W2 Drop-in Turbo Upgrade - Design Pt 2; Turbine Housing Selection
Welcome to part 2 of the W2 turbocharger design blog series. In the last blog we shared with readers our goals for the next turbo kit upgrade for the L15 Earth Dreams engine. The short summary is the W2 needs to..
Make more overall power and torque than the W1 with a stronger power curve
Be a brand new unit
Be a true 100% drop-in
The last goal was going to be the toughest to accomplish because in order for goal 1 to happen you generally need to make the turbo bigger and as we make something bigger it makes our space constraints tighter. But we told you we had the solution and we are happy to share that with you today.
While everyone else has been busy attempting to stuff bigger and bigger wheels into the tiny OE turbocharger housings with little success; the team here at 27WON Performance has been focused on the next step forward. We knew that to truly REDEFINE we would have to go where no one else dared to go: not just larger wheels, but larger housings to support these goals.
For the W2 we created a whole new turbine housing casting.
Take a good look at the cad drawing above. What you see is not an OEM sized turbine housing that was bored out to fit bigger wheels but rather an entirely new turbine housing that is larger, stronger, and more capable of power than any stock frame sized turbocharger out there. That said, this will still be a drop-in and it will fit all the downpipes, intakes, intercoolers, and inlet pipes that we’ve seen so far.
Let’s dive into the meat of this and share with you all what we did and why. There are a lot of terms that are used in the world of turbocharger design and development. One of those key terms is A/R.
A/R is short for Area over Radius and is the mathematical relationship that defines the scroll of the turbine housing (the exhaust gas side), divided by the radius from the turbine wheel centerline (the middle of the round part of your turbine housing). Check out the pic below for a visual explanation.
Bigger A/R = more top end but slower spool, a smaller A/R is the opposite
To help visualize this think of the A/R like a garden hose. The bigger the hose in diameter the more water you can flow through it but the longer it takes to fill the length of the hose when you turn the faucet on. Using a smaller hose will allow you to get the water out faster but you won’t be able to flow as much due to its smaller size.
In the simplest terms, as it regards our turbochargers, a larger A/R will spool slower but will make more power on the top end. A smaller A/R will spool quicker but reach max flow sooner and not flow as much on the top end. This is a classic trade-off scenario and getting both max power and the fast spool is like trying to have your cake and eat it too.
From left to right 0.50a/r → 0.58a/r → 0.65a/r
In order to make more power on pump gas and overall under the curve we needed to help our turbocharger breathe in the upper RPM range. Having already maximized the stock turbine A/R size of the W1 at 0.50 we needed to go bigger for more power. Basically we needed a bigger garden hose. Below is a CAD model comparison of the W1 turbine housing at the left, a mid-sized iteration we entertained in the center, and the final turbine housing for the W2 which utilizes a 0.65 A/R.
Looking at the orange arrows you can see how much larger the cross-sectional area of the turbine scroll is. Because of this increase we can more efficiently flow exhaust gases from the cylinders and through the turbine housing and wheel.
Unlike any other option on the market today...maybe you can have your cake and eat it too?
Stay tuned for part 3 where we dig into the changes we made on the compressor cover side. You’ll learn what was done to improve reliability and longevity while going to a bigger turbocharger.
-Vincent