Why isn't selecting a turbo more straight forward?
 

It just seems to me that you should be able to figure out how much cfm of exhaust your engine is able to push and settle on a target boost or know the max boost a block or internals can take; complete a simple equation and then make your purchase.

But you have to factor in effects of exhaust and compressor housings, compressor wheel trim, ball bearing or not. Then you have to factor in how large your exhaust system is. According to Corky Bell, a quarter of an inch difference between manifolds or headers, a quarter inch from dp on back, quarter inch for axle back all are very critical for power vs. tq.

Then you have to figure in how much gasoline and what minimum octane you're willing and able to push.

Why can't it just be A+B=C?:)

cfm isn't the only thing the turbo actually does need.

Your forgetting a tad bit on the physics side of the force. Mainly the heat the exhaust is producing heat is energy and energy is critical to turbos :D. The weight of the fan you are spinning (more mass means more energy required for target boost you gotta move the fan with somethin eh?).

The back pressure of the system placed on by the turbo and other options like catted or not catted exhuast systems effect spool up times and overall flow.

Far as the headers are concerned i think it was something along the lines of timing the explosions per cylinder fire for smoother flow and less back pressure on the system. Been awhile since i looked at the supremely technical side of this though, i just rely on pre made components now ;)

You're absolutely right, but do you see what I mean?

Seems as if some really smart guy would be able to take the 'constants' of the car...exhaust cfm, a set exhaust system's backpressure....apply every turbo imaginable, and put them all on the dyno.

I know it's asking quite a bit, but boy it'd be nice.:)

[QUOTE=gpatmac]You're absolutely right, but do you see what I mean?

Seems as if some really smart guy would be able to take the 'constants' of the car...exhaust cfm, a set exhaust system's backpressure....apply every turbo imaginable, and put them all on the dyno.

I know it's asking quite a bit, but boy it'd be nice.:)[/QUOTE]

Thats yet another problem. Unfortunately by dumping more air into the car you change also the cfm of the output ;)

So you basically choose a turbo one of a few ways and all of them have to do with application.

1) What kind of setup do you want high hp or fast spool up? For example on a circle track the spool up is sorta useless your never really going to go under the spool up rpm anyways on turns. In rally though where start and stops from really sharp corners it is an issue.

2) How much stress do you want to place on the system. Obviously just increasing boost is going to load up your system maybe to the point of breaking. So choose one that suits you on realiability issues and/or needed mods to make it run correctly.

You just keeping adding to the list.;)

No, seriously, you're right but what I'm talking about is trying to do a ceterus parebus comparison, so those 2 criteria; while most appropriate, would tend to make the comparison that much closer to impossible. They would be exceptional footnotes for each turbo, though.

What I'd like to see is a tool that would give the Impreza consumer the ability to see a table where he could see application/ranking and then after he's selected a couple that look promising, then he can read the footnotes for them to fine tune his selection.

turbos are a tricky ordeal.

[QUOTE=gpatmac]You just keeping adding to the list.;)

No, seriously, you're right but what I'm talking about is trying to do a ceterus parebus comparison, so those 2 criteria; while most appropriate, would tend to make the comparison that much closer to impossible. They would be exceptional footnotes for each turbo, though.

What I'd like to see is a tool that would give the Impreza consumer the ability to see a table where he could see application/ranking and then after he's selected a couple that look promising, then he can read the footnotes for them to fine tune his selection.[/QUOTE]

Sorry, to help you on your quest though just keep in mind a couple things.

Ball bearing or not. Bearings help spool up time which is nice after all.

Look at the rated output, there is a list on a site somewhere telling you how much psi its capable of on the 2.5 and 2.0L model engines. Go with a target psi you plan on running then get a turbo thats either able to handle it but isn't so big your waiting for it to spool up forever.

Definitely get any supporting mod if your going to change the turbo to bigger. This means injecters, bell mouth downpipe, up pipe if you have a wrx, catback, ECU management, fuel pump is a must stock sti injecters work nearly at 90% capacity so i'm told, and oh yes an intercooler upgrade.

[QUOTE=vmar900]turbos are a tricky ordeal. To find the right one for you needs, and also to match the fuel delivery is critical. It is a science when mapping, and if you just buy turbo by looking at cfm #'s, you may be totally dissatisfied. Vishnu performance is currently working on their stage 2 kit, which will include the proper turbo, with proper injectors, and proper mapping which will increase all driving conditions. I have been playing with turbos for years, and know that Vishnu does their homework when it comes to all around performance. So if you call those guys, they are more than helpful, and most of all, they are modest and honest. You will never be mislead by them, and will be more than satisfied with their performance.[/QUOTE]

Dude, can you write one post on this forum that you dont turn into a big gushing testament to how much you love Vishnu?
Just take it down a notch bro.
:rolleyes:

no punn intended. There are a lot of products out there and whichever you choose I'm sure will be a good one. I just wanted to share what I've seen in "real world" at the track.

Welcome to the wonderful world of mechanical engineering.

If found some good information in one of Bell's books. I'll post it later.

If it was easy....it wouldn't be fun.