Rebuilding a Mercruiser 470 for performance?

[newbster]

Is it a good idea? I have a 170hp 2 brl in a 26' Starcraft Islander. It weighs in around 4k or so and is underpowered. Since I just bought the boat I have not had the chance to get it out but a compression test shows 80lbs in the #1. I'm thinking of rebuilding the motor boring 90 over and putting a performance aluminum head in it. Is this a good idea? If I do the 90 over what displacement will it be? I'm hoping to milk upwards of 200 HP out of the motor but more is better since the boat is rated for 270.

I'm looking for the best path to something in the 200-225 HP range while minimizing weight and maximizing efficiency. I'm comparing my cost/benefit of redoing this motor, replacing it or just going with an offshore bracket and outboard. The last option being the most desirable but it would probably be around 20k of which most of the cost I will never recover in resale.

What can I milk out of the 470 block and is it worth it? The cheapest way out would to maximize what I have and overcome the 470 weaknesses by replacing those components.

Ideas?

 

[Don S]

Re: Rebuilding a Mercruiser 470 for performance?

Trying to keep those engine together stock is a challange.
If you want to make a performance engine, go for a Small Block Chevy, not a 470.

 

[newbster]

Re: Rebuilding a Mercruiser 470 for performance?

Are there any good light weight options? IE aluminum..? I'm looking for good power to weight ratio. What can be attached to an Alpha One drive?

 

[180shabah]

Re: Rebuilding a Mercruiser 470 for performance?

The alpha drive is good for up to 300hp. Don't worry about the weight too much, a small block V8 is no big deal in a 26' boat.

 

[Bondo]

Re: Rebuilding a Mercruiser 470 for performance?

Ayuh,........

A 4.3l Chevy will give you the 200hp your looking for,+ a slight Weight savings over the SBC......
And,......
You'll be able to reuse your Outdrive, without Regearing it...........

 

[skeat]

Re: Rebuilding a Mercruiser 470 for performance?

aluminum heads are expensive and probably not going to give you the result you are looking for. First thing is to make sure the engine is healthy and running well. Putting performance parts on a tired moter is a waste of time. If all is good try finding a 4bbl carb and manifold from another 4.3, that should be a noticeable power increase over the 2 bbl. The other limiting factor is the cam. Nothing will change a motor's characteristics as much as different cam profiles, but again the rest of the valvetrain has to be in good shape.

 

[JustJason]

Re: Rebuilding a Mercruiser 470 for performance?

your low compression is more than likely just a valve adjustment.... do that and then recheck it

 

[180shabah]

Re: Rebuilding a Mercruiser 470 for performance?

Skeat, he doesn't have a 4.3, he has a 3.7L 4 cylinder.

 

[newbster]

Re: Rebuilding a Mercruiser 470 for performance?

I purchased the boat from a distance and paid a marina to check it out. I just did a compression check and came up with
1=90
2=145
3=125
4=125

Pouring some 10-40 and repeating on one brought it up to 120 lbs so it seems like a busted ring and the marina screwed me for a motor.

Now I'm stuck trying to decide the best course going forward. The boat is rated for up to 270 HP. Its 26' and about 4-4.5k loaded, 3k empty is list. I have not even got it out on a test run but should expect around 30mph max speed which is a bit low. I want to use the boat for fishing in the Puget Sound and offshore ocean so extra power can be helpful. I think my cheapest way out is to bore the motor .90 over to add cubes and figure out a better cam/head/intake solution. Any clue on what a .90 overbore will add for cubes? This is all fine if the mods I do fix the known 470 issues.

Other options would include another motor such as a 4.3 or maybe bigger? I want to minimize weight as practically possible as I'm trailering a lot and wish to save fuel costs. I'm assuming that I'll get about the same mileage out of a bigger motor at the same speed.

Last option is to replace with an offshore bracket and outboard but outboard prices are scary. They do currently make the 22' version of the Islander with a bracket/outboard so I would copy that design.

Thoughts?

 

[180shabah]

Re: Rebuilding a Mercruiser 470 for performance?

This is all fine if the mods I do fix the known 470 issues.

The majority of 470 issues came from the unusual water pump/cam seal design and the outboard style charging system.

If you like the boat, plan to keep it for a while, and would like to be able to sell it when you are done, the 4.3 swap would be worth the effort. Anywhere from 160 to 250hp is possible depending one the year of the block and components used. A v8 is capable of alot more(only a 100lb difference), just remember to stay under 300hp to protect your drive. Also, aluminum exhaust and intake manifolds can save a bunch of weight.

BUT

The total weight difference from 470 to all iron SBC is only a few hundred pounds, it will not make a noticable difference in your towing MPG. Have you ever noticed how much gas you can save by driving around with the gas tank at 1/4 or less, not carying the weight of a full tank.

It is a non issue.

 

[skeat]

Re: Rebuilding a Mercruiser 470 for performance?

Sorry about that, my misunderstanding. From the previous post I thought that's what we were talking about. That's alot of boat for a 4cyl and 170 hp is pretty respectable for a little motor. I think it's time to repower with a bigger engine.

 

[rodbolt]

Re: Rebuilding a Mercruiser 470 for performance?

that motor is a bit tricky, carefully study the service mnual on valve adjustments. ie they aint really adjustable however they did make several pushrod lengths.
thats why its imperative on that engine to know exactly what your doing or it will die.
the head/block and intake joint also has to be aligned correctly at reassembly.
if you dont have the 4" heat exchanger then its a lost cause anyway.
the open deck block design was **** poor at best.
I dont think merc offered any thing larger than a .060" oversize and aftermarket stuff is mostly non-existant for the 3.7 4cyl.
while its true a few 470's lived, some of techs actually lived the nightmare of the MCM 470.
toss the compression tester and have a leak down test run. that motor had a tendancy to trash headgaskets.

 

[Skid-joe]

Re: Rebuilding a Mercruiser 470 for performance?

To answer your "how many cubes do i get with .090 oversize".

You will only gain about 2 to 3 cubic inches. Hardly the reason for doing it.

If you resize, it is because the engine is now to loose..............

Hope this answers your question...................joe