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Some ideas Posted by Ari [Email] ![]() ![]() In Reply to: performance changes hour to hour, paulmanchester, Tue, 16 Jul 2002 15:39:35 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Ah, yes, the fine English weather.
I think part of my point is made - you say that performance is worse in hotter weather. The hotter it is, the more prone an engine is to knock. There are a bunch of reasons for this - a hotter charge is more likely to detonate itself; probably some affect on the volatitly of the fuel; and the fact that the engine may be running just a bit hotter.
However, you're running between A LOT of boost, and A REAL LOT of boost. Half-way into the red is REALLY boosting. That's actually more than stock boost. So you're running out on the ragged edge.
Most of what you're seeing in terms of the boost tapering off is, well, normal taper.
And running the same grade and brand of petrol is good, but unfortunately no guarantee of repeatability. You know what's listed on the pump, but you really don't know what went into that underground tank. I'm not sure about the UK, but in the US they reformulate fuel four or five times a year to account for seasonal variations. Running the same grade & brand is good, though.
Again, if you're right at the edge of knock, all it takes is one or two very minor things to push you over the edge into knock. Air temp is a big one.
When you nail the pedal hard, you get better boost. That's because the fuel control knows you're looking for lots of acceleration, and dumps in lots of extra fuel. This richer mixture is more resistant to knock. When you 'sneak up' on it, the fuel control doesn't dump in that extra fuel - just enough to keep the mixture right - and that leaner mixture is a little more knock prone.
I would do a few things - first off, check fuel pressure. If your fuel filter is old, replace it. The Fuel Pressure Regulator may be tired. Since you seem to be running well above stock boost, you may need an 'upgraded' FPR.
Next, you might be running a little lean. That would make you more prone to knock. Make sure you don't have any vacuum leaks. Check the plugs to see if the engine is running lean - the plugs live in the engine, and they know the truth. If your car is equipped with an O2 sensor, it may be getting a little slow in response. Again, a cooler heat range plug probably wouldn't hurt, as it seems you press the car hard.
posted by 140.157.4...
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