elementary manifold vacuum question

[Tom Parker]

Stowe, Ted-SEA <StowT at PerkinsCoie.com> wrote:

>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.

Sorry, started this post thinking you had an A series engine, read 1000cc not
1800cc, but maybe it's relevant.

This is with a  5 port head, right? Are you running a twin carburettor or twin
barral carburettor? If so, does your inlet manifold have a balance pipe
running between the two inlet ports? I was told that if you hook up a brake
booster to one of these engines, you must use both inlets, otherwise you get
too much pulsing in the brakes.

Whether the normal balance pipe between the two branches of a twin SU inlet
mainfold is enough to balance the vacuume, I don't know.

>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.

600rpm sounds a bit low for warm up and any type of vacume measurement. You
might be seeing the individual sucks, at 600 per second if the gauge only
sees two cyclinders.

--
Tom Parker - tparker at nznet.gen.nz
           - http://www.geocities.com/MotorCity/Track/8381/