1979 Mercruiser GM120 upgrades?

[JOHN_T_65202]

Hi Guys, I was wondering if anyone knew of anything I might be able to do to maximize the performance of my 79' GM120 engine? Just bought this Cruisers Bonanza (20ft, I think), was able to take it out once before it got too cold and these were the results. GPS'd the boat at 34 MPH @ W.O.T. 3900 RPM. The 14.5 " 19 pitch prop is pretty banged up, so I need to replace that. I understand that this will make a big difference and allow me to get up to the Max RPM of 4400. I had thought of converting to a 4BBL carb, but upon searching the net, I could not locate a combination manifold that would allow me to do this. Is there anything else I might be able to do to soup up this engine? Thanks Guys.

 

[Don S]

Re: 1979 Mercruiser GM120 upgrades?

Keep it tuned up properly is about all you can do.
At 20 feet, you are pushing 120hp engine to it's max.
If you want faster, sell it and buy a boat that has the power and speed you want. There is no quick, cheap, fixes for more Torque and HP. That also means don't be thinking about replacing it with a V6 or V8, then you have to change the drive gear ratio, exhaust, stringers, etc.

 

[JOHN_T_65202]

Re: 1979 Mercruiser GM120 upgrades?

Thank You for the info.

 

[dbraymer]

Re: 1979 Mercruiser GM120 upgrades?

Same question concerning a GM 153 120 HP in an old Sea Ray 185.

If I have to go thru the engine, could I swap in a 3.0 (181) short block and would the rest of the parts fit? The 2.5s are not so easy to find any more, and the HP upgrade is minimal, doesn't seem that you would have to rebuild the vessel structure or anything.

 

[achris]

Re: 1979 Mercruiser GM120 upgrades?

The engine mounting positions are exactly the same for the 2.5 (153) and the 3.0(181). A lot of the parts will move across too. What you'll need to buy new are (not a comprehensive list) exhaust manifold and elbow. I'm not sure if the carb will be just a re-jet or a bigger unit. Most of the ignition system will move, but you'll want to check the advance curve between the two engines. Other than that the starter and alternator should move ok, as should all the mounting gear, flywheel housing and coupler. The wiring harness should be the same also.

Chris.............

 

[dbraymer]

Re: 1979 Mercruiser GM120 upgrades?

Thanks. The old exhaust manifold and the elbow are two of the things I was worried about anyway. If I break something monkeying around with them, I now know the next step!

 

[Haut Medoc]

Re: 1979 Mercruiser GM120 upgrades?

Yep, you'll be good for 140 HP....
Welcome to iboats, btw......​