Hi Folks, a few weeks ago someone told us the June Hot Rod had a tech
analysis of GM's new DOHC Six design. This naturally reminded us
old-timers of the GMC 270/302 of yore, so I secured the mag for a look.
Hot Rod opines that this will be a hot-rodder's engine in time to come,
either in crate form or by the salvage-yard route, but I have some
doubts about that, of which more later.
First off, there's no question in my mind this will be a very successful
production-vehicle engine that will give long and good service in the
sport-utes for which it is designed. Everything about it is
state-of-the-art.
But one thing that jumps out at me is the aluminum block with
shrunk-fitted iron cylinder liners, which we are told are bored after
fitting to a wall thickness of only .060". This is pretty thin, and
obviously leaves very little for clean-up rebores before it gets
super-thin.
We just had some discussion about cylinder bores, torque-plate use, etc.
and obviously it is fundamental that cylinders should be straight and
round, and stay that way under all the heating and pressure forces that
work on them under load.
In this case you bore an aluminum block to size, and fit iron liners in
it with perhaps a .006" interference (at a guess, it doesn't say in the
article) by freezing the liners by some means until they shrink their
O.D. enough that they can be shoved in without grabbing until they
expand in place in the block. Some machine/fixture would do this in
production, of course.
Many years of experience with aluminum motorcycle cylinders with iron
liners shows that aluminum tires of holding an interference-fitted iron
liner, and relieves itself by relaxing its grip so the fit becomes more
like size-to-size. Even though the air-cooled cylinders run probably
double the temperature of the water-cooled here, I imagine the same
thing will apply with the present engine. However, this will be a
harmless situation that would probably not affect performance. We're
not told if it is possible for the liner to drop downwards if somehow it
does manage to come that loose. Maybe there is a shoulder at the bottom
of the bore in the aluminum to locate it. Another point with the liner
is differential expansion vertically. Every time the block heats up it
expands vertically more than the iron liner, so this would seem to be
another factor tending to loosen same.
It strikes me, though, that the sections of aluminum surrounding the
iron liner are going to be variable in thickness, and with heating and
cooling are going to be expanding in different amounts, which could mean
a degree of distortion in the iron liner here and there. Here again
this will probably be of no consequence in a production application
which will not even be running under heavy load 99% of the time. I'm
thinking that if hot-rodders try to put blowers, nitrous or nitro to
this, it might be a different story.
How thick should an iron liner be in a racing engine? The authority P.
E. Irving, in one of his books, says that for air-cooled motorcycle
racing engines, "at least 1/8" thick" and more if possible. To retain
dimensional integrity under extremes of heat and pressure in endurance
engines.
Back to the GMC Six under review, another design item that intrigued me
was that the combustion chamber departs from what seems to be standard
modern four-valve design, in that it exhibits a squish-band on only one
of the four sides, rather than on all four. I have the impression that
the four-squish design with its turbulence is a very key item, but the
GM head seems to bring that into question. I'm sure the GM engineers
have their reasons, of course.
I could go on, but this is plenty and I hope others will comment. Just
a couple more tidbits in conclusion. This new GM engine has 256 cubic
inches, and gives 275 ft. lbs. of torque at 3600 rpm on 87 pump gas. A
fine figure that reflects modern four-valve design.
It happens that at the moment I have in preparation for BRN an Interview
with Don Ferguson Jr., who with his late Dad Don Sr. ran the early GMC
270/302 for years at Bonneville (as well as Arduns). Among many
interesting stories Don tells is that a 302 GMC Six bought for peanuts
30 years ago, with a stock iron block, stock crank, and stock pushrod
OHV iron head, bored out to 320 cubes, with fuel injection pouring 90%
nitro into it, showed 600 ft. lbs. of torque on the dyno they have at
their shop (Paramount Forge) in CA. Carrillo rods were necessary
because the stockers started to bend at 50% nitro, but this makes you
think. And the said engine was able to endure the Long Course at
Bonneville. Cheers from Ardun Bill in the Great Dismal Swamp,
Chesapeake, VA
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