Good slip = Bad tach?

[Rick Stephens]

Just finished up break in hours on a new 1.94 ratio SEI Alpha 1 drive on my 3 Liter. Finally got to go to WOT. Hate to say it, but I got TOO good numbers, if that is possible :^)

Indicated RPMs 4500
GPS Speed 35 MPH
Wheel Pitch 17 inch
Ratio 1.94

This gives me in the neighborhood of 5-6% slip using a couple different calculators. That seems unlikely, especially with a no frills Michigan Wheel 17 inch 3 blade. I can say that is 3 MPH better than I got with the old 1.98 Merc OEM leg and without the Smart Tabs installed. New SEI leg and new Smart tabs seem to make a lot of difference. And that with a pretty tired 25 year old 3 liter.

What do you think? Couple hundred more RPMs than indicated likely? More than that? If so, do I need to replace the tach? I know the tach at idle disagrees with the tach built into a Snapon timing light by a couple hundred on the plus side for the built in. I'm afraid that not knowing actual WOT RPMs is possibly dangerous to the motor. The timing light tach quits reading around 2000, for whatever reason, or I'd make a comparison.

Ideas welcome.

Rick

 

[achris]

Yes, sounds like the tacho is reading low (unless you were running down a river with a 3 knot current)... Dash tachos are inherently inaccurate... Get a 'shop tach' and check again... based on the other numbers, the revs are more likely to be around 4800... But if you're rarely at WOT, be happy with the way it's set up now... I hate over-propping...

Chris...

 

[Bondo]

Yes, sounds like the tacho is reading low (unless you were running down a river with a 3 knot current)... Dash tachos are inherently inaccurate... Get a 'shop tach' and check again... based on the other numbers, the revs are more likely to be around 4800... But if you're rarely at WOT, be happy with the way it's set up now... I hate over-propping...

Chris...

Ayuh,..... I like to see 4800, or so, light load, trimmed out, 'n Flyin',.....

 

[Rick Stephens]

Thx!

I don't run WOT other than for a few minutes of 'thrill' now and then. For the most part cruise is as high as I get. So propin' up isn't the issue...other direction is more the problem as I often run a 15 inch 4 blade when the kids want to wakeboard or ski. Which is where I'd REALLY want to be careful my tach was in the neighborhood. I'll have to find a shop tach to use. No longer own one, for some reason. Must of lent it to someone who needed it more than I.

Much grass gentlemen. I appreciate the feedback.

Rick