I'm beginning to think my tach is incorrect

[Baltimore Sailor]

The consensus on the A4 is that ~2200 rpm is normal cruising rpms. I, and many others, have posted that we are only able to get up to 1300-1500 rpms on our A4s even after fairly extensive maintenance and tune-ups, and ensuring that our props and bottoms are clean.

Today I performed a bit of a sanity test with my A4 by revving it up out of gear to where my tach said was 2200 rpm.

It was screaming. If the prop was turning as fast as that engine sounded, I think it'd be up on plane.

Now, I don't know how tachs work except that they connect to the coil and you set a switch on them depending on whether you have a 4-, 6- or 8-cylinder engine. In my case, the tach is sealed and all I have are the wires coming out of the center of the back of the case. There's no way I can tell how the tach is set.

My question is, if the tach was set for a 6-cylinder engine, would it read higher or lower when connected to a 4-cylinder engine? And, if it is set correctly, why does 2200 rpm sound so excessively over-revved in my case?

[Don Moyer]

You ask a very good question, and I would stick to trying to confirm your tachometer readings before trying to evaluate the sound of your engine at what you are showing to be 2200 RPM.

One thing that comes to mind is that you might find a local diesel mechanic with a digital handheld tachometer that is used to calibrate tachometers on diesel engines.

The reason diesel mechanics are inclined to have a handheld tachometer is that, without a distributor to "count" the number of times the primary ignition circuit opens and closes, diesel tachometers are connected to alternators for an RPM read out. Since alternator fields have different configurations and the alternators themselves operate at different RPMs, depending on pulley size, diesel tachometers have infinitely variable rheostats to set their RPM relative to an external tachometer indication.

Don

[SEMIJim]

Today I performed a bit of a sanity test with my A4 by revving it up out of gear to where my tach said was 2200 rpm.

It was screaming.

If it was truly screaming, then it was turning over a heckuva lot faster than 2200 RPM. 2200 RPM is nothing for a 4-cylinder engine. You have a car? Has it a tachometer? Or a friend or relative with a car with a tachometer?

Now, I don't know how tachs work except that they connect to the coil and you set a switch on them depending on whether you have a 4-, 6- or 8-cylinder engine.

They're a relatively simple thing, actually. With a 4-stroke engine (which the A-4's are), each piston will fire once every-other revolution of the crankshaft. So, on a 4-cylinder engine, the coil will fire twice per revolution. Three times per revolution on a 6-cylinder, four on an eight. All the tach does is count the number of firings-per-minute and divide by half the cylinder count. 2000 firings-per-minute on a 4-cylinder engine yields 1000 revolutions-per-minute. See: Simple .

In my case, the tach is sealed and all I have are the wires coming out of the center of the back of the case. There's no way I can tell how the tach is set.

Unless it is fixed for a 4-cylinder engine, then it cannot possibly be right.

My question is, if the tach was set for a 6-cylinder engine, would it read higher or lower when connected to a 4-cylinder engine?

At 1000 RPM on a 4-cylinder engine the tach would see 2000 firings-per-second. On a 6-cylinder it would see 3000. So if it was set for a 6-cylinder but connected to a 4-cylinder, it would read low by 1/3. In your case: At a tach reading of 2200 RPM you'd really be turning 3300 RPM. That's still not "screaming," btw. Tho perhaps it's pretty fast for the venerable old A-4.

And, if it is set correctly, why does 2200 rpm sound so excessively over-revved in my case?

Because it's not?

Where did you get this tachometer and what leads you to believe it's suitable for a 4-cylinder engine?

Jim

[Baltimore Sailor]

It came with the boat. Given the electrical expertise demonstrated by the PO's, this is not a good sign.

When I am cruising at what my tach says is 11-1200 rpm, it sounds "right." The engine isn't laboring and has a nice hum/throb to it that, if you know anything about engines at all, says "I feel good about myself."

I'll have to find a portable tach, or maybe just buy a new one for the heck of it. The current one looks old, and they're not expensive, so why not?

[sastanley]

BS, (heh heh, I couldn't resist)

I posted some questions in another thread about tachs, and someone reminded me a simple $30 from your local Autozone works just fine. My 1977 Catalina 30 did not come with a tach, and I have a tach laying around in my garage from my old Honda engine. I am just gonna hang it in the engine compartment and see what it reads...it has a switch (if I remove the tach housing) to set it for the number of cylinders, but most of them tell you right on the packaging which ones it is compatible with.

-Shawn

[MikeB.330]

Balt Sailor,

I have to ask, what prop are you using? Depending on the prop you're trying to spin, 1,300 to 1,500 may be all she's got in her.

Mike

[Baltimore Sailor]

It's a 3-blade 10" Indigo with a 7.5 (?) pitch. I'll be over at the boat tomorrow and I'll see if I can get some video of her either out on the water or tied up and in gear. That might help everyone figure out if that's all she'll do.

[MikeB.330]

That's the same prop I run on my fresh water cooled A4. I cruise at 2000 rpm with a max rpm of 2400 +-.The 2400 is when the batteries are topped of and the alternator is hardly putting anything out. Under load with the alternator putting out 70-80 amps I can still cruise at 2000 rpm but the top rpm is only 2300.

At 2000 rpm the engine sounds like it's hardly working and is very smooth.
At 2400 rpm it's starting to sound stressed but in no way screaming or abusive.

Mike

[Baltimore Sailor]

Are you saying that the alternator load can affect the top rpms available? If I'm reading you right, that may be part of my problem. My alternator only charges at about 13v, so is probably going all the time. I know I need to have it looked at, but I never thought it could pull down the rpms.

I did get some cell phone video of my boat under power tied up at the dock, but I haven't figured out how to get it off the phone yet (new phone). I'll be busy tomorrow, but maybe I can get it figured out for Sunday.

[sastanley]

Yes

Baltimore Sailor

One other present I have found with my boat is a busted regulator. What my father did was install a 3-way switch in the charging circuit:
pos 1 = no charging
pos 2 = about 4-5 amps using some random diodes/capacitors/resistors (sorry, I am no electrician)
pos 3 = no restrictions, wide open ~35 amps (original alt)

There is definitely a change in engine speed/load when switching from 4 amps to 35. There is not much of a noticeable difference going from zero to 4. It isn't hundreds of RPM, but you can hear the extra load being put on the engine.