Wasn't aware that's normal for ht to actually request lower afr in higher rpm range. That would make more sense I guess.
Yep, HT and now some AP custom tuners have figured out that on direct injection turbocharged engines with little turbos (K04), you can get in trouble in a hurry by trying to keep boost high at high rpm, high load and lean out AFRs.
You can easily keep the turbo in a good efficiency range at 18-19 psi at lower rpm. But you need to taper that boost as rpm climbs. The little stock turbo just maxes out at anything over about 15-16 psi when pushed past 6,000 rpm. Yeah, you can continue to generate higher flow, but it is hot air. Flamethrower.
But, the benefits of rich AFR's under high load, high rpm conditions plays hand-in-glove with the tapered boost. The cylinder cooling benefit (by Mazda design) helps keep EGT's down and reduces the risk of detonaton. By doing this (taper boost down to say 14 and uber rich AFRs down in the 10's, you can do something else that produces huge power benefits at high rpm ---
very aggressive timing advance. I mean like
18 degrees of timing advance at 6,500 rpm.
There is no way you can get that aggressive with timing if you lean out the AFRs or if you try to keep boost high all across the rpm range. One solid way to gain power at high rpm, high load is timing advance, if you can avoid knock.
I'm not claiming that one approach is necessarily better than the other, but you cannot safely do both. If you are going to go lean, you have to be very conservative on timing. Either way, tapering boost on the stock turbo on pump gas is a necessity. Yes, high (18-19 psi initially), but then a taper. I sometimes see as little as 13 psi boost at redline, but I'm still making over 270 whp all the way up there, almost as much as my peak of about 300-310 with these mods.
I think there is great wisdom in a tuning approach for these cars that uses the rich AFR's for cylinder cooling, starts high on boost but tapers toward redline and cranks up the timing pretty darn aggressively on the top end.
