Calibrating Tachs

Tuna Pursuit

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Got a pair of B series w/VDO Tachs. I'm getting my props done and I need to calibrate the tachs first. I have a tach pick up on the front of the engine. There does not appear to be a cal port on the tach. Do I adjust it on the pick up on the engine?
 

BillD

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Got a pair of B series w/VDO Tachs. I'm getting my props done and I need to calibrate the tachs first. I have a tach pick up on the front of the engine. There does not appear to be a cal port on the tach. Do I adjust it on the pick up on the engine?

Tuna,

I had the same VDO tachs with my 370s.
I don't think there is an adjustment on the tach and adjusting the tach pickup on the engine will not calibrate either.

I may be wrong, but the best you'll be able to do is "photo tach" your rpms on each engine at various rpms and write down how accurate your VDOs are to the actual engine rpms. You can then make a "cheat sheet" for variations if any between "actual" and "reading".

The VDOs on my Blackfin were off +-150 rpms at various actual rpms.

When I repowered with the new 370s I switched to Aetna tachs which are accurate +- 1 rpm.

IMG_3640.jpg
 

BillD

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Got a pair of B series w/VDO Tachs. I'm getting my props done and I need to calibrate the tachs first. I have a tach pick up on the front of the engine. There does not appear to be a cal port on the tach. Do I adjust it on the pick up on the engine?

Tuna,

Just a suggestion.
If you are going to tweak the props to gain some WOT rpms or whatever rpms you are looking for (3100-3150 WOT for the 370s is GREAT with the boat HEAVY).......

take the time to get ALL your present observed WOT rpms for the prop shop accurate to +- 1 rpm.

If you have any doubt using the VDOs, use a photo tach on the front dampener pulley.

I have a photo tach for you to use and could give you a hand.

Bill D
 

BillD

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That's kinda crazy. Were the digital tachs expensive? I have the laser tach coming so I can get a real reading and see how bad they are.

Thanx

What's crazy ? My Aetna tachs?
Or my idea of propping your 370s to 3150 WOT ? ;)

Aetna Engineering Digital Tachometers

$287.50 per tach

You can use the same tach senders as on the engines now.
Simple hookup.
I can give you a hand if necessay. :D
 

Tuna Pursuit

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No No, crazy that they would put tachs you can't calibrate. That's a very generous offer. I had a conversation with Larry at accuprop. He educated me about using the laser tach and the importance of accurate RPM readings. I did buy a laser/photo tach on ebay, $30 good to have. I'm going to try to get it done next week after the weather passes. Gotta be carefull though, probably be alot of stuff pulled out of the marshes out in the bay after the storm & very very high tides.

Thanx for the help.
 

BillD

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No No, crazy that they would put tachs you can't calibrate. That's a very generous offer. I had a conversation with Larry at accuprop. He educated me about using the laser tach and the importance of accurate RPM readings. I did buy a laser/photo tach on ebay, $30 good to have. I'm going to try to get it done next week after the weather passes. Gotta be carefull though, probably be alot of stuff pulled out of the marshes out in the bay after the storm & very very high tides.

Thanx for the help.

The Aetna tachs are used by many many boat owners.
The tachs are "digital" +- accurate to 1 rpm.

The good thing is the tachs eaither work ro they don't.

Simple.

Seriously, if you need a hand with the 370s, let me know. It takes two
"experienced operators" to get the rpms just right.
And you need someone who has the courage tp kneel between a pr. of 370s screaming @ 3000 +rpms measuring rpms!!!:D
You are a very short ride away.

And make sure Larry and his son gets you the rpms your looking for !!

Accutech does nice work, good people. Long ass ride now that he moved to Dover NH from Hampton !
 

Tuna Pursuit

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After further research my tach pickups are on the front of the engine and I beleive is a mag pickup working off the cam gear and counts the teeth on the gear. Therefore the tachs should be right on. I will find out when my laser tach shows up.
 

BillD

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After further research my tach pickups are on the front of the engine and I beleive is a mag pickup working off the cam gear and counts the teeth on the gear. Therefore the tachs should be right on. I will find out when my laser tach shows up.

Sounds like your 370s have "mechanical tachs" and are cable driven.
Same as if you have engine synchroinizers hooked up.
Should be very accurate. BUT double check with the photo tach
 

Tuna Pursuit

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No they have magnetic pick ups off the cam gear on the front of the engine. It generates pulses made by the teeth of the gear. So as long as you know how many teeth on the gear set by the dip swithes on the back of the tach it should be very close. But i will definetly post the results when I hit is with the photo tach.
 

BillD

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No they have magnetic pick ups off the cam gear on the front of the engine. It generates pulses made by the teeth of the gear. So as long as you know how many teeth on the gear set by the dip swithes on the back of the tach it should be very close. But i will definetly post the results when I hit is with the photo tach.

Corrected, understood.

Normally the tach sender on a 6B?BTA is behind the aftercooler just above the flywheel.
Counts teeth.....159 on a Cummins 6B
 

Tuna Pursuit

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Corrected, understood.

Normally the tach sender on a 6B?BTA is behind the aftercooler just above the flywheel.
Counts teeth.....159 on a Cummins 6B

Yes my old 300's had that set up. But my buddy tells me the on on the front does the same thing off of the cam shaft gear. We'll see Bill.
 

captainlarry84

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agreed PHOTO TACH

The only correct way to do it is with a photo tach. I own one. Every year I photo tach the boat to make sure nothing has changed. They are not expensive & easy to us. On boats that I could not read the reduction gear plate, you can use the photo tach on the shaft to count the turns.

Photo tach your boat and give that RPM to the propeller guy. Lastly before doing any of this stuff I always start with fresh fuel filters, clean bottom & full fuel. The less you re- pitch wheels the better.
 

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Photo tached the boat today. Pretty good up to about 1600 but by the time you are WOT off apx 80rpm on one & 125 on the other. That's no load. Since I don't see any adjustment on the tachs I'll have to use the photo tach to give the prop guy the correct readings.

Thanx for all the input.
 

BillD

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Photo tached the boat today. Pretty good up to about 1600 but by the time you are WOT off apx 80rpm on one & 125 on the other. That's no load. Since I don't see any adjustment on the tachs I'll have to use the photo tach to give the prop guy the correct readings.

Thanx for all the input.

Bingo, nice to know the real numbers !
You staying in the water all winter ???:confused:
 

Tuna Pursuit

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-Latest update-

Under load at 2000 rpm the tachs are within 30 turns.

WOT no load one is 3135 the other 3030 with the photo tach.

WOT under load one is 2735 the other is 2675 reading boat tachs.

I was alone & didn't want to crawl down in engine room while WOT on AP. Anyway, not a perfect analysis but I definelty need some prop work. I would say I need another 300 turns min. I will probably wait until next season after the bottom is done and I only had 3/4 fuel but this is a worth while exersise for sure. My engines are way too loaded right now.
 

BillD

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-Latest update-

Under load at 2000 rpm the tachs are within 30 turns.

WOT no load one is 3135 the other 3030 with the photo tach.

WOT under load one is 2735 the other is 2675 reading boat tachs.

I was alone & didn't want to crawl down in engine room while WOT on AP. Anyway, not a perfect analysis but I definelty need some prop work. I would say I need another 300 turns min. I will probably wait until next season after the bottom is done and I only had 3/4 fuel but this is a worth while exersise for sure. My engines are way too loaded right now.

Agreed, you are over propped by a minimum of 150 turns.

btw, I was free to help you ALL day. I would have photo tached the engines for ya, ear muffs and all !:D
 
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