Fwd: Turbo thoughts, was Re: Which transaxle?

[julie at menloparkrandd.com (julie@menloparkrandd.com)]

Fwd from the Great Dan Bubb: (he's too modest to say that himself)

You know how I love to run off at the mouth so hope you don't mind if I share this with the list since there seems to be a lot of interest in turbos lately.

What's important about a turbo is how much air it can flow and how responsive it will be, and secondarily how efficient it is in the range that it will be operated at.
I'll skip the GT series assuming you won't want to spend that much money. I'll also skip any Mitsubishi or KKK turbos cause compressor maps aren't generally available. So that narrows it down to the older Garret turbos which are inexpensive, sometimes available used and are well known.
Basically there are three families:
T3
T3/T04B
T3/T04E

They all have the same center section, the part with the bearings and the oil fittings and possibly any water fittings.
They all also use the same turbine wheels and turbine housings.
The turbine wheels themselves have different sizes that are called stages. Stage 1 is the smallest, then there is stage 2, 3 and 5.
For each turbine wheel there are 4 different turbine housings. They are differentiated by A/R ratio. Area/radius. The available A/R's for the T3 housings are .36, .48, .63, and 1.03 or 1.06 or something. The .36 will be the most responsive and restrictive and the 1.03 will be the laziest but least restrictive. The A/R number is typically cast into the turbine housing on the inside of the exhaust inlet.

The three families have different size compressor wheels.
The T3 wheels are 2.36" diameter, the T04B and T04E are in the 2.75-3.00" range. The T04E is a newer design than the T04B and is typified by peak efficiencies at higher boost pressures, typically in the 15psi range where the T04B's peak is closer to the 10 psi range although both will operate quite well beyond the point of peak efficiency.
Obviously the bigger the compressor wheel the more it will flow.
Each turbo family also has several selections of compressor wheel trim. Trim is the area of the inlet of the compressor wheel, divided by the area of the outlet of the compressor wheel. So, the area of the smallest part of the compressor wheel as you look at the compressor inlet divided by the compressor wheel outside diameter. The higher trims flow more air.
The T3 comes with 40, 45, 50, 60 and Super 60 trims. 
The T04E comes in 40, 46, 50, 54, 57 and 60 trims.
I really don't know shit about the T04B trims cause everytime I compared it to a T04E the E was a better choice. ;^)
The T04E 40 trim flows more than the T3 Super 60.
Compressor housings also have an A/R. The number is generally prominently cast on the outside. This number is useless! Each compressor wheel has a matching housing. You cannot get a different A/R compressor housing for a given wheel trim and a number of different compressor wheel trims can have the same housing A/R. So, quoting that number doesn't tell you trim which is what really matters.

It's also worth pointing out that there are non-hybrid T04E's and T04B's. These have T4 turbine wheels and housings and are much bigger and aren't appropriate for our cars unless you're running a VR6.

So, to completely describe a turbo you need to know the family, turbine wheel stage, turbine housing A/R and compressor wheel trim.
There are also occasionally some variations in the turbine and compressor housing as far as mating surfaces. Mostly this involves turbos used on OEM applications such as Ford T-birds and Volvos.
As an example my nephew's turbo is a T3/T04E, stage 1 turbine, .48 housing and 40 trim compressor.
So, when somebody's offering a T3 with a .50 A/R compressor housing on Ebay you basically know nothing other than it's a T3 and could be one of 4 or 5 different turbos that may or may not work on your application.
General rule of thumb. Don't go smaller than a 50 trim T3 even if you're planning a low power application and probably not larger than a T3/T04E 50 trim unless your car is for racing. Stage 1 turbine wheel unless you're getting to the larger end of the range and a .48 or .63 turbine A/R. .48 if you want low end street response, .63 if your focus is the track. Well, at least that's my opinion.
Having said all that I've seen people run much larger turbos. If you're into huge power above 5000 RPM then that's the ticket. Just the thing for a race car. Personally I like a responsive turbo that's up and running at 3500 RPM and maintains power to redline.

TMI Dan