06-21-2007, 09:08 PM
Join Date: Mar 2006
Total Cats: 1
This is all very budget dependent, what is your turbo budget and what manifold are you going to use?
If you are stuck with a T2 cartridge then that's a constraint, or equally true if you need a T3 based turbo. The cheap chinese turbos are interesting if you get a decent one. There is some hit and miss to this. I am less happy with the newest one (although not disappointed) and was pretty happy with the first one.
So what really matters? Well getting a turbo that is sized correctly is an important part of the equation. Big turbos make more power with less boost but they spool later. Garrett and others focus too much on only the compressor side of the equation in my opinion. The turbine is nearly and sometimes more important than the compressor (especially if you're way off.)
I like larger turbines. Sure they spool slower, but not that much slower, in a Miata any turbo that can fully spool by 4000-4500 is going to be a good match for the car. Once spooled they tend to remain spooled with a good bypass valve.
So you want to use the largest turbine that will allow you to spool the compressor at the RPM required. i.e. If you need to spool at 3500, then you need a turbine that is small enough to meet this requirement, otherwise step up to the next larger turbine... part of this is experience from others and "feel" as most of us don't have the resources to test everything ourselves. You then match that to a compressor that will flow the lbs/min of air closest to the center island of the turbo.
Obviously there is a sweet spot. Take the following compressor maps:
T3/T4 46 Trim and the T3/T4 54 Trim.
You have decided that you want 200 WHP (230 CHP) and to make that much power you will need roughly 21 Lbs/min of air. You want to make ~7 PSI of boost or a pressure ratio of roughly 1.50.
So you need a turbo that will spool freely and flow a decent amount of air at a low pressure ratio. So you'll want a compressor with a larger A/R and the largest A/R turbine that will spool the compressor at the RPM you want while delivering the peak power you want.
The 46 trim can flow 21 lbs/min at a pressure ratio of 1.5 with 74% efficiency. The 54 trim can flow 21 lbs/min at a pressure ratio of 1.5 with 78% efficiency.
It's easy to pick the right compressor now. Unfortunately the turbine is where a lot of the guesswork comes in. Having some experience helps, but bolting the damn turbine in place and having other housings available to "tune" it helps more.