Turbo Tech: Size Matters!



These are the corresponding power curves. Typical turbo street cars will see their power curve flat line in the upper rpm range. A road course car will typically have supporting modifications to improve volumetric efficiency in the upper rpm range which allows the power to continue to climb. Drag cars have a very narrow power band with really big power up top.
Here is a dyno chart from Project STI showing it stock and with a tune. As the car was set up as it came off the showroom floor, notice how the torque hits early and drops off up top. The drop-off of torque up top results in a flat power curve.
This Virtual Dyno dyno chart is from Project Evo X GSR. Looking at just the stock torque and power curves, check out how the torque peaks early and then drops resulting in a flat power curve at high engine speeds. Project Evo X was then modified with the intent of being a street/road course car. With full bolt-ons and a larger GTX3071R bolt-on turbo, check out how the torque peak shifts to later in the rpm range and the horsepower continues to climb at redline.

I’m going to take a moment to deviate a bit from the turbo sizing as Project Evo X is a good example of how to do a quick and dirty power estimation for an engine based on boost pressure. It’s really quite simple; multiply the horsepower of the engine if it were naturally aspirated by the boost pressure ratio. For example, my old Nissan SE-R put down about 140whp naturally aspirated with bolt-ons and a tune. I dyno’d the car with a T25 turbo at 7.5psi and it made 215whp. So, the math to determine the pressure ratio is (boost gauge pressure + ambient pressure)/ambient pressure. 14.7psi is the ambient air pressure at sea level. So, on my SE-R, the PR was (7.5 + 14.7)/14.7 = 1.51. So I take that PR and multiply it by the horsepower the car made naturally aspirated, 140whp x 1.51 = 211whp. Pretty gosh darn close to the 215whp and really within uncertainty.


Now let’s look at Project Evo X again. With most of the bolt-ons with the stock turbo at 7000rpm and ~16psi, the car made about 310whp. So that is about a pressure ratio of 2.09 which means the car would make about 148whp naturally aspirated. With all the bolt-ons and the bigger turbo, the car is running ~23psi of boost at 7000rpm making about 375whp or so. (23 +14.7)/ 14.7 x 148whp = 380whp. Pretty darn close again. So that’s how you do a back of the envelope calculation on how much power an engine will make. This does assume a similar state of tune and modification (so intake, exhaust, similar fuel, etc) and similar volumetric efficiency which requires proper turbo sizing. It is by no means perfect but it gets you in the right ballpark.

For an example of a drag car dyno chart, you’ll have to look elsewhere. Sorry, we don’t have any drag cars in the MotoIQ stable.

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