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 loss?

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 configuration.

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.