ZF gearbox ratio

[Jon Tisdale]

Hi guys I have a 40 express with Cummins 480’s and a pair of ZF 280-1A gearboxes. Boat runs great at 1980 rpm at about 20 kts full load at around 40k lbs, burn the typical 12.6 gph a side at that speed in a good sea, and up at 2200-2230 I can see around 23 kts at 15 gph a side, so I know I’m propped correctly. I’m wondering if anyone has installed new gear boxes with a higher ratio for more top end speed? I have all electronic shift and throttle, micro commander helm unit, is it possible to swap transmissions to a higher ratio since the idle speed can be controlled electronically with the electronic trolling valve settings? Processors are cruisecommand from ZF. Frequently run offshore and to get 100 miles, 5 hours is a haul each way. I would like to be able to run just a bit faster. Also the boat seems to handle and track better at 23-24 kts than it does at 19-20, would love to see 28-30 kts. Just not sure if it’s feasible. Any input is appreciated

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[Jon Tisdale]

Thanks for the input

Thanks Steve, looks like I’ll be living with the speed I run now, til I find that Cabo 45 with 1100’s… ha thanks again for the response

[Steve Lewis]

Jon,

When I bought my boat, I had a thread that Tony contributed to.

https://www.sbmar.com/community/topic/we-bought-it/

His recommendation to me for these engines is that 15gph each @2300rpm is about the most you want to burn in a sustained fashion. Assuming longevity is the primary goal. In fact, he recommended adding some alarms as a precaution if we are going to run like that. You appear to be getting a decent turn of speed in return for that 30gph total @2300. If you were to change the gearing ratio you will change the engine loading and the fuel burn.

If I went from 1.77 to 1.5 on the gear ratio, I would be effectively loading the engine more like if we had more pitch. Therefore you will burn more fuel. Not to mention the amount of money that you would expend to acquire and refit the gears.

If you have the upgraded heads and want to run faster, you might be better off adding the water flow alarms, Pyrometers to measure EGT and get the 190degree coolant temp alarms. Also verify that you have the 160F thermostats.

That would be much less intrusive, more cost effective and you can run @2300rpm or slightly higher by keeping an eye on all of the life saving parameters for this model engine. It will be a trade off of life span for more performance. Do you want more speed or do you want these engines to last longer? That, ultimately is up to you.

This winter is going to be a busy one for us. New navigation electronics, Pyros and 1 more inch of pitch removed from the props. We are hoping, but not expecting, that heads will be available. I am mentally prepared to go another season with the existing heads and just take it easy.

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