S300 Turbo Flange Guide: Marmon, T4 and V-Band Fitment
An S300 needs three flanged connections: the turbine inlet where it meets the manifold, the turbine discharge where it meets the downpipe, and the compressor discharge where it meets the charge pipe. Get any one wrong and the turbo is a paperweight until the right part arrives.
This guide covers all three, how to measure what you have, and the one mistake that catches more S300 buyers than any other.
The Mistake That Catches Everyone: 4.25" vs 4.62"
Start here, because it is the single most common ordering error on this frame.
- S200 / S300 turbine discharge: 4.25" marmon
- S400 T4 turbine discharge: 4.62" (117mm) half marmon
Those two numbers are 0.37" apart. In a photograph on a marketplace listing they are indistinguishable. Described in a forum post as simply "marmon," they are identical. They will not clamp together.
You are most at risk if you are stepping down from a 400-frame turbo, buying used hardware, or ordering a flange separately from the turbo. Measure the turbine housing outlet before you order. If you cannot measure it, send us photographs through the Turbo Spec Form.
Turbine Discharge — 4.25" Marmon
The marmon connection is a concentric, clamped joint that seals the turbine outlet to the downpipe without a gasket. It handles heat cycling better than a bolted flange and lets you clock the downpipe to whatever angle the install needs.
The Part
| Variant | Material | Tube Size | SKU |
|---|---|---|---|
| 3" SS | 304 stainless | 3.0" | BSI0030TURDISMARMFL |
| 3.5" SS | 304 stainless | 3.5" | BSI0035TURDISMARMFL |
| 4" SS | 304 stainless | 4.0" | BSI0040TURDISMARMFL |
| 4" Aluminum | 6061 aluminum | 4.0" | BSI0040ALTURDISMARMFL |
The flange and its quick-release clamp are sold together, so the joint is right the first time rather than assembled from two suppliers' tolerances.
Choosing a Tube Size
Size to the downpipe you are building, not to the turbine housing. A 3" downpipe is adequate on smaller S300 builds; 3.5" and 4" are the common choices as power climbs. Undersizing here creates backpressure that costs you exactly the response you bought the 300 frame for.
Stainless or Aluminum
304 stainless is the default for the turbine side — it lives in the exhaust stream and handles the heat. The aluminum option is lighter and is available in 4.0".
Turbine Inlet — T4 Bolt Pattern
Every True North Turbos TNT3R uses the standard T4 bolt pattern, a genuine four-bolt flange. Note the terminology: bolt pattern is correct here. The oil drain flange is specified by bolt centre, which is a different measurement entirely — see the drain section.
Open vs Divided
- T4 open (undivided), 0.88 A/R — one inlet, pairs with an open manifold
- T4 divided, 0.91 A/R — split inlet that keeps exhaust pulses separated, pairs with a divided manifold
Match the housing to the manifold you have. A divided housing on an open manifold carries the divider without delivering the pulse separation that justifies it.
What About T3?
Many S300s in service use a T3 inlet. TNT3R does not offer a T3 turbine housing, so replacing a T3 S300 means a new T4 manifold. See the cross-reference guide for everything that changes.
Weld Flanges and Gaskets
Compressor Discharge — V-Band
TNT3R turbos use a compressor housing with a 4" inlet and a 2.5" outlet.
The important principle: size the v-band to your charge pipe, not to the compressor housing outlet. The discharge flange is the transition between the two, and the pipe diameter is what matters for flow.
Common charge pipe sizes on 300-frame builds are 2.5" and 3.0". Larger pipe reduces pressure drop between the turbo and the intercooler, but there is no benefit to going larger than the rest of the charge air path supports.
How to Measure What You Have
Turbine Discharge
Measure across the outside diameter of the marmon flange face on the turbine housing, at its widest point. 4.25" means the 300 frame. 4.62" means a 400-frame T4 half marmon. 5.75" means a 400-frame T6. Use calipers or a good rule — the difference between 4.25" and 4.62" is small enough that a tape measure held by hand will mislead you.
Turbine Inlet
Measure the bolt pattern across the four bolt holes on the manifold face. T3 and T4 patterns are visibly different once you have both in front of you, but if you are working from memory or photographs, measure.
Compressor Discharge
Measure the outside diameter of the charge pipe you are connecting, not the housing outlet.
Complete Flange Checklist
| Connection | Spec | What You Need |
|---|---|---|
| Turbine inlet | T4 bolt pattern, open or divided | Matching T4 manifold; weld flange if fabricating |
| Turbine discharge | 4.25" marmon | 4.25" marmon flange and clamp, sized to your downpipe |
| Compressor discharge | 2.5" housing outlet | V-band flange and clamp sized to your charge pipe |
| Oil drain | 50mm (1.97") bolt centre, mid frame | 10AN or 12AN drain flange — see the drain guide |
| Wastegate | Application dependent | External wastegate and dump tube on open or divided housings |
Why Buy Flanges From Black Sheep Industries
- Machined in-house in Canada, not resold imports
- Flange and clamp sold together as one part number
- 304 stainless and 6061 aluminum options
- Three tube sizes on the turbine discharge so the downpipe suits the build
- Priced in CAD with US customs and duties covered
Related Guides
- S300 Turbo Guide: Sizes, Specs, Flanges & Fitment
- S300 Turbo Oil Drain Guide
- S300 Turbine Wheels and Housings Explained
- S400 Turbo Flange Guide
- The Origins of the Marmon Flange
- The Evolution of the V-Band Clamp
Frequently Asked Questions
What size marmon flange does an S300 use?
S200 and S300 turbine housings use a 4.25" marmon discharge. This is not the same as the S400 T4's 4.62" (117mm) half marmon, and the two will not clamp together despite looking similar.
Will an S400 downpipe flange fit an S300?
No. The S400 T4 half marmon is 4.62" and the S400 T6 is 5.75", against 4.25" on the 300 frame. None of the three are interchangeable. Measure the turbine housing outlet before ordering.
Does the 4.25" marmon flange come with a clamp?
Yes. Black Sheep Industries sells the flange and its quick-release clamp together as one part number so the joint is correct on the first assembly.
What tube size should I choose for the turbine discharge?
Size it to the downpipe you are building rather than to the turbine housing. 3" suits smaller S300 builds; 3.5" and 4" are common as power climbs. Undersizing creates backpressure that costs low-RPM response.
What size is the compressor outlet on an S300?
TNT3R turbos use a compressor housing with a 4" inlet and 2.5" outlet. The compressor discharge v-band should be sized to your charge pipe, not to the housing outlet.
Is it "bolt pattern" or "bolt centre"?
Both, depending on what you are describing. The turbine inlet uses a genuine four-bolt pattern — T3 or T4. The oil drain flange is specified by bolt centre, the distance from the centre of one bolt hole to the centre of the other on the drain pad.