At peak TQ the MAF ratio is (17.5/14.6=) 1.199.

The absolute pressure ratio is the same amount, which assuming 14.7 ambient gives (1.199=[?+14.7]/14.7 >> 1.199*14.7-14.7=) 2.9 psi.

At peak HP the MAF ratio is (22.68/19.13=) 1.186.

The absolute pressure ratio is the same amount, which assuming 14.7 ambient gives (1.186=[?+14.7]/14.7 >> 1.186*14.7-14.7=) 2.7 psi.

Neither of these seem to match folks' reported boost numbers, even on stock tunes. I don't have one in my truck, so I can't verify. At all of these I figured the boost pressure would be more. Then again, since you seemed to be hinting at AFR: as a general rule I've read 16- for max power, 18-19 for normal, 20+ for low EGT towing. So, for a sanity check, let's plot several different AFR:

HP | AFR | MAF | MAFR | PSI

162 | 14:1 | 13.608 | 0.9337 | -0.9742

162 | 16:1 | 15.552 | 1.0671 | 0.9866

162 | 18:1 | 17.496 | 1.2005 | 2.9474

162 | 20:1 | 19.440 | 1.3339 | 4.9082

162 | 22:1 | 21.384 | 1.4673 | 6.8690

162 | 24:1 | 23.328 | 1.6007 | 8.8299

HP | AFR | MAF | MAFR | PSI

210 | 14:1 | 17.640 | 0.9222 | -1.1437

210 | 16:1 | 20.160 | 1.0539 | 0.7929

210 | 18:1 | 22.680 | 1.1857 | 2.7295

210 | 20:1 | 25.200 | 1.3174 | 4.6661

210 | 22:1 | 27.720 | 1.4492 | 6.6028

210 | 24:1 | 30.240 | 1.5809 | 8.5394

If this is accurate, the turbo only hits max efficiency at 3000 RPM, with full throttle, and leaned way out to 24:1. I don't know if that is realistic, although I don't any experience in tuning.

Notably, the turbo here doesn't have wastegates or vanes, and aren't externally controlled at all. Its output is simply a function of the engine exhaust, engine draw, and its turbine and compressor characteristics. Perhaps the way to look at it is that, for a given RPM a diesel engine's power output is predominantly dependent on the timing and amount of fuel injected, which is ultimately the only thing the system "decides." The turbo speed changes in response to the laws of physics, with the dominant factors being its design characteristics, and the engine's exhaust flow. The resultant balance can be quantified by calculating the AFR, and there happen to be side effects to different balances of AF, the primary positive one being lower EGTs with higher AFR.

For kicks, the same thing with our predecessor:

1993 7.3 IDI Turbo was rated

[email protected] and

[email protected]
At torque peak it produces (HP=TQ*RPM/5252=395*1400/5252=) 105 HP.

If the turbo were disconnected, and assuming 80% VE at 1400 RPM, the motor naturally pumps:

(444cuin/1728/2*1400*0.8=144CFM >> 144CFM*0.0709lb/

[email protected]°F=) 10.21 lb/min MAF

Garrett's suggested 18:1 AFR and 0.36 BSFC for 105 HP give:

(MAF=HP*AFR*BSFC/60 >> 105*18*0.36/60=) 11.34 lb/min MAF.

Let's also try to calculate boost. At peak TQ the MAF ratio is (11.34/10.21=) 1.111.

This is achieved by an absolute pressure ratio of the same amount, which assuming 14.7 ambient gives (1.111=[?+14.7]/14.7 >> 1.111*14.7-14.7=) 1.6 psi.

If the turbo were disconnected, and assuming 70% VE at 3000 RPM, the motor naturally pumps:

(444cuin/1728/2*3000*0.7=270CFM >> 270CFM*0.0709lb/

[email protected]°F=) 19.13 lb/min MAF

Garrett's suggested 18:1 AFR and 0.36 BSFC for 190 HP give:

(MAF=HP*AFR*BSFC/60 >> 190*18*0.36/60=) 20.52 lb/min MAF.

At peak HP the MAF ratio is (20.52/19.13=) 1.073.

Thich is achieved by an absolute pressure ratio of the same amount, which assuming 14.7 ambient gives (1.073=[?+14.7]/14.7 >> 1.073*14.7-14.7=) 1.1 psi.