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i have a 340 viper whit 2x502 mpi now i had stroke them to 540 .i wonder how many hp can i have ?? and what cand of propellers shuld i have my rpm now is 5900 to mutch .sorry my spelling soo bad i am from sweden / tanks Micke

Depending on your choise of parts, around 600hp.
What props and gear ratio do you have (assume you have bravos)? 5900rpm is way too much if you got stock 502mpi's, are your tachs correct, rev limiters should kick in.

I am not sure wich ratio i have but it is Bravo one stern. what kind of props should i have? i think my tachs is showing correct. i have 28" mirage props today. The manufacturer says that the boat should do 80 mph but the top speed we have reached is 68 mph, and we have got more hp than original so its getting me even more confused. but i have not really pushed it cause everything is new and the rpm is so high. What should i do?

thanks micke

I'm no expert, but it sounds like you need a lot more prop with that HP. I'm spinning 30" Bravo 1s and I'm turning about 530 HP. Of course gear ratio comes into play. I think a 4 blade Bravo would wk for you. See if Brett at BBlades can give you some advice.

thanks i did that. called brett very nice guy he helped me whit every ting i bought two 32" bravo from him...

That should work!
Happy Boating