SC vs Turbo for max safe HP limits on stock internals

[gimmie11s]

If drive pressure is 2x boost pressure you have a severe restriction with the exhaust or the turbine housing itself. You won't run into this issue on any of the popular mainstream turbo kits in the 700-800 range. Most all of them support 1000+ hp with the base turbos.

No.

2x exhaust pressure vs boost happens all the time on turbo coyotes.

1:1 or lower is ideal, but damn near impossible to achieve on a coyote system.

This is the reason you cannot run any real boost pressure on a turbo coyote (yes, twins included) with the stock cat back. They ALL require 3” turbo back exhaust or larger.

Ask me how I know

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[beefcake]

i usually tell guys i'm comfortable putting a customer car out at 750 on e85 with a s/c and 800 on a turbo, on a stock engine, we have customers doing more, and others doing more, but thats my rule of thumb that i go buy

 

[Ryan_s550]

No.

2x exhaust pressure vs boost happens all the time on turbo coyotes.

1:1 or lower is ideal, but damn near impossible to achieve on a coyote system.

This is the reason you cannot run any real boost pressure on a turbo coyote (yes, twins included) with the stock cat back. They ALL require 3” turbo back exhaust or larger.

Ask me how I know

With a free flowing exhaust and properly sized turbines, why should it be difficult to achieve a 2:1 backpressure to boost ratio? The stock catback is only 2 1/4" so that is understandable.

 

[sigintel]

Consider googling Adiabatic and Isentropic Turbine expansion.

Some work/hp comes from the turbine expansion of hot gas converting heat into shaft power.

Some work/hp comes from converting expansion of gas via the pressure drop across the turbine.

If we were going to calculate the ideal(max) shaft hp produced, we would likely sum 3 enthalpy change calculations:
Fixed pressure: Temperature change only:EGT to post turbine temp
Fixed temperature: pressure drop from output pressure to 1 atmosphere
Constant temp compensation: calc temp constant at output temp and from 1atm back up to outlet pressure
Enthalpy change sum equals shaft hp idealized.
Sum these and you'll get a total of about 150-200 hp at 1200 crank. Roughly 37-50hp is "free" via the heat scavenging, the rest it from pressure as seen by the motor as back pressure.

The HUGE advantage of parasitic back pressure vs front crank supercharger: the parasitic load is distributed evenly all along the crank instead 95% on the front journal and snout.
The belt drive is WAY stiffer than the compressed air drive; SC adding substantially higher harmonic load/shock peaks, but slightly more efficient at power transmission if isolated (no heat or pressure work derived included).

Going for max whp: turbo has some free hp, and a huge advantage in distributed load across crank. However, both will allow you to find the physical and thermal limits of the block. In this regard, other factors like power delivery and tuning ease come into play and could easily offset a 50 hp advantage.

 

[sigintel]

With a free flowing exhaust and properly sized turbines, why should it be difficult to achieve a 2:1 backpressure to boost ratio? The stock catback is only 2 1/4" so that is understandable.

Only way to move more mass is higher velocity.
Once you hit sonic (max compression wave speed in that fluid i.e. air), only way to move more air is to increase density of fluid such as accomplished by increasing pressure.
Basically, you are creating a standing shockwave at the exhaust port fighting you.

 

[Ryan_s550]

Only way to move more mass is higher velocity.
Once you hit sonic (max compression wave speed in that fluid i.e. air), only way to move more air is to increase density of fluid such as accomplished by increasing pressure.
Basically, you are creating a standing shockwave at the exhaust port fighting you.

So if you have a standing shock wave at the exhaust port shouldn't you theoretically be able to solve that by opening up the port and slowing the velocity? Does anyone have any actual data showing the boost pressure vs drive pressure on a turbo coyote. I'm not talking about a 110% max effort setup. I'm referring to something properly designed being pushed within its designed capabilities. I have a hard time buying that drive pressures are so high (2:1+) like it is being implied. I'm not saying its not true, Id just like to see some data, rather than theory.

 

[engineermike]

Conversely, why do you think it’s low?

Everything I’ve read and tested tells me that if you get much below 2/1, response starts getting a lot worse.

 

[80FoxCoupe]

So if you have a standing shock wave at the exhaust port shouldn't you theoretically be able to solve that by opening up the port and slowing the velocity? Does anyone have any actual data showing the boost pressure vs drive pressure on a turbo coyote. I'm not talking about a 110% max effort setup. I'm referring to something properly designed being pushed within its designed capabilities. I have a hard time buying that drive pressures are so high (2:1+) like it is being implied. I'm not saying its not true, Id just like to see some data, rather than theory.

2:1 is common. Hot side pressure has to exceed cold side for a pressure differential to exist. Without that differential there is no drive.

 

[Ryan_s550]

Conversely, why do you think it’s low?

Everything I’ve read and tested tells me that if you get much below 2/1, response starts getting a lot worse.

I dont think 2:1 back pressure: boost is low. Based on what I understand thats starting to get high, and causes a lot of pumping losses. I'm not saying its excessive, but it is less than ideal. In most cases there is probably some power left on the table at 2:1, it just becomes increasingly less efficient.

2:1 is common. Hot side pressure has to exceed cold side for a pressure differential to exist. Without that differential there is no drive.

Agreed, there must be a differential. I was recently watching Gale Banks testing a Duramax. He was saying the turbocharger was pretty much done at a ratio that wasnt even 2:1. Its not that there wasnt a little more power on the table, but shaft speed was at max and any additionaly boost wouldn't have added much power, and definitely not efficient power.

 

[engineermike]

I dont think 2:1 back pressure: boost is low. Based on what I understand thats starting to get high, and causes a lot of pumping losses. I'm not saying its excessive, but it is less than ideal. In most cases there is probably some power left on the table at 2:1, it just becomes increasingly less efficient...

Ok but I’m asking what are you basing this off of?

 

[sigintel]

... shaft speed was at max and any additionaly boost wouldn't have added much power, and definitely not efficient power.

What happens when turbine blade tip speeds exceed speed of sound in the fluid they are in?

 

[Jay-rod427]

How exactly is a rainbow made, how exactly does the sun set, how exactly does a positrac rear end on a plymouth work? It just does.

 

[Rjames18]

850 is pretty safe on e85 for turbo applications. After that it's a gamble. I've been sitting around 1000whp and beat the hell out of mine everytime I take it out. No motor probs but driveshafts dont like me

 

[engineermike]

What happens when turbine blade tip speeds exceed speed of sound in the fluid they are in?

Relative or absolute speed? I’m pretty sure the turbine tip speeds are close to the gas velocity so the relative speed is very low.

 

[Ryan_s550]

Ok but I’m asking what are you basing this off of?

There is a lot of good data in this video, and it shows the actual performance of the turbo and engine at max shaft speed. Mass flow and and power is not increasing as its is boosted higher. 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, but a lot of the same concepts still apply. I'm not calling anyone a liar or wrong. I would just like to see some actual data showing what the boost:backpressure ratio is on a properly designed coyote kit that is ran within its upper design range. If everything says 1:1 is ideal (likely not a real world result in most cases) and other turbo systems are on falling off the map sooner, are we concluding that all coyote systems are inefficient being that the boost:backpressure ratio of 2:1 is very common? Are we saying that up to even 3:1 is ok? Obviously that is not the case of coyote kits being innefficient because most of the kits make very good power per psi of boost. I would just like some actual data assuming it exist, not just turbo theory.

What happens when turbine blade tip speeds exceed speed of sound in the fluid they are in?

I'm not a fluid dynamics expert, but you will likely kit some kind of cavitation at the area where your tip speed surpasses the speed of sound. Are you asking because you know or trying to tie something together?

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