Install Larger Engine on Merc Stern drives ?

[ritkecek]

Hi all..
noob here....
New to site and fairly new to boating. Live in Colorado.
From what I've read so far, this looks like a knowledgeable & interesting bunch.
I look forward to learning a lot.

Just bought (cheap) 1974 Starcraft 18' with 120 or 140 IL4(can't tell, yet) Mercruiser sterndrive.
Is it feasable / easy to install small block chevy engines on these drives.
What are true maximum HP ratings of these drives ?
I've yet to put boat in water. Ordering manual. I feel that I should go through and check most everything out , first. I'm trying to pick up, from your site, what all I should check/test/get into... the first I've found is slack in the steering. I'm assumming I'll find a lot more. LOL
I might find that existing engine is powerfull enough ?? nah !!! LOL.
Look forward to it.
thanks in advance for any and all ideas
Kit (Old Fart)

 

[180shabah]

Re: Install Larger Engine on Merc Stern drives ?

First - Welcome to iboats.

Can you put an SBC in front of your pre-alpha?

Yes and No.

You currently have a 2:1 gear ratio, a v8 would require changing the drive(or atleast the upper half of it) to a 1.5:1 ratio. The current alphas are rated for 300hp, and the many pre alphas had 260hp 350's in front of them, so they will hold up(provided the correct gear ratio is used). There is also the matter of mounting the engine. Your 4 cyl has a front mount, while the V engines use side mounts. More than likely you will need to fabricate and glass in a new engine bed to support a v8.

All in all there will be alot of time and effort involved here as well as a small fortune spent, that you will never recover if you sell the boat.

Unless there is an attachment to this boat, or it is a rare beast, then typically the best approach is to sell it and find something that is powered the way you want.

 

[Chris1956]

Re: Install Larger Engine on Merc Stern drives ?

Old, The previous poster pointed out the problems installing a SBC into that rig. You can get around some of the issues easily. For example, there are a lot of prop pitches available for that outdrive. You may be able to get a new prop with the correct pitch to compensate for the increased power. The motor mount issue is more complicated. You need to measure the distance from the front mount to the outdrive to see if the SBC will fit in that space. Likely a 4.3LX V6 (205HP) motor could fit. You will need to fabricate side motor mounts to hold the V6. The Alpha outdrive is OK for up to 300HP, so that is not an issue. You may need a new outdrive coupler.

 

[Haut Medoc]

Re: Install Larger Engine on Merc Stern drives ?

First - Welcome to iboats.

Can you put an SBC in front of your pre-alpha?

Yes and No.

You currently have a 2:1 gear ratio, a v8 would require changing the drive(or atleast the upper half of it) to a 1.5:1 ratio. The current alphas are rated for 300hp, and the many pre alphas had 260hp 350's in front of them, so they will hold up(provided the correct gear ratio is used). There is also the matter of mounting the engine. Your 4 cyl has a front mount, while the V engines use side mounts. More than likely you will need to fabricate and glass in a new engine bed to support a v8.

All in all there will be alot of time and effort involved here as well as a small fortune spent, that you will never recover if you sell the boat.

Unless there is an attachment to this boat, or it is a rare beast, then typically the best approach is to sell it and find something that is powered the way you want.

What he said.....^^^^^
Not worth the money or effort IMHO.....
Nice avatar!......
Welcome to iboats, btw.....

 

[Bondo]

Re: Install Larger Engine on Merc Stern drives ?

First - Welcome to iboats.

Can you put an SBC in front of your pre-alpha?

Yes and No.

You currently have a 2:1 gear ratio, a v8 would require changing the drive(or atleast the upper half of it) to a 1.5:1 ratio. The current alphas are rated for 300hp, and the many pre alphas had 260hp 350's in front of them, so they will hold up(provided the correct gear ratio is used). There is also the matter of mounting the engine. Your 4 cyl has a front mount, while the V engines use side mounts. More than likely you will need to fabricate and glass in a new engine bed to support a v8.

All in all there will be alot of time and effort involved here as well as a small fortune spent, that you will never recover if you sell the boat.

Unless there is an attachment to this boat, or it is a rare beast, then typically the best approach is to sell it and find something that is powered the way you want.

Ayuh,........... What He said,..........
You'd need a 40somethingish" prop to overcome the gear ratio issue......

What he said.....^^^^^
Not worth the money or effort IMHO.....
Nice avatar!......
Welcome to iboats, btw.....

And what He said too,........................

 

[ritkecek]

Re: Install Larger Engine on Merc Stern drives ?

Thanks all for input.
I'll have to ponder.
I have to build a new cover, anyway.. so... space is not an issue.
Motor mounts should be easy.... and ...
....the engine is just begging to be put to work.
If just changing prop ( changing gear ratio sounds like good money into an old beast) and coupler were the main remaining issues...
.....not to bad.... but ....
maybe it'll just be fine, just like it is. LOL. A lot to be said for being lazy and just going fishing.
..... It would be nice to go skiing with it, though.....
......Heck... I might find that 120-140 horse is enough, anyway.
Thanks again
Kit (Old Fart)

 

[Don S]

Re: Install Larger Engine on Merc Stern drives ?

If just changing prop ( changing gear ratio sounds like good money into an old beast) and coupler were the main remaining issues...

You are not going to find a prop that will prop that ratio for a V8. You might find some specialty one that will keep the rpm in the proper range, but it sure won't make the boat run normal. The blade pitch would be so steep as to kill off any hole shot. Of course if you ever did get it moving, it would really go fast.

 

[Bry21317]

Re: Install Larger Engine on Merc Stern drives ?

I have a friend that has the same 4 cyl Merc Engine on a 17 foot Invader. It will run in the low 40s fully loaded. He can keep up with my 19 foot Crownline with a V8 305. Now I can still smoke him, but he can hang in a 4 cyl, so its all about how that little motor is geared to give you the performance.

I can run in the low 50s, but my cruise speed is about 35mph, and his is more like 20 something. So when we are both trying to run our cruise speeds, I will leave him sitting and then have to stop to wait for him.

Bryan

 

[heyttown]

Re: Install Larger Engine on Merc Stern drives ?

I have a small 4 cyl 3.0 mercruiser in my 18' boat...Has no problem at all with the water toys....Im a big guy and I tubed my buddy who was around 300lbs,plus a spotter in the boat....Im perfectly happy with my little gas saver.

I say clean it up,get it running good and go see what she has....You may be suprised..

Top speed is about 42 on the dial, but a good cruising speed like mentioned above is the mid 20's

 

[ziggy]

Re: Install Larger Engine on Merc Stern drives ?

this question seems to be ask a lot.....moveing up to a larger engine......
it sounds like a lota work to me.....i'd rather fish. and i don't fish........never catch anything....

it's also an old boat. it may have issues that ya aint aware of yet..... that'll keep ya busy if ya do..

imho. i'd work on getting what ya got in fine tune..... it's a drag workin on an i/o and ya may find ya have yer hands full as is.....i have my hands full trying to keep on top of mine and it stock.... moneywise and skill to repair.. both of which are nessessary.....

i'd think that 4 cyl. engine would do ya watersking too. just don't load her up with a bunch of buddys and beverages.....

have fun with what ya got. any boat is better than no boat if it gets ya on the water....hang on 'the' board and learn learn learn......then enjoy, enjoy, enjoy.........

i'm in the same boat as Bry21317 talking about. my buddys all got faster boats...... they just have to wait for me.......oh well, they all like blowin by me like i'm not moveing. makes them feel good i think and it don't bother me... cause my boats cooler than theres are, slow or not.... if 43 is slow?

 

[fyrfightr]

Re: Install Larger Engine on Merc Stern drives ?

My first question is, What type hull is it? Aluminum or fiberglass. Next is engine size? I almost bought a late 70s Aluminum Starcraft with a 170hp 4cyl. This boat was a high 50s mph on gps. I would think that if the boat you bought is aluminum even with a 120hp, it will be a good running hull...

 

[ritkecek]

Re: Install Larger Engine on Merc Stern drives ?

Thanks again all... for the input.
Ima clean it up and see what I've got.
You're right... it might be perfect.

The wife and I don't have a lot of friends with boats, to try to keep up with, anyway. LOL
And It'll spent WAY more time fishing and exploring than skiing.

It's a fiberglass tri-hull. I'm guessing it's fairly heavy for size ?
Can't nail down the engine, (Seloc manual is ordered and on the way) exactly.. but... 120-140 hp.

... besides... there might be a bit of tuning/additions/etc.. to make the 4cyl a bit more productive. The exhaust systems look a bit limiting, though. I'm just ignorant of available options in boat engines... obviously.

thanks all
Kit (Old Fart)

 

[ziggy]

Re: Install Larger Engine on Merc Stern drives ?

(Seloc manual is ordered and on the way)

get the real mccoy

it's free

looks like ya need service manual #2. it's a big file 1.08 gb....be patient, it'll load......

"http://www-alt.mercurymarine.com/mnetdata/sisdata/srvcovr.pdf"

 

[whywhyzed]

Re: Install Larger Engine on Merc Stern drives ?

to tell a 120 from a 140:

the 120 has only a pulley bolted to the front of the crankshaft, whereas a 140 has a vibration dampener with the pulley