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Tach's - R. P. M.

3K views 25 replies 11 participants last post by  red65mustang 
#1 ·
Rev's ( Crank or Cam? ) Per. Minute
 
#6 ·
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 tach..............now, i'm wondering, did i miss something here? i dunno.........too damned early for me in the morn........ :D
 
#7 ·
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:
 
#8 ·
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.
 
#11 ·
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:
 

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#13 ·
predator carb guru said:
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 :

http://www.woodyg.com/fairlane/finfo/mph.html

proof that your tach is showing crank speed, because its the crank thats conectd to the transmission
 
#15 ·
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.
 
#17 ·
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 :
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
But to check Tach. You Drive 60 M.P.H. / Know the Cir. of your rear tires (Dia. divided by 2 = Rad. X 3.14 = Cir.) [ in Inches! ] Divide 63,360 ( inches in a Mile.) by Cir..! X "Rear end Rato." X 1:1 Trans Hi Gear. = R.P.M. of Crank. { If that number is the same as the tach. is showing; Tach is right!
 
#20 ·
AmericanPackMule said:
But to check Tach. You Drive 60 M.P.H. / Know the Cir. of your rear tires (Dia. divided by 2 = Rad. X 3.14 = Cir.) [ in Inches! ] Divide 63,360 ( inches in a Mile.) by Cir..! X "Rear end Rato." X 1:1 Trans Hi Gear. = R.P.M. of Crank. { If that number is the same as the tach. is showing; Tach is right!
yea i know the long way to do it :p

i just didnt feel like typing out all the details ... lol
 
#22 ·
predator carb guru said:
Well, I did some research on the subject and I stand corrected............now i feel really dumb..............................

NO!!! absolutly NOt so
this is how we learn
my nick name is "slowride66" for a reason ( i guess)
I sometimes get off slower then the rest soon as I know what in doin' i figure a way to catch up

if you came off like 'A know it all' than i'd probally let you wolly in it for a little while :)

hell I had to look it up ! :thumbup:

American Pack Mule is the one at fault here(just kidding)

we all learned something so ... nothing to feel dumb about

I'm Really Slow in the morning!!!

SR66 :thumbup:
 
#26 ·
a tach is simply a "spark" counter, the "timer interval" chip on the tach compares the total number of sparks per time interval and puts out an analog signal (0-5V, 4-10ma as ex) to control the pointer movement

you set the dip switches for 4-6-8 cylinders so the chip will divide the total sparks count so that the pointer will show the correct complete firing cycles (per minute on the dial)
 
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