First off, all sleeves are made of nodular
iron, as it is the only material that is compatible with breaking
in (seating) rings. Therefore, all sleeves have virtually the
same physical strength. (The real differences in sleeved blocks
are who does the install, how good does the installer holds
his machined tolerances and what is the installing procedure).
Taking that fact into account, bore size in any sleeved block
is limited to the center line of the stock bores in the block
and how much meat is left BETWEEN the cylinders. I always tell
my average customer (using his motor on the street) to do no
more than 83 mm on a turbo application and 85 mm all motor.
Obviously these numbers can be increased and the motor will
live but the question is for how long and how many rebores are
left in the block. We are testing an 86mm turbo block with the
thought that the block must live and survive in a racing environment.
I would assume that same block would go 87 in all motor and
be able to live also. This block uses Prelude sleeves but of
course is still limited by bore centerlines.
I believe claims of an 87+mm are probably true. The question
becomes how long did the block survive and did the extra bore
make more power or cause a power loss because of the sleeves
distorting from being too thin and actually cause compression
Benson is also working on offset sleeve project that changes
the centerline of the cylinders. This can increase potential
bore size available. The first tests will be in an all motor
I know I didn't give you a definite conclusion on bore size
because I don't think there has been enough real testing by
ANYONE. Any company that will tell you that their sleeves are
good for X amount of boost or Y amount of bore, is really just
blowing smoke. It is all based on the thickness between the
sleeves. The reason I preach 83mm for turbo blocks is that it
allows room for several rebuilds before you will need to sell
your block to your all motor buddy. Tuning errors and part failures
can cause the need for a fresh bore from time to time. For those
that say I can make more power at 84mm bore rather than 83,
I say just turn up the boost an extra pound on your 83mm and
enjoy the power.