Why Rev Limits Are… Limits

I am frequently asked, “Why doesn’t Mercury Racing increase peak rpm on its engines?” The desire is understandable: A higher revving engine would carry higher boat speed and improve getting on plane. But it is not that easy…

Pushrod valve train components at peace.

We electronically limit engine rpm on our pushrod sterndrive engines primarily due to valve train dynamics. A cam spanks a push rod; at its other end, the rocker gets a nudge; the rocker pivots and rolls onto the valve stem, compressing the valve spring and opening a hole to gas flow – either fresh air in, or spent combustion by-products out – and closes again. At our rev limit, each valve sees that input and return 46 times every second! As engine rpm increases, it reaches a speed where the chain of valve train hardware simply cannot keep up – even with good design and very exotic hardware.

With the evolution of components, we’ve taken GM-based, big block engines to outputs once only dreamed about. Cam profiles are continually refined. Hollow, end-hardened push rods replaced their heavier predecessors. Roller rockers replaced stamped ones. Screw compressors are fitted to stuff more air into the combustion chambers. Every change we make is thoroughly tested.

The rev limitation could be valve float – where the rocker loses contact with valve stem; however, often before float, physical loads on components cause an exponential decrease in life – if not outright, catastrophic failure. That’s unacceptable to us.

We have an obligation to provide durable, reliable products for recreation. We back them with limited warranties. That’s why we offer complete propulsion systems – from controls to prop nuts – all designed to work together. That’s why, and how, we validate them: as systems. Modifications to any component of the propulsion system may degrade its function, reduce product life or threaten your safety.

We are aware that an aftermarket “tuner” goes into our Engine Control Unit and modifies its map to increase the rev limit. We also know that the valve train will fail in these modified engines. It’s not if, it’s when. If you make that choice, know what you’re getting into. If “Mr. Tuner” offers warranty, it’s not likely to cover collateral engine damage, so demand it – and get it in writing. (You’re not under our warranty after a modification.)

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