who knows what "rpm" means ? .... not THIS guy ....

uuuum … yea … enjoy

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.


what was that from and what was he trying to get at?

from a hot rod forum, he was trying to explain what “rpm” means to a car noob …

a simple Revolutions Per Minute would suffice


I want my minute back.


wow that guy was retarded. The tach gauge def reads off of crank speed. Because if it read off of cam what happens on variable valve timing cars where one cam is spinning faster than the other? Toyotas vary their intake cam but not their exhaust.

My car is a Honda so god measures my RPMs for me.

at least he was right with the cam(s) and crank being at different speeds

He’s likely talking about cams in domestic, push rod engine. Yes, the cam spins at a 2x’s speed of the crank… hence a compression stroke followed by a combustion stroke. The cam drives the distributer at a 1:1 ratio where one revolution of the distributer = 1 rpm. Every time a point contact touches…the spark is delivered.

1 full revolution/cycle is a complete compression stroke and combustion stroke. 2 spins of the crankshaft.

Some engines DO read RPM off of the distributer/coil. That guy is correct, you got to learn something today.

EDIT: Variable timing does change it’s rotational speed, but it never get’s lapped by the crank revolutions. Your timing changes by a few degrees… not 360

are most of you guys realy mising whjat hes saying ?

hes saying that when your tach reads 5000 rpm, your cam is doing 5 k and the crank is going TEN THOUSAND rpm …


I thought that’s what he said, but he’s over-explaining shit so it’s really confusing. The guy is a dumbass.

yes, i understand what he said… rather the jumping on his back and calling him a dumbass… I reiterated what he ‘wanted’ to say.

still, i’m with xander and want my minute back.

i steal other peoples minutes so that i can live longer … you can not have your minute back

The camshaft does NOT spin at 2X the crankshaft, it spins at 1/2 the RPM, not other way; due to the 4 strokes of the 4-stroke engine. If the cam spun at 2X the crank, then the valves would actuate 2x every revolution of the crank; wouldn’t even run.

Take a look at any SBC timing set, you will notice that the cam sproket of the timing chain is 2X the size of the crank sproket; AKA the crank spins 2X for every one revolution of the camshaft. This doesn’t matter what car it is, AFAIK all camshafts run at 1/2 cranks speed.

He was good in his argument, but he screwed up the 2X Vs 1/2X; and the old-school shaft-driven tachs did actually double the RPMs to obtain the true RPM of the engine; my buddies old Vette still has the shaft driven tach.

Now a days, and even when the first electronic tachometers were started they read the ignition coil pulses to determine RPM, nothing else; this is still how almost every tach to this day operates.

I know that it can read off of the coil thankyou, i’m pretty sure I go to school to work on cars and am being trained by Lexus/Toyota so i’m pretty sure i understand how VVT-I works ;). What I said was it reads off of crank speed. What I meant was that it goes by how fast the crank is turning even if its calculated by using the cam signal. And actually the cam spins at 1/2 the revolutions of the crank on a pushrod right? And just for some extra info in a distributor every time the contacts touch it doesn’t create a spark, its when the contacts are broken that it sparks.

Correct, the cam doesn’t spin twice as fas as the crank… i typoed

I got caught between trying to explain a “Cycle” (compression/combustion strokes) vs. a rotating crankshaft.

I contradicted myself when i was going back and forth editing. thanks for catching it.

I misread what you said then… the crankspeed IS the rpm, but the tach inwhich this “guy” was referring takes readings from ignition system… not from a crank sensor. I thought you were saying the sensor was a camshaft sensor.

as far as the points distributer system is concerned… when the rotor passes the contact (even though it doesn’t acutally ‘contact’ or touch) the spark does jump to the contact closing the circuit… not after the contact is passed.

now i want the 3 minutes of my life back…somehow.

Hotrod: you owe me for them somehow.

Yeah, I figured you just typo’d about the cam going 2x the speed. I just wanted to make sure we were all on the same level here. And I agree, Hotrod owes us all a few minutes back. They owe me 180 extra hours because I was forced to go through their training program haha.

For some reason this issue get’s easily confusing when you try typing it out…

Bottom line to summarize.

1.) the crank speed is the actual RPM
2.) a 4-stroke cycle is repeats itself every 2 RPM’s
3.) Hot Rod stole 10 minutes of my life for me.