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Discussion Starter · #1 ·
I decided to look at the map for my 1996 F350 7.3L Powerstroke and it doesn't make sense. Even at full tilt my math shows the turbo barely spinning and not doing much. I had the same conclusion for the 1993 7.3 IDI turbo, the 2001 Silverado Duramax, and the 2001 Dodge Cummins. What am I doing wrong here???

1994 PS was 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/[email protected]°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.

Assuming 70% VE at peak HP 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 210 HP give:
(MAF=HP*AFR*BSFC/60 >> 210*18*0.36/60=) 22.68 lb/min MAF.

Slope Rectangle Font Line Parallel

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.

I must be doing something wrong. I'm wasting all my free time (and work time... oops) thinking about it because it's bugging me. Can someone please tell me where I've gone wrong???
 

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Yes factory turbo is too "big" because the injectors are too "small."

I'll have a detailed look later but i'm not sure you are considering the diesel motor efficiency factor vs fueling in real world scenarios.
 

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Discussion Starter · #3 ·
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.

Slope Font Triangle Line Rectangle
 

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I'm not sure you are accounting for the turbo ratio. There is no MAF sensor for these motors so it is not a factor to consider.

Stick with MAP, consider the exhaust gas expansion through the exhaust housing, and the worthless pancake downpipe these trucks came with.

I think you're over thinking 25+ year old technology scroll compressors but it is interesting seeing your calculations. The 14.7PSIA does not apply to the exhaust housing and also turbine variances between inlet and exhaust are to be considered.

Don't forget the rotating mass of the turbine wheel and efficiency loss there. Let me know what you come up with since exhaust gas pressure is not 1:1 with the cold side of the turbo.
 

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Discussion Starter · #5 ·
There may not be a sensor, but that is what the X-axis of those compressor maps show, in units of lbs/min. Therefore, in order to plot any point, you have to assume, know, or calculate, the mass air flow.

14.7 is applicable here because the plots, and my calculations, deal solely with intake. The plots show at what pressues the turbo can flow a certain amount of air, and make no comment on how the force required to spin the wheel is generated. In fact, if I cut the turbo in half and spun it with an electric motor, this plot would be exactly the same, and my conclusion would also be the same: the air requirement for 210hp is very low compared to the capabilities of this turbo in its efficiency range. It seems much too big, and a terrible choice for the application.

So either:
A) my math is right and everyone agrees FoMoCo's decision was inexplicably awful
B) my math is right but there was a very good reason they chose this turbo and I simply don't know it
C) my math is wrong and therefore so are my conclusions
 

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These motors last forever due to the injectors and turbo size and tuning.

There was some serious diesel wars in the 90s and this was Fords answer to the Cummins. Not too much thought went in to performance.
 

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Discussion Starter · #7 ·
I thought diesel was just getting warm in the 90s. I was a kid then, though, so I wouldn't know. Wasn't the F-series with a 6.9 the first mass-produced diesel pickup? My grandpa bought one, I think in 84, the first day they sold. That's how he tells it, anyway.

I don't suppose anyone can definitively say if my numbers are right or wrong?
 

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I don't suppose anyone can definitively say if my numbers are right or wrong?
I'm not saying the numbers are right or wrong, I just don't care to check them.

Yes Ford and Chevy were going at it but the "Powerstroke" name itself introduced in 94.5 was due to the soaring popularity of the Cummins. It still wasn't until the late 90's and early 2000's when the true power potential was discovered. Ford answered with the Superduty then along came the EPA.

Remember, the 7.3 is an International motor so Ford did their thing with it and made it user friendly so it could be a DD for average Joe but hold its own while hauling hay or cattle for the country folk. Computers weren't very advanced to monitor EGT's etc. so there had to be wiggle room in performance vs reliability.
 
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