detonation

[Mark H.]

I've been reading some past posts on timing curves, cetrif. advance, etc., and see many references to using as much advance as possible w/o detonation. How do you know if your engine is experiencing detonation? any good descriptions of what to listen for? Can you tell by pulling the plugs and looking at them? I hear a rythmic knocking or rapping every so often, but don't really know what is causing it.

Thanks.

[Michael W.]

Re: detonation

Most common occurance is during had accelertion or climbing a steep hill and lugging the engine. Sounds like somebody rattling a coffee can full of marbles.

[Michael W.]

Re: detonation

Most common occurance is during had accelertion or climbing a steep hill and lugging the engine. Sounds like somebody rattling a coffee can full of marbles.

[Duke W.]

Re: detonation

Detonation is the technical term. It is also referred to as "knock" or "ping", and it sounds like you may be experiencing it by your description of a "rythmic knocking or rapping sound". It usually happens on acceleration and is most prevalent at high load/low revs.

Detonation is an abnormal combustion phenomenon. Instead of the flame front progessively consuming the mixture, the mixture is partially comsumed by normal combustion, and then the remaining unburned mixture suddenly reacts in a literal explosion. This generates shock waves that bounce around the combustion chamber and "rings" the engine structure, and this is the "knock" or "ping" we hear.

It's not the mechanical pounding of shock waves that can damage the engine; it's local overheating. Shock waves increase the rate of heat transfer to the combustion chamber boundaries by up to a factor of ten, which can rapidly overheat surfaces, and the piston crowns and exhaust valves operate closest to their limiting temperatures where they rapidly loose mechanical strength, so they are typically damaged first.

Brief transient detonation is usually not harmful, but sustained detonation, such as what might occur climbing a long hill can do cumulative damage.

If the engine detonates, shift to a lower gear. If detonation is sustained under some operating conditionn, the ignition advance map should be modified to reduce timing under the detonation conditions. This can consist of reducing initial timing, slowing the centrifugal advance, or a combination of both. Also, the vacuum advance should be checked to make sure it is not too aggressive. For example, using the 8" can on a 300 HP engine could cause detonation. The 300 HP engine only needs a 16" can since they pull about 18" at idle. A L-79 or L-72/71 only needs a 12" can since they pull about 14" of idle vacuum, if set up correctly. The mechanical lifter SBs need the 8" can because they only generate 10-12" of idle vacuum. The can only needs to sufficiently aggressive to provide full advance at 2" less than idle vacuum.

All of the above assumes you are using the highest octane fuel. Octane number is just a relative measure of detonation resistance. The higher the number, the more detonation resistant the fuel.

If you aren't sure you can recognize detonation, find someone who can and take them for a ride. Load the engine at low revs because this is usually the most severe condition. Even light detonation can usually be heard over a relatively loud exhaust, but not with open exhaust, which is way racing engines can suffer damage fairly quickly if they get into detonation.

You can also "drive around" detonation. For example, my '88 Mercedes 190E 2.6 inline six "requires" 91 RON fuel according to the manufacturer, but it only has a 9.2:1 CR. A few years ago I tried 87 RON fuel, and it only detonated briefly if I short shifted and applied throttle rapidly. I learned to "drive around" the detonation by either shifting at higher revs or feeding the throttle in more slowly after upshifting. Since I only drive the Merc in the winter, the cool temperatures help, but if we get a warm spell the engine has a much higher tendency to detonate. Back when I drove it year round it would sometimes even detonate a bit in hot weather (A/C on, engine temp over 90C) on premium.

Modern engines have "detonation sensors" that detect detonation, and the engine control electronics will retard timing until operation is detonation free. Most modern cars that "require" premium can be operated on regular. For example GM's stated octane requirement for the LS2 engine in the C6 is "93 octane recommended, not required".

If you run regular in a modern "premium fuel engine" you might notice a loss of power, particulary at low revs and might also notice a drop in fuel economy in normal suburban driving, especially if you have a heavy foot, because the engine will have less advance, which lowers fuel efficiency; however, freeway cruising fuel economy will likely be unaffected.

At my suggestion, several friends have switched their modern "premium fuel" cars to regular and none report audible detonation, reduced performance, or reduced fuel economy, but none of them are heavy footed drivers.

Duke

[Duke W.]

Re: detonation

Detonation is the technical term. It is also referred to as "knock" or "ping", and it sounds like you may be experiencing it by your description of a "rythmic knocking or rapping sound". It usually happens on acceleration and is most prevalent at high load/low revs.

Detonation is an abnormal combustion phenomenon. Instead of the flame front progessively consuming the mixture, the mixture is partially comsumed by normal combustion, and then the remaining unburned mixture suddenly reacts in a literal explosion. This generates shock waves that bounce around the combustion chamber and "rings" the engine structure, and this is the "knock" or "ping" we hear.

It's not the mechanical pounding of shock waves that can damage the engine; it's local overheating. Shock waves increase the rate of heat transfer to the combustion chamber boundaries by up to a factor of ten, which can rapidly overheat surfaces, and the piston crowns and exhaust valves operate closest to their limiting temperatures where they rapidly loose mechanical strength, so they are typically damaged first.

Brief transient detonation is usually not harmful, but sustained detonation, such as what might occur climbing a long hill can do cumulative damage.

If the engine detonates, shift to a lower gear. If detonation is sustained under some operating conditionn, the ignition advance map should be modified to reduce timing under the detonation conditions. This can consist of reducing initial timing, slowing the centrifugal advance, or a combination of both. Also, the vacuum advance should be checked to make sure it is not too aggressive. For example, using the 8" can on a 300 HP engine could cause detonation. The 300 HP engine only needs a 16" can since they pull about 18" at idle. A L-79 or L-72/71 only needs a 12" can since they pull about 14" of idle vacuum, if set up correctly. The mechanical lifter SBs need the 8" can because they only generate 10-12" of idle vacuum. The can only needs to sufficiently aggressive to provide full advance at 2" less than idle vacuum.

All of the above assumes you are using the highest octane fuel. Octane number is just a relative measure of detonation resistance. The higher the number, the more detonation resistant the fuel.

If you aren't sure you can recognize detonation, find someone who can and take them for a ride. Load the engine at low revs because this is usually the most severe condition. Even light detonation can usually be heard over a relatively loud exhaust, but not with open exhaust, which is way racing engines can suffer damage fairly quickly if they get into detonation.

You can also "drive around" detonation. For example, my '88 Mercedes 190E 2.6 inline six "requires" 91 RON fuel according to the manufacturer, but it only has a 9.2:1 CR. A few years ago I tried 87 RON fuel, and it only detonated briefly if I short shifted and applied throttle rapidly. I learned to "drive around" the detonation by either shifting at higher revs or feeding the throttle in more slowly after upshifting. Since I only drive the Merc in the winter, the cool temperatures help, but if we get a warm spell the engine has a much higher tendency to detonate. Back when I drove it year round it would sometimes even detonate a bit in hot weather (A/C on, engine temp over 90C) on premium.

Modern engines have "detonation sensors" that detect detonation, and the engine control electronics will retard timing until operation is detonation free. Most modern cars that "require" premium can be operated on regular. For example GM's stated octane requirement for the LS2 engine in the C6 is "93 octane recommended, not required".

If you run regular in a modern "premium fuel engine" you might notice a loss of power, particulary at low revs and might also notice a drop in fuel economy in normal suburban driving, especially if you have a heavy foot, because the engine will have less advance, which lowers fuel efficiency; however, freeway cruising fuel economy will likely be unaffected.

At my suggestion, several friends have switched their modern "premium fuel" cars to regular and none report audible detonation, reduced performance, or reduced fuel economy, but none of them are heavy footed drivers.

Duke