This may sound like a really dumb question but I have always wondered this.

When a manufacture makes a turbo, they list the turbos general "horsepower" range".

1. does that mean WHP? or BHP?
2. based on that answer, why do they not list the max PSI the turbo can hold instead of the horsepower?

Let's take a a subaru. Lets throw on a GT30 and push 30lbs of boost and let's say we get 350whp.

Now let's take an evo and put on that same exact turbo, run 25lbs, and get the same exact 350whp.

Lets say the max "horsepower" this turbo can hold is 500whp. if we turned up the boost in both turbos to acheive the same whp, the PSI would be different in each car. so (work with me here, i know the numbers are totally make beleive) if the evo has 500whp on 40psi and the subaru has 500whp on 45psi, isnt that a dramatic increase of stress on the turbo and thus resulting in blowing the turbo faster?

I just dont get how one car can be 500whp and the other be 500whp, but the turbo is rated at 500whp but one car is pushing the turbo MUCH harder, even though the turbo is rated through horsepower. shouldnt it be PSI? how do they base it off of horsepower? or am i being a retard?

Thanks - Jake

 

It takes a "set" amount of air and fuel to make a given amount horsepower. They rate the turbo with HP because that's how much air it flows. A 600hp turbo may hold 35psi on a 2.0L 4 cylinder. But on a V8, may only make 10-12 psi but still make 600hp because the V8 is consuming all of the air. (numbers are completely theoretical for examples sake). Boost is simply a measurement of restriction.

In your question, the Evo engine is more efficient, and can take in more air, so boost pressure is lower for a given hp level, but the same amount of air is going into the engine due to the engines ability to flow more..

 

I think there rated at BHP, but from what I understand is you take the lb/min rating of the turbo and multiply by ten and that gives you a rough estimation of what the turbo is good for. For example the fp red is a 57- 58 lb/min turbo so about 570-580 hp est. . Now I would think that's WHP as we have seen red's make that. I'm sure there will be others who comment on this.

 

One would assume they use BHP for your generic non-Evo specific turbos due to variables in drivetrain losses (AWD v RWD v FWD etc) ?

 

it's BHP, not WHP.

as mentioned above: WHP ratings are based on the CFM. some turbos can make more, obviously, but then you are pushing hot air, spinning the turbo too fast, etc.
it's always safest to stay within the suggested parameters otherwise you eventually kill the turbo.

my turbo would make more power at 30psi, but i'm told not to go above 27, so no way will i go over.

 

The turbo manufacturer estimates BHP, or how much theoretical power the turbo should be expected to support based upon the mass of air it can flow efficiently. The turbo manufacturer has already measured the mass of air (lb/min or kg/sec), or how many air molecules its compressor sucks into it in a given amount of time when operating at its design limits, and uses that figure to estimate power potential. This is how optimum turbo size is approximated for a given application.

WHP includes a host of variables specific to a particular application, such as actual drivetrain losses, volumetric efficiency of hardware, tuning, etc. The manufacturer knows how much airflow (power) a particular turbo can support, but the manufacturer doesn't have a clue as to what you're doing with a turbo or how efficiently your application converts that airflow into power. The turbo manufacturer assumes that you know your own application well enough to estimate what a given BHP translates to in WHP for your application.


Think of a turbo as an air compressor, and the engine as an air leak. If we set the air compressor to run at a constant speed, and we gradually make that air leak larger, the pressure in the line will be less, while the amount (mass) of air the compressor is consuming remains the same. PSI is simply a function of the capacity of the turbo, the size/efficiency of the engine, and how much power the turbo is being asked to support.

To maintain a fixed amount of power at a given rpm, if we reduce the size or efficiency of the engine, the turbo will have to provide increased pressure to maintain the same number of air molecules the engine ingests with each stroke.

Conversely, if we set the turbo to maintain a fixed pressure at a given rpm, and we increase the size or efficiency of the engine, power will increase at that same pressure - as long as the turbo can support it.

This is a very basic picture, but is intended to help you understand that intake system pressure is just a factor of other variables, and doesn't determine anything on its own any more than a certain tire pressure tells us anything about the capability of a tire.


Just as an engine has a practical rpm limit, turbos have practical limits to their shaft speed. As you predicted, forcing a turbo to work past its practical design limits does adversely impact turbo life.


Had you been a retard, instead of asking the questions, you would have made up the answers, then regurgitated them as gospel truth in a Subaru forum.

 

Woah, more than enough information that I was expecting to get! Thanks a lot guys!