Suggestions on rebiulding my (79) 260 Merc to get more HP.

I have a (79) 260 Merc and want to build it for more HP to push my 23' Nova Wellcraft. I have read some of the comments using Vortec head. Not sure that I want to do that. How much will the stock heads effect to outcome?

No substitute for cubic inches when it comes to pushing a boat...I'd suggest you look into getting a 502 crate engine if you are serious.

The Vortec chambers will certainly help, but they will perform best when accompanied by the correct piston profile
The full dished piston can not offer this.

Build yourself a good quench style combustion chamber, and pull some torque from this engine.
Or.... turn this into a 6.3L (383 cu in) with a quench style combustion chamber.

However, and you may or may not know this, but with the A drive, you are limited to approx 300 hp and/or up to a 26 foot hull, so the Merc experts say.

FWIW, I've done several Merc A drives to Volvo Penta Duo Prop conversions over the years.
Quite honestly, the V/P Duo Prop performance is similar to that of an additional 80 or so horse power in terms of over-all performance.
Now you can have both your increased HP and Torque without "A" drive worries.
Plus.... no more Dog Clutch gear engagement.

Plus, it's more fun when spending other people's money!

Just a suggestion!

.

*Well I just got back on here. Sorry for the delay.
I tore down the 260 Merc and found too many problems to worry about it at this time. I also tore down another 350 small block motor that I had, It has a small knock in it. I found that it already has a .30 over bore. Heads looked good, but need to have them checked. I'm not sure of the size of crank or cam, but I know that crank needs to be turned or replaced.
Right now I'm looking at using two engines to make one good one. Of course with the proper machine work done. I keep hearing of these quench pistons. I guess there is alot of opinions on these pistions. Why is the deep dish better than a flat top. I'm assuming it's a better swirl cup.
I'm going to port and polish the intake and exhaust. Most likely open up the exhaust. Don't know what to do with the cam yet. Mainly because I don't know what it has in it. What size cam is in a 383, Marine motor? I know I going to stay with the stock lifters. Might put it roller rockers. I have a high rise eddlebrock torquer intake. Don't know how good this intake is or isn't. Right now full of more questions than answers....

If this was a smaller and lighter weight hull, we could possibly get away with a few more Non-Cruiser type build techniques.
However......, with your 23 foot hull....... we'd best stay with a Cruiser type build...... so that's what I'd recommend for you!

Danger136 said:

I found that it already has a .30 over bore.
Have your machine shop check the cylinders. If they make 4.030", you may get by with simply cleaning them up.

Heads looked good, but need to have them checked.
Again, regarding the combustion chamber volume, these must compliment the piston profile, or visa-versa.
IOW, I'd not assume that the last build was Marine Correct!
Check casting numbers!

I'm not sure of the size of crank or cam, but I know that crank needs to be turned or replaced.
Nothing wrong with a turned crankshaft!

I keep hearing of these quench pistons. I guess there is alot of opinions on these pistions.
Yes, lots of opinions I suppose.
Do some homework and reserch, and learn what the Quench Effect or Squish Zone accomplishes in the SBC.
If you find some good articles, they'll even go into details on Detonation control, increased Ignition Advance, how this affects LPCP, low end torque, etc...... all of which is good stuff that the full dished piston just cannot accomplish.

The quench or squish is nothing new with any engine whereby a portion of the chamber is wedge shaped.
The Chrysler Wedge Head boys have been doing this for years!
The original 1955 265 cu in GM SBC engine was designed using a flat top piston coming up under the wedge area of the cylinder head, which gave this engine a quench area.

In the 70's, GM screwed this all up by going to the full dished piston.... and they continue doing it today for the GM SBC line that we see in our boats!
Machine shops are so familiar with seeing this silly piston, that most don't even question it..... let alone know about the Q/E or S/Z.
Some will even give you that "deer in the headlights" look when you ask about a quench build!
If the customer does not specify that they want a Q/E build.......... then before you know it, they've grabbed new full dished pistons, and your build end up being no better than what GM did originally.
Now we're back to using a very conversative Ignition Advance (as to throat off "Detonation Potential"), our LPCP ends up being on the weak side, and we don't get the low end torque potential, etc. etc.

After you research this.... I'll offer a Gentleman's Wager that you become on board with the concept.


Why is the deep dish better than a flat top. I'm assuming it's a better swirl cup.
Again, it's the piston deck profile that we want to "mirror" the wedge area of the combusion chamber. When this deck area mirrors the wedge, and when we bring this dimension into approximately .038" or so...., we gain the "quench" or "squish" that we are looking for.
IOW..... we do not want any dished area underneath the wedge section.... the full dish piston cannot possibly create a quench/squish.

• The F/T piston works well in a 5.7L with the 76cc chamber heads.
• The low compression Quench, D-dish, or a Reverse Dome piston (with the correct dish volume) works well with the smaller 64cc chambers.
• The full dished piston is the piston that we should avoid..... especially in a Marine build.

Try not to confuse the "dish" of the full dished piston, with that of a quench style piston "dish" volume.
Apples/Oranges!
The dish volume (in either style piston) is simply there to control compression ratio.



Don't know what to do with the cam yet. Mainly because I don't know what it has in it. What size cam is in a 383, Marine motor?
Deside upon your build, and then call someone like Comp Cams for a suggestion.
These people know better than us as to what profile to use.

I have a high rise eddlebrock torquer intake. Don't know how good this intake is or isn't.
The Torker is a single plane manifold, and would not be a good choice for a cruiser style build.
A good Dual Plane intake is what you'll want, IMO.

Also..... if this is Raw Water Cooled, the aluminum Edelbrock automotive manifold will eventually fail.

This is a single plane intake manifold:

Click to expand...

Have fun..... make good parts selections..... and you won't regret it!



.

Rick
Well, thank you for your help. I've desided to buy the Vortec heads, but I'm going to have to find out what the valve sizes are. I'm not sure.... 1.94 intake and 1.5 exhaust? with what chamber 76cc? I'm looking into a .450cam. The pistons that are in it now are 4.030 Flat tops and in good shape. So, this is a good thing. Going to look into the other pistons. I'm going to have the engine line bored and the clyinders de-glazed and block decked. I guess this will give me a good start at putting it back together. Not going to build a 383. (?) I think I going to put it back together with the stock crank. So, the build would be as such.
vortec heads, .450cam, .030 over pistons, roller rockers and stock crank. How does this sound to you? I'm thinking it would put me in the low 300hp with a good torque rating...????? Thoughts????

Danger136 said:

Rick
Well, thank you for your help. I've desided to buy the Vortec heads, but I'm going to have to find out what the valve sizes are. I'm not sure.... 1.94 intake and 1.5 exhaust?

1... with what chamber 76cc?

2... I'm looking into a .450cam.


3... The pistons that are in it now are 4.030 Flat tops and in good shape. So, this is a good thing.


4... Going to look into the other pistons.


5... I'm going to have the engine line bored and the clyinders de-glazed and block decked.

Click to expand...

1... The GM Vortec combustion chambers are 64cc, and you may find some at 65cc.
Check the casting numbers.

2... The lift is only one aspect of the cam profile.


3... I have to say NO..... not with the Vortec 64cc chambers.
Go to this C/R Calculator, and plug in your numbers.
Be sure to include the correct compressed head gasket thickness and your piston deck height (the quench target for a Marine SBC can be .038")
Vary the dish volume for a Quench Effect piston, and see what C/R you come up with.
Example: if you use a cmp h/g thickness of .022", a piston deck height of -.016", a dish volume of -16cc..... you'll come up with a static c/r of approx 8.5:1.

NOTE: with a Q/E build, the C/R can be a tad bit higher (than that of a full dished piston build) without detonation concerns.
It's the quench/squish that allows for this, and is what makes better low end torque when the ignition TA is correct.


4... You can find a good quench style piston that nicely mirrors the pointed wedge area of the Vortec chamber.
You then select the dish volume for your desired C/R.

(example only.... you won't want the short skirts or raise pin)



5... If cutting these with a hone does it, then you will be OK.
If not, you'll be looking at a .040" over bore, or a fresh block core.

***************************

Here is a D-dish piston suitable for the Non-Vortec cylinder heads.
You can see that the larger piston deck mirrors the cylinder head wedge area.
(example image only)




See piston in #4 above, and the shape of the Vortec cylinder head wedge area below.






Here is the piston to avoid.
While the dish volume brings the C/R into spec, the full dished piston simply cannot offer a quench or squish area.





******************************

This change or modification does not need to break the boat bank.
Do your research, and you can find an affordable quench style piston set.

NOTE:
When you go to a Quench Style piston, due to chamber configuration, piston deck shape, Right/Left bank wrist pin offset, you are now required to use 4 p/n's. The person helping you will explain this.
(the full dished piston allows one p/n to be used for all 8 cylinders... a cheezy means of keeping cost down)



.