Gents, I'm pulling out my hair trying to understand something here, and I'm hoping someone can help. I've started looking at turbo upgrades, and therefore had to look up compressor maps. I believe I understand how they work, and I've followed several tutorials for plotting on maps to determine suitability. Generally, you have a HP goal in mind, and you calculate the Mass Air Flow required to support it. Then, use your engine's displacement and desired RPM to calculate the required boost. Hopefully that point lands in a good efficiency "island." If it doesn't, pick a different turbo, or change your HP or RPM goals.
It seems reasonable for performance applications, as everyone wants mo' powa, and picks points on the high right of the graph. But, when I try to plot factory engine ratings on factory turbo maps, even at full tilt, the points end up way low left, not even beginning to reach the high-efficiency islands. It seems like such a poor match, that the turbo is barely even spinning even at full output. This doesn't line up with other folks' boost claims on stock tunes. I assume I'm doing or assuming something wrong. The Garrett turbo mfg website has a tutorial for calculating this stuff, and I have attempted to follow it. Can anyone correct me?
1993 7.3 IDI Turbo was rated 190hp@3000rpm and 395ft@1400rpm.
At torque peak it produces (HP=TQ*RPM/5252=395*1400/5252=) 105 HP.
If the turbo were disconnected, and assuming 80% VE at that RPM, the motor naturally pumps:
(444cuin/1728/2*1400*0.8=144CFM >> 144CFM*0.0709lb/cuft@100°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.
Next the same deal, but at peak HP instead of TQ, and at this RPM the motor breathes worse.
If the turbo were disconnected, and assuming 70% VE at that RPM, the motor naturally pumps:
(444cuin/1728/2*3000*0.7=270CFM >> 270CFM*0.0709lb/cuft@100°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.
So at max torque, which is supposed to be the engine's most efficient, with the pedal to the floor, the turbo is barely spinning, and at max inefficiency. And even at max hp, full tilt, pedal to the floor, the turbo barely touches the high efficiency island. Wouldn't it make more sense to have the turbo at maximum efficiency in the midrange, dropping off on the high end as the motor does as well? To my idiot mind it seems like the factory turbo is way too big.
Same thing with the Power Stroke:
1994 7.3 turbo rated 425 lb-ft (162HP) @ 2000 RPM and 210 HP @ 3000 RPM.
Assuming 80% VE N/A at peak TQ RPM, the motor naturally pumps:
(444cuin/1728/2*2000*0.8=206CFM >> 206CFM*0.0709lb/cuft@100°F=) 14.6 lb/min MAF.
Garrett's suggested 18:1 AFR and 0.36 BSFC for 162 HP give:
(MAF=HP*AFR*BSFC/60 >> 162*18*0.36/60=) 17.5 lb/min MAF.
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.
Assuming 70% VE at peak HP RPM, the motor naturally pumps:
(444cuin/1728/2*3000*0.7=270CFM >> 270CFM*0.0709lb/cuft@100°F=) 19.13 lb/min MAF
Garrett's suggested 18:1 AFR and 0.36 BSFC for 210 HP give:
(MAF=HP*AFR*BSFC/60 >> 210*18*0.36/60=) 22.68 lb/min MAF.
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.
Really? Huh. At all of these I figured the boost pressure would be more. Then again, for a given HP, MAF (boost) and AFR balance each other. 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
Well now I'm just confusing myself more. Hmm. Plus, the turbos here don'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.
It's sort of a chicken-or-egg thing, I suppose. Anyway, I'm still confused why the air requirements don't seem to match the turbo map.
Any time I try to understand something it becomes a quest, and the rabbit holes never end. Sigh. I did not sleep at all last night, and I'm wasting all my free time (and work time... oops) because it's bugging me. Can someone please tell me where I've gone wrong???
It seems reasonable for performance applications, as everyone wants mo' powa, and picks points on the high right of the graph. But, when I try to plot factory engine ratings on factory turbo maps, even at full tilt, the points end up way low left, not even beginning to reach the high-efficiency islands. It seems like such a poor match, that the turbo is barely even spinning even at full output. This doesn't line up with other folks' boost claims on stock tunes. I assume I'm doing or assuming something wrong. The Garrett turbo mfg website has a tutorial for calculating this stuff, and I have attempted to follow it. Can anyone correct me?
1993 7.3 IDI Turbo was rated 190hp@3000rpm and 395ft@1400rpm.
At torque peak it produces (HP=TQ*RPM/5252=395*1400/5252=) 105 HP.
If the turbo were disconnected, and assuming 80% VE at that RPM, the motor naturally pumps:
(444cuin/1728/2*1400*0.8=144CFM >> 144CFM*0.0709lb/cuft@100°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.
Next the same deal, but at peak HP instead of TQ, and at this RPM the motor breathes worse.
If the turbo were disconnected, and assuming 70% VE at that RPM, the motor naturally pumps:
(444cuin/1728/2*3000*0.7=270CFM >> 270CFM*0.0709lb/cuft@100°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.
So at max torque, which is supposed to be the engine's most efficient, with the pedal to the floor, the turbo is barely spinning, and at max inefficiency. And even at max hp, full tilt, pedal to the floor, the turbo barely touches the high efficiency island. Wouldn't it make more sense to have the turbo at maximum efficiency in the midrange, dropping off on the high end as the motor does as well? To my idiot mind it seems like the factory turbo is way too big.
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Same thing with the Power Stroke:
1994 7.3 turbo rated 425 lb-ft (162HP) @ 2000 RPM and 210 HP @ 3000 RPM.
Assuming 80% VE N/A at peak TQ RPM, the motor naturally pumps:
(444cuin/1728/2*2000*0.8=206CFM >> 206CFM*0.0709lb/cuft@100°F=) 14.6 lb/min MAF.
Garrett's suggested 18:1 AFR and 0.36 BSFC for 162 HP give:
(MAF=HP*AFR*BSFC/60 >> 162*18*0.36/60=) 17.5 lb/min MAF.
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.
Assuming 70% VE at peak HP RPM, the motor naturally pumps:
(444cuin/1728/2*3000*0.7=270CFM >> 270CFM*0.0709lb/cuft@100°F=) 19.13 lb/min MAF
Garrett's suggested 18:1 AFR and 0.36 BSFC for 210 HP give:
(MAF=HP*AFR*BSFC/60 >> 210*18*0.36/60=) 22.68 lb/min MAF.
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.
You must be registered for see images attach
Really? Huh. At all of these I figured the boost pressure would be more. Then again, for a given HP, MAF (boost) and AFR balance each other. 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
Well now I'm just confusing myself more. Hmm. Plus, the turbos here don'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.
It's sort of a chicken-or-egg thing, I suppose. Anyway, I'm still confused why the air requirements don't seem to match the turbo map.
Any time I try to understand something it becomes a quest, and the rabbit holes never end. Sigh. I did not sleep at all last night, and I'm wasting all my free time (and work time... oops) because it's bugging me. Can someone please tell me where I've gone wrong???
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