Tac Speed Cyl Adjustment Calculation?

[HuskerTx]

I have a 1990 4.3L. 190 HP. New 14X19 prop. Max rpm wot has been showing 3400. Found that the switch on the back of the Tac was dialed to 4cyl. Tried to change it to 6 but the tac does not work at all at that setting. At 8 cyl setting it reads way to high ( 5-6K- at 3/4 throttle).

Does anyone have any idea what the tac at 4 cyl setting calculates to a usable rpm speed at wot for this engine? What I'm trying to do is see if the engine is operating at its best at wot.
In the 18'8 boat I'm getting around 38mph at wot with that prop and two adults on board if that helps.

Thanks for your vast amount of wisdom here.

 

[Bondo]

Re: Tac Speed Cyl Adjustment Calculation?

Why not just Replace the Tach,..???.......

 

[bomar76]

Re: Tac Speed Cyl Adjustment Calculation?

Why not just Replace the Tach,..???.......

Convoluted solutions to simple problems are the norm here.....ddd

 

[bamadave]

Re: Tac Speed Cyl Adjustment Calculation?

I know I should bite my tongue and say nothing, BUT what the he!!, are you kidding? Why would anyone think that someone here would know what the tach would show if incorrectly set. Buy a new tach, install it, and guess what, you can figure all the calculation problems yourself.

 

[HuskerTx]

Re: Tac Speed Cyl Adjustment Calculation?

Buy a new tach, install it, and guess what, you can figure all the calculation problems yourself.

You guys are brilliant. I am always amazed at the help one can find on the internet. I NEVER would have come up with the "install a new tach" on my own. :^
Either way thanks for the responses and pushing me to the obvious. As a new powerboater I was of course looking for the simple and economical answer. Then again I now own a boat and there are no longer ANY simple and economical answers.

 

[hullofalottatrouble]

Re: Tac Speed Cyl Adjustment Calculation?

as far as the costs for boat repairs.. buying and installing a new tach can be about as easy and economical as it gets...

ebay up yourself a new tach and drop that thing in there..

8)

 

[Silvertip]

Re: Tac Speed Cyl Adjustment Calculation?

Or -- take the 4 cylinder reading, take the 8 cyilinder reading and 6 cylinder would be in the middle of that range. OR 4 cylinder to 6 cylinder is +50%. Or 4 cylinder to 8 cylinder is double. Or 8 cylinder to six cylinder is -25% or 8 cylinder to 4 cylinder is -50%. I think I covered them all eh???? But then I agree with the others -- a new or at least working tach is in order. But 3400 +50% = 5100 so the 4 cylinder setting +50% says you are spinning the engine a little fast. On the other hand 6000 (8 cyl reading) -25% = 4500. Since you said thats at 3/4 throttle, you still are spinning the engine too fast. Sorry to make you read all the other nonsense to get to the answer to your question but after all that you still need an accurate tach or you'll need a new engine when yours develops ventilation holes. Now - aren't forums fun? 8)

 

[HuskerTx]

Re: Tac Speed Cyl Adjustment Calculation?

Best response so far. I appreciate it. I have priced a new tach and plan to drop one in this weekend if time permits.
Thanks again.

 

[Silvertip]

Re: Tac Speed Cyl Adjustment Calculation?

Just funnin' with you. Sometimes we just can't help ourselves. Glad you took it that way. Some folks don't take kindly to that at times.