knock, knock! Who's there?

[Tony Hoffman]

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