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From: "Hapgood, Matt" <Matt.Hapgood@funb.com>
To: lancair.list@olsusa.com
Subject: Manifold pressure sensor
Date: Wed, 4 Apr 2001 12:38:14 -0400 
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Reply-To: lancair.list@olsusa.com

          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
          <<  Lancair Builders' Mail List  >>
          <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> 
>>
First Union Securities, Inc.

Question to the list -  I am trying to determine the most appropriate spot
to pick up manifold pressure, and it isn't as simple as I thought it should
be.

Background - my engine is a turbocharged (turbonormalized) Lycoming IO360,
and I am using the VisionMicro VM 1000 for manifold pressure.  I am also
using manifold pressure sensors for my engine control (it's an electronic
injection/ignition system).

I had been picking up manifold pressure at a port immediately behind the
butterfly of the throttle body.  Most people (engineering types) agree that
this should be an accurate measurement of the manifold pressure, even though
the port is only about 1/4 of an inch behind the throttle plate.  This is a
Ford throttle body, and it is where they pick up the manifold pressure.

But the two test pilots that have flown the plane both indicated that the
pressure indicated didn't seem right.  Both said it acted more like deck
pressure.  It appears to indicate too high a manifold pressure at about 1/2
to almost full throttle.

So I pulled out the Lycoming technical drawings, and they show picking up
manifold pressure at the #2 or #3 cylinder head.  So I did that, and the
reading was definitely lower than what is being reported by the port at the
throttle body.

Unfortunately, the electronic engine management doesn't like using a single
cylinder port due to large pressure variations (nor did the manufacturer
like the idea of teeing the #2 and #3 cylinder together).

So... where are most people getting manifold pressure?  What about the turbo
guys?  are there any other easy alternatives?  At this point I think I will
likely machine a port into a small spacer that is about 1.5" behind the
throttle plate...

Any thoughts?

Thanks,

Matt


>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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Please send your photos and drawings to marvkaye@olsusa.com.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>