Own a turbocharged car with a pneumatic boost gauge? Well, it's
likely you'll be interested in this - a do-it-yourself 'peak hold' facility.
Here's what you need - a one-way check valve as found in the
vacuum hose arrangement of various cars. If you're feeling rich, you can
purchase a brand new one-way valve (identical to the white/black example
pictured) for AUD$2.30 via Jayair distributors. Alternatively, you should be
able to scam a second-hand one-way valve (such as the pictured black/orange
example from a 1984 Daihatsu Charade Turbo) at no charge from a wrecker.
According to our tests, the new and second-hand one-way valves perform identically. Both allow air to flow in only one direction and have a
cracking pressure (the pressure at which air can begin to flow) of around 1 kPa.
In other words, they'll start flowing almost as soon as positive pressure is
applied to the inlet port.
Here's how you set up the valve...
First, gain access to the back of the boost gauge and cut the
rubber hose that connects it to the engine.
Insert the one-way valve into the hose as shown. The side of
the valve that is coloured differently to the main body connects to the hose
from the engine, while the opposite end of the valve connects to the hose that leads to the boost gauge. If in doubt, blow though the valve to identify its flow
direction and insert it so that air can flow to the boost gauge but not in the
opposite direction.
And here's the key point - the hose connections must be
absolutely airtight.
It's essential to use hose clamps both sides of the valve. With an airtight line between the one-way valve and the boost
gauge, a peak boost reading can be maintained for many hours.
But keep in mind that even a tiny air leak will cause the boost
needle to gradually fall back to zero.
The peak hold facility is extremely valuable when calibrating a
new boost control system; it lets you watch the road without having to keep one
eye on the boost gauge. Note that you will need to 'zero' the boost gauge
between each incremental adjustment of the boost system. The gauge can be zero'd
by releasing the pressure stored between the one-way valve and the gauge -
simply release one of the clamps and temporarily remove a hose end.
If this step sounds a bit awkward - or maybe you want the peak
hold function permanently on demand - you can plumb a tap in parallel with the one-way valve.
Regardless of whether you use the peak hold facility only for
calibration purposes or as a permanent tap operated function, it's a very
cheap and effective mod that anyone can do!
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The Heavy-Duty Option
If
you have any reservations regarding the strength of the OE one-way valves in
very high boost applications, here's another option... The Norgren T55C1800
non-return valve is a rugged aluminium bodied unit designed for use up to 10 Bar
(147 psi) and has a cracking pressure 0.05 Bar (0.735 psi). The valve retails
for AUD$38.80 (plus GST) and you'll also require a pair of screw-in male barbed
fittings, which cost a couple of dollars each.
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