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Speedwell

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mgbcca

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I have bought a 1975 MGB with some modifications which include twin SUs, hotter road camshaft and increased compression ratio. My instruments include a Speedwell dial, which measure vacuum (I think). The needle moves around in an entertaining way, but I would like to know how to interpret it.

Any suggestions?
 
Does the gauge actually say anything on it, like vacuum, pressure, pounds squared, or psi? or just numbers and a needle jumping around? If it's vacuum, it's probably measuring intake manifold vacuum, which is useful, I guess. I figure it'd act a bit like a boost gauge. Interesting that it's from Speedwell-they made accessories and complete racing packages-even a few cars- in the late fifties and early sixties. Lots of stuff for Austins, like A30s and A-H Sprites (a Speedwell Sprite is high on my automotive wishlist). Formula One World Champion Graham Hill was an enthusiastic user of Speedwell's stuff, mostly because he was a director of the company!I'd no idea the name Speedwell was still being used as late as 1975.
-William
 
I bet it was an add on
driving.gif
 
iagree.gif
I guarantee it was an add-on!
 
Of course it was an add-on, but it's interesting it exists at all. I was just reading about the Speedwell Sprites earlier today. I gotta get me one o' them.
 
Hello MGBCCA,

a vacuum gauge is normally used as an aid to economic driving. The higher the vacuum reading, the better the fuel economy will be. A bit of a waste of time in a sports car in my view, as economy and DRIVING a sports car do not go hand in hand.
They can be useful, if a little difficult to interpret, as a guide to adjusting your engine tune. Normally vacuum gauges for that purpose are larger in diameter and calibrated in inches of mercury rather than the coloured bands that a lot of dashboard gauges are.

Alec
cheers.gif
 
To parapharse Col. Kurtz: "The vaccum gauge is your friend, or an enemy to be feared".
Well, not realy feared, but it can reveal both good and bad news.
Here is a link to a page on tuning with a vaccum gauge that demonstrates that these are very handy indeed. https://www.centuryperformance.com/vacuum.asp
 
Thanks for all the help - especially the century performance link. The guage reads in inches of mercury from 0 to 30. It has a movable marker set at 11 ins, which is where it usually is at idle. The needle moves smoothly. So all seems OK. Right?
 
A reading of 11" is a bit low. If I recall, when I put a gauge on mine at the manifold it reads around 17. Can you trace the connection on yours? The numbers on that site are for manifold vacuum.
 
Thats manifold vacuum, as opposed to ported vacuum from the base of the carb.

[ 09-30-2003: Message edited by: Andy Blackley ]</p>
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by mgbcca:
It has a movable marker set at 11 ins, which is where it usually is at idle. The needle moves smoothly. So all seems OK. Right?<hr></blockquote>

MG,
While 11 inches of vacuum is quite low it may not be unreasonable with your hotter cam. Make sure that your idle timing is not retarded. Also, vacuum will drop a few inches as altitude increases.
D
 
Also check your valve lash. If a little tight, this will also cause low vacuum, however you would probably notice poor idle and acceleration.
A stock engine usually develops about 20" manifold vacuum at idle, however as Dave states, longer duration/ higher lift camshafts effect this directly.
Pete H
 
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