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Old 10-12-2003, 12:51 PM   #12
Scooby Newbie
Member#: 19979
Join Date: Jun 2002
Location: Rockledge, FL
2002 WRX

Default Re: Rich Knock

In lame men's terms.
There are multiple factors working to create/prevent knock. But, knock is most influenced by intake charge temperature. So, if you increase boost, or it's a real hot day, the odds of knock occuring are higher. Dumping in more and more fuel (richer mixture) helps to cool the intake charge from the fuel vaporizing in the charge. This works up to a point, since when you add fuel, you are essentially adding more and more points in the combustion chamber for knock to begin from (increasing the odds of knock). This factor isn't quite as strong as the intake temperature though, so it comes in later. Research by Ricardo in the 30's with a non-intercooled supercharged engine suggests that you can keep adding boost, by using more fuel to prevent knock up to about 9:1 afr, and then after that he saw that more fuel actually caused more knock. Usually if you are running 9:1 afr, your car will reek of gasoline at and your oil will smell like it too. Taking this a bit further, this is why water injection works well. Since it only cools the charge and doesn't add more sites for knock.
That's my 2cents.

And I always thought the extra bung in the uppipes was for aftermarket EGT sensors, which is the wrong place for that too.
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dnolek is offline   Reply With Quote