Higher compression rate on a ford 351W

Hello folks!
I Have a 20 feet boat with a Mercury ford 351W engine. It had a water leakage problem with exhaust manifold witch spilled water on the oil and now it needs a complete overhaul.
I am considering using hipereutetic pistons with a smaller recess (-12cc , instead of -28cc). That is going to give me a 9.8 compression rate, instead of the original 8.0 CR. I think this will increase engine torque and give me a better fuel efficiency. Am i right? Is there any specific problems with increasing CR on a boat engine, besides requiring higher octane gas ? Any other tips on overhauling this engine??
Thanks!!!!!!!!

 

Increasing C.R. will have minimal effect - perhaps 10 to 15 HP at best. This translates to probably 1 to 3 mph with premium fuel.

In order to get apprecible power, you need to also change Heads (larger ports & valves), Intake manifold & carb,as well as Cam with More lift - not necessarily more duration. You need the Truck like characteristics of the marine cam.

All of this might get you 80 -100Hp additional, perhaps 8 -10 mph at the expense of Engine durability.

 

A 9.8 comp ratio is really dependent upon the quality and octane rating of the gas that you use. Much over 10 and you start to need 91 octane (which also depends on a whole host of other factors - cam timing, manifold, tuning, advance, etc.)

Won't yield enough HP to really matter. Better off sticking with stock specs, IMHO.

 

There is a huge difference between 9.8 and 8.0 CR
Work with a competent engine builder and you will be amazed at the power increase. Cost won't be too bad. I just had a 350 Chevy rebuilt, dart iron eagle heads, bored for new pistons and a custom ground cam from comp cams. Whole thing assembled with marine gaskets was $2500.

 

I'm going to say this with carefullness, you can run 8.5:1 just as easily as you can 11:1 compression. Ford Windors are my specialty infact thats all I have built. I have done blown, nitrous, strokers all in the windsor family. The key here, let me say it again the "KEY" is to build a quench motor in the .034 to .035. This requires building the motor usually with pistons .006inch over deck height. An unquenched windsor will ping more at 9:1 then it will at 11:1 having a proper quench. The theory comes into play that by using tighter clearances between the piston top and bottom of the head that it creates air disturbances, with the additional air flow this creates in the combustion chamber it actually cools the piston surface and keeps hot spots from developing. This one of the main reasons that using shorter rods to lower compression ratio is not used. A few things to exclude are blowers, and turbo's motors as valve overlap causes cooling under forced induction. Here are some examples using 93 octane in automotive applications I would assume although I dont know for sure that with marine limitless access to fresh water cooling you could push the numbers a little higher

quenched at .034-.035
351w with aluminum heads 10.8:1
351w with iron heads 9.9:1
non quenched
351w aluminum 10.2:1
351w iron 9.5:1

 

You can run the clearance up to abt .050 and still get the benefits of the "Quench", but .035 is better unless you're in the 7000+ RPM range.

Also No need to "deck" the block in order to get positive Deck Height. I use Solid Soft Copper head gaskets, .021 thk. from Jegs.

Brainsboy is exactly right.

 

Thanks Brainsboy and Jango!!!

Actually i have already purchased my pistons, they are keith Black hipereuthetic , -19cc dish, with iron heads, 57cc combustion chamber, that will give me a 9.5 compression rate. But i can yet take my block to the machine shop for modifications that would help me.
I am sorry for my lack of knowledge, but how do you measure the "Quench"?
Is it the distance between the deck surface and the top of the piston, or is it the distance between the botton of the head and the top of piston, including head gasket height?
The head gasket , by itself, would measure 0,035 in after torqued.
Regards!!!!

 

Quench is the distance from the Flat part of the piston to the Cylinder head. It is measured by adding the gasket thickness to the Deck Height (distance FLAT part of piston is below or above the block surface)
The Deck height is a positive no. when the piston is below and negative when it is above the block surface.
Because Head gasgets are normally thicker than the desired Quench, It is often necessary to remove material from the block surfaces in order to cause a "negative Deck height" -or simply getting the top of the piston ABOVE the block surface.

The deck height of KB Pistons are normally .010 to .015 assuming your block has the stock 9.503 height. (Stroke/2 + connecting rod length + Piston compression height = distance to top of piston from crank CL) Subtract this total from 9.503 and you will have Deck height.

With a 9.5 : 1 comp ratio, I believe a quench of up to .050 will be adequate for use with reg. 87 - 89 oct. fuel.

 

before you put all that money in your windsor why not find a cleveland? much better engine,,longliner