Twin-Scroll Large A/R Turbine Housing

Understanding Turbine Housing A/R Ratio

A/R (Area over Radius) is a geometric ratio that defines a turbine housing's exhaust flow capacity. Instead of getting lost in complex geometry, it’s easier to think of it as a funnel for your exhaust gas. The A/R number tells you how large that funnel is.

kinugawa turbo turbine housing a/r

This creates a fundamental trade-off between spool time and peak power:

Smaller A/R Housings: These have a "tighter" funnel, forcing exhaust gas to move faster. This spins the turbine wheel quicker, leading to faster spool-up and better low-RPM response. However, at high RPM, this tightness can become a restriction, creating engine backpressure and limiting top-end horsepower.
Larger A/R Housings: These have a "wider" funnel, allowing for much greater exhaust flow. This supports higher horsepower levels and reduces backpressure at high RPM. The trade-off is that it takes more exhaust gas volume and time to spin the turbine wheel, resulting in slower spool-up (more turbo lag).

A/R Conversion Chart (cm² to A/R) Here is a quick reference for converting common cm² housing sizes to the A/R ratios used by brands like Garrett:

6 cm² = 0.41 A/R
7 cm² = 0.49 A/R
8 cm² = 0.57 A/R
9 cm² = 0.65 A/R
10 cm² = 0.73 A/R
11 cm² = 0.81 A/R
12 cm² = 0.89 A/R

(Generally, TD04-based turbos use 5-7cm² housings, while TD05 series turbos use 6-10cm² housings.)

An excellent question that gets to the heart of modern turbocharger efficiency. The choice between a single-scroll and a twin-scroll turbine housing is a critical one, as it fundamentally changes how your engine responds and delivers power.

The primary difference lies in how each design manages the engine's exhaust pulses before they hit the turbine wheel.

Performance Comparison: Single-Scroll vs. Twin-Scroll

Single-Scroll: The Open Funnel

Think of a single-scroll housing as a simple, open funnel. It collects all the exhaust gas from every cylinder and channels it through a single, undivided passage (a "scroll") to spin the turbine wheel.

- How it works: All exhaust pulses are merged into one large stream. While effective, this merging causes turbulence and interference between the pulses from cylinders that are in different phases of the 4-stroke cycle. This interference reduces the energy and focus of the exhaust stream hitting the turbine blades.
Twin-Scroll: The Divided Highway

A twin-scroll housing is more sophisticated. It features a divided inlet and two separate scrolls inside the housing. This design pairs specific engine cylinders (based on their firing order) into two separate channels.

- How it works: For a typical 4-cylinder engine, exhaust from cylinders 1 and 4 might go to one scroll, while cylinders 2 and 3 feed the other. By separating these pulses, they don't interfere with each other. This creates a more consistent, higher-energy, and focused stream of gas hitting the turbine wheel, much like coordinating traffic onto two separate highway lanes instead of a single chaotic on-ramp. This design requires a specific "twin-scroll" or "divided" exhaust manifold to maintain the separation from the engine to the turbo.

This design difference leads to significant trade-offs in performance:

Feature	Single-Scroll	Twin-Scroll
Spool & Turbo Lag	Slower Spool. The less efficient energy from merged exhaust pulses means it takes more time and RPM for the turbo to produce boost.	Faster Spool. The focused, high-energy pulses spin the turbine wheel much more quickly, resulting in significantly less lag and better low-RPM response.
Power Band	Narrower. Often delivers a more sudden "kick" of power once it spools, but can feel sluggish at lower RPMs.	Broader. Delivers strong, usable power across a much wider RPM range, from low-end torque to high-end horsepower.
Engine Efficiency	Less Efficient. Pulse interference can hinder exhaust scavenging (the clearing of exhaust from cylinders), slightly reducing overall engine efficiency.	More Efficient. Greatly improves exhaust scavenging, leading to better combustion, potentially lower EGTs, and improved fuel economy.
Peak Horsepower	Excellent. For all-out drag racing applications, a large single-scroll housing is very effective at producing maximum top-end horsepower.	Excellent. Can achieve similar peak horsepower to a comparable single-scroll, but with the added benefit of a much wider powerband.
Cost & Complexity	Simpler & Less Expensive. The housing and required manifold are simpler to design and manufacture, making it a cost-effective choice.	More Complex & Expensive. Requires a more complex and costly divided exhaust manifold and a more intricate turbine housing.

Still Have Questions? We're Here to Help. Choosing the right turbo housing is a critical decision. If you're unsure what's best for your specific application, contact our team of turbo experts. We'll be happy to guide you through the selection process.