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The Bosch 280-150-036 injectors that were originally supplied with the D jet system are listed in the Bosch Motorsports catalogue as 364 gm/min (492 ml/min) injectors at 43.5 psi. Actual testing by me and others indicates that the flow rate is more like 555 ml/min at 43.5 psi. All vendors generally list their flow rates at 43.5 psi even if they don't operate at 43.5 psi.
Within reason, it is permissible to adjust the flow rate through an injector by altering the base fuel pressure. The relationship between flow rate and fuel pressure is
New flow rate = sqrt(new pressure/old pressure) x old flow rate
If the VP Auto injectors have a flow rate that is higher than the Bosch 036 injectors, it is acceptable to reduce the fuel pressure to a level where the flow rate matches the Bosch 036 injector. However, I would be surprised if the VP injectors are so oversized that they require a pressure reduction to 18 psi to make them work. The Bosch 036 injectors are already grossly oversized for the size of the B20 engine and I am not aware of anybody who made a barb fitting style injector larger than the Bosch 036. .
One way to resolve this would be to send one of the VP injectors out to RC Fuel Injection or WitchHunter (or whoever you hoose) and have it flow tested. If the test results come back saying it has a flow rate in the 500 - 550 ml/min range, then the injectors are probably Bosch rebuilds or one of the aftermarket equivalents. In such case, you have some other problem causing the high fuel flow rates and dropping the fuel pressure is only masking that problem. You need to address that problem whatever it is rather than continue to run at 18 psi.
If the test results come back indicating an injector flow rate much higher than 550 ml/min, then within reason it is fair game to adjust the base fuel pressure down to get a flow rate that would match the Bosch 036 injectors. Once you know the injectors actual flow rate you can calculate what the correct base pressure should be using the preceding formula. You need to be cautious with this approach because the Volvo fuel pressure regulator was designed to operate with a base pressure around 28-30 psi. It may or may not operate reliably at a much lower pressure (control pressure may fluctuate depending on how much fuel is being by-passed).
If the VP injectors come back from testing with a flow rate higher than 500 - 550 ml/min, I would be inclined to complain to VP and request a refund. I would replace them with the Beck Arnley 158-0438 injector which a number of users have confirmed as a perfect replacement for the Bosch 036 injector. If you shop around, you can find them on sale for less than $40 each.
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