The facts.
| Brand | Akrapovic |
|---|---|
| Part number | E-K4R2 |
| Fitment | Kawasaki Ninja 400, Ninja 500 and Z400 (2018–2026) |
| Type | Optional header — requires the Akrapovic Slip-On Line |
| Materials | Stainless steel |
| Weight | 1.59 kg where the standard header assembly is 4.08 |
| Power | 49.5 PS with the slip-on, from a standard 44.3 |
| Noise | 98.9 dB against 86.6 standard, at 5,000 rpm |
| Installation | Sixty minutes |
| Road approval | None — closed course competition use only |
DescriptionThe whole story▼
Five point two horsepower on a 44 PS motorcycle. That is nearly twelve per cent, and it is the largest relative power gain of any header in this catalogue — because a small Kawasaki twin loses more to its emissions equipment, proportionally, than a big engine ever does.
Key Features
- 49.5 PS with the matching slip-on, from a standard 44.3
- 38 Nm, gaining 3.4 measured at 9,300 rpm
- 1.59 kg where the standard header assembly weighs 4.08
- Stainless steel throughout
- Static noise of 98.9 dB against a standard 86.6
- An hour of workshop time
- Closed course competition use only
Twelve Per Cent
44.3 PS to 49.5. On a litre bike five horsepower is a rounding error; on an A2 machine it is a different motorcycle out of a corner. Kawasaki's standard header carries a large catalytic converter sized for Euro 5 on a 399 cc twin, and taking it out is where nearly all of this comes from.
Torque follows the same pattern: 35.8 Nm to 38, with the gain read at 9,300 rpm.
Two and a Half Kilograms
4.08 kg down to 1.59. That is 61 per cent, and on a motorcycle of about 168 kg it is a meaningful fraction — taken from the lowest and most forward point of the chassis, under the engine.
On a small twin used for club racing or track days, weight in that position is worth more than the same number bolted anywhere else.
Twelve Decibels Is the Price
86.6 dB standard to 98.9 at 5,000 rpm. Kawasaki builds these bikes quiet, and this header undoes that comprehensively. Circuits with a 95 dB static limit will refuse it, and no insert is offered to bring it back down.
That figure, more than the approval status, is what decides whether a given rider can use this part.
It Needs the Slip-On
Akrapovic builds this header as an upgrade path for machines that already wear the Slip-On Line. Bolted to Kawasaki's standard silencer it has nothing sensible to work with, and the published figures assume the pair.
Three Names, Nine Model Years
Kawasaki's 399 cc twin powered the Ninja 400 and the naked Z400 from 2018, and the 451 cc Ninja 500 replaced the 400 for 2024. The exhaust route did not change, so one header covers all of them.
That is unusually broad coverage for a performance part and it makes the Ninja 400 platform one of the easier small bikes to build up.
An Hour
Sixty minutes of workshop time. A small twin has two header pipes rather than four, and there is little in the way, so the job is straightforward once the fairing is off.
Road Approval
None. This product does not meet emission compliance requirements for street or highway use.
Specifications
- Part number: E-K4R2
- EAN: 3831126413156
- Bikes covered: Kawasaki Ninja 400, Ninja 500 and Z400, 2018 to 2026
- Type: optional header, for use with the Akrapovic Slip-On Line
- Made of: stainless steel
- Weight: 1.59 kg against 4.08 kg standard
- Power: up to 49.5 PS from 44.3 standard; 5 PS more at 9,550 rpm
- Torque: up to 38 Nm from 35.8 standard; 3.4 Nm more at 9,300 rpm
- Noise: 98.9 dB against 86.6 standard, measured at 5,000 rpm
- Workshop time: about an hour
- Approval: none; closed circuits only
FitmentWhich bikes this fits▼
- Kawasaki Ninja 400 (2018–2023)
- Kawasaki Ninja 500 (2024–2026)
- Kawasaki Z400 (2018–2026)
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Measure your head circumference about one centimetre above your eyebrows and compare it with the size chart on this page. Between sizes? Choose the smaller one — the padding beds in with use.
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