elementary manifold vacuum question

[Bruce Plecan]

-----Original Message-----
From: Stowe, Ted-SEA <StowT at PerkinsCoie.com>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Sunday, November 08, 1998 6:04 PM
Subject: elementary manifold vacuum question

Dampening or restricting the MAP reference would be a last ditch effort in
my opinion.  With the cam I got in my car and running a 747, the one thing
I wound up not fooling with afterall was the MAP+ it's reference line.
It's a pita but even playing with a big cam, by careful doctoring the
timing/
fuel at idle, I was able to do just fine.
Just don't get inna hurry, and keep good notes.  Trust me on the notes
issue.
Cheers
Bruce


>I have a question for the collective. I think I have forgotten vacuum >101.
I am trying to make sure everything is hunky dory before I >strap my gm
throttle body onto my car.I have a 77 mgb 1800 cc.
>this has siamesed intake and exhaust ports unfortunately.
>when it is cold and I hook up a vacuum gage to the intake manifold, >at
about 600 rpm the needle fluctuates between 15-19 inches. >meaning that the
needle itself is nearly invisible I can see the trend. >if I pinch the hose
to the gauge, I can dampen down the 'pulses' >and get it to stay steady. at
higher rpms this effect goes away >completely.  as it warms up ,at 1000 rpm,
it will fluctuate between >16-18 and eventually just settle down to a buzz
at about 17+.
>the engine runs fine and idles fine by all appearances, no shaking >or
anything.
>I have no idea if the cam is stock or not.
>I remember that one is supposed to actually restrict the vacuum line >to
resolve this issue, I am just wondering if this sounds like word >valve
guides or seals.
>I don't know if the gm 1800 cc I have this from had this type of issue >and
I am wondering how this would affect the sensors on the >throttle body/ECU.
I don't seem to remember this as an issue on >other engines.
>thanks for all the great support btw.
>Ted Stowe