Do-It-Yourself Peak Hold

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!

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.

Contacts: Jayair +61 3 9896 7300 http://www.jayair.com.au IMI Norgren Pty Ltd (see website for nearest sales and service centre) www.norgren.com.au

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