CP what i didnt get across to you is that the St 45 when compared to the other will turn higher numbers on the jett muffler. try one your self. you will be supprised how well it turns. The reason for this is St was first to run the 1/4 wave mufflers. Jett and Nelson both used this design until jett put a patient on the design. The ST 45 perfectly matches the ST/Jett/Nelson mufflers perfectly where as all the other engines out there have very wide exhaust openings in the case. MFG designed these engines to accept muffler that had the bolts go thru the engine case. if they kept the exhaust port small then the engine weight more. OS even put the E port on in the 5 o clock position in the sleeve yet kept the 90 degree outlet for muffler. very poor for tuned exhaust applications. sure they work but not as well as they can. You need a smooth transition for the exhaust to go in and out of the engine. The St does this well as it shows on the gains.