Tachometer

[StevensMan]

I'm still playing with gears on my alpha one outdrive - so far it looks like with 1:1.50 gear ratio and 3X16X16 prop 5.7 liter spins a little bit too fast - at least 4900 RPM WOT and I believe it may go a little bit faster. I'll continue experimenting with props and also I want to check if my tachometers reads accurately.
As I understand most tachometers are simple - they read negative pulses of coil. If so tachometers for 4, 6 and 8 cylinder engines should be different - 4 stroke 4 cylinder will have 1 pulse per revolution, the same 8 cylinder should have 2 pulses. Is this true or I misunderstand how tachometer works? Can I just get auto tachometer from junk yard and verify that tach on my boat reads correctly or there is a better way to acheive the same thing?

Thanks,
Anton

[heymagic]

I don't think they work quite like that, but it doesn't really matter. New tachs are about $50, I've seen plenty that are out of spec. Mine is about 8 years old, has less than 200 hours and reads from OK to 500 rpm fast. My buddy Ed's reads about 500 rpm slow. I would invest in a decent DVOM that has a tach function, not a lot of money. Sears, NAPA somewhere. You will find plenty of uses for the meter over time !! If you get down this way with the boat I'd be happy to check it against my shop equipment.
IF your tach is correct you need to add at least an inch of pitch. Gene

[SomeSailor]

I've got one if you want to validate your tach Anton.

That's along the lines of what I'd told him Gene. Getting it within 4800 would be a good idea. Not so much that you'll ever be running it that fast, but so your power band shifts down a couple hundred RPM.

You want that power in the mid-band, not way out at 5000 RPM.

[StevensMan]

Well, thanks for this great idea, yes, it is much better to check tach operation with multimeter.
I looked at mine, it does not have tachometer function but has frequency meter. I think it is pretty much the same. So I measured frequency at tach terminals. Results were ~62hz at ~1000rpm, 122hz at 2000rpm, 153hz at 2500rpm and not very reliable reading from 3000rpm and up.
So based on my calculations 122hz means 122X60=7320 pulses a second, there should be 4 pulses per revolution on 8 cylinder 4 stroke engine (4 stroke is 2 revolutions, 8 cylinders fire during 4 revolutions = 4 fires per revolution). So 7320 / 4 = 1830 rpm. That means that when my tach reads 2000 rpm engine has 1830 rpm in reality. 5000rpm indicated on tach means 4575 - this is my real WOT RPM.
It is right within the spec unless there is something wrong with my calculations. In fact I believe it was within the spec with 1:1.65 gears and cupped 16X16 prop. It is great to learn something new every day :lol:
Also, there is a switch at tachometers back side - 4 / 6 / 8 cylinders. So they probably work just like that - read negative pulses and adjust for number of cylinders.

Mike, 3300rpm at ~30MPH, I know you had 5.7 2 barrel carb and 16X16 prop, what was your WOT RPM and WOT speed with this prop? Do you think your tachometer was dead accurate or possibly was reading low?

Thanks,
Anton

[SomeSailor]

Ignition coils are pretty straightforward. Once the primary windings of the coil ramp up (primary) they'll "saturate" during the dwell period (the time that the points or pickup are closed). When the points (optical in your case) open the magnetic field that built up through the dwell cycle, collapse inwards and cut across the secondary windings. This large flux induces the higher output you see as a spark.

What you're measuring with a frequency meter is the opening of the primary (negative or positive... depends on your meter). A four-stroke V-8 engine will generate 4 pulses per revolution.

Each cylinder is fired once for every two revolutions of the crankshaft. This means that on a 8-cylinder engine there will be four normal firing pulses per revolution of the crankshaft; a 6-cylinder engine will be have three normal firing pulses per revolution of the crankshaft, a 4-cylinder engine will be have two normal firing pulses per revolution of the crankshaft, etc.

Whatever value you're getting, simply divide it by 4 (to get revs per second) and then multiply by 60 to get RPM.

I do believe that your tach could easily be off by that amount.... but I don't think you can assume you would have still been within range using your old upper gears. No matter how you calculate it.... you would be off by a pretty large percentage (too high).

Mine operated right at the top end of WOT with a 15.25x15 prop. You have the largest prop available for that drive and are just in the middle margin (4500). You were WAY high before... enough to blow oil out the dipstick. Switching down to the factory prop would likely get you at the upper end of WOT RPM now as well (4800).

My boat would run just at 42 MPH with a light load and no camper. That was just at 4800 RPM.

[StevensMan]

Great, thanks. I think your calculations are the same as mine.
When I said I was in correct range with old gears I mean with 16X16 cupped prop. With this prop I had 39mph (camper and crusing load) at 5100 WOT RPM indicated on flat water and no wind which should be ~4600 true RPM if my caclulations are correct. By "cupped" prop I mean the prop which I got for testing from PropShop, it was 3X16X16 and I could see that there were some extra cupping applied. The prop I have now is 3X16X16 without any extra cupping. With stock prop and old gears RPMs were way too high.

I'm glad that I changed gears, I feel that boat runs better this way and I'll have more prop options. I do not want to replace my tachometer unless I can find exaclty the same looking one for reasonable price. I plan experiment with different props more just for the hell of it to see what differense 4 blades could make.

Anton