There is a difference between what your gage reads and what the waste gate sees for pressure. The dash gage is connected to the engine side of the throtle body. That means that it reads the pressure/vacuum in the intake manifold. When the throttle is wide open, it will read pretty much want the turbo is putting out, however at any other position, it will read a lower pressure. The waste gate sees the pressure at the turbo outlet (for non intercooled cars) or the outlet of the intercooler for intercooled cars. The waste gate has a spring and a diaphragm - the spring acts to keep the waste gate closed and the diaphragm acts with pressure to open the waste gate. The waste gate itself is simply a round hole with a cover that allows exhaust to bypass the turbine wheel when it is open.

The actuator actually has a range of pressure where it starts to open, and will be fully open at a higher pressure. You are not likely to see or feel its effects, except that it limits the maximum boost you can get out of the turbo.

One thing that I have wanted to do is get a dual pressure gage and measure the boost both at the I/C outlet and at the manifold. That will tell me what the available boost is. One thing that happens is that at lower rpm, there is not enough exhaust flow to keep boost up - or rather that the turbine is not spinning rapidly enough to maintain full boost AND max airflow, although the pressure might be there at a low flow rate (partial throttle). Then when you open the throttle, the available boost pressure drops until the turbo gets going fast enough to supply the higher flow rate (turbo lag). I have often wondered about building a control system that used both mas air flow AND MANIFOLD pressure to control the waste gate. In other words, at low mass flows, you could allow much higher turbo outlet pressure as long as the manifold pressure did not exceed set limits, but at the higher mass flow (closer to WOT), the turbo outlet pressure would be limited to the same pressure as the manifold. A system like this would go a long way toward eliminating problems of turbo lag in the mid rpm ranges when going from partial to full throttle. --
Steve Seekins Re: Wastegate Question[200/1983] posted by philip bradley on
Wednesday, 21 October 1998, at 10:50 a.m.

If it was simple and could be done for not too much money, wouldn't some manufacturers of performance oriented turbo car have done something like that? The closest things I have seen are the Saab APC system, which a few Volvo Turbo owners have retrofitted to 200 and 700 series Turbos, and the Porsche 944 system. Both use a cycling valve that opens and closes many times per second and that is electronically controlled, to regulate boost more accurately. HKS, GReddy, Apexi and others make these for the aftermarket. These devices seem to reduce lag. The boost comes in sooner. Years ago, IPD sold the HKS and ran back to back tests on cars with the same boost level. The car with the HKS EVC made it to 60 mph 6/10 second faster than the car with the same boost level, but achieved by the ordinary bleed off method. The second device I have seen that helps is the compressor bypass valve. These recycle turbo output to the turbo input during vacuum situations and also relieve the surge of air that backs up down the turbo plumbing and into the compressor when the throttle is suddenly closed after there has been a lot of boost. Compressor bypass valves are widely reported to help throttle response, particularly between shifts, by helping to keep the turbo spooled up and not stalled out. Volvo saw fit to add these to the 700 series turbos. A few 240 owners have retrofitted them. They are widely used on other turbo cars.

Philip Bradley