30Lbs. vs 30Lbs. ?

[turbocamino1]

I'm having a debate with a buddy of another board. So before "open mouth,insert foot" ... Given the same engine, is the 30lbs that say a 6262 makes the same as the 30lbs a 6766 or 6466 (etc) make? I imagine as the turbo gets close to the end of its range/map,,that while it will still make the 30 ( or even say 34 +) that it's a weaker charge in general. Maybe hotter air..and other factors i can't articulate?

 

[Reggie West]

No its not the same. The physical characteristics of the compressor wheel come into play. There is a maximum amount of air flow a wheel will generate at any shaft speed and it will be done. A 62 mm wheel will not flow the same amount of air that a 67 will. The compressor wheels will flow a certain amount of air to make a set amount of hoursepower. Airflow plus fuel = horsepower. Boost is resistance to flow but we use it as this indicator of how well the turbo works but it is misleading. That 67 turbo can go past 30 psi and still make additional power or airflow. The 62 will not make any more power past 28 psi, why? It cannot flow any more air.

As for the hot air point yes its all hot when it's compressed and ran through a manifold at the speed of sound and then some. That is why the intercooler is important.

Lets say you make 550 hp with a 62mm turbo at 30 psi and it is maxed out. The 67 can make the same 550 hp at 22psi. Airflow......

This is when the converter, fuel and timing all come into play but that is another story.

 

[Nigel]

If the air exiting the compressor is the same pressure and the same temperature on the same engine, then it is the same flow given the hot side is also at the same conditions. If you are at the same efficiency on the compressor map for both compressors, any horsepower difference would be due to the hot side. The larger turbine wheel would likely make more shaft power with less back pressure to drive the compressor, which will improve the engine's volumetric efficiency and reduce pumping losses.

 

[Mike T]

Nice job explaining that without turning it into 2 pages long

I almost replied earlier but couldn't figure out how to sum it up nearly as well as you did

 

[V6sleeper]

Nice job explaining that without turning it into 2 pages long

I almost replied earlier but couldn't figure out how to sum it up nearly as well as you did

x2 lol

 

[V6sleeper]

And compressor wheel design comes into play also if your not using the same design on all 3 sizes

 

[turbocamino1]

Nigel quote: "If you are at the same efficiency on the compressor map for both compressors, any horsepower difference would be due to the hot side. The larger turbine wheel would likely make more shaft power with less back pressure to drive the compressor" < This is why i chose 30,or so, lbs. as the smaller turbo is weezing up there ,, where the bigger wheel is in a happy place. As the air (boost) get consumed from a the smaller wheel turbo and shaft speed lessons the bigger wheels are creating a shitton more velocity at the same psi ..and this velocity ,i think, makes the 2 30s very different ,yes? The force of what is waiting to get in the door. or do i have something twisted?

 

[Nigel]

It starts to run into a bunch of if conditions as to what becomes limiting. The larger turbo will be able to run more boost, given similar technology wheels, on the same engine. The boost number where the two diverge is obviously based on the two selected. The behavior will also be based on the engine it is bolted to. So it is really a hypothetical and bounded, applying to a static condition like on a break dyno. It discounts the actual real world combination required to make a good working car.
While the shaft speed and compressor tip speed may be different on the two turbos at 30 psi, if the air is at the same pressure and temperature entering the intercooler/up pipe/manifold it will be at the same velocity if on the same engine at the same volumetric efficiency.
If you had an engine optimized to run 30 psi with the small turbo, and put the bigger turbo on and run 30 psi, not much gain likely.
If you optimize the engine to run 30 psi on the larger turbo, you may not even be able to get 30 psi out of the smaller one.

 

[Mike T]

Hypothetical Small Turbo VS Larger

Given this scenario....at the track we can leave with 20psi and maintain that though the entire run. Why would it matter if we were running a 60 compressor with a 63 housing or a 70 compressor with a 85 housing? Other than the obvious of possibly more heat out of the smaller set up. Also...

www.turbobuick.com

 

[Mr.Spool]

I'm having a debate with a buddy of another board. So before "open mouth,insert foot" ... Given the same engine, is the 30lbs that say a 6262 makes the same as the 30lbs a 6766 or 6466 (etc) make? I imagine as the turbo gets close to the end of its range/map,,that while it will still make the 30 ( or even say 34 +) that it's a weaker charge in general. Maybe hotter air..and other factors i can't articulate?

If both turbos are capable of making 30psi on the same engine
30psi is 30psi.
It's the same
It cannot be different because 30psi is the measure of what did not get in.
Boost is just a number and a representation of pressure waiting to get into the motor.

 

[Mr.Spool]

I'm having a debate with a buddy of another board. So before "open mouth,insert foot" ... Given the same engine, is the 30lbs that say a 6262 makes the same as the 30lbs a 6766 or 6466 (etc) make? I imagine as the turbo gets close to the end of its range/map,,that while it will still make the 30 ( or even say 34 +) that it's a weaker charge in general. Maybe hotter air..and other factors i can't articulate?

Going further Your scenario is an impossible one.
A 6262 would not get to 30psi if the 67 could

 

[Mr.Spool]

And going further
If the motor was inefficient and a 62 could make say 30psi and you make 30psi putting a 67 on it.
They would use the same amount of fuel cause its the same amount of air waiting to get into the motor.
Now put squeeze it more and the 67 would make more boost requiring more fuel therefore making more power.

 

[Mr.Spool]

Now take an efficient motor.
The 62 would hit a point where it cannot make more boost while the 67 would make more boost before it runs out.

 

[turbocamino1]

I see there are many variables at play...but comparing apples to apples the 30s are equivalent . thanks for all the clarifications.

 

[forcefed3.8's]

Manifold pressure is just that. Doesnt matter what it took to make it. If a smaller turbo making the same boost is running the hot side hard with high backpressure, but the bigger turbo has considerably less backpressure, then the larger turbo will make more power.