All kinds of tricks have been used over the years in an attempt to gain a performance advantage - tapered bores are probably one of them.
In actuality, rings seal at both the face and on the sides. Depending on the ring configuration, the rings' side sealing capability/potential is enhanced by chamfers and bevels - these cause the rings to "twist" in the groove when they get squeezed into position.
THus, when you combine piston tilt due to clearance, there is travel potential that can occur between the rings and grooves. Lots of wear or movement potential can atually cause the rings to friction weld to the grooves - this can really compromise real sealing potential.
Granted one could experiment with tapered bores BUT the downside risk(s) is/are NOT one that I'd try to encourage in any way, shape or form.
THe last thing you need is for a MORE difficult environment of ring-to-groove sealing potential - a tapered bore would surely be detrimental to side sealing potential all things considered. The extra wear potential from additional ring movement as it tries to conform to a tapered bore would surely NOT help the ring-to-groove relationship.
As far as I know, round smooth untapered bores still offer the best sealing potential than avante garde alternatives.
The tricks with deck plates, block heating and/or bolting bell housings, motor mounts, etc to the engine have CLEARLY demonstrated positive effects with regard to bore sealing potential. Any shop that does NOT use at least a deck plate is almost asking for a comeback due to poor oil control and/or ring sealing.
I know of a case where a stripped block measured round and then went 0.008" out of round when the main caps, heads and other crap was bolted into place. Real hard for any ring to seal in that environment whether the bore is either a straight or tapered bore.
As long as I"ve been involved in racing (since 1965), guys always seem to look for magic when it comes to bore preparation. However, good, sound basics (which some guys lack or short cut) really need to be followed, actually mastered, FIRST before you start trying exotic non-traditional bore finishing techniques.  |