How does a stock car control boost?

ttribejr

Member
I have been searching so don't roast me. Im a bit confused.

87 GN, digital dash car

I read that the MAP sensor on the passenger fender does nothing but turn on lights on the dash

I found the WG solenoid on the right side valve cover

If the computer has no access to MAP signal, how does it know what load its at (boost/vacuum) ?
 
There is a small table in the stock chip that controls the duty cycle of the solenoid based on rpm and load. It's basically and open loop system that worked pretty well in stock form matching the duty cycle in the solenoid with the spring in the wastegate. It all worked out pretty well...
Until we started mucking around in the chip and blowing head gaskets, etc. :eek: ;)
 
It doesn't know the load. A bone stock car only makes 12.5 psi and a bone stock engine is designed to live with that, so it works.

Fast forward to the present. Boost is set as wastegate spring tension + additional boost from bleeding off manifold pressure out the boost control solenoid. In other words if you're making 15#s of boost and the solenoid dumps 2PSI the wastegate will boost to 17 as the diaphragm itself only sees the 15 it takes to find it's 'happy place'.


If you run 'tuner style' with a hose straight to the wastegate, then the spring tension is the boost.
 
So the wastegate solenoid doesn't know how to bleed off 2 psi to make 17 psi (in your example) ... but the chip burner knows to use X duty cycle and it typically makes around X boost ?
 
Yep. Burn enough chips and the end up with experience. The chip burner also assumes the end user knows how to monitor knock and set their cars exact desired boost level.

Keep in mind the ECM knows if you're in 3rd or 4th. The solenoid duty cycle can be lower for the higher gears. That way you can have higher boost on lower gears.


Keep in mind mass flow and boost are NOT he same thing. One is an indicator of how much fuel you need to add, the other is how much CAN'T get into the cylinder.

What I find odd while looking at my powerlogger logs, is that when major knock starts up, the chip doesn't slam the solenoid shut.
 
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