Wastegate mod.....

[avos at cochlear.com.au]

     Simon, Robert,
     
     In my experience, Electronic boost controllers (EBCs) can considerably 
     improve the boost curve of a car, but my experience revolves around 
     cars do not have the wastgate directly connected to boost pressure, 
     but instead have a solenoid in the line controlled by the ECU. Many 
     Japanese cars have this setup. The boost is controlled by the ECU 
     outputting a PWM waveform to the solenoid. Most ECUs that have this 
     setup do not have closed loop control, but instead calculate the pulse 
     width from the revs and throttle position. This setup means that if 
     exhaust/induction mods are made, the boost curve can change due to the 
     control system being open-loop. Also, atmospheric conditions can alter 
     the boost curve. Also most manufacturers will attempt to make the 
     power curve of the car come on smoothly for a 'refined feel', which is 
     not always optimal for the performance enthusiast who wants as much 
     power as possible at any operating point. The EBC improves the bottom 
     end boost of such cars, and also provides extremely accurate closed 
     loop control of an adjustable boost level. I admit that many cars with 
     an EBC can feel very peaky as torque can come on extremely quickly, 
     but this feels great, and as long as you stay in boost revs, it is 
     quite responsive.
     
     For cars with the boost connected directly to the solenoid, EBC can 
     offer a few advantages- if the wastegate is not adjustable, it may be 
     the only means to alter the standard boost level, and a rise in boost 
     will give a good increase in power, and an EBC provides adjustable 
     boost, so it can be altered for various reasons from within the cabin 
     of the car (most of the time). 
     
     As far as what Robert mentioned about simple wastegate control 
     operating over a narrow boost range, well this may be the case, and 
     the boost curve may be close to optimal, I don't know. This is why it 
     would be good to see what the results of Simons project are. I am 
     interested to see if an increase in performance can be achieved 
     without adjusting the boost level, but instead just stopping any boost 
     getting to the wastegate at low boost levels.
     
     Simons case is unique case as he wants to optimise the boost curve 
     without adjusting the maximum boost level. Most people who purchase 
     EBCs have the main aim of achieving higher boost, and the secondary 
     aims are to have adjustable boost from within the cabin and to have 
     the boost curve optimised. If you only want to optimise the boost 
     curve and forget the other advantages, perhaps an EBC is not the most 
     economical method, and as Robert said, perhaps there is no point in 
     changing the standard setup.
     
     Regards,
     
     Adrian


______________________________ Reply Separator _________________________________
Subject: Re: Re[2]: Wastegate mod.....
Author:  diy_efi at coulomb.eng.ohio-state.edu at INTERNET
Date:    13/10/97 17:59


At 04:22 PM 10/13/97 +1300, you wrote: 
>Hi All
>
>>      Unfortunately its not (quite) a perfect world, 
>
>;-) too true
>
>>      and if you do not let 
>>      the pressure get to the wastegate a few psi before the set boost 
>>      level, you will find that boost may fluctuate (oscillate), 
>
>This I know, so just to make sure I have this right your micro device is 
going to 
>do the same my compariter device, which is only open the wastegate at the last 
>possible moment to achieve a stable boost set point.
>
>The 2 promo's of the boost controles I saw weren't very clear all they talked 
>about was programmed boost control to achieve more power. All I'm trying to 
find 
>out is is the boost controlled just doing what I want to do? 
>
>Cheers
>
>Simon
     
 Adrian and Simon
     
I have been following your discussion and am confused. I understand that all 
systems that use feedback may oscillate but why is it necessary to use an 
electronic device to control the waste gate? Don't waste gates work by 
simply applying boost pressure against a diaphragm to open a valve against 
spring pressure? The greater the spring pressure the greater the amount of 
boost required to unseat the valve and vent a portion of the exhaust, 
thereby reducing the pressure driving the turbo?
     
As I understand it, when the force of the exhaust on the waste gate valve 
plus the force of the boost pressure on the diaphragm equals the spring 
force, there will be no seat pressure and a slight amount of exhaust will 
leak past the waste gate valve. Additional boost pressure will compress the 
spring lifting the valve off its seat. This will vent some of the exhaust 
gases, reducing the amount of exhaust available to drive the turbo. 
Depending upon the spring rate and the area of the diaphragm, an increase in 
boost pressure of 1 psi could lift the valve 1/4 inch off its seat. (4 inch 
diameter diaphragm and 50 lb/in spring rate) A 1 1/2 inch diameter valve 1/4 
inch off its seat would vent quite a bit of exhaust. Thus, it would appear 
that normal waste gates would control boost pressures over a fairly narrow 
range and that until the boost pressure limit was reached all exhaust would 
be forced to flow through the turbo. Am I missing something?
     
Robert