Does this RPM sound right?

[fordhistory]

I'm running a '53 Merc flathead in my '40 sedan delivery. All stock engine, except I'm using a MSD electronic distributor. The neat thing about this distributor is that it has a tachometer lead. So, I bought an electronic tachometer and hooked it up. Now, I'm wondering if the tach is reading correctly.
At 50 mph, the tach reads 4,000 rpm. At 60 mph, the tach reads 5,000 rpm. The engine does seem to be "singing" at 60 mph, but is it really turning at 5,000 rpm? The rear end is a 3:54.
Can somebody please confirm these rpm numbers?

Henry

[40cpe]

You should be turning around 2800 at 60 mph with a 3.78 rear, so slightly less with your 3.54. Look for settings on the tach that indicate 4 cyl, 6cyl, 8 cyl.

[RalphG]

Sounds way high. I'd guess there is a setting for different numbers of cylinders. I have an old tach here like that.
On a related subject, would my old 12 volt tach work on the 6 volt Mercury system?

[Ross F-1]

I also have the MSD Ready-to-Run distributor. I run my tach off the (-) terminal of the coil.

[fordhistory]

Oh, I'll try hooking it up to the (-) coil terminal.

I'll let you know what happens.

[CarlG]

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Sounds like your tach is set for a 4 (or maybe even a 6 cylinder) application. My tach has settings for 4, 6 or 8 cylinders. -- and is hooked up to the negative coil terminal.

[Mart]

If you can state your tyre size there are online calculators that will tell you the revs. I'm just about to go out but can help later, but someone ought to be able to help.

[JSeery]

It's about double what it should be so most likely incorrect number of cylinders setting. Is your car positive or negative ground? The tach normally connects to the coil connection that goes to the distributor (points). It is counting the number of times the points open and close per unit of time. I'm a little confused as you stated your MSD system had a tach connection, if your connecting the tach to the coil where does the MSD come into play??

[fordhistory]

Those are all good questions! But I have no answers!
I disconnected the tach from the distributor's "tach line," and connected the tach to the (-) side of he coil. NO difference.
I initially had the switch on the back of the tach switched to "4," which was wrong. But when I switched it to "8", still NO difference.
This makes no sense. Maybe the tach is defective. The needle responds to engine rpm. It moves smoothly. But nothing changes its reading: always 5,000 at 60 mph.

[john in illinois]

With rear tire diameter this calculator will give rpm.

http://advanced-ev.com/Calculators/TireSize/index.html

John

[40cpe]

There are probably wires on the tach for light, ground, positive voltage, etc. Did instructions come with the tach? Can you give the make/model of the tach?

[fordhistory]

OK, I think the tach is bad. Based on the calculations at John's website (see above), at 60 mph, the old flatty is/should be turning about 2600, which makes more sense. So the tach is reading about twice that amount. AND, no matter where I put the switch on the back of the tach to: 4, 6, or 8, it still reads the same. By switching that switch, I should probably see quite a change in rpm reading, but I don't. So, I'm going to exchange it and try that. Also, I called MSD, and they said the tach should work off their tach wire or off the coil. No need for any kind of adapter/module to make the tach work. At least not on this "plug-and-play" distributor. I'll keep you posted. And thanks for all the good advice and info.

[Mart]

Yeah, If switching the switch makes no difference, there's something (obviously) wrong. on an eight, having it switched to four should double the revs, and pro rata for 6.

Mart.

[Talkwrench]

Could be you need a converter on some systems
https://www.mpsracing.com/instructions/MSD/8920.pdf

there is this type as well again depending on your system.
http://www.ebay.com/itm/MSD-8910EIS-...-/131454942397