This is an exerpt from Richard Turner of Turner
Engineering fame. Believe the figure they are talking
about is the amount to mill a 8-1 head.
I would like to reiterate that we cater for the day
to day motorist who generally wants a vehicle that
drives better, returns improved fuel economy and is
reliable. We do not cater for the racing market as
this is a totally different area of the market place.
However one of our cylinder heads was used with great
success in the Mintex ralley by an Army entrant. The
only difference to our gasflowed spec was that the
head was also skimmed by .080". This was their 2nd
entry. The previous year they ran a LR with a head at
-.080 but no gasflowing. They said the improvement by
gasflowing was tremendous and that the vehicle also
used significantly less fuel per stage than the
previous year. We can produce this kind of head in
large numbers at little additional cost to the
customer, but decline to do so because of difficulties
that customers will experience with push rod geometry,
heads fouling waterpump housings, detonation on poor
quality fuel and other technical queries that we
simply do not have time to deal with.
There is also some doubt as to what thickness of
cylinder head represents 9:1. I have a customer from
the racing industry that wants a 9:1 head and has done
some research with an American company that has been
involved with LPG since 1967 who claims that the best
CR for LPG is 9 to 9.25:1. He has instructed us to
remove .067" from his cylinder head, which he has
advised us represents 9:1CR . Imagine the embarassment
of supplying large numbers of cylinder heads into the
market that are in actual fact significantly higher
than 9:1 if you have got your sums wrong. Especially
the American market is unmerciful towards perceived
poor quality or technical difficulty.
The following is a post by Hank Rutherford from some
years ago on the different heads and dimensions.
"H.Rutherford" <ruthrfrd@borg.com>
A while back I modified a 7:1 head to 8:1 specs. In
doing so, I collected
some interesting info. Here goes:
7:1 head is 3.700 in thick, chamber volume is 91
cc.
8:1 head is 3.600 in. thick, chamber volume is 75.2
cc
7:1 pushrod length overall, 7.300 in
8:1 pushrod length 7.175 in overall (some parts
suppliers are not able to
tell you the difference between pushrods, just that
they are different)
Within the range of our interest, there is about
1.2 cc change in volume
per .010 in. change in thickness
Milling .100 in. makes a 7:1 head 8:1. Almost
exactly.
Head gasket volume and volume above the piston
crown is 4.25 cc.
Swept volume of one cylinder is 571.5 cc.
I sawed a 7:1 head into slices to measure casting
thickness. I discovered that there is little material
left between the waterways and the head face after
milling the required .100 in. IMHO, I would NOT dvise
going any further as I doubt the head would hold up
for long, I imagine cracks would develop on the head
face. If you want a 9:1 CR, use an 8:1 head. The
castings are definitely different, a stock 8:1 head
has the same metal thickness in critical areas as a
stock 7:1 before machining.
I would advise obtaining a burette for measuring
combustion chamber volume to be sure you don't go too
far with CR. I can provide more info on
CC'ing if you need it.
If you modify a 7:1 to 8:1, use 8:1 pushrods.
Otherwise the rocker geometry is off, and the
possibility of fouling the rocker exists. By rocker
geometry, I mean the angle that the rocker arm
contacts the tip of the valve stem. If the longer
pushrods are used in place of the shorter (correct)
ones, the rocker arm depresses the valve with it's
very tip, concentrating wear and forces at a place
where the rocker was not designed to be operated.
This can cause premature wear of the valve stem,
unusual wear of the rocker
arm, and failure (breakage) in extreme cases.
Jim Allen is the guy who has the most info of
anyone I have talked with, and I believe he monitors
this list. Hope this is of some help.
H. Rutherford
'62 SerIIa SWB
"Gromit" ported 8:1 CR
AFAIK, there is no difference in the 8-1 and 7-1 heads
in the later (after '62 or so) castings. Rover just
mills an extra .100 off to raise the compression. The
later castings can be identified by an extended head
bolt boss in the area of the carburetor. Believe Hank
is talking about the non extended boss head when he
says 7-1 head in the above. It is doubtful that many
of the early heads are still around as they seemed to
crack fairly easily even when milled to 7-1 specs.
I've got one but it was cracked and welded. I haven't
been able to find any of the longer 7-1 pushrods
despite all my engines originally being 7-1
compression. Might be that the longer push rods were
phased out with the Series two engines or maybe even
earlier.
There may be several later type castings, btw. Seem
to remember a post from ACR that mentions several
different casting numbers with the latter being more
robust and better for going beyond 8-1 compression.
Might want to check with ACR about that or possibly
Bill has the info.
Aloha
Peter O.
--- Monika & Steve Rochna <mns@oasisol.com> wrote:
>
> All:
>
> Does anyone know what the highest safe ratio to have
> a head shaved to for
> general purposes? Here at four thousand feet any
> bit of power addition
> would be appreciated but I don't want to jeopardize
> my engine - I know it's
> not a race car.
>
> Thanks - Steve Rochna, Fallon, NV
> 1972 SIII 88"
> 1968 SIIB 110" FC
>
> ---
> [This E-mail scanned for viruses by Oasis Online]
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