Re: overrev protected by fuel cut-off..NOT!

["Bret Luter" <bretluter@xxxxxxxxxxx>]

***Mark ERAM writes:
Date: Thu, 26 Nov 1998 15:14:19 -0500
From: mark kibort <mkibort@domain.elided>
Subject: over rev protection is done by fuel cut off.

Isnt this covered by the  rev limiter that is engaged  when the
revs go
over the certain limit.   (fuel cut off will stop a over rev
dead in its
tracks.  No fuel, no more rev , it is that simple)

Mark
*********************

Sorry to disagree with Mark but the fuel cut off will only protect an 
over-rev if the revs are being increased by "Normal" engine acceleration 
(caused by increasing the amount of fuel/air entering the combustion 
chamber). Many of you will recognize this as the "rev-limiter" (also 
controlled by ignition cut-off via DME). The rev-limiter WILL protect 
against over-revs under these circumstances. It is entirely possible to 
over-rev an engine by "missing" a shift thereby causing the momentum of 
the car to over-rev the engine via the drivetrain. This can (and will) 
happen regardless of any fuel or spark cut-off. The engine does not need 
any internal combustion to increase revs under these conditions, it is 
being literally "dragged" past redline into over-rev and possible 
piston-valve contact!

Imagine running your car at 80mph in 3rd gear (on a closed course, of 
course!). Now you are approaching redline and attempt to shift to 4th 
gear but you catch 2nd by mistake! (damn, don't you wish you had 
installed that BL/SS ERK UUC shifter?) At this point it doesn't matter 
if you cut all fuel and spark to the engine, it will spin past redline 
unless you realize your mistake and press the clutch pedal back in and 
shift to 4th. Instead of being driven by the internal combustion 
process, the engine is being driven by the rear wheels via the diff to 
the driveshaft, etc, etc.....

My 0.02 cents worth.
Now back to the soon to be AMSOIL thread and the mystery of the missing 
Bubbas........AAAAGH!

Bret Luter, Raleigh, NC
BMWCCA 52894
BMWMOA
Lots of BMW cars + a few bikes too!

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