Bones232

As the title says. Anybody know why I would see more knock counts when reducing SA?

The attached pic is of two relatively identical runs. They both are 2nd gear WOT pulls. In the second bin I began pulling timing above 3600rpm to see what would happen. To my surprise knock counts increased. I've often wondered if a rich condition will throw counts.

Graph Legend:
Dark Blue - Commanded AFR
Magenta - Actual AFR
Yellow - TPS (scaled to fit on graph)
Light Blue - AEPW (scaled to fit on graph)
Purple - SA
Maroon - Knock Counts (scaled to fit on graph)

Any insight would be appreciated.

91L98Z28

In the past, Grumpy and others have posted some real gems on dealing with knock. I'll bring up a few things I remember:

First, knock takes time to get started. In other words, the conditions for the knock exist for some period of time before the knock actually develops. Chamber temperature and pressure is rising with each firing stroke, the plug is getting hotter, etc. etc.

Second, once the knock has occurred, you actually have to pull MORE timing out to get it to go away, than you could run if you had never gotten into knock in the first place.

What I see when I look at your chart, is that your knock is starting at 3700rpm, +/-. What I also see is that your timing pullout starts around the same RPM - possibly too late, as the conditions for knock have already been present long enough to start the knock going so pulling a few degrees past 3700 is too little, too late.

I would modify your WOT timing curve and instead of hitting it with 34* of timing at 2800rpm, do something like this:

2400: 26
2800: 28
3200: 30
3600: 31
4000: 32
4400: 32
4800: 32
5200: 32
5600: 32

Then, go for a datalog drive. You'll probably find that you no longer have knock, AND you might feel a power gain as well.

Vortec heads are efficient heads, and don't need a ton of timing to work well. Plus, you have a relatively small cam which helps build cylinder pressure, which also means you need less timing. You don't mention what fuel or compression ratio you're running, but that has an effect too. You're also running 3.23 gears which reduces your ability to be aggressive with the timing as compared to, say, 3.73's.

If a timing curve like the one above doesn't get rid of the knock, then I'd increase the octane of fuel. I'f you're already at 91 or better, then consider that your balancer timing marks may have slipped and/or you may be running more timing than you realize (either way, check it with a timing light).

Bones232

Thanks for the post. I'll load up the timing curve and try it out.

I'm running 87 octane fuel. Compression should be around 9:1. I have stock dish style pistons, the heads are not shave or anything like that.

Bones232

Interesting results.

I did a couple of second gear pulls with the new SA. The results seem promising. Both pulls registered significantly less KnkCts. Of course I'll have to keep an eye on it to be sure the KnkCts stay away.

I appreciate the help.