build diary

May 6, 2007: I received an email today containing a comment I've seen a few times before, and it has to do with the troubles I have with my engine management when I change altitude.
The Link system I use doesn't usually have a problem with altitude changes, but I have two things about my car that makes it more sensitive than normal. First, my cams. I don't get a whole lot of vacuum at idle because of their overlap. Second, the throttle bodies. They make the car run in a very narrow range of vacuum. The end result is that when I change altitude - and that's not driving to the top of a hill, but 4000' of change - the range of vacuum my car operates in changes quite a bit. It's a problem that's particularly apparent with a Colorado car that has a tendency to travel around a lot. All of the Sevens on the cross-USA tour a couple of years ago were suffering badly from the change in air pressures - none of them had any way of compensating because they tended to run purely off throttle position.

A different ECU might be able to compensate better with a blend of throttle position and pressure sensing. Perhaps a bit more adjustability in the breakpoints in the fuel map would help as well. But it's always going to be hard to be my particular setup to behave with massive altitude changes. At least it's easy to change out the program in the ECU (providing I remember!) and it's a whole lot easier than messing with carbs.

I should also point out - in case it's not clear for those reading along - that the problems I was experiencing in Texas for the Car and Driver test had nothing to do with tuning. It was due to the broken PCV valve, something I didn't diagnose until I got back to Colorado.

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