Hey Guys. As many of you know I have been playing around with
turbocharging the NA 10.5:1 CR engine for a few years now. I hit a new
milestone this morning hitting 13.1 psi on a nice cool morning.From Nistune: Air/Fuel Ratio: 11.5ish
EGTs: 862 Celcius
Injector Duty Cycle: 54 percent (Nismo 740s) So this has been an interesting week and I have learned some
interesting stuff that might shed some insight as to past failures with the
NATT configuration. Here are some thoughts: 1) At 10 psi, the NATT is using the last (rightmost) column of the fuel
map. Once you exceed 10 psi you are off the map and simply using the
last cells to try to maintain AFR. (This is really bad for several reasons -
read on) 2) At 12 psi, the injector duty cycle of the 740 cc injector is at 50
percent. So, extrapolating, 370 cc injectors would be a) off
the fuel maps as described above, and b) running flat out.
Definitely leaning out = bad. 3) Given that the GT28RS turbos are a more efficient ball bearing turbo
than the stock turbo (in this particular application) I assume that the
EGTs are likely higher with the stock turbos. That, combined with
leaning out = bad. 4) At 13 psi, the injector duty cycle of the 740 cc injector rises to 54
percent to maintain a constant AFR. This reaffirms point 1) above, that
even if the 370 cc injectors did not max out, the fuel map needs to be
expanded to exceed the 10 psi level. The last cell on the stock map will
not hold the correct AFR once 10 psi exceeded. 5) The stock TT timing map has too much (too advanced) timing for this
setup. I haven't dyno tuned yet - so this is a guess - but I am currently
running NAish timing at the low end that is retarded even further through
boost onset and then lower than stock TT timing in the higher RPM and
load ranges. I won't be advancing the timing until I can get on a dyno
and closely monitor knock so we'll see where we end up. So the bottom line (as we already know empirically) is "just bolting up
some stock turbos and 370cc injectors and running a stock TT map" is a
bad idea at 10 psi and above. However, there seems to be room to push
beyond the 10 psi level to ???? with appropriate supporting mods and
tuning :) Oh yeah - Nistune FTW !!! (BTW: ConVerTT 1.0 - stock turbos, 370cc injectors, custom tune - still
runs great at 8 to 8.5 psi - I don't think that I will be cranking the boost
now that I have the data)
 
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