knock, knock! Who's there?

[Grant Lenahan]

Yes, and vacuum advances and variable engine maps can change timing 
w/r/t rpm, so, under malfunction, they may well cause knock at one rpm 
range and not another.

Grant
On Jul 22, 2007, at 1:46 PM, Tony Hoffman wrote:

> And, to add to that, because it's pre-detonation, it tends to hapen
> more at lower RPM's or where the "dynamic" compression is higher. As
> an engine turns faster, it actually pulls in less ari per RPM. This is
> because the valves, intake system, exhaust, etc, is not a limiting
> factor for airflow at lower RPM's. But, that becomes an impedence at
> higher RPM's. Which is exactly where bigger cams, bigger exhaust, high
> flow air intake systems, ported heads, etc all come in.
>
> As stated, the knock sensor will stop an engine from doing this.
> However, it does not prevent it. In fact, to work, it requires knock.
> IOW, the sound preignition produces is a frequency that vibrates the
> crystal in the knock sensor. It turns this into a "short" that it
> sends to the ECU, where the timing is retarded accordingly.
>
> There are several factors to consider here. Octane rating, which you
> have mentioned already. Also, mixture. The leaner an engine runs, the
> greater chances for preignition to occur. Also, carbon buildup inside
> of an engine can cause preignition through "hot spots". If you run
> your engine hard, you will not have to worry about this one.
>
> One thing you didn't mention is if your car is equipped with a vacuum
> pump driven off the camshaft. This is a very common noise maker (quite
> often a knock sound) on the NA Audi engines. A lot of people have just
> taken it off and removed the plunger.
>
> Hope this sheds a bit of light on the subject,
> Tony Hoffman
>
> On 7/22/07, Tess McMillan <tessmc at drizzle.com> wrote:
>> So what actually is a "knock"?
>>
>> How can you have a knock only at certain rpms?
>>
>> In response to my query about a CEL that was going off (and which is 
>> no
>> longer doing so), someone responded they thought I could have a faulty
>> knock sensor. Would this by itself cause "knock"? What about if 
>> you've had
>> cam modifications? The knock is about where the power band starts; 
>> beyond
>> that, and below that, it is not present.
>>
>> For some reason I thought poor gas caused knock.... The minimum I use 
>> is
>> 92, sometimes I'll opt for 100 when I can get it.
>>
>> As usual, thanks for the education and advice &:-).
>>
>> Tess
>>
>> in Bellevue, WA USA
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