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Acceleration Junkie
Join Date: Aug 2007
Location: Laguna Beach, CA
Posts: 263
Originally Posted by billjam View Post
. . .The gauge was reading vacuum OK, but as boost came on, the needle moved up to zero then moved back into the vacuum range until boost got towards 0.5 bar (on the electric tacho gauge). As boost continued to rise to about 1 bar on the electric gauge, the needle would move eventually up into the boost range, but never to more than about 0.5 bar.


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.
Old 01-21-2009, 06:32 PM
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