difference between 75 and 90

[some day]

Hello all,<br /> This is my first post. This seems to be the place to go for answers. I have a 96 mariner(mercury) 75hp mounted on a bass tracker. Its been a great motor since day one. Compression is even and the motor runs smooth. However my intention is to mount the motor on a different hull with requires a little more power. The bore,stroke and displacement are the same on the 90 and currently the 115 as it is on the 75. So my question is how does a person go about raising the hp a little? The hull is max recievable 135 and as a general rule of thumb Id like to have at least 66% of max. This seems to be the formula most dealers are using.

 

[Laddies]

Re: difference between 75 and 90

The increase in HP comes mostly from port timing

 

[bluewater19]

Re: difference between 75 and 90

Laddies,<br />How do we increase our port timing? CAn we do this ourselves? I have a 1996 merc. The same as his mariner. My boat is rated for 125 and I would like a little more pop!<br />Someday,<br />I thought the 115 was a 4 cyl only the 75 and 90 share the same powerhead that year.

 

[Laddies]

Re: difference between 75 and 90

Most service personal are not equipete with either the tools or the knowledge to correct port timing on a engine and rather than ruin what you have trade up with your engine is your best bet. You are right the 90 is the largest of the 3 cyl engines

 

[some day]

Re: difference between 75 and 90

Thanx for the input Laddies. Theres a couple thousand dollar difference between the three apparently due mainly to port timing. Mercury website has all three listed in the optimax with same bore and stroke. The 115 came out only lately. Im a little curious just what the added hp does when there is a minimal change in torque.

 

[Silvertip]

Re: difference between 75 and 90

There is more to it than just port timing. Exhaust tubes, carb jetting, ignition timing, sometimes compression. These changes are easy for the factory to make but are very expensive for the consumer to make due to the cost of the parts. In very few cases is it simply a matter of bolting on a bigger carb or inserting bigger jets. Engines make power by "breating". That means port size, port timing, exhaust, intake and ignition modifications.

 

[dsrchamp04]

Re: difference between 75 and 90

nobody said it was just port timing

 

[Silvertip]

Re: difference between 75 and 90

Quote: "Theres a couple thousand dollar difference between the three apparently due mainly to port timing." <br /><br />That indicates the poster is convinced port timing is key and the other stuff is really not a big deal. If he elects to spend a bunch of money getting an engine ported thinking thats all there is to it he will be very disappointed and likely to come back saying "why didn't you tell me there was more to it! That's what I did.

 

[Motor Boater Bill]

Re: difference between 75 and 90

Difference between 75 and 90 is 15.<br /><br />Sorry, couldn't resist!<br />

 

[dsrchamp04]

Re: difference between 75 and 90

lol at bill.....that pretty much sums it up...