Mercruiser 120 Help

[79TEMPEST]

Hey guys i have a 79 Tempest 18' with a mercruiser I/O 120 in it, then engine runs strong and i have been using it all summer with no issues ( new starter, alternator, full tune-up. But the engine sounds a bit iffy at full RPM (about 4100) so this winter im gonna yank her out and do a full rebuild now im new to boats but i am a 4th year car mechanic so not new to engines, just wondering how i can get more power out of this little 4cyl ie(cams, carb, Bore?) just wondering how i can get more power out of it because if im tearing it down for a FULL rebuild (machineing everything) what can i do to add some noticable HP? Thanks alot for any info i just wanted to ask the gurus if its worth it? im fine with the power now but more is allways better so if theres a way to squeeze some more power out of it i would love to know thanks alot guys!

 

[zbnutcase]

Re: Mercruiser 120 Help

Not much you can do due the poor cylinder head design, but a good multi angle valve job, along with some minor pocket porting will make the head about as good as it's ever gonna get. Leave the rest stock for reliabilty.

 

[daydreamer1252]

Re: Mercruiser 120 Help

As already stated not much you can do to make the 120 better.

You may want to consider installing a 140hp

(181 cu in, 3.0L) version of the same engine. Not much, if any, extra cost and no real issues to the install....physical outside of engine is the same so no "shoehorning" to get the extra power. You do have to pay attention to the years as there were some changes in the engines later, (in the late 80's or 90's) An easy way to get 17% more power.
Someone here can tip you as to the specific years when changes occurred.

Good luck