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Was thinking about this again today and realised there is one major factor that I missed that makes restricting the APV method preferable to reducing boost.
*When the APV opens you get a sudden drop in flow followed by a rapid and large increase in flow. You don't actually see this in the g/s log because the air is actually filling up the APV runners so in that instant it's not flowing through the engine. This results in a rich then lean spike that is extremely difficult (near on impossible with a stock ecu) to tune out in every gear. This is a dangerous condition and restricting the APV reduces the danger............... a lot.
Chart below shows what happens to EMAP. Can see APV is opening when blue line goes berserk , this this exhaust backpressure with no damper on the line measuring pressure. First you see a reduction in EMAP as apv opens and air rushes in to fill the void . Then you see the increase in EMAP as the turbo has to catch up . That increase in emap combined with the lean spike is particularly dangerous at elevated boost.
I'm planning on taking this idea further with my 'built for boost ' engine yet to be installed. This will involve increasing boost to around 15-16 psi while restricting apv and dumping boost past 7500rpm . The hope is to make a flat torque curve over 300lbft tapering down past 7500 to allow safety at higher rpms and some gearshift leeway.
The car is already a roll race weapon and with these changes I aim to upset even more doubters at the track.
I'd not worry about the sudden APV induced airflow change.
I've found that the opening of the SSV to be much more troublesome... the APV was a breeze. Then again, I tune my maps down to an individual cell in the VE table so that there are no lean pockets. Rich ones I can tolerate, as long as I don't feel it bogging or spitting unburnt oil out the rear bumper.
It's worst in 1st and 2nd. Whoever made this ECU on this rather lackluster hardware knew damn well how fast will RPMs change and as such they figured that they are covered for an NA engine.
Add boost in 1st gear and that ramp-up rate doubles. Fast RPM changes are difficult for this antique MCU powering the ECU; so to cope with that I've toned down boost to spring pressure for speeds under 70 kmh. Not a big deal for me as I'm daily-ing this thing across rain and snow. Also, flooring it in 1st and 2nd is rarely a use-case for me.
Well .... you can see the benefit of restricting the APV then. Not worth doing in your case, but with 100 more whp doing multiple track days/roll racing etc, it's definitely an improvement to a setup that was never designed to run boost.
07-18-2023, 05:37 PM
