Some things that jump out at me when I read bottom end/hp and 62T.
When it comes to Blasters and Superjets, the OEM cylinder is 61X with a 81mm bore.
There is a 62T cylinder with a 81mm bore too, however this was standard in couches. Then there is the 64X cylinder, found on 760's with an 84mm bore.
The last one, the 760 cylinder has 62T stamped on the cylinder. So it causes a bit of confusion. All the 760 cylinders I have seen have two extra gussets on the intake side. The part no for the cylinder starts with 64X.
So, davidescu_radu, the reason I bring this up is that your have asked what is the max bottom end/hp you can get from a 62T 780cc (85-86mm bore). The 61X has port timing to achieve more bottom end power than a 62T or 64X. When you bore a 61X to 84mm the bottom of the sleeve, the skirt part that fits into the crankcase gets pretty thin. And I think 84.5mm is the absolute max you can bore a 61X OEM cylinders to. So to keep the port timing of a 61X and use 85 or 86mm pistons, you can buy a larger cylinder sleeve to replace the stock one.
When you resleeve a 61X the cylinder requires boring to get the sleeve in. Then the cylinders need sizing to the pistons. So some folks take a cheaper path and buy a used 64X cylinder, that at 1mm oversize is 85mm. But the exhaust ports are higher up the sleeve than the 61X. Thats why people say a 61X hits harder down low than a 64X.
This then leads to Wax's point about the 62T centres. When you put bigger sleeves in you start to descrease the transfer port volume as you can't move the position of sleeves relative to each other and the crank to try to preserve the transfer volume.
Then you have Tim's observation that max horsepower cannot be achieved with a B-pipe. The B-pipe uses water to change the temperature of gases in the pipe, which changes the speed of sound moving through the gases. The point of this is to send a sound wave back at the exhaust port when the new fuel air mix is being introduced through the transfer ports but the exhaust port is open. Otherwise the new charge flows out the exhaust port.
So, do you want max hp or max bottom end?
Are you free riding or racing?
This affects the way you should configure your engine.
I think it was on the other TNT thread that Tim pointed out 2 stroke 500cc bikes produced 200hp in Grand Prix. And the major difference between them and pwc engines is the exhaust. So if you start chasing max hp, you really need to look at the exhaust.
Then there is this thread, about the yamaha triples with a 3-1 header and single expansion chamber. Easy to tune a pipe when there is one per cylinder (easy for an experienced engine tuner, of which I am not). Hence the power a triple piper can produce. When tuning a pipe for three cylinders how do you time that exhaust signal to work for all three cylinders? I don't know. At the limit of my knowledge.
Then Big Kahuna points out some people should be listened to. Others who were on the forum should have been listed to as well but have left because they got sick of arguing with people who told them they didn't know what they were talking about.
Then there is only so much you can get from a forum. I know an engine builder that can build crazy power from a 65U for putting into Balsters and Superjets. He won't give away 20 years of engine building experience on a forum. He always keeps a little up his sleeve. The info I put at the top of this thread is easily obtained from google and forums etc. When I talk with my engine tuner mate, the more I shut up and listen, the more he shares and the more I learn.
So enough waffling from me.
If anything a have written above is wrong, Im happy to be corrected.
