6BTA max rpms under load

[J Dragon]

315 6bta, denso pump, 36′ downeast.

I want to go from a 4 blade to a 5 blade prop to get more speed at cruise rpms.

I understand that I want to be at 2925rpm fully loaded, WOT.

I discovered yesterday with a strobe that, at cruise rpms between 1800-2400, my tach consistently reports 150 rpms higher than real engine rpms (ie, 2400 at gauge is 2250 by laser tach on harmonic balancer)

My tach is showing 3200rpm at WOT (so assume 3050ish real rpms, but want to verify)…maybe even more. I didnt push past those rpms because I was worried about spooling the engine up too much. And so I did not get a photo tach reading at WOT either, pending this question:

Any harm running the engine up over 3000-3100rpms under load for 30 seconds or so to get my “real” WOT rpms? I believe not because of how the governor works…but I wanted confirmation.

Thanks all

Topic

[David Wesner]

Thanks. But, I don’t think that is an adjustment. THat’s the set up for a mag pickup tach.

D

[J Dragon]

I have my tach adjusted to spec per this document:

Beede Tach Settings & Cummins Flywheel Tooth Count

Still waiting for the right friend along with me to get my WOT phototach ##s.

Will just use the tach card for now and fiddle with the tach later.

[David Wesner]

Is the tachometer adjustable? Or, do you just have to have your “tach card” handy.

[pwrobert]

Tach Callibration from another view

You might consider installing a Noland RS11 and a magnetic flywheel pickup on your engine. Not only will your RPM display in digital format on your chartplotter or PC you can add digital info for other critical things like Engine Temperature and Oil Pressure.
It’s an easy and exact cross check between the Digital reading and your analog in dash tach. Many analog tachs have an adjustment feature on the back that can get them synched.

I feel you can trust the digital reading because it’s setup only asks for the number of teeth on the flywheel (159 for the 6BTA).

I have in dash tachometers and the Noland setup and it allows me to create a page in my Garmin that is all instruments. Oil pressure, Engine Temp, fuel info, trip info etc. While I don’t run with that screen up all the time, I do pop it up from time to time to double check my in dash gauges. All through NMEA 2000 output from the Noland RS11 and my Maretron fuel tank and fuel burn sensors. But] even with only the basic RS11 you could have your RPM and more.

[J Dragon]

Clark I agree and will do.

Details to follow for those on the edge of their seats.

[Clark Leighs]

For proper propping you need to KNOW the actual revs. Tachs are not allways linear in their errors.
Put in the effort to take the boat out for a real warmup run and then do the WOT throttle test unloaded AND loaded.
Take a friend to operate the boat for the few seconds each will take.

For the change you wish to make you need accurate info, not guesstimating.

I am suggesting worst case but haywire info could leave you with an expensive door stop.

[J Dragon]

Thanks Rob. I don’t have to bring it to WOT neutral if everyone thinks I can assume the delta between real and tach rpms is the same at 2600 as it is at 3000. At 2600, the tach is off 150 rpms (real is 2450).

Last question, is there a max RPMs under load I should not exceed when testing for WOT under load? I didnt think so based on the governor, but want confirmation before I ring her neck…as it’s looking like my real WOT rpms are 3050+.

Thx

[Rob Schepis]

You can at the dock if at FULL operating temperature – both the coolant and the oil, and the only way to get to FULL operating temperature is to get her out on a real sea trial under cruise load. My car has an oil temperature gauge and my office is 11 miles of back roads from home. If I drive to work “casually” the oil temp only gets to about half of normal temp that is sees under real driving. If for some reason you feel the need to go WOT neutral best would be to be out on the bay and bring her to rest after a good cruise load run and do it … if you are so inclined.

[J Dragon]

Thanks Tony

Can I run it up “wot” no load at dock in neutral or just run to say 2400 and see the delta between tach and real then extrapolate out when running WOT under load?

And, if I’m over 3000/3100 real RPMs under load … any issue pushing her there to confirm max RPMs?

[Tony Athens]

Calibrate your tach iN NEUTRAL at the dock….Make a “tach card, ‘Real vs not Real” if needed………………………Then it’s easy and safe to CHECK REAL WOT under load.

Real RPM is REAL RPM , whether in neutral or under load is the same. And when at WOT under load, it should only take 10-15 seconds max to see how high the engine really goes. Making an honest 2900-2925 is what you want and want it EASY.

[Author]

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