RPM...Have to laugh!

Hey all. Anyone need a good laugh?

I bought an old boat 6-7 years ago.

At full speed the tach read about 4500rpm. I figured I had a bad tach. Engine sounded great.

Well two years ago the tach died completely. I don't use it too much, so who cares.

I finally got around to replacing it with a custom one I made. I calibrated it to V8 standards for hei...1 pulse per spark, so rpm = signal frequency*60/4 (sparks per rpm) I did this with a bench source to be super accurate.

So, I installed it, and at full throttle it read 4krpm! What the heck??


I just oscilliscoped the current into the dist, the 'tach' output and checked the rpm at different points. The engine is actually going that slow!!!!

So, I did have a bad tach initially, just not in the way I thought! I suspect I have the wrong ratio in the leg, 'cause it's a alpha 1 and should be a prealpha. I'll have to check it out the next time I pull it off.

At 4k I'm doing 35 knots (21' Glastron) so not bad...wonder how fast she'll go spooled up!


Chay

Re: RPM...Have to laugh!

Glastron_V210 said:

So, I did have a bad tach initially, just not in the way I thought! I suspect I have the wrong ratio in the leg, 'cause it's a alpha 1 and should be a prealpha. I'll have to check it out the next time I pull it off.

At 4k I'm doing 35 knots (21' Glastron) so not bad...wonder how fast she'll go spooled up!


Chay

Click to expand...

There isn't any difference in a pre-alpha/alpha I that would cause a gearing difference. As long as you have a 1.50 gear ratio, it should be the same. I'd say you have the wrong prop. If 4K rpms is all you are getting, you are over-propped. Most V8's have a WOT range in the mid 4K's.

Re: RPM...Have to laugh!

The other variable is the engine itself. You said its an old boat but you didn't really give any details about the engine itself. If its not "hittin' on all cylinders" and completely tuned, it may just be incapable of turning it's rated rpm. Just a thought.

I suspected my 2004 200LX Stingray's tach when I bought it used. I installed an Autometer tach mounted to the steering wheel column just to compare readings. They were 200 rpm apart. I guess I'd need a third tach to actually know which one is reading correctly. Your homemade, calibrated tach sounds like a quality piece of equipment. I'd bet it's correct.

Re: RPM...Have to laugh!

Using a "Scope" is FAR more acurrate than any analog tach!! (assuming it's calibrated)

You are definitely "over-propped". What ever prop you have reduce the pitch to the next (2") lower pitch and try it. Your V-8 (if it's a V8) should be turning about 4600-4800 RPM at WOT.

Re: RPM...Have to laugh!

This was a while ago, but I wanted to thank you for your responses.

It's a merc 260 (350 chev).

I guess I should prop down eh.

When I scrape together a few bucks I will.

Chay