How much horsepower will slip-on mufflers add?
Simulated rear‑wheel horsepower and torque — measured after the primary drive, clutch and transmission, the same way a chassis dyno reads a bike. These are not flywheel numbers, and they are not directly comparable to a manufacturer’s crank rating. Rev ceiling for this combination: 5500 rpm, limited by engine.
Where each one wins
| RPM | slip-ons | stock with catalyst | diff |
|---|---|---|---|
| 2000 | 78 | 76 | -1 |
| 2500 | 83 | 81 | -1 |
| 3000 | 87 | 85 | -2 |
| 3500 | 89 | 87 | -2 |
| 4000 | 86 | 84 | -2 |
| 4500 | 81 | 78 | -2 |
| 5000 | 75 | 73 | -3 |
| 5500 | 70 | 68 | -2 |
Torque, lb-ft, at the rear wheel. Everything except the part named is identical between the two runs.
Slip-ons and the stock head pipe, with the catalyst gone. This is the single most common first modification on a Twin Cam and the one with the widest gap between expectation and result.
The gain is small. The restriction that mattered was never only the muffler — the head pipe, the cam and the factory calibration are all still in place, and each of them caps what the exhaust change can deliver.
What you buy for the money is mostly sound, plus a small amount of top-end. That is not an argument against doing it. It is an argument against expecting a cam-sized result from a muffler-sized change.
The build
| Engine | TC103 (2010 CVO / 2012+) 3.875 × 4.375 |
|---|---|
| Crank / balancer | A motor — non-balanced (Dyna / Touring / CVO) |
| Pistons | OEM cast piston (stock dome) |
| Head gasket | STOCK OEM MLS (.030" compressed) |
| Cylinder heads | STOCK 2006+ (-06 casting) · 1.805/1.575 |
| Porting | As-cast / factory (no additional work) |
| Camshaft | Stock TC EFI (B) · 216/220 dur · 0.473" lift |
| Valve springs | Stock springs (as fitted to the heads) |
| Rocker arms | Stock rocker arms — 1.625:1 |
| Pushrods | Stock pushrods |
| Lifters | Stock hydraulic lifters |
| Induction | Stock 50mm Delphi EFI (2008+) |
| Exhaust | Stock head pipe + slip-on mufflers |
| Air cleaner | Stock airbox |
| Tune | OEM map (untouched) |
| Power adder | None (naturally aspirated) |
The other one
| Engine | TC103 (2010 CVO / 2012+) 3.875 × 4.375 |
|---|---|
| Crank / balancer | A motor — non-balanced (Dyna / Touring / CVO) |
| Pistons | OEM cast piston (stock dome) |
| Head gasket | STOCK OEM MLS (.030" compressed) |
| Cylinder heads | STOCK 2006+ (-06 casting) · 1.805/1.575 |
| Porting | As-cast / factory (no additional work) |
| Camshaft | Stock TC EFI (B) · 216/220 dur · 0.473" lift |
| Valve springs | Stock springs (as fitted to the heads) |
| Rocker arms | Stock rocker arms — 1.625:1 |
| Pushrods | Stock pushrods |
| Lifters | Stock hydraulic lifters |
| Induction | Stock 50mm Delphi EFI (2008+) |
| Exhaust | STOCK with catalyst |
| Air cleaner | Stock airbox |
| Tune | OEM map (untouched) |
| Power adder | None (naturally aspirated) |