GM 8.1L Ring References - Engine Builder Magazine

GM 8.1L Ring References

Type the word "ring" in the Google search engine and get beyond all the
stuff that is being sold and you will be amazed at how many occurrences
there are. It is mind boggling. There is even a site that monitors and
archives data relevant to planetary rings.

You can’t talk about rings and not mention the Olympic rings, which
stand for the coming together of five continents; I’d like to see that.

I started thinking about all the places that rings are used in an
engine. Piston rings, ring gears, snap rings, "O" rings, ring grooves,
ring lands and of course reluctor rings, that provide a signal for the
firing event. If you’re not already aware of this information, it is
interesting to know that nearly all of today’s engines control spark
timing based upon increase and decrease of crankshaft rotation speed,
not by vacuum and distributor curve advance springs.

Today, each time a spark plug fires there is an instantaneous increase
of crank speed. It’s an "every action has an equal and opposite
reaction" kind of thing. That is what the sensor detects from the
crankshaft reluctor ring, whose signal information is processed by the
ECM. Spark timing is then altered based upon a large collection of
sensor data according to each nanosecond of conditions.

Hence on initial start up the "learn" process must occur, to alter
spark based upon correct crankshaft increase and decrease of rotational
speed at the correct moment in time. At that point all is right with
the world of combustion.

That said, we have a GM engine application we should start to talk
about; the 8.1L big block came on line in 2001 and was used as a V10
beater and/or diesel alternative in HD trucks, vans and commercial or
personal busses. Production numbers are now approaching one-half
million so we are going to start to see them.

This engine has the same valve and bore centers as the familiar 7.4L
but a .370" longer stroke. Funny, quarter-inch big block strokers have
been built forever by many performance enthusiasts but a 3/8" stroker
was only for those who were real serious and with deeper pockets. Now
you get that behemoth configuration right out of the box from GM. Talk
about progress!

One of the primary differences of this "Big Rat Motor" is a new firing
order of 1-8-7-2-6-5-4-3 which gives that perfect crankshaft increase
and decrease speed ratio and that is finally what I want to make you
aware of. Previous Gen III articles should have you prepared for what I
am about to show you: the reluctor rings.

That’s right, there are two of them. The Gen III crank has two pieces
spot-welded together while the 8.1L has two separate pieces as you can
see in Figure 1.

Each ring is clearly marked FWD for forward (or to the front of the
engine) and AFT (or to the rear of the engine). I’m thinking that the
engineer involved must have had some naval experience.

Figure 2 shows the two rings installed on the crankshaft. Now for the
best part: in Figure 3 you can see that there is an index key slot,
which indexes with tabs on the two reluctor rings.

I haven’t seen any procedures for installation and assume that we won’t
because it is presently considered a non-serviceable part, which is the
same way the Gen III started. I would
recommend heating them to 350° F to allow them to fall on the back of
the crankshaft. I would also find an old cylinder sleeve or similar
thing to do that bottoming ricochet tap with a hammer. To date I have
only found one resource for these rings: Precision Diversified,
Detroit, MI. I also want to thank to Larry Eriksson, Crankshaft
Rebuilders and Chip Helderman, Jasper Engines, for their help with this bulletin.
figure 1figure 2Figure 3

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