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I originally posted this and got no responses in the engine performance column. Since posting this I have also read that boost controllers can cause your catalytic converter to break apart and destroy your turbo. Is this true? There isn't much saying adding a boost controller is risky, and if it is true, could you get away from that problem with a catless downpipe?

If anyone has read my post about premium vs. regular they will know where I am coming from when I ask;
Can you make the STi slower/more fuel efficient with a dual stage boost controller? I went home on leave a couple of days ago and took the old man out in a STi and he was "scared of it" due to traffic, cops, and his urge to haul ass, plus he would prefer to stay in the upper range of the cars gas mileage to buy as little premium gas as possible.
If I set one boost level lower than factory, and one higher, is it possible to have the best of both worlds and let the old man be a miser while driving my car? If it would work, I would just set it as low as possible anytime anyone wanted to drive it!
 

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Theoretically, what your are asking for is possible, although my recommendation is to do it with some form of engine tuning. Messing with the STi's boost without some form of air/fuel managment (other than stock ECU) can be a bit tricky.

Based on what I know of how the ECU responds to boost modification (manual or electric) I'm going to go out on a limb hear and say that your fuel mileage won't be affected by a lower boost setting using a manual contoller. The ECU will probably just keep pumping in the same amount of fuel it think you should be getting for the flow of air it is receiving, probably resulting in a rich AF ration. With a boost controller set to a higher than stock level, the ECU tends to retard timing to compensate for the lean condition it sees. I'm wary of this as detonation is merely a form of slow death for our engines.

I'd give TurboXS or Godspeed a call and talk to them. From what I remember, Cobb also provides some sort of "valet" mode in one of there reflash stages, so you might want to speak to them too.

Good luck, and let us know how you end up addressing this.
 

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Its also kind of hard for a cat that is after the turbo to destroy the turbo. Where ever you heard that from was talking about on a wrx the cat in the uppipe can break up from the increased egts that you could get from running lean.
 

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Discussion Starter #4
Re

Thanks for the info. I didn't think it was the same with the STi as the regular WRX but since I am new to the turbo world, I figured I would ask.
 

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Well hear is what I have experienced with my STi and I have a bit of experience with tuning/wideband watching with my old 02 WRX as well.

I really don't know about turning the boost down much what will happen. I know the ECU does have the load tables for lower boost levels but I have never tried it to monitor it. I DO know that in stock from up here at 6000ft asl the af on a friends STi was a nice 11.5 at peak torque going down to 11.0:1 at redline. Then an TD05H-18G was added and rerun on the dyno and the afs dropped to 9.5-10.0:1 across the board after 4500rpm. This was with about 3psi more than stock across the board. This tells me the stock ECU was seeing more air coming in and was in a higher load cell in the fuel map = more fuel in that load cell = richer.

I have personally tried a 18G and now a 20G on the STi with stock ECU and fuel system. The TD05H-18G would draw in more air across the MAF sensor (as recorded by delta dash - 265g/sec at redline @ 16psi) and I would run pretty rich 10.4-10.7:1 across the board at about 20psi peak down to 16psi at redline (this would be like 16.5psi peak down to 12.5psi at sea level since we are high alititude - subtract about 1psi for every 2000ft you go up for atmoshperic pressure losses).

Then I added the TD06H-20G and the a/f went even lower at a slightly higher boost level (peak of 22psi down to 18.5psi at redline). With that boost curve the a/f is around 10.3-10.4:1 until about 6000rpm where the af quickly leans out to 11.6-11.8:1 range by redline. Meaning the fuel system is finally getting maxed out up here with the 20G. A fuel pump upgrade is in the future to see to help with this. But even with leaning out that much at redline no knock has been heard or detected with a knocklink.

Sorry to ramble above but my point is the stock ECU has a bit of mapping for timing and fuel across a fairly wide range of MAF loads so I would personally bet that if you turn the boost down at the WORST you would maintain the same af as you have at normal boost and not go any richer. But again I have yet to try this so I cannot confirm. THe only potential issue you would have would be some learning detonation when you go from low -> high boost as described next...

I also had a friend who tried what you are talking about on his 02 WRX Wagon. He would turn the boost down to wastegate level (~7-8psi) for good mileage and the car ran fine (didn't know what the af was though). The only downside to that was when he turned the boost up to 14psi he got some hesitation the first couple of WOT runs which was probably some detonation brought on by high timing multipliers from running low boost. But after 1-2 WOT runs the car readjusted those and the hesitation went away. Meaning the ECU probably ramped the timing WAY up since it wasn't getting any knock (from running low boost) and then when he did turn the boost up the ECU had to compensate by seeing a few det events. For comparison I personally vary my boost levels on the 20G from a daily driven 18psi peak to 22psi peak and I see no detonation events switching between those two boost levels, but this is only a 4psi/22% swing instead of 7psi/100% swing in boost.
 

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Nice details in a real life situation David. Can you provide some details on your data logger and knocklink? What kind, etc.? I'm interested in the knocklink and what kind of specs it provides.

BTW..did your friend with WRX who turned his boost down to essentially wastegate pressure improve his gas mileage by doing so? And I didn't see it, but are either one of you using a piggyback or reflash?

Thanks,
Scott
 

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YBNormal07 said:
Nice details in a real life situation David. Can you provide some details on your data logger and knocklink? What kind, etc.? I'm interested in the knocklink and what kind of specs it provides.

BTW..did your friend with WRX who turned his boost down to essentially wastegate pressure improve his gas mileage by doing so? And I didn't see it, but are either one of you using a piggyback or reflash?

Thanks,
Scott
data logger = http://www.ecutek.com/products/deltadash/
knocklink = http://www.link-electro.co.nz/knocklink.html

Honestly I don't know if my friend noticed improved mileage. I think you would but that is also related to how many times you go WOT. If you are on a road trip/cruise sitaution I would think your mileage would be slightly impacted if at all. Around town you might see more of a difference.

I am not using any reflash/piggyback currently on my stock STi ECU. The reason for this is I am simply trying to compensate for atmospheric conditions to get my car to sea level stock STi power levels.
 
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