Vacuum advance/intake question (long) - Saab 900 Bulletin Board - Saabnet.com
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Vacuum advance/intake question (long)
Posted by Mark J (more from Mark J) on Wed, 10 Apr 2002 15:06:21
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On both of my 900s, there are ports on the intake manifold and throttle body for various vacuum hoses. The ports on the throttle body are a smaller diameter than the ones on the intake manifold.
While setting the timing the other day, I discovered that at idle, the small throttle body ports do not provide much vacuum, while the larger ports provide plenty of it. I confirmed this on both cars.
Since the vacuum advance is attached to a smaller diameter port, this means that the timing does not advance until the engine is off-idle. I confirmed this with a timing light on both cars.
As an experiment, I switched the vacuum advance hose to the larger throttle body port to see what would happen. This was on the 1984 900T. I confirmed that the timing was advanced at idle with the timing light and went for a test drive. I was surprised to find the car noticably peppier off the line, requiring less clutch slippage and generally much easier to drive. I switched the line back and forth a few times to confirm that I was not imagining this. I really like the way the car behaves this way and am considering leaving it.
I am hoping someone can shed some light on exactly what I am noticing. Why does the timing not advance until the engine is off-idle in the stock setup? Thanks.
- Mark
1985 SPG
1984 900T
1978 99T
posted by 12.144.1...
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