Physics: Boost pressure vs displacement vs HP
10-04-2016, 11:26 PM
#16
Focusing on the turbine wheel, which is driven by exhaust gases (hot side)... I hope I understand this correctly:
Think of two steering wheels - one with a small diameter and the other with a larger diameter. The smaller steering wheel feels very responsive at speed due to the smaller diameter - less input to get the desired response. The larger steering wheel takes longer to make a full revolution, but is easier to turn when facing resistance. I suspect this shorter revolution of a small steering wheel is analogous to the smaller turbine wheel spooling faster than a larger one. A larger diameter wheel has to travel more distance for a full revolution than a smaller one.
On the other hand, the larger steering wheel has more leverage against resistance. You can exert less force on it to get more work - think of having no power steering and which steering wheel would be easier to turn against the resistance of the pavement. The big steering wheel. This is ability to overcome resistance easier than the smaller wheel is probably analogous to be able to sustain higher boost pressure longer.
With a turbo, as the exhaust turns that turbine wheel (our steering wheel), we are building increasing pressure (resistance) as the compressor wheel opposite the turbine wheel pulls air in and builds boost. The turbine wheel sizes are about leverage against resistance. The smaller wheel will get harder to turn sooner than the larger wheel against increasing resistance. Think of our example with no power steering sitting in the driveway. For the same resistance (the driveway), we can turn turn the steering wheel easier with the larger diameter.
Now imagine if the resistance provided by the driveway to the tires kept increasing. Our ability to turn the smaller steering wheel soon becomes futile and we're killing ourselves trying to turn it. We are not in the efficiency range of the small steering wheel anymore. We can keep turning our large wheel as resistance increases until we are out of its efficiency range.
Once we're past the efficiency range for the smaller wheel, the exhaust gas pressure begins to raise because the wheel is no longer able to turn quickly enough, which is the reversion discussed, where pressure mounts on the exhaust side which is greater than the intake side. And as we build that undersirable pressure on the exaust side, our airflow throughout the whole system is impacted.
I'm not sure how to bring it all back in, but the whole turbo calculation is tricky. I think there is probably too much focus on the compressor wheel and not enough on the relationship between the two wheels and the desired amount of boost.
Think of two steering wheels - one with a small diameter and the other with a larger diameter. The smaller steering wheel feels very responsive at speed due to the smaller diameter - less input to get the desired response. The larger steering wheel takes longer to make a full revolution, but is easier to turn when facing resistance. I suspect this shorter revolution of a small steering wheel is analogous to the smaller turbine wheel spooling faster than a larger one. A larger diameter wheel has to travel more distance for a full revolution than a smaller one.
On the other hand, the larger steering wheel has more leverage against resistance. You can exert less force on it to get more work - think of having no power steering and which steering wheel would be easier to turn against the resistance of the pavement. The big steering wheel. This is ability to overcome resistance easier than the smaller wheel is probably analogous to be able to sustain higher boost pressure longer.
With a turbo, as the exhaust turns that turbine wheel (our steering wheel), we are building increasing pressure (resistance) as the compressor wheel opposite the turbine wheel pulls air in and builds boost. The turbine wheel sizes are about leverage against resistance. The smaller wheel will get harder to turn sooner than the larger wheel against increasing resistance. Think of our example with no power steering sitting in the driveway. For the same resistance (the driveway), we can turn turn the steering wheel easier with the larger diameter.
Now imagine if the resistance provided by the driveway to the tires kept increasing. Our ability to turn the smaller steering wheel soon becomes futile and we're killing ourselves trying to turn it. We are not in the efficiency range of the small steering wheel anymore. We can keep turning our large wheel as resistance increases until we are out of its efficiency range.
Once we're past the efficiency range for the smaller wheel, the exhaust gas pressure begins to raise because the wheel is no longer able to turn quickly enough, which is the reversion discussed, where pressure mounts on the exhaust side which is greater than the intake side. And as we build that undersirable pressure on the exaust side, our airflow throughout the whole system is impacted.
I'm not sure how to bring it all back in, but the whole turbo calculation is tricky. I think there is probably too much focus on the compressor wheel and not enough on the relationship between the two wheels and the desired amount of boost.
10-05-2016, 05:07 AM
#17
The balance between the cold and hotside is indeed critical.
As the 2V head breathes roughly as well as a manhole cover, increasing the size of the cold side retaining the same hotside won't cause too much adverse effect at upper rpm as back pressure is already higher than it should/could be with a stock K26/6. Results are typically an increase in mid range torque but the engine keeps falling on its knees near 6000 rpm, which was already the case with a stock K26/6. That's not even mentioning that the usual 951 rev counter overreads by 10%...
When using a 4V head that allows a level of VE comparable to most modern 4V engines, both sides should be sized up and turbo selection is no more the headache it can sometimes be with a stock 2V engine.
That probably does not answer the original question, but I do not think the VE on a stock-ish 2V 951 engine is anywhere close to 90%.
As the 2V head breathes roughly as well as a manhole cover, increasing the size of the cold side retaining the same hotside won't cause too much adverse effect at upper rpm as back pressure is already higher than it should/could be with a stock K26/6. Results are typically an increase in mid range torque but the engine keeps falling on its knees near 6000 rpm, which was already the case with a stock K26/6. That's not even mentioning that the usual 951 rev counter overreads by 10%...
When using a 4V head that allows a level of VE comparable to most modern 4V engines, both sides should be sized up and turbo selection is no more the headache it can sometimes be with a stock 2V engine.
That probably does not answer the original question, but I do not think the VE on a stock-ish 2V 951 engine is anywhere close to 90%.
10-05-2016, 06:39 AM
#18
Rennlist Member
My "VE" at peak power is around 90% on my VE table. But it's not true VE because that's tuned to about 12.8:1 AFR (NA engine) and my air density curve is centered around 80* F (average temp when I tuned that curve). It's probably closer to 70% but I'm spitballing. On a 951 it's even farther away because at peak power your VE is well over 100% (by definition of VE, and considering the pressure differential is flipped)
10-05-2016, 02:15 PM
#19
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nobody has mentioned back pressure yet....you can have high boost with low flow if you have enough back pressure!
Boost pressure does not equate to flow.
Boost pressure does not equate to flow.
10-05-2016, 05:05 PM
#20
10-06-2016, 04:54 AM
#23
Rennlist Member
What is the desired b/pressure range nowadays? Would seem with superior turbos we can run less bp than in the past.