The Right Turbo

Most of the manufacturers producing turbos do a pretty good job. Your choice of turbo should not be decided based on one manufacturer over another just because you like their brand or the name better. What really matters is which turbo is best suited for your engine. I’ll give you an example. When I first built my RB25DET I was using a Bullseye Power (Borg Warner) S362 Turbo with a .85 a/r. This same turbo was producing 650hp on a Toyota Supra, but it was maybe hitting 400hp on a good day. When the turbo seals blew I went with a .70 a/r and had a bit better spool but still only about 421hp. So I started looking at compressor maps a little more closely and with a little help from a friend and some additional reading I determined that a smaller turbo like a GTX3076R by Garrett would be better matched with the RB25DET engine based on the math. Sure enough, the planning paid off because I hit 534hp with a SMALLER turbo that was spooling quicker.

So the key here is that a bigger turbo does not always automatically mean more power. Yes, if I would have continued to push the limits of my Bullseye Power turbo with race fuel and tons of boost, it would eventually make more power than the smaller Garrett. But that is just not practical for a street/track car hybrid and that power is not in the usable RPM range.

Be smart when selecting your turbo.  I highly recommend reading this book and running the numbers yourself. Maximum Boost

GT35R Knockoff, GTX3076R, Bullseye S362

2 thoughts on “The Right Turbo”

  1. I think you would start to push the limits of the GTX3076R around 600hp.

    Here are some notes from a Garrett employee about my choice of turbos before I bought the GTX3076R. You might find them insightful.


    It is actually lucky for Andrew that he asked now, since we’re running simulations comparing the results from our prototype iPhone app and our more involved, application-matching spreadsheet program. This example gives us a good simulation. This means, not only did he have a developmentally-challenged marketing guy run the numbers, but also a real, live aftermarket engineer double-checking everything! Woot.

    So, we made some assumptions about the engine.

    It is running an air-to-air intercooler.
    4 valves/cylinder for .9 volumetric efficiency
    It will be using pump gas (though I did try the match with a few different A/F ratios)
    It will be used near Louisville with an average barometric pressure of 14.37 and an average temp of 55* F
    Actual crank horsepower is approximately 675 accounting for a 20% drivetrain loss to give the 550 RWHP target
    Brake Specific Fuel Consumption (BSFC) of 0.46

    From what we can see, a 2.5L will have a difficult time making 675 HP at 20 psi without running really rich (I got it there at 9.2:1). However, a more sober 10.5:1 A/F ratio delivers the power at a pressure ratio of 2.8, or 25 psi gauge. This will give the target 675 crank HP at 7500 RPM. Intake manifold temps should hover around 129*, and torque should be 473 lb/ft.

    Now, which turbo? Both of the choices Andrew listed would work. The GT3582R will make the power, but the bigger wheels are going to make it a little laggier. Plus, I don’t feel like the added volume is totally necessary given that it is a 2.5L and back pressure shouldn’t be an issue. My choice would be the GTX3076R, not only because it is the hawtness right now, but also because it will be faster reacting and will still deliver the power at ~73-75% efficiency. You’ll be closer to the choke flow line of the map, so the real power will be right smack dab in the heart of the map where you want it. Also, there are three A/R’s for turbine housing sizing. I’m feeling like the larger 1.06 might be the best way to go, but it is kind of guess and check with the turbine housing A/R sizing. The only caution I would have in using the GTX3076R is to watch for surge. I don’t think that it will happen, but there is a better chance of it occurring with this turbo than the GT3582R.


    So there you have it. I hope that helps you in your decision process.

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