pro stock snowmobile engine advice

Q I’m thinking about stroking my 809 pro stock motor. Will I be able to use the existing engine or should I start over? I want to go to a 70mm stroke. Does anyone know what kind of port timing would be required? Also, will I have to change my pipes? – Robmacdoo

A After stroking an engine the cc displacement will go up, and typically out of the range for the class you are asking about. This will require a smaller bore in order to legally run in the 800 pro stock class. As for port timing degree information, the changing of stroke will change the current port timing you have, thus requiring new porting. The current exhaust pipes you have should be able to accept the 2mm increase in stroke with no big changes. It still may be required to shorten of lengthen the pipes to match the engine design. – Jason Houle, Straightline Performance


Q I’m in the process of building a high rpm pro stock engine. I think I will be needing carbs somewhere in the 44-48 mm size. The engine is 197.7cc’s / cylinder, and the plan is to run it at 10,000-plus rpm’s. I have a lot of tuning experience with Mikuni carbs, but going this big I am looking for other options and suggestions. What carbs seem to be the most consistent and predictable for tuning when getting this big? – jabber

A The carburetor size for your machine you have chosen will work just fine. Changing to a new flat side will have throttle response and acceleration, but may not be necessary depending on your racing application. – Jason Houle, Straightline Performance