Douglas Quebbeman wrote:
[stuff delete]
> So, prior to the fuel crisis and emission controls, what was
> the motivating factor for the use of EFI?
Cost, weight and reparability. Mechanical injection pumps have to
be mechanically timed to the engine (either gear or belt driven off
the cam), require their own oil supply and return lines, and have
about 973 parts ranging in size from large to itty-bitty. When the
3D cam starts to wear (and it does) it effectively changes the map of
the engine, and anytime you modify the engine in terms of
displacement or compression (or even futz with the exhaust too much)
you have to re-profile that weird 3D cam -- something that requires
taking the pump to bits and having a Really Good Time in a fairly
specialized machine shop.
With D (and L) Jetronic there are knobs that can be turned (although
there are fewer on D -- and the D MAP sensor, while clever, is also
just plain weird), meaning that engine tweaks -- or completely
different engines -- can be accommodated with more or less the same
hardware.
> And why bother with CIS once you have EFI (spraying fuel into
> cylinders at off-times never maid sense).
Actually, it makes perfect sense when you realize that you're not
spraying fuel into the cylinder, you're spraying it onto the back
of the (presumably warm) intake valve, which increases the
thermodynamic efficiency of the injection process. It's not as if
there's bucketloads of time for fuel to pool given how fast the
valve is cycling, even at idle. As for "why CIS", it's basically
the same answer as "Why D-Jetronic" -- save for the fact that the
system is almost completely mechanical and therefore doesn't have
the failure modes on 1960's electronics stuffed into a hostile
automotive environment. Essentially the same hardware works on
everything from VWs to BMWs to the Porsche 930 -- in general the
only real differences are the size of the calibration slot in the
fuel distributor (which is a distinct, changeable unit), the profile
and diameter of the airflow horn (which is a simple and inexpensive
part to make -- and if you're crazy enough can be tweaked with
tape-a-weights) and the bias pressure applied by the warm-up regulator
(which is also pretty much the same across most products).
CIS was great because it was bone-simple but produced results at least
as good as D-Jetronic with far better reliability. L-Jetronic fixed
many of the problems of D-Jetronic, but wasn't good enough to displace
it from existing designs. It took Motronic to do that...
--
Chris Kennedy
chris_at_mainecoon.com
http://www.mainecoon.com
PGP fingerprint: 4E99 10B6 7253 B048 6685 6CBC 55E1 20A3 108D AB97
Received on Mon Aug 06 2001 - 17:35:39 BST