F/A Ratio?

From: Dkota4by4@aol.com
Date: Sat Mar 20 1999 - 18:19:23 EST


This is off the DiRT list. Does this sound like it would work to anyone?

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Wanna richen your air to fuel mixture to get more horsepower, even when
your PCM is in 'closed loop' and getting feed back from the O2 sensor?

Get two 5-6 inch pieces of one eighth, or three sixteenths, inch diameter
tubing made of stainless steel. Stainless steel brake line stock are a
good source.

Bend a smooth 90 degree curve in each of the stainless tube near one end.

Drill an appropriate sized hole in both of your right and left exhaust down
pipes for the tubing to fit snugly through. Choose a spot for the holes
fairly near to where the cast iron manifolds or headers end. Insert the
tubing with the 90 degree bend going 'downstream' so that the open end of
the tube goes toward the catalytic converter end. Push enough tubing in the
hole so that the tube is about one quarter to half an inch away from the
exhaust pipe wall. Weld the stainless tube to the exhaust pipe in this
position.

Now you have what engineers call a 'pitot' tube pointing backwards in your
exhaust pipe. As the high speed exhaust gases rush by the backward looking
tube opening - they create a partial vacuum and suck air into the exhaust.

The O2 sensor will sense that added oxygen and think that your engine is
running too lean an air/fuel mixture. It will then 'richen' the mixture.
You may need to pinch the outside opening of the tube to keep from
richening it too much.

( If you have a 1996 or later Ram with the OBDII system, if you richen the
mixture too much the O2 sensor behind the catalytic converter may give you
a 'check engine' light. But you can run a richer mixture up to 13 and
still meet the pollution specs - and anything richer than about 13.5 to one
doesn't make any more horsepower. This is why Dodge can sell a smog-legal
"Performance PCM Computer" ! )

A neat thing about this trick, according to the guy who told me, is that it
is almost self-adjusting once you get the tube hole pinched to the right
size. The more rpm the engine builds up, the more the exhaust gas velocity
increases, and the more 'O2-sensor-foolin air' you draw in.

------------------------------------------
Not interested in power tricks,
but want to lean out your air to fuel mixture to get more mpg? If you lean
the mixture out to about 16 to one, the economy should go up 5-10%

How do you fool the O2 sensor to go lean?

Make the stainless tube modification just like above, but intead of leaving
them open to the air, connect them with a tee to the PCV valve hose.

The 'blowby' gases in the crankcase always have some unburned gas. If you
draw this gas into the exhaust pipe ahead of the O2 sensor, the blowby will
combine with what excess oxygen is in the exhaust pipe and reduce the
oxygen content further before the exhaust gases reach the O2 sensor. The O2
sensor will sense a 'rich' mixture and tell the PCM to lean out by
shortening the injector pulses.

If the mixture gets too lean, pinch the rubber line to the PCV valve a little.

An added benefit of this trick is that it may create a partial vacuum in
the crankcase. Racers sometimes do this on purpose.

A crankcase vacuum does three small but good things for power, economy and
reliability:

1. It reduces crankshaft 'windage' or air drag.

2. It reduces the pressure on the bottom side of the pistons. Reducing the
pressure on the bottom of the pistons has the same effect as increasing the
pressure on the top of the piston. It makes a little more power.

3. A crankcase vacuum slows down, or even stops, engine oil leaks !



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