ive tried 3 different types of boost controllers: bleed valve, ball and spring valve, and an EBC
BLEED:
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by far the crappiest boost controller ive used, was the bleed valve. around 2 years ago when i hooked one of these up, they were fetching around $80 for the turbosmart ones. they simply bleed off boost from the vacuum line to the wastegate through an adjustable orifice and thats how you got your boost increase.
-boost buildup/wastegate creep was pretty average because the wastegate saw SOME boost at all times.
-top end boost control was no good, it tapered off, and trying to bleed more air just made boost spike and overboost in the midrange.
-at the time, it was relatively cheap at $80 for boost control
-simple installation
BALL & SPRING:
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a relatively new form of boost control, i picked up my one for ~$30 off ebay, from some guy in australia. i first came across this reading up a post about someone who purchased one and had success on the SAU forums. works on the same principle as the bleed valve by bleeding off air in the wastegates vacuum line to fool the wastegate and increase boost. the difference is that its NOT always bleeding off air like the bleed valve, but only after a user adjustable boost pressure, which is the point where the ball in the valve lifts off its seat and allows air to pass through to the wastegate. this gives much better boost response, as wastegate creep is greatly reduced.
-extremely cheap @ $30
-better then any bleed valves out there, including all brand names ones such as turbosmart
-simple install
-simple adjustment with a screw fitting to adjust the springs compression/pressure.
-still got some top end taper but not as bad as the bleed valve
-decent wastegate creep control, alot better then the bleed valve
IEBC - EBC:
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this is what im currently running at the moment, and i find it to be an extremely good form of boost control at a very good price. ive done a write up on it before, for those who havent seen it, heres the link:
http://www.silviansw.com/forum/viewtopic.php?t=18327
but i'll give a quick run down on here as well. below is a pic of how its all hooked up.
again the same concept of reducing what the wastegate sees by means of bleeding off air in the vacuum line to the wastegate is how this works. it has the same ability as the ball and needle valve, where a user adjustable point can be set where it actually starts allowing air to pass through to the wastegate actuator, though now its an electronic form of control which is done via the standard boost control solenoid.
basicly the solenoid stops any air from reaching the actuator until you want it to, and the point it does this is based upon a selected duty cycle of the injectors. the duty cycle of the injectors (0-100%) is represented by 64 points on the boost controller ( 0% = point 1 & 100% = point 64), and the value of each load point can be 0-100%, which gives us a 2D map - an x and y axis, one of load points 0-64, and the other the points value between 0-100%.
this 2D map is where the user tunes his boost curve. the amount of pressure the actuator sees, is based up on the duty cycle of the solenoid, or the value of the load point which you have set - this determines how much air is being allowed past the solenoid to the actuator. there is also a bleed post solenoid that lets air out so that its not trapped in there, but it also has a restrictor in it, so that it allows only a tiny bit of air out. in the section of the hose post solenoid, it becomes pressurised only when the amount of air being let through the solenoid is more then what the restrictor is capable of flowing. now by varying the duty cycle of the solenoid and hence the amount of air passing through, you can vary the pressure at the actuator, and from that you can control your boost.
rather then setting a target boost pressure, and adjusting 'gain' settings which control wastegate creep, you have full control over the wastegates operation over 64 points, which are referenced to the duty cycle of your injectors. and thats the best i can explain it, if its not enough, read the construction book
-great boost control
-doesnt taper in top end as you can reduce pressure at wastegate up high to to make more boost
-good value @ ~$150
-must be built, prebuilt ones are more expensive
-can be fiddly, and you really do need to have a good grasp of the concepts used to make full use of it - recommended you get the electronics book for it ~$20
-restrictor sizing is dependant on setup ie wastegate spring has an influence, as does the solenoid used etc.
-doesnt have boost control feedback correction
-can run/store 2 completely different maps
-no wastegate creep at all, due to the ability to precisely hold the wastegate closed to a certain point.
-can very the boost pressure over the whole range, rather then only have a set target ie run more boost in the mid range then in the top end.
-need to size the restrictor so that you attain max boost @ 70-80% duty cycle of the solenoid for good accuracy.