dont bother changing the springs unless your going with a big turbo and u want to increase your useable rpm to take advantage of the power curve with 272's and a big turbo, i ran my car with 272's and an sp74 for about 60000mi with no problems, redlining at 8000 rpm all day long , the main advantage of dual springs is to decrease return time of the valve, so u can increase redline and decrease chance of valve float, but if i had stock twins i would go with 264's, i previously had that setup on another supra with great success, even an sp74 with 264's is fuc...in awesome for the street. i am now running a sp76 with 272's a fully ported and polished head and dual valve springs and i am redlining at 9500rpm, i believe i can raise it up more maybe to 10000, but once u get over the stock 8000rpm limit u take on a great huge task, when toyota balances their parts they have a tolerance of like three grams, at 8000rpm this will not be much of a problem however at 9500rpm it becomes a huge problem, i already cracked one of my crank's at this rpm because of this, i just put my car back together and we balanced all the parts with a tolerance of half a gram, it worked out beautiful, i also installed a unortodox crank damper, but we are doing this ourselves, if you dont have the equipment and a shop is doing the work it can be very expensive just for an extra 1500 rpm, my advice is talk to larry at sound performance, i know him personally and he has more hands on experience with supras than anyone. off the top of his head he can tell you for sure.
