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Regarding your recent post on your 20psi boost mod.
>You do need a remapped chip for increased fuel delivery to maintain the optimal
>14:1 air to fuel ratio under high boost.SOME TEXT DELETED: If you increase
>the air flow without the adequate increase in the fuel delivery you are doomed to
>overlean the mixture. Have you ever seen the burned valves/pistons?
Well, the stock CIS fuel system is mostly a mechanical fuel injection system
with the exception of the frequency valve that can tweak the mixture.
Higher boost means more air flow which raises the air flow plate in the
fuel distributor which raises the metering valve and exposes more of
the metering slot which gives more fuel. In wide open throttle mode
under full boost you don't want to be at a lean 14:1 air fuel ratio
(14:7 to 1 is great for reducing emissions but not ideal for max power).
The computer actually tweaks the mixture slightly above and below the
14.7 to 1 to allow the cat convertor to oxidize HC and CO and reduce NOx.
The Computer goes into open loop when it detects the 2/3 (full throttle)
switch closed and then sets the frequency valve to a duty cycle around
68% to richen up the mixture under full throttle. Using an expensive
portable 4 gas analyzer I have measured the air fuel ratio to be closer to 12.8 to 1
(upstream of the cat convertor) under sustained full throttle at 1.4 bar
and 1.8 bar boost on the 5000 MC and the later 89 200 MC engines
with the stock frequency valve duty cycle settings.
Orin located the full throttle duty cycle locations in the Mac11
ECU's so this could be tweaked if needed.
You may be pushing the system limits at 20psi,
Ned has mentioned the limits of the stock CIS fuel system
before and I think it was at the 350HP mark, others (PDQSHIP)
may want to comment on where the stock system runs
out of gas (literally) .......
>I would highly recommend installing an ANALOGUE(!) boost gauge. Mine is from
>(don't laugh) J.C.Whitney catalogue, was only $15 on sale. It covers the range
>SOME TEXT DELETED: So get the better unit from Whitney for less price!
Have you or anyone else out there had these various boost gauges
(VDO, AutoMeter, JC Whitney etc) tested for accuracy? It is nice to
have an analog gauge but I wonder whether some of these gauges are giving
>I have also installed a device that analyzes the mixture at any given moment and
>displays it as a moving colour LED bar. ORANGE for RICH, GREEN for NORMAL and RED
>for LEAN. This device cost me nothing, 'coz I designed and built it myself. It is
>amazing, how much more accurate can this device show the lean mixture under full
>boost by the tell-tale RED LED bar. Far more accurate and way before my ear can
SOME TEXT DELETED.
If this device is based on the O2 sensor signal then it can only really tell you
accurately when you are above the 14.7 to 1 air fuel ratio (too lean) or below
this ratio when you are at full throttle. In other words the O2 sensor is
designed to switch high/low or low/high at the 14.7 air
fuel ratio but goes non-linear above or below that ratio. You can check
several SAE articles that describe the operation of these standard O2 sensors.
There are other O2 sensors that can operate under wider A/F ratios but they
are in limited use. These O2 sensor A/F LED devices are useful for indicating
a lean condition at full throttle (open loop) , and whether you are close to 14.7 to 1 when
you cruise at part throttle (closed loop) You should be no where near the 14.7 to 1
under full throttle conditions, if I remember correctly 12.6 to 1 ratio gives max