How to Calculate a Turbo’s Trim



trim = inducer²/exducer² * 100

ex. take a TD05H-20g compressor wheel:
inducer diameter: 52.56mm
exducer diameter: 68.01mm
trim: ?

trim = (52.56mm)² / (68.01mm)² * 100
= 2762.5536 / 4625.3601 * 100
= 59.7…
~= 60

ex. take the turbine of a Garrett GTX2867R where Garrett doesn’t list the inducer diameter
inducer: ?
exducer: 53.8mm
trim : 76

76 = inducer²/(53.8mm)² * 100
=> 76 / 100 = inducer² / 2894.44mm²
=> .76 * 2894.44mm² = inducer²
=> inducer = sqrt(2199.7744mm²)
=? inducer = 46.9mm

I am getting turbo compressor surge; how do I fix it?

I am getting turbo compressor surge; how do I fix it?

There are at least two kinds of compressor surge:
1. the intake path is being too restrictive so the boost is hitting too much back pressure causing a surge
2. -or- the turbine/turbine housing is mismatched with the exhaust flow such that the exhaust is out flowing the compressor pushing it to the left of the surge line. You can see this at higher RPMs.

For #1:
– If having your throttle closed cause a surge or boost spike, then I think you blow off valve is not working correctly. Using a recirculating blow of valve is normally the best.
– if your BOV is working, then maybe there is a restriction in your intake path.

For #2:
– if you are getting boost creep, you could consider porting out the waste gage hole turbine section so that it is smooth a little bigger. Make sure the flapper still shuts it off completely when closed.
– you can also try a bigger hotside, or external wastegate
– a bit of compressor surge here may be acceptable

Guage Wiring Using the Clock Wiring Harness

Planning on adding some gauges to your WRX? There are several methods to get the proper voltage sources to get the job done, but just using the wiring harness for the clock is the simplest in my opinion. It has all of the voltage sources you need, and it is a great location.  This is especially if you are planning a gauge pod in the clock’s spot, or cluster bezel.  On my ’05, I personally used the SMY ClusterMaker Dual Gauge Pod 52mm that replaces the factory cluster bezel. I personally like keeping my clock and don’t favor the look of having the gauges on the a-pillar.

Here is the breakdown on the wiring harness for the clock:
Clock Wiring for Gauges

I spliced right into the wires I needed and soldered in the power harness for the the boost gauge I was installing.  Before buttoning everything back up, it is a good idea to make sure everything is working as expected.
Test fitting the bezel gage pod