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AmericanPackMule 01-05-2006 09:20 PM

Tach's - R. P. M.
Rev's ( Crank or Cam? ) Per. Minute

Rob Keller 01-05-2006 09:31 PM


SR66 :thumbup:

AmericanPackMule 01-05-2006 10:04 PM

1 - 5 - 4 - 2 - 6 - 3 - 7 - 8 -- 1 is a Rev. ( Cam! )
You are Right --- BUT! I Mentaly must Divide by 2 when I want to know How often # 1 is Igniting per Minute. ( 1,000 times / Min. @ 60 M. P. H.) { 16 1/2 times per Second.} Amazing

AmericanPackMule 01-05-2006 10:11 PM

Which reminds me of the little o'l lady that was pulled over in her Corvette by a Hiway Patrol Man - He sayed Don't you know this is a 60 M.P.H. Zone? She sayed I was - Pointing to her Tach.

Rob Keller 01-06-2006 06:29 AM

:D wish that would work for me :)

here this will tickle you imagination some!!!! :D

lemme no whatchoo think bout this.

SR66 :thumbup:

predator carb guru 01-06-2006 09:18 AM

tachs reading crank rpm? nope, they read cam rpm (via ign signal from distributor module, cam sensor signal, etc.......). on all four cycle engines, rpms are determined by camshaft rpm..........crank rpm will be TWICE what you see on the, i'm wondering, did i miss something here? i dunno.........too damned early for me in the morn........ :D

Rob Keller 01-06-2006 06:08 PM

and what about a old time mechanical tach? with the cable its still measuring the RPM'S of the crankshaft

I know the electric tach comes off the dist but its measuring the crankshaft revolutions or showing the cranks RPM's

the crank turns 2:1 of the cam, so the cam is turning 1/2 the speed of the crank there for the tach must double time to produce a accurate RPM reading

SR66 :thumbup:

predator carb guru 01-06-2006 09:34 PM

even the old cable driven tachs are measuring camshaft rpm. all the ones i've seen are a part of the distributor. the distributor spins at CAMSHAFT rpm, which is 1/2 of crankshaft rpm. (distributors are not driven by the crank. they are driven by the cam, and turn at camshaft speed) all four cycle engines turn the cams at 1/2 crankshaft rpm. i'm not aware of ANY four cycle engine that takes a reading of crank rpm for the tach signal, unless it's a newer engine using the crank sensor for the signal, however, this signal will still be "conditioned" to read 1/2 of crank rpm. so when you look at your tach and you see a reading, say 5000 rpms, that's CAMSHAFT speed, not crank speed. tachs do not convert the signal they get into a "1/2" speed signal for rpm. they read the pulses sent to them and display them as rpm. crank speed will be TWICE what your tach says.............two cycle engines, or two-strokes are a different beast, and those engines will read crankshaft rpm, due to most of them not having cams. (old detroit diesels are the exception here, they are two strokes that have cams, but they only actuate exhaust valves. those cams are driven at crank speed, and the tach signal for those is either mechanical via a cable, or electronically via a cam position sensor). one thing to remember here is it takes TWO complete turns of the crankshaft to complete ONE cycle of operation, and only ONE TURN of the camshaft. camshaft speed is what we measure as rpm.

61bone 01-07-2006 07:42 AM

turn of events
Now this is interedting. Does anyone have a shaft tach to check this out?

BRONCO_78 01-07-2006 08:23 AM

Just The New Guy
I Know I'm Just A New Guy But I'm With Rob Keller It's Crank Rpm.

Rob Keller 01-07-2006 08:51 AM

1 Attachment(s)
I don't have any formal :D eduamakatuns :D {makin' a funny..OK?}but if a cam is spinning @ 1000RPM @ a fast idle that means the crank @ 1/2 time is only @500RPM way to slow to run,

I know very little about electronic cars but I do know that Ford had a 302{5.0} that had a timingless distributer that worked off the crank


(distributors are not driven by the crank. they are driven by the cam, and turn at camshaft speed)

now further more IMO EVERYTHING works off the crank without a crank you got nothing
starter ~ crank
crank ~ timing chain& gears~cam~dist~tappets~push rods~rocker arms~valves

also front external all the accessories ..water pump ~power steering Etc. off the crank
external rear ~transmission~ drive shaft ~ differently ~wheels

another example would be distributer less ijnitions but I think im gettin" off topic

SR66 :thumbup:

Rob Keller 01-07-2006 08:53 AM

still fingering out how to use a "sreen shot"!

SR66 :thumbup:

lowROLLERchevy 01-07-2006 09:06 AM

66 Attachment(s)

Originally Posted by predator carb guru

corection here

your right that most tachs use the ingition signal as the pickup for the tach, but tachs are designed to cut that speed in half to display crank speed

next time your out in you car do this

research and figure out the gear ratio of your car
drive down the road at a steady speed, say 40 mph
right down what gear your in and what the rpms are

multiply rpm's X gear ratio X final drive ratio X tire size ..... youll get the speed you were driving ... heres an online calculator that will do it for you :

proof that your tach is showing crank speed, because its the crank thats conectd to the transmission

cliff tate 01-07-2006 09:09 AM


Originally Posted by BRONCO_78
I Know I'm Just A New Guy But I'm With Rob Keller It's Crank Rpm.

look at a 1955 to 57 t bird they all had mechanical tacks driven frome the ditributor, i think corvets as well

onovakind67 01-07-2006 09:11 AM

A typical tachometer that hooks to the ignition system isn't reading distributor rpm, it's reading pulses per unit of time. A tach set up for 6-cylinder engine will read 33% high if you hook it to an 8-cylinder engine. If your V-8 engine is misfiring and the pulses aren't getting to the tach, the rpm reading will be off. The numbers you read are only printed on the face of the meter, so you can adjust them to represent anything you want.

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