SC vs Turbo for max safe HP limits on stock internals

[engineermike]

Well I posted some real world data I took from a well-performing combo. I started at 2.3 and got it down to 2. There’s a lot of real data in the following thread, with some just below unity and others as high as 3.3/1.

https://www.turbobuick.com/threads/boost-vs-back-pressure.144094/

Keep in mind also that high backpressure/boost ratio isn’t necessarily a due to the turbo being max’d Out, though that is one possible cause.

Next, supersonic tip speed isn’t a problem, as long as it’s absolute speed not relative speed vs the gas. Cavitation is not a thing when dealing in gases. You are not likely to encounter the tip speed being supersonic vs the gas speed because they are both traveling the same direction and similar speeds.

And finally, reaching sonic velocity is the limit of gas speed through a passage. If you’ve heard of stonewall, that’s what it is. On the compressor side, the inlet size will determine the max possible air flow rate it can pass no matter what happens downstream in the engine or in the turbine.

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[sigintel]

Turbines are freakn amazing. Turbine meaning the part that converts energy from the fluid (exhaust) into shaft power. Compressors convert the shaft power back into compressed fluid (intake air). Nice blade comparison:
https://www.quora.com/What-is-the-d...ine-and-compressor-blade-design-and-why-is-so

Are you asking because you know or

trying to tie something together?

I find aerodynamics, fluid flow, turbines, etc hella interesting. Thermodynamics is

'real' magic Harry Potter would approve of. Kinda like RF engineering. Amazing stuff. Dont mean to come across as anything other than a totally enthusiastic thermodynamics dork. lol.

You totally answered it:

...power is not increasing as its is

boosted higher.

Unless designed for supersonic flow on the turbine blades: the shock waves produced decrease the energy 'available' to be converted into shaft horsepower by the turbine blades. The turbine sections in our turbos are 'conventional' to be able to function at wide range of flows from off idle to redline. Supersonic turbines are cutting edge in constant flow rate power generation turbines; they have horrible speed ranges over which they can function well and would be terrible for automotive turbos.

https://ars.els-cdn.com/content/image/1-s2.0-S0020740314002793-gr13.jpg

https://ars.els-cdn.com/content/image/1-s2.0-S0020740314002793-gr11.jpg
https://www.sciencedirect.com/science/article/pii/S0020740314002793

Shock waves and turbine blades near walls:

https://www.grc.nasa.gov/WWW/BGH/oblique.html

https://www.grc.nasa.gov/www/k-12/airplane/crosshock.html

https://www.grc.nasa.gov/www/k-12/airplane/reflects.html

Keep in mind that the turbine blade is designed to extract a substantial amount of energy from purely expanding the gas and reducing the temperature EVEN IF INLET AND OUTLET PRESSURE IS THE SAME. CRAZY STUFF!!
This is called Isentropic Process. This is 'part' of the energy turbos get from the exhaust by recovering pure heat energy via gas expansion. Pressure drop yields additional shaft power; however, it loads the crank evenly via increased exhaust back pressure.
https://www.ecourses.ou.edu/cgi-bin/eBook.cgi?doc=&topic=th&chap_sec=06.5&page=theory
https://www.ecourses.ou.edu/cgi-bin/eBook.cgi?doc=&topic=th&chap_sec=06.4&page=theory

If you think about it, it is insanely freakn cool that a turbine blade can extract power by providing a region of expansion where the heat energy in the gas can be converted into shaft power. This is even if there is 0 pressure drop!; purely accomplished by providing a flow region where gas expansion cools the air to extract power! There is a slight compression region on the blade immediately in front of that gas expansion portion of the blade. When you hit sonic, you destroy the gas expansion fluid flow region of the blade and shaft power generation.
Supersonic Expansion is a way different animal vs subsonic:
https://farside.ph.utexas.edu/teaching/336L/Fluidhtml/node209.html

Spinning a turbo faster may get more flow through it; but the ability of the turbine section to extract heat and pressure from the exhaust decreases (requiring higher pressure drop as increased exhaust valve backpressure for the same amount of shaft power developed). Then as a double whammy, the compressor section becomes less efficient and you are now adding more heat to the compressed air where you are forced to take higher IATs even with an inter cooler.

I think @engineermike covered choked flow.
https://en.wikipedia.org/wiki/Choked_flow
Yeay.. restrictor plate racing.. lol..
That turbobuick thread is pretty sweet.

Interesting read on history of studying supersonic flow:

https://history.nasa.gov/SP-440/ch5-2.htm

Have a great weekend and thanks to all our Veterans for their Service!

 

[sigintel]

This is all at a boost:backpressure ratio that is quite a bit lower than what it seems that the general though of 2:1 being ok is in here. I realize that it is on a diesel engine...

Diesel is pretty big difference. Optimizing the turbine and compressor wheels for a specific flow rate and temperature can greatly increase the ratio of shaft power produced from Isentropic expansion versus the power generated by the pressure drop.
The diesel turbo setup is engineered to operate in a MUCH narrower flow rate/rpm range and the OEM engine and turbo combined development is highly optimized.

If you look at the size of the turbos being used versus the flow/hp, you can see the diesel may have another huge advantage versus what you see on Coyote motors due to our packaging limitations. Increasing the turbine blade section per amount of flow can increase the Isentropic power recovery.

Street turbos are typically restrictive AR to get fast enough spool at low rpm. Its embarrassing to roll around at 5000 rpm with low AR just to be ready in case someone messes with you. Thats why Whipple rules the street for authoritative traffic anarchy without a downshift needed in most cases.

Lol, I know I am not the only one who stripped the trunk liner out and spent hours staring at the trunk trying to figure out how to cut out the entire tub to fit a massive single turbo in there. It can be done, but figuring out how to do it clean enough to be able to sell the car later is.. uh.. tricky.

 

[Ryan_s550]

Diesel is pretty big difference. Optimizing the turbine and compressor wheels for a specific flow rate and temperature can greatly increase the ratio of shaft power produced from Isentropic expansion versus the power generated by the pressure drop.
The diesel turbo setup is engineered to operate in a MUCH narrower flow rate/rpm range and the OEM engine and turbo combined development is highly optimized.

If you look at the size of the turbos being used versus the flow/hp, you can see the diesel may have another huge advantage versus what you see on Coyote motors due to our packaging limitations. Increasing the turbine blade section per amount of flow can increase the Isentropic power recovery.

Street turbos are typically restrictive AR to get fast enough spool at low rpm. Its embarrassing to roll around at 5000 rpm with low AR just to be ready in case someone messes with you. Thats why Whipple rules the street for authoritative traffic anarchy without a downshift needed in most cases.

Lol, I know I am not the only one who stripped the trunk liner out and spent hours staring at the trunk trying to figure out how to cut out the entire tub to fit a massive single turbo in there. It can be done, but figuring out how to do it clean enough to be able to sell the car later is.. uh.. tricky.

Understandable. I could see where the RPM range could have a significant effect on the balance of back-pressure and response. I agree on your Whipple comment, which is one of the main reasons I personally went that direction. However, the turbo car I have driven several times is very responsive, with very little lag. Boost comes in very fast, especially with the a10 auto.

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