Basic Tachometer Info Needed

O

odSteve

New member
Hi, I plan to install a cable-driven tachometer. I have a Delco distributor with the cable drive option for my 392 V8. There are two different types of cable-driven tachs available from Isspro. I noticed for every full revolution of the distributor's rotor the distributor's cable drive would make one full revolution. Isspro's tech support could not tell me which tachometer to use. One version states .5:1 ratio and the other version is 1:1 ratio. I'm not clear - does .5:1 mean that the tach shows half the rpm's or shows double the rpm's because it expects to see the drive to spin at half the rotation speed of the rotor? I'm so confused I don't even know how to ask the question properly, especially taking into consideration the rotation of the crank in relation to the distributor. Which version of tachometer should I use and why?
Thank you.
 
You'll want the tach that matches the gear drive in the distributor. Put a paint dot on the tach and spin the engine 1 revolution. It should be easy to see if the dizzy rotated a full or half turn.
 
My engine is froze up and I'm waiting for delivery of another truck with a good engine I have on order. While waiting thought I'd work on the interior's dash. I have the Delco distributor I plan on putting in the other engine. I'm probably just too lazy to figure out how many revolutions the crank shaft makes for each one full revolution of the distributor's rotor, then figure out if I want a tach with a .5:1 or 1:1 ratio.
 
Unfortunately I've had several discussions with Isspro's tech support. We've determined the proper cable to go between the distributor and their tachometer. (Two different types in three different diameters of cable are possible). They have no info on using their tach on an old International V8. And that's about all they know - what their book says to use for each of the big 3 manufacturers' engines.

Just trying to figure out which ratio of tachometer reads properly. If the crank spins twice for every one rotor revolution, that means for one complete revolution of the rotor I would have 720 or two rotations of the crank shaft, correct? That means I would need the distributor's cable drive to rotate twice per one revolution of the rotor. Or have one cable rotation seen as two rotations by the tach. I would assume I need a .5:1 tach for a V8 because the cable is only going half the speed of the crank, or a 1:1 tach for a 4-cylinder? Does that make sense?
 
All 4-stroke engines (intake-compression-power-exhaust): the camshaft and distributor rotate once for every two crankshaft revolutions.
That's why the cam gear is exactly twice the diameter/ twice the teeth as the crack gear.....and the cam drives the distributor.
Engine RPM's are the same as crankshaft RPM's.
IE. 700 crankshaft revolutions = 700 engine RPM.
The number of engine cylinders (4 or 6 or 8) have no bearing on it.
 
1975IH200 said:
All 4-stroke engines (intake-compression-power-exhaust): the camshaft and distributor rotate once for every two crankshaft revolutions.
That's why the cam gear is exactly twice the diameter/ twice the teeth as the crack gear.....and the cam drives the distributor.
Engine RPM's are the same as crankshaft RPM's.
IE. 700 crankshaft revolutions = 700 engine RPM.
The number of engine cylinders (4 or 6 or 8) have no bearing on it.
Click to expand...
Ok, then should I be ordering an Isspro tachometer that the book lists as .5:1 ratio or 1:1 ratio? Keep in mind the distributor's cable tach drive turns one full revolution for each full revolution of the distributor's rotor.
 
OK Steve, I guess you just want the quick answer.
You want the .5:1 ratio tachometer head.

All the info needed is in your first post.
"I noticed for every full revolution of the distributor's rotor the distributor's cable drive would make one full revolution." and 392 engine which is a 4-stroke cycle engine.

Distributor with ONE full revolution with the drive of ONE revolution = two crankshaft revolutions of the 392 4-stroke engine.
The above ratio of 1:2 equals a ratio of .5:1

Understand? It's pretty basic, no other info is needed.
 
Thank you, yes I understand now.
I don't know why Isspro's tech support did not know which tach to recommend, but I know now. Perhaps I need to refer them to these posts so they learn.
Thanks.
 
Yeah, "Tech Support" isn't what it used to be.
I seems Isspro TS deals with GM/Ford/Mopar and are stumped by a reference to any other manufacturer.

So, age of the engine and manufacturer of the engine are irrelevant, as are number of cylinders. In regard to a CABLE DRIVEN tachometer.
 
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