Turbos, air flow, and boost... turbo newb in need of clarifiaction

[Thor06]

I was just reading the thread on someones 14b install and I was reading about the flow numbers. They are in lbs per minute right? I was wondering... what the flow numbers were for some of the common turbos specifically the 14b, evo III 16g, and the 20g? I tried to search, but I didnt find much probably because I dont know where to start. I did find a writeup of Kevins on turbos that I think should be stickied, thanks for the info Kevin. The boost pressure is how much force is being put on the air going into the engine and the more pressure the more air until the turbo cant force more air in right? So what would be better, to run a big turbo at low boost or a smaller turbo at higher boost? My friend and I are both wanting to build up our (when I get mine) DSM's and we are trying to get our mods planned or atleast have an idea of what we are up against. We are planning on upgrading the internals, but not until later so we are planning on getting our cars to around 350 and then build the internals and go up some more. Since my first thread, I have wanted a evo III 16g, but I know that they are only good to around 400 hp. Now I was wondering with my new found knowledge on air flow, I was thinking about a 20g at lower boost until the internals and then turning it up some for more power. Is that a good idea? The car will be daily driven, but I want to run some low times in the 1/4. Also, where about would the spool time be at on a 20g? Thanks guys!

[kjewer1]

Holy shit batman, where to start...

First with some airflow numbers and spool times (boost will be 20 or 25 psi depending on the turbo)

14b - 30 lbs/min - 3000
small/big 16g - 40 lbs/min - 3300
EVO3 16g - 42-44 lbs/min - 3300
20g - 48 lbs/min - 3700 (tdo6H, tdo5h will be ~3500)
50 trim - 50-51 lbs/min - 3900
60-1 - 60-62 lbs/min - 4200-4400 (depends on hotside)
56 trim GT40 (GT35R) - 65 lbs/min 4400
T67 - 75 lbs/min - nearly 5000 on a 2.4

These are all measurements taken on my own car FYI.

In the case of the 16g vs 20 and movingmore airflow at the same boost pressure. Unlikely. The turbine sides are the same in the case of the TDO5 20g. The difference in compressor efficiency is almost impercievable in the real world. If the 20g is a TDO6H 20g, then you will get more airflow at the same boost thanks to the increase in turbine flow/volumetric efficiency. I have not done that comparison however. But to illustrate the point, when I ran a tdo6H 20g and a TDO6H 56 trim at 25 psi, airflow was exactly 48 lbs/min in both cases. No gain at all, despite a much larger and very efficient compressor wheel

The key to making power, especially on pump gas, is to move more air without raising boost. Airflow is power, boost is heat and heat is knock (laws of adiabatic compression/ideal gas law).

Some good examples of increasing airflow without raising boost, in addition to the larger turbine side, are cams and intake manifold. At 20 psi on a normal setup cams are worth 3-4 lbs/min. An intake manifold is typically worth ~3 lbs/min. This is all free power as far as pump gas goes, assuming the turbo has the extra flow capacity to meet these numbers. A camed car should move 40 lbs/min with cams at 20 psi. ~36 without cams. 1 psi is roughly 1 lb/min on a 2 liter, so thats the difference between getting 40 lbs/min at 20 psi or 24 psi. Good intercooling is the other half of the pump gas puzzle.

Airflow is worth roughly 10 hp per lb/min. I like to size the turbo based on the airflow/boost I intend to run. For example, a 2 liter at 30 psi and 8000 rpm is only going to move ~55 lbs/min at 100% Volumetric Efficiency, so a 65 lb turbo in this case is not going to help a whole lot. But that 65 lb wheel will be maxed out at 30 psi on a strocker. so it depends on the goals and setup. Going from the 20g to the 56 trim at the same 25 psi actually made the car slower because it was moving the same amount of air, but was laggier. At higher boost the 56 trim will become worth while though. The 20g is nearly maxed out at 25 psi in my example (most poeple run it at ~27-28 psi with boost falling off), while the 56 trim would be good for that 55 lbs/min at 30 psi... More headroom, and at that level it begins to counter the added lag with tons of extra power.

This is obviously a pretty involved topic, but I'll end it here for now before I get fired...

[Thor06]

Wow, thanks Kevin! That really helped, and I REALLY appreciate it!