Motor rebuild and regear

I have a 2002 Monterey with a 5.0 Mercruiser and alpha one drive. It has 300 hrs on it and I am going to tear it down take it to the machine shop do head cam and bore on the block and head work. So my question is do I need to re gear the drive? I'm not going extremely excessive but it will be way past stock numbers. Thanks for any info!!

Re: Motor rebuild and regear

With a low hour motor why not sell it and get a 350ci? As for re-gear I doubt it. Might have to re-prop though.

Re: Motor rebuild and regear

Personally it's too much hassle. I have a running account at the machine shop for my race motors and I can drop it off and pick it up in a few days. This was my first year boat and was very very pleased with boat and motor. Not one single mechanical issue what so ever. But I'm a gear head and want to hot rod it a little. If I could get a fair price for the motor and the same for buying another I would. It's amazing how much this stuff goes for just because it says marine on it. Lol. That's the biggest issue. The wife would kill me if I dumped to much into this toy.

Re: Motor rebuild and regear

I know this doesn't specifically answer your question, but if you look into past 305 vs 350 threads I believe you will see the concensious is that a 305 is not worth messing with vs a 350.

Re: Motor rebuild and regear

Get a 350 truck engine and throw all the 305 marine bits on it... Job done. The basic block for a 350 and a 305 are one and the same. Look in the Merc parts catalogue and you'll find that the 305 and 350 even run the same camshaft! If you want to 'hotrod' it, be aware of the pitfalls.

1. More top end power usually means less low end torque. In a boat that's a bad idea. Unlike your cars, a boat has no gearbox with selectable gears on the fly. You need the low end torque to push you up and out of the 'hole'. If it's not there, the boat is a dog to drive.
2. More power can also trade reliability. More stress on parts ect.

Better than producing more horsepower, try and get more torque (and a 350 is fantastic at that), particularly low end... You find the boat even better then.

As long as you don't go over 300hp, then the drive will be fine. It looks like the drive ratio for the 305 and the 350 are the same (1.47:1), so all you'd need to do is change propeller.

Chris........

Re: Motor rebuild and regear

If the cam is radical you can have problems with water reversion although there are anti-reversion options to help prevent this. I would get a marine 350 and be done.

Re: Motor rebuild and regear

I like loud, fast boats as much as the next guy...but...

I'd leave it alone. "Hot rodding" engines doesn't translate well from the track to the water; the parameters are different. As someone mentioned, you need to build torque in a boat and there is no substitute for cubic inches. Couple that to the fact your boat isn't meant for (or maybe even safe for) higher speeds and you're really taking some unnecessary chances here with little chance for any real improvement in performance.

Reliability is critical. Unlike a race car, you can't just push it off the line when it breaks. The 5.0 is a very reliable powerplant when left alone....not so much when people try to tweak it.

If you want a faster boat, buy a faster boat. If you still want to hot rod it, be prepared to bleed money to do it right. For example, marine headers are somewhere in the neighborhood of $5k/set. Your wife (and your wallet) won't be happy.

....To answer your question: it isn't likely you're going to build enough power at 5k rpm to necessitate a gearing change, but even if you did that's an easy/cheap fix compared to what you'll spend getting there.

Re: Motor rebuild and regear

Montereyjohn02 said:

I have a 2002 Monterey with a 5.0 Mercruiser and alpha one drive. It has 300 hrs on it and I am going to tear it down take it to the machine shop do head cam and bore on the block and head work. So my question is do I need to re gear the drive? I'm not going extremely excessive but it will be way past stock numbers. Thanks for any info!!

Click to expand...

Ayuh,.... Welcome Aboard,... I truly doubt it,...
A 305 can't be built to build enough power to call for a re-gearin'...

I'd Guess ya got a 1.5:1 ratio now, which is as tall a gear ratio as you can get anyways...

Just what have ya got in mind that'll " be way past stock numbers",..??..??

Most automotive Hot Roddin' ideas don't work so well with a boat motor,...
What ya really need is a Stump pullin' Truck motor...
Big torque, down low, 'n outa power at 5000 rpms...

Re: Motor rebuild and regear

sell the 305 and drop in a 350

Re: Motor rebuild and regear

You can't do a 350 if it's EFI without injectors and an ecm change. Carbs can be updated easily, the primary jets are a tiny bit bigger but the other stuff is as on the 305. A 350 would be a nice torquer. On the other hand if you are going to do an upgrade just do a 383 stroker with a marine cam and you have maxed out what can be done with it.