trx2.5 ain't gonna spin at no 50,000 rpm even at no load. And the ns15s5 isn't hitting no 43,000rpm in a car. Those RPM ratings are deceiving.

Just look at the HP rating and don't assume any engine makes more than 33,000rpm. If you're calculating gearing for a given speed, use the 30,000 rpm assumption. It's realistic.

Also, I believe the S3 and S5 series engines have been discontinued. But i could be wrong. I know novarossi hasn't had much distribution in the US since they dropped team trinity as their importer. I heard some other company is planning, or may have started importing the engines.

Also, there's A LOT better engine than the TRX2.5...get something italian.