Hello all. This is a question for the techey engine flow experts. The application is a stock ZZ-4 with Crane roller rockers installed in a 24' SkipJack boat. The engine is connected to a Mercruiser Bravo-II outdrive with the stock water cooled exhaust that is pretty restrictive. The exhaust alternative of thru-transom is not an option..... to loud. The induction system is a larger Q-jet and a reasonable cool air intake. The RPM operating range is 2600-5000. Currently with a set of Fast Burns installed max RPM is 4710 and top speed of 44.1...... not bad for this heavy boat. The option is to install a new set of ZZ-3 heads (same as ZZ-4 but have the orange springs). My theory is that the smaller valves and smaller intake runner along with a slightly smaller chamber of the ZZ-3 head will improve velocity and burn at my RPM range. Thus improving torque fuel mileage and perhaps HP at this under 5000 RPM application. What do you guys think?
Thanks for the input,
JR  

I think, get a 454. But if that's out of the question smaller ports will stimulate higher low end velocities for more torque. Horsepower is really unimportant in your case, moving that heavy boat requires lots of torque to get on plane quickly and keep it there. Smaller comb. chambers equal more cylinder pressure which equals more torque. I fully agree about the stock manifolds and elbows being very restictive. Have you thought of using mufflers with a thru-transom exhaust? You would still need to use a set of water jaketed headers to make it worth your while. Retaining the stock exhaust and not useing the thru-transom is actually in your favor. Its said that the vortex of the prop helps to evacuate the exhaust system.  

Thanks for the reply. A BB just won't fit in the engine compartment...... or it would be there. Truth is, the SB does a pretty good job. I think you are correct. The smaller heads just might give her a little more torque and equate to better fuel economy at cruise..... about 3000 RPM. I was concerned about the top end. 1 MPH can mean the difference of having to buy the beers or enjoying the fruits of the last 1/2 mile run in. Use to have the wet thru transom with muffelers. Sounds really good..... except when you have to listen to it 12 hrs a day while trolling for billfish. The current exhaust uses a collar affair and not a solid hose to attach to the exhaust to the outdrive leg. Merc claims it is better but you eliminate the vacume effect of the prop vortex. However, I can re-install the flex hose to hook it back up. That will make for an interesting comparison test. May try it next week prior to any serious rework.
thanks again for your comments.
JR  

The smaller valves may equate to more fuel economy depending on engine load but they surely won't mean more torque.

A heavy deciding factor of output is volumetric efficiency or how much air (and properly mixed fuel) we can cram into the cylinder per cycle. Since these engines are already severely undervalved reducing the valve size more will only inhibit cylinder filling, So...

If you're talking about midrange power under wide-open throttle conditions making the induction smaller may not be such a good idea.  

Gib is right about the smaller valves being restrictive. Larger valves wont hurt torque and will help in the mid and top end.