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billjam billjam is offline
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Join Date: Aug 2007
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Originally Posted by 356-930 View Post
Because the vac/boost pressure sense tap point is fundamentally looking directly into #2 cylinder's intake "plenum," it is subjected to pressure pulses in the 'plenum' that occur with cylinder intake/exhaust valve overlap. And, the higher the rpm, the faster the pulses.
Classic vac/boost/ mechanical “C-type” pressure gages (like NHW's boost gage) cannot keep up with some and can't even "see" most of the pulses but they raise hell with the C-tube – you saw it.
The brake booster line should be a good source to tap. At the engine, a good pick off point is a T in the line to the WUR’s boost enrichment port.
One should use a line/hose from the engine to the gage with a minimum ID of 5/16" (8mm).
The hose is a mechanical filter that pneumatically damps engine pressure change to a frequency the gage can handle.
Thanks - most of this makes sense.
Although I don't see why the larger vacuum connection for the booster (which is in a very similar location - close to inlet for #1) wouldn't also suffer a similar problem with pressure pulses. Maybe size has some effect resulting in pressure fluctuations getting damped.
1988 Carrera - 3.6 engine (with ITBs, COPs, MS3X) and a whole set of turbo body panels waiting in the attic.
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Old 01-21-2009, 07:45 PM
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