Tach settings update (medallion)

I spoke with Scott Frye at BorgWarner Instrumentation, formerly Medallion, and here's what he had to say about the Tachometer settings:

1 - 4 cyl.
2 - 6 cyl.
3 - 8 cyl.
4 - 10 pole
5 - 12 pole

Everybody has given me a different answer up to this point. The problem is, now that I have an answer, I'm even more baffled. My tach was set at 3 when I bought the boat, and it was telling me 7200 @ 75 mph with 28" chopper. Sounded like bull****, so I set it to 4, under advice from a S&F member. the 4 setting makes it read approx. 5700 @ 70 mph, same prop, which sounds correct. But, according to Scott, and what others have told me, the "12 pole" setting is the right one, and Scott says that "5" is 12-pole. I'm curious what the Tach will read tonight when I take it for a ride... -Chad

[Euroski]

when I purchased my STV it had the same tach as you and the former owner was running a 93 2.5 merc. It was set to "5".

[Techno]

My guess is 4800 RPM.
An 8 cylinder is like an 8 pole so it equals 57,600 pulses
a 10 pole = 57,000 pulses
57,000/12poles= 4,750 rpm
That is if my math assumption is correct.

That's what I thought, too, but it's almost physically impossible. I found an invoice that says "add pitch." I would assume the prev. owner had some extra cup added. But, it would require something like 4" more pitch to produce those figures at the speeds it's running. Something isn't right. This has me FRUSTRATED. But, oh, hell, the thing is fast, and that's what matters. I would like to get to the bottom of this, though. Thanks -Chad

>==FFWD====>

[Dan Dillmann]

Well, my tach is set to 1 which thinks thinks the engine is a 4 cylinder which means when I show 7200 rpm I'd have 28800 pulses and with the tach set to 5 I would only be turning 2400 rpm. I checked my tach and it was right on, so something does not seem right here. Also, my tach only had 4 positions to choose from. I will double check the accuracy of my tach.

'87 Vegas XT/'87 Merc 220 Laser EFI

[Techno]

Dan, you must have a different model from Pyros because you only have 4 positions. He has 5 and I have 6 positions(OMC). So the #s must have a different meaning on the different tachs.
With the pulses thing, I'm assuming the tach has to divide a set amount of pulses from the engine.

On your other post you mentioned using an inductive pickup tach. How did you mount this? I might be able to get one from work (if I ever get my boat done) but wouldn't want to take the flywheel off.

Well, I was out on the lake again today running the **** out of the merc. My girlfriend CAN'T SWIM, if you can beleive that. But, she trusts me with her life, and she enjoys traversing the lake at 70 mph. I am beginning to doubt whether that medallion guy knew what he was talking about. Apperently, they must have made several different tachs, and he was talking about the one he's most familiar with. I don't care what he says, my tach seems to be giving me correct-looking figures at the "4" position. I can't verify this, but consider the following: I set it on the 5 position (12 pole according to the medallion guy) and it reads 600 at idle, and the motor starts to howl right around "1800." Bull****! A couple of S&F members say that 2.4 / 2.5 mercs start to howl loudly as you come up on 3000 rpm, which is exactly where it starts to sound like a merc with it set at the 4 position. Finding answers to this issue is harder than getting Hewlett-Packard scanners fixed. I was delighted to notice that my bow light is nowhere in sight. The side comparments on the VegasXT don't have any stops at the stern, so it must have slid to the back when I was planing off, went behind the gas tank, and into the bilge! S.O.B.!!! I don't feel like taking the gas tank out. I'm sure ther's a couple of beer bottles back there to keep it company. -Chad

[Ray Neudecker]

Medallion has made many different tachs over the years.The best trick we have found to calibrate is to hook up a tach which you have the proper info with to set correctly and then change the position on the old one to read the same.

[Dan Dillmann]

Techno,
I use a Blue-Point (Snap-On) MT586AK multimeter which has a RPM reading function. I have also used a Snap-On inductive pickup tach. They both work the same, you just clip the inductive pickup around one of the sparkplug wires and switch the tach to read 2-stroke which calculates RPM based on one pulse per rev. I borrowed these tachs from work.

What if I wrapped a couple of turns of wire around a spark plug wire to pick up a weak inductive signal, and hooked my Fluke Multimeter up to it on the "Hz" setting? Cycles per second would be RPM. Would that do the job? -Chad