Read an interesting article today about alot of the myths of engine tuning. One had to do with timing. Basically, if I can remember now, related the slower burn rate of gasoline and the speed of pistons. Of course. Detonation happens when the burn time of gasoline is less than the travel time of the piston to TDC or so. Advance too much and the piston is still traveling upward when the fuel is fully combusted. (This might be timing 101 to some, but somewhat clarifying to me). So, is it appropriate to manually, slowly advance timing until detonation occurs at WOT? If not clear, can I just keep playing with the timing until I hit detontation?

 

Interesting post...

I'll throw a stupid question here: what does detonation (ping) sound like? Can anyone describe it? Is it something that you need a tuned ear to hear, or something any moron like me would notice?

Thanks,
-Pedro

 

Like metal hitting metal, but not really loudly. Almost resembles a rattle that's coming from your engine.

 

If you want your motor to run well do this: Disconnect the vac hose to dist and plug. Buy a set back timing light and turn dial to 36 degrees. Have someone to start car and hold at 3000 rpm's. Turn dist till your timing marks on motor line up. Tighten dist, hook up vac hose and go to the races. This is know as the lars method. It works great!

 

Things to think about--detonation and pre-ignition are two different things. Most of the pinging we hear is actually pre-ignition which is the mixture igniting before the plug fires. Detonation is a to do with the shock waves ( lots of theory I don't have in detail off the top of my head ) of ignition not travelling from one side of the combustion chamber to the other in the correct manner.
bob

 

Wow, now this is getting too deep... :crazy:

Chris, thanks for the description. Hope it's something really noticeable for someone with no experience like me...

-Pedro

 

Pedro'74--you will know it if you ever hear it. If it ever happens fix it fast ping can damage an engine quick.

Cruise on!!! :seeya

 

Thanks, Super! :cheers:

-Pedro

 

In my experience, pinging doesn't occur much at WOT, but rather on the way there, especially under load. If your engine is on the ragged edge of advance timing, you can often get it to ping by putting it in 4th and stepping on the throttle. You can gradually advance timing until it doesn't ping under load. I think pinging sounds like marbles being dropped on the engine block - not real loud, but loud enough to be heard from the drivers seat with the window down.

Good luck...

 

I wish Lars would post more. He has a wealth of info. Im going to try his full advance timing trick.

 

Vetters, Interesting. I have notice I have been ping on occassion when really getting into the throttle. I just put a Holley 670 on it. Before it was fine. My timing is set at 8 BTDC. If I understand correctly. I can advance my timing until the ping goes away? True?

Randy

 

Randy, I'm no expert, but if you have it set at 8 BTDC, you aren't close to pushing the limit on overadvanced timing. Did you check your total advance?Carb shouldn't be the cause of that, but wondering what octane gas you are using? May be a simple as changing to a higher octane or retarding the timing a little so that it goes away. Putting the engine under load is an excellent test and the reply from ZMan is right on, in my opinion. Let the engine lug in a higher gear, then stand on it. Retard timing gradually until it goes away. Hope you don't have to do that, because you'll kill performance as well as pinging.

 

Z-man, now you have really confused me. My experience has always been that I get pinging with timing advanced too much. If I am running premium, and still get pinging, I retard and pinging goes away. Consistent with my understanding, the gasoline has been fully burned but the piston is still in the compression stroke. Help me out, because I seem to have it backwards.

 

Pedro,

Detonation, engine-ping, whatever you want to call it sounds like marbles hitting each other really hard. :hat

 

Z-man, drsierra and others: thanks!

What is the frequency in which "the marbles hit each other"? Is it something isolated, or is it like you're throwing a box of marbles on the floor at once?

Thanks,
-Pedro

 

To answer Larry's initial question....

Most GM V8s, including small block and big block Chevys, will produce peak torque (and power) when the TOTAL mechanical timing (full centrifugal timing is slammed all the way in without the vacuum advance contributing) is at or about 36 degrees at 2500-or-so rpm. The 36-degree spec is a general spec that is a good starting point to do some final fine-tuning from: you can optimize it in one of two ways:

1. If you have access to a track or to a performance computer (G-Tech or Vericom, etc.), starting from the 36-degree total spec, advance or retard timing in 2-degree increments until trap speed (speed at end of a fixed distance) is maximized. You will find in virtually all cases that trap speed will be maximized when the TOTAL timing is set somewhere between 34-38 degrees. The new "fast-burn" style heads require less total timing. Higher elevations require more total timing.

2. You can advance your timing until you just barely get audible detonation ("engine knock") while lugging the car under load and wide open throttle. I usually check for this by putting the car in 2nd gear at about 1500 rpm and flooring it up through about 3500 rpm. If any audible detonation occurs (which sounds like putting a bunch of marbles in a blender), retard the timing in 2-degree increments until all detonation stops. Likewise, if there is no detonation under this test, you can advance the timing in 2-degree increments until you hit detonation, then back off 2 degrees. This procedure is commonly known as "power timing." You are setting up the timing to produce the maximum amount of cylinder pressure that your engine/fuel will support.

A critical note on power timing, however: Power timing only works if you use the grade (octane) of fuel that is recommended for your engine. If you use high-octane fuel in a low-compression engine, detonation will be fully suppressed, and you will obtain an over-advanced ignition condition before you ever hit detonation. The over-advanced condition will result in a power loss. If you are doing the power timing method, always check to see what total timing spec you end up with: if you end up with much more than 38 degrees total timing, I would question the validity of the spec you ended up with...

Finally, there are two terms relating to "engine knocking" that are commonly interchanged. Pre-ignition and detonation.

Pre-ignition occurs when the air/fuel mixture in the cylinder ignites before the spark plug lights the mixture. It can be caused by hot-spots in the cylinders, or by high volatility of the fuel causing the fuel to explode from engine compression prior to spark plug ignition. The early ignition of the fuel causes excessive cylinder pressures and temperatures before the piston hits top center.

Detonation occurs when the fuel is ignited by the spark plug, but instead of the fuel burning at a nice, steady pace across the combustion chamber and across the face of the piston (fuel, when buring properly, does not "explode" - it burns at a steady, although very rapid, pace from one side of the combustion chamber to the other, this producing a predictable steady "push" on the top of the piston), the fuel suddenly explodes all at once. This produces a massive and sudden impact on the top of the piston, much like hitting the piston with a sledge hammer instead of pushing the piston with a steady force.

Both pre-ignition and detonation are audible as a "rattling sound" in the engine, as if the engine were full of marbles. Both pre-ignition and detonation cause a masive power loss, since the fuel is not being burned properly in the engine. And both pre-ignition and detonation can cause massive engine damage due to melting and errosion of the tops of the pistons, and impact damage to wrist pins and rod bearings. Higher octane fuel has a resistance to detonation. High octane fuel will not solve a pre-ignition problem caused by hot or glowing points in the cylinder. High-octane fuel does not have any more energy than low-octane fuel. However, high-octane fuel can allow you to produce more power by eliminating the power-robbing symptoms of detonation and by allowing you to set your timing for 36 degrees total without getting into detonation. If you can obtain the 36-or-so degree total spec without detonation on low-octane fuel, you will not gain more power by using high-octane.

I know, I'm rambling off subject now... I better quit.

 

As usual lars answers all. We should all bow to him for his knowledge. I know my car runs better due to his carb and timing articles. I'm still working on getting it faster. :yesnod:

 

Gordon -
You're much too kind, and I certainly don't want anybody bowing.... the best reward I get is when somebody from the Forum stops over with a cold beer and trusts me to do a little tweaking on their Vette...! Glad to hear some of the info has come in useful - keep up the good work!

 

I'd love to stop over with a couple of beers but they would be a little warm after the trip. :cheers:

 

Lars - thanks alot!

At least I'm not crazy ( I don't think :crazy: ) and advancing timing will ultimately lead to detonation/pinging, retarding will alleviate. Can't tell you how many times I have had others say the opposite.

I'm not crazy am I? :lol: :yesnod: