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The DI is used for knock detection
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Sat, 6 Dec 2008 05:49:09 Share Post by Email
In Reply to: Re: Yes and no, Aom, Fri, 5 Dec 2008 10:42:44
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The DI provides a pre-ignition signal to LH2.2 (pin 20) and LH2.4 and up (pin 28 on the ECU). The DI can actually detect the conditions for pre-detonation; so the ECU can adjust timing before you get to knock. Once knock occurs, the ECU uses fuel enrichment. Yes, the earlier cars have knock sensors, but the DI can actually prevent knock by detecting the precursors.

I'll agree with Dr. Boost - don't use a Black DI on a T5 car if you can at all avoid it.

The longer spark is a nice idea - based on the description, the 400 V source needs to be AC. Note that the spark isn't any bigger, it just lasts longer. I can see that as being useful if there are fuel-burn issues; the spark only impacts the fuel in the immediate area of the plug. Fuel beyond the plug is burned by the advancing flame front.

The other big difference in the DI's is how the pre-detonation circuitry signals the engine - Dr. Boost also notes that. "However, the "signal interface" are slightly different, or in ither words, the combustion and knock signals are different. " And LH equipped cars do use the signal for knock detection, in addition to the knock sensor.

I've been going with the recommendations from John Moss, who has been the chief Saab technician trainer for a few decades. His feeling is to avoid the black DI on 'red' cars if at all possible.

I'll stand by my recommendations - probably OK, avoid it if the car has been modified for more horsepower, better for a non-turbo than a turbo. And YMMV. We're talking about an engine old enough to vote (but not drink) - it doesn't perform the way it did when new (none of us do.) That's why I'm hedging with 'probably.'

I'll put it this way - the fleabay price difference isn't much, so I'd stick with red. If I'm buying new, the price difference is greater. Thankfully I'm in a position where the price difference doesn't mean if I get dinner this week, so I avoid the possible downside and buy the red for my 'red' cars. If someone offered me a black DI cheap, I'd take it, and use it as yet another spare for limp home.

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