Regular gas?
ud
Joined: Feb 2010
Posts: 1,595
Likes: 151
From: USA
If your ride has computer controls, then the computer makes adjustments to make up for this. With variable cam timing, the intake valve closing is retarded to make your dynamic compression ratio drop down to around 8 to 1.
Knock can also come from pre-ignition which isn't controled by the spark timing.
Remember, your computer will be trying to advance the cam timing for more low end power and then the knock sensors will cause it to retard the timing. This will go back and forth and it isn't so swell for the motor.
Knock can also come from pre-ignition which isn't controled by the spark timing.
Remember, your computer will be trying to advance the cam timing for more low end power and then the knock sensors will cause it to retard the timing. This will go back and forth and it isn't so swell for the motor.
Last edited by steve11; Oct 23, 2011 at 08:41 AM.
Senior Member
Joined: Feb 2011
Posts: 273
Likes: 75
From: California
I didn't put down the whole process of how a ECU deals with knocking and everything else.
The point is that with variable cam timing; at low rpms the cam is advanced to give high torque. Of course this raises the dynamic compression ratio and low octane gas can knock. Small block engines have less pre-ignition problems than big blocks. Generally, a small block can run on regular if the compression test reads around 160 psi. When you get up to around 180 psi, then premium is needed.
When the knock sensors pick up knocking the ECU has instructions on how to stop it. Retarding cam timing to drop the dynamic compression ratio works, but cuts the low end torque. There are other things that can be done like adjusting spark timing, or feeding in exhaust gas.
The point is that with variable cam timing; at low rpms the cam is advanced to give high torque. Of course this raises the dynamic compression ratio and low octane gas can knock. Small block engines have less pre-ignition problems than big blocks. Generally, a small block can run on regular if the compression test reads around 160 psi. When you get up to around 180 psi, then premium is needed.
When the knock sensors pick up knocking the ECU has instructions on how to stop it. Retarding cam timing to drop the dynamic compression ratio works, but cuts the low end torque. There are other things that can be done like adjusting spark timing, or feeding in exhaust gas.
Joined: Oct 2009
Posts: 11,057
Likes: 2,272
From: Perth Ontario Canada
Not to be rude, but twice you've used the term 'pre-ignition' in place of 'detonation'. The two are very different and essentially unrelated phenomena and the terms are not interchangeable. Variable valve timing, altering ignition advance or inducing exhaust gas recirculation may be effective in dealing with detonation, but not with pre-ignition.
ud
Joined: Feb 2010
Posts: 1,595
Likes: 151
From: USA
I didn't put down the whole process of how a ECU deals with knocking and everything else.
The point is that with variable cam timing; at low rpms the cam is advanced to give high torque. Of course this raises the dynamic compression ratio and low octane gas can knock. Small block engines have less pre-ignition problems than big blocks. Generally, a small block can run on regular if the compression test reads around 160 psi. When you get up to around 180 psi, then premium is needed.
When the knock sensors pick up knocking the ECU has instructions on how to stop it. Retarding cam timing to drop the dynamic compression ratio works, but cuts the low end torque. There are other things that can be done like adjusting spark timing, or feeding in exhaust gas.
The point is that with variable cam timing; at low rpms the cam is advanced to give high torque. Of course this raises the dynamic compression ratio and low octane gas can knock. Small block engines have less pre-ignition problems than big blocks. Generally, a small block can run on regular if the compression test reads around 160 psi. When you get up to around 180 psi, then premium is needed.
When the knock sensors pick up knocking the ECU has instructions on how to stop it. Retarding cam timing to drop the dynamic compression ratio works, but cuts the low end torque. There are other things that can be done like adjusting spark timing, or feeding in exhaust gas.
"the ECU has instructions"
"There are other things that can be done"
The rest is either partially or completely incorrect.
Last edited by steve11; Oct 23, 2011 at 08:42 AM.
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