Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Tach's - R. P. M.
View Single Post
  #8 (permalink)  
Old 01-06-2006, 09:34 PM
predator carb guru's Avatar
predator carb guru predator carb guru is offline
Mark Goulette
Join Date: Feb 2005
Location: Conifer, Colorado
Posts: 149
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
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.
Reply With Quote