Low tachometer readings

[Jimxb02]

I have a XB2002 with a 225 ProMax and a 7800 RPM rev limiter. I am running a 26" stock chopper II at a speed of 93.5 GPS. I show 7200 RPM on the tach ; but if I do the math, the tach should be showing 7600 RPM. The tach is a Merc performance products 10000 RPM tach. Could it be that far off, reading 400 RPM low ????

Jim

 

[chad202]

Stock props are almost never what's stamped on them. If you throw 27 in there, it comes out to 5% slip which is what my numbers show on my XB2003.

 

[John Richied]

Jim, Your tach does seem to be off around 200-300 rpm’s, but it could be a verity of things. The prop pitch, the tach, the Gps?

I had the Merc 10K tach in my XB-2002 hull when I bought it. I also had another Merc 10K tack that came with my Merc engine. I traded both the Merc 10K tachs for Allison tachs (not sure what brand they are) that only goes to 8,000 and it seemed to read a few hundred less. I could be wrong but I think it is more accurate, plus it says Allison on it

If you want to trade your Merc 10K tack for one of my Allison 8K tachs you could give that a try.

I think I may need a tach that reads over 8K soon anyway.

 

[John S]

John Richied wrote


I think I may need a tach that reads over 8K soon anyway.



Why is that? :twisted: :twisted:

 

[John Richied]

Well because my engine never revs over 7750, so my 8,000 tach works great… IF it does ever rev over 8K in the near future, and it just might…I will need a tach that goes over 8.000. :wink:

 

[Jimxb02]

Thanks for the offer John, but I think I will hang on to my 10K tach. I may also need the higher reading for a future project.

What I'm going to do is calibrate my tach using a friends tach. It also sounds like from your experience that these 10K tachs may read high in the upper mid range.

Thanks again for your response!!!

Jim

 

[John S]

As I said about a month ago, I wish I had a cd with a merc spinin 10 grand, because it was the coolest thing I ever heard. Good luck with whatever you do.

John S


Also, on my sled I used to turn the tach sideways for a quicker view of my max r's, and had to recalibrate it for that. I was told most cheaper tachs are off a few. Just something to think about.

 

[froggy]

My Allison tach is a Medallion Comp unit (its printed on the back). Pretty good tach overall.

froggy

 

[Blueally XB21]

A guy I used to race with said the old Merc Tach's (non-digital) are off ~400. He said Merc does that on purpose. Personally, I don't know. Just something to maybe call Merc about?
Good luck. BA

 

[Jimxb02]

Thanks Blue!!!!


That's about the kind of error I think I'm getting. With different props, GPS's, and weather conditions an error indication of something around 300 -- 400 RPM too high is what I'm getting when I do the math.

Thanks to everyone who responded.

Jim

 

[wj225]

Merc. Tach

When I had a Merc 10k tach and the stock 6750 limiter on the ProMax, the tach was dead-on at 6750 everytime it hit the limiter.

Bill