like that would need premium in stock form(i doubt her step dad did anything to the timing, though i could be wrong) so i wanted to know if cars gain the tendency to knock as the miles pile up for whatever reason
XZeroII
Lifer
- Jun 30, 2001
- 12,572
- 0
- 0
It can also be caused by wear in the engine. If you have an overhead cam, the cam needs to push down on some lifter things (I can't remeber what they are called). I think they are called rocker arms. As the rocker arm gets worn down, the cam and the rocker arm will not be positioned correctly and the lobe will slam into the rocker arm instead of steadily pushing it down. I don't know if you understand what I'm saying, but I know that it happens in old cars with an overhead cam. Replacing the thing that I'm calling a rocker arm will help eliminate the noise. It takes a day or so and with the right tools (lots of beer) can be done by a person with average knowledge.
It's possible that carbon buildup has reduced the available volume of one or more combustion chambers, thereby increasing the compression ratio which would in turn require higher octane fuel.
<< If the engine is knocking on 89 octane than the car does require 93 octane, unless you retard the timing/reduce the compression.
Short version: engine knock occurs when more combustive force is exerted on the piston than can be controlled. Contrary to popular belief, your car will have more power on 87 octane as long as it does not cause knock/cause the car to reduce its own timing (OBDII).
Parts needed:
Some cash for gasoline, or a gas can filled with a few gallons of 87 octane.
Ratchet wrench, sockets (standard and metric, most likely standard needed), and ratchet extension (Might need, might not, depends on the car), flex attachment (Again, might need, might not)
Timing degree ruler (Auto parts store)
Marker that is not the color of the car's engine. I like red.
To fix the car so that it will run on 87 octane, run the car (nearly) empty, then you need to find the distributor, and retard its timing quite a ways since you won't know its original setting from the factory. The distributor is the piece that all of the spark plug wires run into. Fill the car up with 87 octane and take it back home. Use a timing degree marker to mark off a few degrees plus and minus above center. Advance the timing forward some, rev the car. Repeat until you hear knock (Then stop immediately), then reduce the timing two degrees (To give allowance for the lower octane you will eventually have without the 93 in the tank). Give the car full throttle. If you get no knock, you have cheaply fixed your car and prolly gained some torque/HP in the process. >>
Bullsh1t
Back in the early 60's, Honda raced a coupla 250cc motorcycles that had 20K rpm engines that had to run regular gas because Premium burned to slow...
<<
<< If the engine is knocking on 89 octane than the car does require 93 octane, unless you retard the timing/reduce the compression. Short version: engine knock occurs when more combustive force is exerted on the piston than can be controlled. Contrary to popular belief, your car will have more power on 87 octane as long as it does not cause knock/cause the car to reduce its own timing (OBDII). Parts needed: Some cash for gasoline, or a gas can filled with a few gallons of 87 octane. Ratchet wrench, sockets (standard and metric, most likely standard needed), and ratchet extension (Might need, might not, depends on the car), flex attachment (Again, might need, might not) Timing degree ruler (Auto parts store) Marker that is not the color of the car's engine. I like red. To fix the car so that it will run on 87 octane, run the car (nearly) empty, then you need to find the distributor, and retard its timing quite a ways since you won't know its original setting from the factory. The distributor is the piece that all of the spark plug wires run into. Fill the car up with 87 octane and take it back home. Use a timing degree marker to mark off a few degrees plus and minus above center. Advance the timing forward some, rev the car. Repeat until you hear knock (Then stop immediately), then reduce the timing two degrees (To give allowance for the lower octane you will eventually have without the 93 in the tank). Give the car full throttle. If you get no knock, you have cheaply fixed your car and prolly gained some torque/HP in the process. >>
Bullsh1t >>
I agree, you often won't hear knock when just revving the engine; only under a load (like moving the beast while its in gear) will you be able to hear it.
Freejack2
Diamond Member
- Dec 31, 2000
- 7,751
- 8
- 91
The hampster running the wheel that powers your engine has been trapped in there for 16 years. He wants out and is knocking on your engine to let him out.
Let him out for a few days and he'll come back and stop knocking for a few years.
All kidding aside TerryMathews probably has the best answer. I remember reading on here that on cars formulated for 87 octane, one shouldn't put premium in too much as it'll do nothing performancewise and I forget exactly, but I think it causes buildup in the engine and if you try to switch off it after using premium too much the 87 octane will cause knock.
Let him out for a few days and he'll come back and stop knocking for a few years.
All kidding aside TerryMathews probably has the best answer. I remember reading on here that on cars formulated for 87 octane, one shouldn't put premium in too much as it'll do nothing performancewise and I forget exactly, but I think it causes buildup in the engine and if you try to switch off it after using premium too much the 87 octane will cause knock.
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