Boost control questions

Pete_89T2 · 10-16-2011, 09:46 AM

Before asking my questions, here's some relevant background info. Car was basically a stock '89T2, that I restored to near factory newness over the years. Last winter I replaced my tired stock turbo with a BNR stage 1 rebuild, replaced all FIs with Bosch 720cc injectors, dropped in a ND Supra TT fuel pump, and shipped out the stock ECU for the Rtek7 2.x job so I could tweak fuel/spark maps & do datalogging. Exhaust system at that time was still a Bonez DP into the stock main cat, and into a Borla catback system (i.e., very similar to the RB stock replacement catback, duals w/2-1/2" piping). Then I had Dave at Speed1 tune it on his dyno, end result was about 209rwhp, which was less than expected, but we hit a brick wall since the car wouldn't make more than 5.5~6psi boost, which Dave attributed to a restrictive stock main catalytic converter combined with the now more effective ported wastegate (part of the BNR stage 1 turbo package).

This summer, I was able to get historic plates for the car, which does away with the MD sniff test. Time to make some exhaust mods & delete some emissions junk.... So I picked up the RB 2-1/2" downpipe & pre-silencer combo that is designed to replace the stock cat(s) and bolts up to my Borla catback. Also removed the air pump & ACV, added a dual belt alternator pulley. When Dave last tuned my Rtek, he basically richened all cells above 6psi MAP to ensure I wouldn't blow up my engine with this exhaust mod; plan is to go back to Dave soon, get the car on the dyno and optimize this setup.

Which brings me back to my questions on boost control. Since making the exhaust mods, I've been using the Rtek datalogging to characterize MAP vs RPM and a bunch of other parameters. The logs are telling me that I'm hitting a maximum boost of 7~8 psi, which happens to be where BNR presets the adjustable wastegate actuator on the Stage 1 turbo. Also, I still have the stock boost control solenoid wired in and plumbed into the system per the FSM diagrams. If I understand the FSM correctly, boost pressure from the high side of the compressor gets routed to the solenoid, which when energized by the ECU, routes boost pressure back to the suction side of the compressor, effectively shunting the wastegate actuator. That way if the solenoid fails, the wastegate will still see boost pressure and work as it should. Unfortunately, the Rtek v2.x does not allow you to tweak the maximum boost set point that is hard coded into the stock S5 N370 ECU.

I like the way the stock system & BNR modified wastegate as currently configured controls boost - it comes on quick & linear, then hits its control point and doesn't spike/creep/overboost beyond that. Don't want to loose those characteristics. What I'd like to be able to do is stretch that control point up to 9~10psi, before investing in some more dyno time with Dave.

What are my options for this, and I'm thinking simple is best? First thing I thought of was trying to play with the WG actuator adustment rod to see where that gets me; if that doesn't cut it, next option might be one of those simple manual boost control valves. If I understand these correctly, these are simply ball valves that are spring loaded to open above an adjustable psi setting? So then if I plumbed one of these suckers inline (series) with the WG actuator, the WG would only start to open whenever you're above the set boost threshold? Any other solutions, thoughts or recommended products to consider here?

RETed · 10-16-2011, 01:44 PM

An electronic boost controller (EBC) is best, especially when it's closed-loop with it's own dedicated MAP sensor.
Of course, this will tend to be the most expensive option.

A manual boost controller (MBC) does allow increases in boost, but this preset level changes with ambient temperature, humidity, etc.
The EBC will always hit it's preset boost level no matter what the ambient conditions are.

Adjustable wastegate actuator rods are a waste of time, as this only changes how fast (or slow) the boost ramp hits when the wastegate is triggered.

-Ted

Pete_89T2 · 10-16-2011, 09:30 PM

Thanks Ted, I sort of figured tweaking the WG actuator rod wouldn't buy me much. I understand a closed loop EBC would yield the best and most consistent results, but how much a difference are we talking when compared to a high-quality MBC in practice? Since my FC is generally a fair weather car, it generally operates over a predicable range of ambient temps, so shouldn't it be possible to tune an MBC for a median set point and call it "good enough"?

Too bad the Rtek folks didn't include a hack of the stock S5 boost control firmware to allow some adjustability, that would have been the best solution.

10-16-2011, 09:46 AM	#1
Pete_89T2 Lifetime Rotorhead Join Date: Apr 2010 Location: Elkton, MD iTrader: (1) Posts: 874 Rep Power: 15	Boost control questions Before asking my questions, here's some relevant background info. Car was basically a stock '89T2, that I restored to near factory newness over the years. Last winter I replaced my tired stock turbo with a BNR stage 1 rebuild, replaced all FIs with Bosch 720cc injectors, dropped in a ND Supra TT fuel pump, and shipped out the stock ECU for the Rtek7 2.x job so I could tweak fuel/spark maps & do datalogging. Exhaust system at that time was still a Bonez DP into the stock main cat, and into a Borla catback system (i.e., very similar to the RB stock replacement catback, duals w/2-1/2" piping). Then I had Dave at Speed1 tune it on his dyno, end result was about 209rwhp, which was less than expected, but we hit a brick wall since the car wouldn't make more than 5.5~6psi boost, which Dave attributed to a restrictive stock main catalytic converter combined with the now more effective ported wastegate (part of the BNR stage 1 turbo package). This summer, I was able to get historic plates for the car, which does away with the MD sniff test. Time to make some exhaust mods & delete some emissions junk.... So I picked up the RB 2-1/2" downpipe & pre-silencer combo that is designed to replace the stock cat(s) and bolts up to my Borla catback. Also removed the air pump & ACV, added a dual belt alternator pulley. When Dave last tuned my Rtek, he basically richened all cells above 6psi MAP to ensure I wouldn't blow up my engine with this exhaust mod; plan is to go back to Dave soon, get the car on the dyno and optimize this setup. Which brings me back to my questions on boost control. Since making the exhaust mods, I've been using the Rtek datalogging to characterize MAP vs RPM and a bunch of other parameters. The logs are telling me that I'm hitting a maximum boost of 7~8 psi, which happens to be where BNR presets the adjustable wastegate actuator on the Stage 1 turbo. Also, I still have the stock boost control solenoid wired in and plumbed into the system per the FSM diagrams. If I understand the FSM correctly, boost pressure from the high side of the compressor gets routed to the solenoid, which when energized by the ECU, routes boost pressure back to the suction side of the compressor, effectively shunting the wastegate actuator. That way if the solenoid fails, the wastegate will still see boost pressure and work as it should. Unfortunately, the Rtek v2.x does not allow you to tweak the maximum boost set point that is hard coded into the stock S5 N370 ECU. I like the way the stock system & BNR modified wastegate as currently configured controls boost - it comes on quick & linear, then hits its control point and doesn't spike/creep/overboost beyond that. Don't want to loose those characteristics. What I'd like to be able to do is stretch that control point up to 9~10psi, before investing in some more dyno time with Dave. What are my options for this, and I'm thinking simple is best? First thing I thought of was trying to play with the WG actuator adustment rod to see where that gets me; if that doesn't cut it, next option might be one of those simple manual boost control valves. If I understand these correctly, these are simply ball valves that are spring loaded to open above an adjustable psi setting? So then if I plumbed one of these suckers inline (series) with the WG actuator, the WG would only start to open whenever you're above the set boost threshold? Any other solutions, thoughts or recommended products to consider here?

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10-16-2011, 09:30 PM	#3
Pete_89T2 Lifetime Rotorhead Join Date: Apr 2010 Location: Elkton, MD iTrader: (1) Posts: 874 Rep Power: 15	Thanks Ted, I sort of figured tweaking the WG actuator rod wouldn't buy me much. I understand a closed loop EBC would yield the best and most consistent results, but how much a difference are we talking when compared to a high-quality MBC in practice? Since my FC is generally a fair weather car, it generally operates over a predicable range of ambient temps, so shouldn't it be possible to tune an MBC for a median set point and call it "good enough"? Too bad the Rtek folks didn't include a hack of the stock S5 boost control firmware to allow some adjustability, that would have been the best solution.