Modifying a Mariner 8HP


How are you? I have an 8 hp. Mariner (1995, 2 cyl.) which I want to rev up. So far no one seems to have a clue what to do, the Mercury dealer just want to sell a bigger engine so they say it canít be done. There must be a way since this engine has 210 cc. while some Japanese engines standardly deliver 10 hp out of 160 cc.  

I hope you find the time to answer my mail and appreciate it beforehand. 

Best regards,
Alex van Heugten



While it is true we could mod the little 8hp, the real question is do we want to. Most any two stroke can be modified to produce more power but only at the expense of some of the qualities that make the motor desirable. If we raised the exhaust and did a little intake "flow enhancement", we would only make more power at a higher rpm. Probably 1000rpm would be required to gain maybe a horse to a horse and a half. But now we would have less low end so we would need a smaller prop to get to the rpm where the power is. If this is beginning to sound complicated it's because it is. We haven't even talked about the lack of a tuned exhaust system and the intake system that is not made for elevated rpm.  

I'd take the route of just buying the 15hp and be several hundred (or thousand) dollars ahead. Try to find the book: Two Stroke Tuners Handbook by Gordon Jennings. This book will give you all the formulas and theory that you'll ever need to study and understand the how's and why's of two stroke engines. The book is from the 60's and 70's but the stuff is still true today. 

Hope this helps you, 




Please submit whatever questions you have to  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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