S400 Turbine Housings & A/R Explained: Picking the Right Ratio

Compressor size gets all the attention, but turbine housing selection has just as much say in how an S400 turbo actually drives. Two identical S476s with different A/R housings behave like different turbos.

This guide explains what A/R means, how to read the wheel numbers, and how to pick a housing that matches your engine and your driving.

What A/R Actually Means

A/R is a ratio, not a size. It compares the cross-sectional area of the turbine housing scroll to the radius from the turbine centreline to the centroid of that area. Because it is a ratio, it has no units — a 1.25 A/R is just 1.25.

What it controls in practice is exhaust gas velocity into the turbine wheel:

  • Smaller A/R — narrower passage, gas accelerates, wheel spins up sooner. Earlier spool, but the housing becomes a restriction at high flow and drive pressure climbs.
  • Larger A/R — wider passage, gas moves slower, wheel takes longer to light. Later spool, but far less restriction at the top of the RPM range.

Every A/R decision is that trade. There is no A/R that does both.

Reading Turbine Wheel Numbers

S400 turbine housings are usually specified as two numbers, such as 96/88. Those are the turbine wheel's exducer and inducer diameters in millimetres — the outlet and inlet sides of the turbine wheel.

Wheel Typical Housing Character
83/74 T4 Smallest of the range, quickest response
87/82 T4 Step up in flow, still street-friendly
96/88 T6 The common high-power choice
103/92 T6 Maximum flow, dedicated race

Bigger turbine wheel means more exhaust flow capacity and less backpressure at high RPM, at the cost of response — the same trade as A/R, on a different axis.

Picking an A/R

0.90–1.00 — Earliest Spool

Street trucks, towing-adjacent builds, and sled pulling where getting off the line matters more than trap speed. Expect drive pressure to climb at high RPM.

1.10–1.25 — The Balanced Middle

Where most street/strip S400 builds land, and the safest default if you are unsure. Enough response to drive, enough flow to make the power the compressor is capable of.

1.32–1.45 — Top-End Flow

Drag racing and high-RPM applications where you can afford to wait for boost and want the turbine out of the way at the top. Not pleasant on the street.

Rule of Thumb

Larger displacement and higher operating RPM tolerate a larger A/R. A 6.7L that lives at 3,000 RPM will happily use a 1.32 that would make a 5.9L street truck miserable.

If you are torn between two compressor sizes, taking the smaller compressor with a larger A/R often drives better than the larger compressor with a small A/R.

Divided vs Open Housings

  • Divided (twin-scroll) keeps exhaust pulses from different cylinders separated all the way to the turbine wheel. Better pulse energy recovery, better spool. Requires a divided manifold to be worth anything.
  • Open (undivided) merges everything into one scroll. Simpler, and fine on high-RPM race applications where pulse separation matters less than raw flow.

A divided housing on an open manifold gains you nothing. Match them.

Shop S400 Turbine Housings

400 T4 Turbine Housing

83/74 and 87/82 wheels, A/R 0.90 through 1.25, T4 divided.

Shop the 400 T4 Turbine Housing

400 T6 Turbine Housing

96/88 and 103/92 wheels, A/R 1.15 through 1.45, T4 and T6 divided options.

Shop the 400 T6 Turbine Housing

Other Frames

Replacing a Housing vs Replacing the Turbo

If your compressor is right but the truck drives wrong, a housing change is far cheaper than a new turbo. Common reasons to swap housings:

  • The truck spools later than you expected — try a smaller A/R
  • Drive pressure exceeds boost pressure at high RPM — go larger
  • Cracked or warped housing after hard use
  • Changing the truck's job, from street to competition or back

Housings also change your downpipe requirement if you move between T4 and T6 — see the S400 Flange Guide.

Related Guides

Not sure which housing suits your build? Send the details through the Turbo Spec Form.

Frequently Asked Questions

What does A/R mean on a turbine housing?

A/R is the ratio of the housing scroll's cross-sectional area to the radius from the turbine centreline to that area's centroid. It is unitless. In practice it controls exhaust gas velocity into the turbine wheel: a smaller A/R spools earlier but restricts at high RPM, a larger A/R flows better at the top at the cost of response.

What A/R should I run on an S400?

Most street/strip S400 builds run 1.10 to 1.25. Use 0.90 to 1.00 if early spool matters most, and 1.32 to 1.45 for drag racing or high-RPM applications where top-end flow is the priority.

What do the numbers like 96/88 mean?

They are the turbine wheel's exducer and inducer diameters in millimetres — the outlet and inlet sides of the wheel. A larger wheel flows more exhaust with less backpressure at high RPM but responds more slowly.

Does a smaller A/R make more power?

No. A smaller A/R makes power sooner, not more. At high RPM it becomes a restriction and drive pressure climbs, which usually costs peak power compared with a correctly sized larger housing.

Should I use a divided or open turbine housing?

Use divided if you have a divided manifold, as it preserves exhaust pulse separation and improves spool. Use open on high-RPM race applications or where the manifold is undivided. A divided housing on an open manifold provides no benefit.

Can I change just the turbine housing instead of the whole turbo?

Yes, and it is far cheaper than a new turbo if the compressor side is already right. Note that moving between T4 and T6 housings changes your downpipe flange requirement.

Does a bigger turbine housing hurt spool?

Yes, that is the trade. A larger A/R or larger turbine wheel lets exhaust move more slowly through the housing, so the wheel takes longer to accelerate. You gain top-end flow and lose low-end response.