How to Mock Up a Turbo Manifold Before You Cut Pipe

A tubular turbo manifold is a three-dimensional puzzle. Every primary has to leave the head, clear the steering shaft, chassis rail and engine mounts, and reach the collector at the right angle, ideally matching its neighbours in length. Then the turbo has to land where the oil drain, downpipe and intake can live with it. Here is how to plan a manifold (or header), mock it up, check it and build it in metal.

Why mock up a turbo manifold first

Every mistake on a manifold is paid for in stainless. An elbow cut to the wrong angle or turned the wrong way is scrap. A mockup settles the design where changes cost nothing:

• Clearance: does each route clear the rails, steering shaft and bulkhead?
• Equal length primaries: compare lengths properly, not with a tape wrapped round a bend.
• Collector angle: see how the primaries meet at the merge.
• Turbo position: the manifold decides where the turbo sits, and so the oil drain, downpipe and intake routes.
• Wastegate: an external gate needs a take-off, room for the gate and a dump pipe route.
• Cut list: you order the right elbows first time.

Plan before you start

Pipe or tube

• SCH10S stainless pipe with butt-weld elbows, sized by nominal pipe size (NPS). 1.25" NPS is 42.2 mm OD and 1.5" NPS is 48.3 mm OD, both with a 2.77 mm (0.109") wall. The thicker wall is favoured for high temperature turbo manifolds.
• Thin-wall mandrel-bent tube, sized by actual OD. 1.75" tube is 44.45 mm, with a wall typically 1.2 to 2 mm. It is the standard for naturally aspirated headers and plenty of turbo work.

The two don't mix: 1.25" NPS and 1.75" tube are only 2.25 mm apart on the outside, but need different blocks and fittings. Our guide to SCH10S pipe vs OD tube sizes explains the numbers.

1D or 1.5D bends

On butt-weld elbows (ASME B16.9), D is the nominal size. A 1.5" NPS short radius (1D) elbow has a 38.1 mm centreline radius (CLR), smaller than the pipe's own OD, and a long radius (1.5D) elbow 57.2 mm. On tube, D is the OD, so 1.75" tube at 1.5D is 66.7 mm.

Use 1D bends to get round obstructions and 1.5D bends on open sections, where they flow better. Most manifolds mix both. More in 1D vs 1.5D vs 2D bend radius.

How to mock up a turbo manifold, step by step

Work on the engine in the car, sitting on its mounts, with the steering shaft, lines and wiring fitted. The blocks are plastic and won't take any heat, so work on a cold engine and keep them away from anything hot.

1. Fit the flange adapters

Bolt the head flange to the head. Support the turbo (or just its inlet flange) roughly where you want it, on a stand or temporary bracket. Fit the flexible flange adapters into the ports, either in the head flange or straight into the head, whichever suits your setup. Every primary then starts from its real position.

2. Route each primary

Snap bends and straights together from the head outwards, one primary at a time. The joints are a friction fit with embedded magnets, so a run holds its shape while you build the next. Start with the awkward primary, such as the one fighting the steering shaft; the rest fit round it.

3. Rotate joints to clear obstructions

Every joint turns freely through 360°, so a 45° bend can point any direction around the pipe axis. Turning joints is how you tuck a primary under a rail or clear the oil filter without changing parts. If it still won't clear, swap a 1.5D bend for a 1D.

4. Balance the lengths

Primary length is measured along the centreline, from the head flange face to the collector. Two primaries built from the same set of blocks are the same length, whatever the order. When they differ, add up the pieces. Once joined, a straight is exactly its stated length, and each bend block is a pure arc, so its length is its CLR × the angle in radians.

Size	Radius	CLR (mm)	30° (mm)	45° (mm)	90° (mm)
1.25" NPS	1D	31.8	16.6	24.9	49.9
1.25" NPS	1.5D	47.6	24.9	37.4	74.8
1.5" NPS	1D	38.1	19.9	29.9	59.8
1.5" NPS	1.5D	57.2	29.9	44.9	89.8
1.75" OD tube	1D	44.45	23.3	34.9	69.8
1.75" OD tube	1.5D	66.7	34.9	52.4	104.7
1.75" OD tube	2D	88.9	46.5	69.8	139.6

Pure arc lengths at the centreline. Measure factory 45° elbows rather than assuming they are half a 90°.

To lengthen a short primary, add a 15 mm straight or swap a 1D bend for a 1.5D.

For a twin-scroll turbo on an inline four with the usual 1-3-4-2 firing order, cylinders 1 and 4 share one scroll and 2 and 3 the other, so each scroll receives pulses 360° of crank apart. Balance the lengths within each pair.

5. Plan the collector

Decide how the primaries merge: four into one, in pairs first, or two groups for a twin-scroll turbine. Bring each primary in at the same angle to the collector centreline, and avoid ending one with a tight bend right at the merge. The blocks take each primary up to the merge; plan the collector itself from where they finish.

6. Check the turbo, wastegate and downpipe

Then check the turbo end:

• The oil drain must fall steadily to the sump, with the centre section within the turbo maker's tilt limit.
• Oil and water feeds need a route.
• The compressor needs room for its inlet and intercooler pipes.
• An external wastegate needs a take-off that sees flow from every cylinder, usually at or near the collector. An internal gate needs actuator clearance.
• The downpipe needs a path past the bulkhead, steering and subframe.
• A heavy turbo on a long manifold may need a support bracket.

7. Record it

Before anything comes apart:

1. Photograph the mockup from every side, with a tape measure in shot.
2. Number the primaries and label each block with masking tape, in order from the head.
3. Note each joint's rotation relative to the one before, using the tick marks round each block end. They're spaced every 15°.
4. Write the cut list: every bend (angle and radius) and straight, in order, for each primary. The flat measuring gauges supplied with the kits are for marking those straight lengths onto the pipe when you cut.

Clearance checklist

The blocks match the pipe's OD, so what fits in plastic fits in metal. Still allow for the engine rocking on its mounts, heat shields or wrap, and weld beads. Check:

• Steering shaft and joints, at full lock both ways
• Chassis rails and subframe
• Bulkhead (firewall)
• Engine mounts and their brackets
• Oil filter, with room to change it
• Starter motor, with room to remove it
• Brake and fuel lines
• Wiring and sensors
• Bonnet (hood) line, with the bonnet shut
• Heat-sensitive parts: brake master cylinder, clutch hydraulics, plastic tanks, rubber hoses and boots

Transferring the mockup to metal

1. Cut the elbows to angle. Mark each cut square to the elbow's centreline, cut on a bandsaw or with a thin disc, then deburr. A 90° elbow gives two 45s or three 30s, but each cut loses a little angle (more on 1D elbows), so check every piece against its block. Wear eye protection. For a radius no elbow offers, pie cut it.
2. Work one primary at a time. Remove one primary's blocks and build its metal replacement in the same port, using the rest of the mockup as your reference.
3. Tack onto the flanges. Tack piece by piece from the head flange, checking angle and rotation against the blocks before each tack. Take the blocks off the primary you're building before any heat goes near it; they're plastic and won't survive tacking. Finish at the turbo flange, braced in position.
4. Check the fit against the clearance list, then move to the bench. Keep the head flange bolted to something flat and rigid so it doesn't pull out of true.
5. TIG weld with a back purge. Without argon inside the joint, the root oxidises ("sugaring") and ends up weak and rough. Seal the ends, purge, and weld in a sequence that spreads the heat. Use fume extraction for stainless.

Choosing a kit

Pipe size

Blocks from different sizes won't connect, so buy the kit that matches your material: 1.25" or 1.5" NPS SCH10S for schedule 10 pipe, or 1.75" for mandrel-bent tube. 2D bends are available as 1.75" add-ons.

Kit size

Contents are the same in every pipe size and change with the kit size:

	Starter	Builder	Workshop
Pieces in total	73	136	219
Bends (30°, 45° and 90°, in 1D and 1.5D)	36	72	115
Straights (15 to 100 mm)	28	51	83
Flexible flange adapters	2	6	8
Flat measuring gauges	5	5	10
Pipe guides	1	1	2
Carry case	Yes	Yes	Yes

The Starter suits one manifold with a straightforward routing, such as a simple four-cylinder layout; tight bays use more. The Builder, the standard shop kit, covers a full manifold with blocks spare to trial a second routing side by side, or to model six cylinders. The Workshop adds enough for V8 header work, or for keeping several projects mocked up at once. Add-on packs of straights and 30°, 45° and 90° bends (5, 10 or 20 per pack) top up whatever you run short on.

Tools that help

• Mockup kits: all sizes, split into full kits and add-on packs.
• 1.5" NPS SCH10S Builder kit: for turbo manifolds in schedule 10 pipe.
• 1.75" tube Starter kit: for a four-cylinder header or manifold in mandrel-bent tube.
• Mandrel bend angle marking guide: clamps over a mandrel bend (1.5" to 4.5" OD tube, in 1D, 1.2D or 1.5D) and marks cut positions every 7.5°.
• Variable mitre angle jig: for 1.5D mandrel bends in OD tube (1.5" to 4"). It clamps round the bend, and its angle key sets a stop anywhere from 10° to 80° for repeatable cuts. It is sized for tube, not SCH10S weld elbows.
• Y piece jig sleeves, SCH10S: mark 60° Y merges in 1.25" and 1.5" NPS, if you merge primaries in pairs.

FAQ

What are exhaust mockup blocks?

Also called header mockup or exhaust modelling blocks, they are plastic straights and bends made to the outside diameter of your pipe or tube. They join together so you can model primaries in place before cutting metal. Ours use a friction fit with embedded magnets.

Can I use the same method for a naturally aspirated header?

Yes. A header simply ends at a collector instead of a turbo flange. The 1.75" tube kits are sold as header kits, and the NPS kits work for headers built in pipe.

Do turbo manifold primaries have to be equal length?

No. Equal lengths keep exhaust pulses evenly spaced at the turbine, but plenty of good manifolds are a packaging compromise. A mockup shows exactly how far apart the lengths are, so the compromise is a choice, not a surprise.

Can I use 1.5" NPS blocks for 1.5" tube?

No. 1.5" NPS pipe is 48.3 mm OD, while 1.5" tube is 38.1 mm. Buy the kit that matches the material you will weld.