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"VTEC" Idea in a boat?

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Old 02-26-2003, 10:25 AM
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Don't knock variable valve timing. It rocks.

In the Honda VTEC application, I beg to differ with you, but the VTEC motors DEFINITELY have more low end torque than they would if they were not equipped with the VTEC system.

Benz used a variable intake cam system on its inline 6's and 4-cam V8's thru 1999. Those motors have killer bottom and screaming topend. Those motors have been replaced with a 3-valve dual plug motor that uses a dual tuned intake for a similar effect. Lexus/Toyota uses an electronic version of valve event timing variation (they still do).

Cars, however, spend a good deal of time under 2000 rpm. There needs to be adequate, or even impressive, response and pulling power at those low engine speeds. Otherwise, buyers drive them and feel like the cars aren't powerful even though they might spit out 300+ hp on top.

Variable valve timing/selective intake valve actuation both offer the engine designer a useful tool to restore usable or impressive low end torque while being able to utilize significant valve overlap for topend performance.

Boats (not talking about tournament ski boats) never need to display big torque numbers under 2000 rpm. Most boats see the planing load at 2500-3000 rpm and top out at around 5000 rpm.

It is easy to cam a boat motor for a flat torque curve in a 2500 rpm window. This is easy enough to do without valves that turn on and off. Since we use single-cam OHV motors, it makes it impossible to offer variable intake timing. The best we could do in a domestic V8 is to offer total camshaft advance and retard which would prove effective, but not nearly so much as moving the intake separately from the exhaust.

As far as VTEC style selective valve actuation, exactly which valve would you choose to not operate? A boat motor has one exhaust valve and one intake valve per cylinder - it needs them all operational. To be able to use the VTEC system, it would need to be a multivalve motor.

Boat motors are always based upon easily available automotive longblocks. It keeps the costs down.


To rehash:
Boat motors need to produce torque within a relatively narrow rpm range when compared to an automobile application.
Boat motors do not need to produce prodigious torque right off idle.
Boat motors do not offer a feasible way to provide variable intake timing or selective valve actuation.
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