ANyone with a good explanation for knocking..

[ae1969]

So I understand detonation..............but I have also read many things regarding engine knock in an engine being due to the octane rating (which doesn't seem right to me)....or are they one in the same??

Any gas representatives among us that can shed a little light???

[divezic]

I'll give my shade-tree version of what I know about detonation and octane.

Octane rating refers to the amount of molecular octane rings are tied up in the fuel. These octane rings tend to stabilize fuel, meaning the fuel can be exposed to more heat before it burns.

Engine knock or detonation is when your engine, for some reason, detonates the fuel instead of burning it in a controlled manner. Usually inadequate cooling, hot-spots, excessive compression or excessively advance timing cause this.
Cooling problems: I have a 1966 Corvair. In the summer time in Florida I had to go to 93 octane and then an octane booster to stop the knock. What I need to do is drop the engine and clean out the mud and flashing from the cooling fins.
Hot-spots: Carbon build-up inside the combustion chamber gets hot enough to detonate the fuel before the spark plug ingnites it. The fix...remove the cylinder head and clean out the carbon deposits. Another trick to clean out the carbon is to slowly inject a gallon of water into the carburator via a fine mist while the engine is running at 3000 rpm. This is called water injection and is sometimes used on old carburated cars. I don't know if it will work on FI.
Excessive compression: High compression cars like performance stroked rebuilds can't take pump gas. A lot of these guys run racing fuel or sneak some aviation 100LL (100 octane Low Lead) into the tank. Illegal but it works.
Excessively advanced timing-Modern FI have a knock sonsor in the engine. If the FI senses a knock then it automatically retards the timing to the point where the knock (detonation) stops.

Since the higher octane gas is more stable, it will not produce as much heat as a lower octane gas. This Super-Premium gas junk is just a gimick. Higher octane does not increase your power. Just the opposite. On the other hand, if you use too low of an octane and your engine detonates, you lose power and you could damage your engine. When the FI retards the ignition, you lose power. Trick is to find the octane that works for your car. I start with the lowest mfgs recommended octane and work from there.

Let me throw out a question. Diesel fuel is relatively very stable. Can I mix a part of diesel fuel in my Corvair's gas to help out with the summertime knock? Will I screw up my engine or fuel lines/seals?

[ae1969]

Thanks divesic

I ended searching the library and answering my own quesition (knocking and detonation are the same).. mechanical knocking is different and pre-ignition (although similar to detonation) will cause early detonation

Yes adding diesel will ruin the car.........

diesel (is quite oily does not evaporate very fast)

fuel filters, fuel lines, create carbon deposits (cause pre-ignition) etc.....................

I wonder if you could rig a knock sensor on your car???
Maybe carbon deposit build up as you mention?

The excessive heat is probably causing the pre-ignition (which causes early detonation)

Have you done any engine work on the car? Is it relatively in stock form?

[divezic]

My knocking car is a 1966 Chevy Corvair. The only engine work done that I know of was a re-seal job. Otherwise it is in stock condition. I suspect a valve train problem that is causing the knocking.

I know that there are after-market fuel injection kits that come with a knock sensor. I will go for a less drastic (expensive) approach and just end up rebuilding it. Perhaps if I increase the displacement to 3.1 liters and................so much for less expensive

[Allldi]

You're right, higher octane doesn't equal more power. However, if you run a higher octane, the ignition timing can be advanced more which can create more power. For cars without knock sensors that doesn't matter (unless you advance the timing yourself). But on fuel injected cars with electronically controlled timing and a knock sensor, the computer can advance the timing to the knock limit to take advantage of the higher octane for more power.