Most mechanical pressure gauges will react very quickly, the problem
usually is in the lines that supply them. On a Triumph you have a very
restrictive banjo that takes the oil pressure bleeding around a bolt and
transmits it via a fine-diameter tube to a squishy rubber hose, then back
to a restrictive hard line. The ID of the gauge inlet is 3/8". Feeding it
with capillary tubes is a problem.
A dinky tube or kinks in the line will trap pressure and require time to
bleed or show increases. Likewise, a soft tube will balloon and damp
pressure changes. You can solve the problem with either a large diameter
hard line (bendable brake lines work well) or -4 or -6 armored tubing. I
used -6 for my oil gauge since I had a good piece and the right connectors
on hand. I tap the oil pressure from the gallery. There might be better
places but this is the pressure the bearings see (I think). For those
confused by the Army/navy nomenclature, just remember that it's
sixteenths, i.e. -4 is 4/16 = 1/4 inch inside diameter tubing. -6 = 3/8.
Avoid any adapters with a restricted ID or that have a sharp bend unless
they make up for it with larger diameter than the pipe they connect to.
I think I'm getting kind of weird in my old age, worrying about arcane
technical trivia with old cars. But some of these things really make a
difference. Spiffing up my oil pressure line led me directly to refitting
my accusump. A must-have device for racing with wet sump engines in my
humble opinion.
-----Original Message-----
From: Robert M. Lang [mailto:lang@isis.mit.edu]
Sent: Monday, February 24, 2003 11:10 AM
To: fot@autox.team.net
Subject: Re: Gauges
Hi,
The recent flurry of messages about Smiths gauges reminds me of a minor
quandry that I have. When I first started up my race car motor three years
ago, I had a minor problem with the pressure relief valve being stuck
partially closed. This produced copious amounts of oil pressure, the
extent of which was two dead Smiths oil pressure gauges.
I installed a VDO mechanical gauge. I selected VDO because they had a nice
line of 2-1/16 gauges that fit the stock dash layout without modification.
This mechanical unit worked fine, but was seemingly slow to "react" to
changes in pressure.
So, I tried a replacement VDO gauge that was electrical. The sending unit
is a standard 100 PSI unit with one output connection.
Again, this works fine, but the numbers are suspect and the gauge reacts
(seemingly) very slowly. I'm concerned about this reaction time because it
seems that in a corner if I did starve the engine for oil, it's possible
that the bearings would be long gone before the gauge would indicate that
there is a problem.
I guess it's safe to say that there is a lot of "damping" in the guage or
the sending unit.
Okay - I'd like to fix that.
My recollection is that years ago (when I was in my VW Beetle phase), was
that I installed a VDO oil pressure guage that gave me _instant_ response
when provoked, and this saved the motor many times as I tried my best F1
accumen driving on/off ramps and generally terrorizing my home town (e.g.
on long sweepers that gauge would "dip" if the sump had emptied itself
into one of the valve cover areas...)
I'm looking for recommendations for gauges that will react quickly to
problems but are easy to install in our quirky cars. I'm okay with
electrical guages or mechanical, but with the mechanical sending units, I
want to be absolutely sure that I don't need machine the various housings
to accomodate them. For example, I have a VDO coolant temp guage with
mechanical pickup (wet bulb and long armor clad line) - but the fitment to
the engine is via a 1/2 NPT connection. To use this example, I'd have to
drill out the water pump housing to accomodate the sending unit and I
don't want to do that. Hence the $50 VDO gauge sitting on a shelf in the
garage.
I'd like _any_ suggestions, but the more specific the better.
Oh - and upgrading to 2-5/8 dial might be in the cards because now that I
am advancing into the bi-focal age, I find the 2-1/16 gauges are downright
tiny!
:-0
regards,
rml
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