Spark knock?

[Dark_Horse]

Over the last two trips to the drag strip, I’ve been noticing an issue with the motor on the top end – i.e. between 5K and 6K RPMS. It sounds like a popping or crackling noise which I can’t figure out. My initial thought was that it may be the alternator or one of the other bearings on the pulley system. My other thought was maybe it was a slight exhaust leak coming from the header to midpipe connection. Somebody on the other forums suggested that it might be spark knock, which I think may be the problem.

I’m running a stock 2V 4.6L setup with catless h-pipe and an SCT 93 tune and some octane boost. Last summer, I changed out the spark plugs and bought a set of used coils on eBay to replace the stock ones for really cheap. I did this as a preventative measure really. However if the problem is spark knock at high rpms, my first thought would be a bad coil or some fault in the ignition system.

So my question is this: what is spark knock and how do I identify it? Is there a way to test each of the coils for functionality? I have read tons of threads about keeping the stock coils because they work just as good as the MSD or Accel aftermarket ones. I don’t have an A/F gauge but I’m thinking of pulling the plugs just to see if any combustion problems exist. Is it possible I have a bad coil then? Thanks for your help guys.

 

[tonytaylor85]

The coils would not cause spark knock. Try adding 2% more fuel with the tuner. If that doesn't fix it, pull timing @ 4k-8k. Depending on how much ethanol is blended in your gas, the fuel may or may not help. Timing will solve it for sure, but may cost some power.

 

[Dark_Horse]

Okay yah ill try adding fuel and see what happens then. This may be a dumb question, but how do you pull timing? I see I have a timing adjustment on the tuner, should I retard timing until the spark knock goes away? I'm really green when it comes to tuning so any help I can get would be tremendous.

 

[bhuff30]

First, ethanol adds octane. I run E85 in my 2.3 and it has about 104 octane.... or enough octane for an 8:1 engine running 24psi and a 35 shot.

That rattle sound is most likely detonation. Detonation can sound like rocks in a can or like a hammer beating on your engine. Just depends on the severity.

It is possible it is from running lean in the upper rpms... maybe a weak fuel pump. It is also likely that your 93 tune is just a bit too aggressive and needs a little bit of timing pulled out. A few degrees of timing isn't going to make much difference on your engine.

 

[tonytaylor85]

Ethanol has about 63% the energy per unit mass. Ethanol is heavier than gasoline so you end up with around 66% less energy per unit volume- which is what the EEC uses. E10 therefore has 6.6% less energy available at WOT compared to pure gasoline. Lean is lean and will cause detonation. 93 octane is 93 octane and you aren't getting anything for free from the gas station.

 

[bhuff30]

I ran gas with up to 30% ethanol for several years in my 97 GT. The wonderful thing about modern ECUs, is that they can compensate for varying conditions and fuels (to a limit). Even with E30, I didn't get a check engine light. When I looked at the AFs, the lambda was identical to running pure gasoline, even at full throttle where the O2 sensor is in open loop: so not leaner and no change in power!

Yes, it has less energy, but that just means you need a little more of it. The computer automatically adjust it. My mileage was down about 1 mpg, but it was cheaper, so I was ahead.

Try it for yourself.

Ethanol has about 63% the energy per unit mass. Ethanol is heavier than gasoline so you end up with around 66% less energy per unit volume- which is what the EEC uses. E10 therefore has 6.6% less energy available at WOT compared to pure gasoline. Lean is lean and will cause detonation. 93 octane is 93 octane and you aren't getting anything for free from the gas station.

 

[tonytaylor85]

The LTFT can only do so much. Why else would we need widebands and dyno tuning? I agree that a stock tune on a stock car is designed with varying fuel quality in mind, but the 93 tune he has is probably pretty close to the edge.

 

[GDawg]

I love these kinds of threads...

 

[jgorm]

Check out this link. It will clear up a lot for you.
Detonation Explained