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Was the weather really the same?
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Tue, 16 Jul 2002 16:28:33 Share Post by Email
In Reply to: performance changes hour to hour, paulmanchester, Tue, 16 Jul 2002 15:39:35
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I'm not sure where you live, but where I am, it's 60's in the morning, and mid to upper 80's in the afternoon.

Two issues here - turbocharged engines are very sensitive to ambient temperature. Hotter weather reduces boost. It also reduces the effectiveness of the intercooler. Now, the difference isn't between Fantastic! and Running on Three cylinders; but there is a noticeable difference between a 40 degree dry day and a humid 90 degree day.

That's the normal loss due to temperature - I'd guess at it as a 10 hp loss going from 60 to 85, worse if it's a humid 85. However, if you're getting a very noticeable loss, it can be other issues.

An engine is more prone to knock at higher temperature. The intake charge is hotter, and the intercooler isn't as efficent. If your engine is knocking, you're going to see a major loss in power as the APC system reduces boost to compensate. Let's say your engine is pretty close to knock, but a cool 65 degree morning is just cool enough that it isn't knocking. You get full boost. But at 85, you're over the edge, and the engine starts to knock. The APC dials back the boost, and you get lousy performance.

Do you see the boost gauge only going to mid-yellow or so when the car is running poorly? If so, I'd guess it's compensating for knock. Run a higher grade of fuel (higher octane) if possible and see if that helps; make sure your spark plugs are the exact right heat range, and aren't getting old. If so, replace them. You may want to run the next lower heat range. As engines age, some become more prone to knock, others less so.



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